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In the previous unit you have learnt that the element carbon has the unique property called catenation due to which it forms covalent bonds with other carbon atoms. It also forms covalent bonds with atoms of other elements like hydrogen, oxygen, nitrogen, sulphur, phosphorus and halogens. The resulting compounds are studied under a separate branch of chemistry called organic chemistry. This unit incorporates some basic principles and techniques of analysis required for understanding the formation and properties of organic compounds.

GENERAL INTRODUCTION

Organic compounds are vital for sustaining life on earth and include complex molecules like genetic information bearing deoxyribonucleic acid (DNA) and proteins that constitute essential compounds of our blood, muscles and skin. Organic compounds appear in materials like clothing, fuels, polymers, dyes and medicines. These are some of the important areas of application of these compounds.

Science of organic chemistry is about two hundred years old. Around the year 1780, chemists began to distinguish between organic compounds obtained from plants and animals and inorganic compounds prepared from mineral sources. Berzilius, a Swedish chemist proposed that a ‘vital force’ was responsible for the formation of organic compounds. However, this notion was rejected in 1828 when F. Wohler synthesised an organic compound, urea from an inorganic compound, ammonium cyanate.

Ammonium cyanate               Urea

The pioneering synthesis of acetic acid by Kolbe (1845) and that of methane by Berthelot (1856) showed conclusively that organic compounds could be synthesised from inorganic sources in a laboratory.

The development of electronic theory of covalent bonding ushered organic chemistry into its modern shape.

TETRAVALENCE OF CARBON: SHAPES OF ORGANIC COMPOUNDS

THE Shapes of Carbon Compounds

The knowledge of fundamental concepts of molecular structure helps in understanding and predicting the properties of organic compounds. You have already learnt theories of valency and molecular structure in Unit 4. Also, you already know that tetravalence of carbon and the formation of covalent bonds by it are explained in terms of its electronic configuration and the hybridisation of s and p orbitals. It may be recalled that formation and the shapes of molecules like methane (CH₄), ethene (C₂H₄), ethyne (C₂H₂) are explained in terms of the use of sp³, sp² and sp hybrid orbitals by carbon atoms in the respective molecules.

Hybridisation influences the bond length and bond enthalpy (strength) in compounds. The sp hybrid orbital contains more s character and hence it is closer to its nucleus and forms shorter and stronger bonds than the sp³ hybrid orbital. The sp² hybrid orbital is intermediate in s character between sp and sp³ and, hence, the length and enthalpy of the bonds it forms, are also intermediate between them. The change in hybridisation affects the electronegativity of carbon. The greater the s character of the hybrid orbitals, the greater is the electronegativity. Thus, a carbon atom having an sp hybrid orbital with 50% s character is more electronegative than that possessing sp² or sp³ hybridised orbitals. This relative electronegativity is reflected in several physical and chemical properties of the molecules concerned, about which you will learn in later units.

Some Characteristic Features of \(\pi\)Bonds

In a \(\pi\) (pi) bond formation, parallel orientation of the two p orbitals on adjacent atoms is necessary for a proper sideways overlap.

Thus, in H₂C=CH₂ molecule all the atoms must be in the same plane. The p orbitals are mutually parallel and both the p orbitals are perpendicular to the plane of the molecule. Rotation of one CH₂ fragment with respect to other interferes with maximum overlap of p orbitals and, therefore, such rotation about carbon-carbon double bond (C=C) is restricted. The electron charge cloud of the \(\pi\) bond is located above and below the plane of bonding atoms. This results in the electrons being easily available to the attacking reagents. In general, \(\pi\) bonds provide the most reactive centres in the molecules containing multiple bonds.

PROBLEM 1 

How many \(\sigma\) and \(\pi\) bonds are present in each of the following molecules ?

(a)HC \(% MathType!MTEF!2!1!+- % feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeyyyIOlaaa!37BF! \equiv \) CCH =CHCH₃ (b)CH₂ = C = CHCH₃

SOLUTION

\(% MathType!MTEF!2!1!+- % feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGceaqabeaacaGGOa % GaamyyaiaacMcacaaMc8UaaGPaVlabeo8aZnaaBaaaleaacaWGdbGa % eyOeI0Iaam4qaaqabaGccaGG6aGaaGinaiaacUdacqaHdpWCdaWgaa % WcbaGaam4qaiabgkHiTiaadIeaaeqaaOGaaiOoaiaaiAdacaGG7aGa % eqiWda3aaSbaaSqaaiaadoeacqGH9aqpaeqaaOWaaSbaaSqaaiaado % eaaeqaaOGaaiOoaiaaigdacaGG7aGaeqiWdaNaam4qaiabggMi6kaa % doeacaGG6aGaaGOmaaqaaiaacIcacaWGIbGaaiykaiaaykW7caaMc8 % Uaeq4Wdm3aaSbaaSqaaiaadoeacqGHsislcaWGdbaabeaakiaacQda % caaIZaGaai4oaiabeo8aZnaaBaaaleaacaWGdbGaeyOeI0Iaamisaa % qabaGccaGG6aGaaGOnaiaacUdacqaHapaCdaWgaaWcbaGaam4qaiab % g2da9iaadoeaaeqaaOGaaiOoaiaaikdacaGGUaaaaaa!6FB5! \begin{array}{l} (a)\,\,{\sigma _{C - C}}:4;{\sigma _{C - H}}:6;{\pi _{C = }}_C:1;\pi C \equiv C:2\\ (b)\,\,{\sigma _{C - C}}:3;{\sigma _{C - H}}:6;{\pi _{C = C}}:2. \end{array}\)

PROBLEM 2

What is the type of hybridisation of each carbon in the following compounds ?) 

(a) CH₃Cl, (b) (CH₃) ₂CO, (c) CH₃CN,

(d) HCONH₂, (e) CH₃CH=CHCN

SOLUTION

(a) sp³ (b) sp³,sp²  (c) sp³, sp 

(d) sp²  (e) sp³, sp², sp

PROBLEM 3

Write the state of hybridisation  of carbon in the following compounds and shapes of each of the molecules.

(a) H₂C=O , (b) CH₃F, (c) HC\(% MathType!MTEF!2!1!+- % feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeyyyIOlaaa!37BF! \equiv \)N.

SOLUTION

(a) sp² hybridised carbon , trigonal planar; (b) sp³ bybridised carbon, tetrahedral; (c) sp hybridised carbon, linear.

STRUCTURAL REPRESENTATIONS OF ORGANIC COMPOUNDS

Complete, Condensed and Bond-line Structural Formulas

Structures of organic compounds are represented in several ways. The Lewis structure or dot structure, dash structure, condensed structure and bond line structural formulas are some of the specific types. The Lewis structures, however, can be simplified by representing the two-electron covalent bond by a dash (–). Such a structural formula focuses on the electrons involved in bond formation. A single dash represents a single bond, double dash is used for double bond and a triple dash represents triple bond. Lone- pairs of electrons on heteroatoms (e.g., oxygen, nitrogen, sulphur, halogens etc.) may or may not be shown. Thus, ethane (C₂H₆), ethene (C₂H₄), ethyne (C₂H₂) and methanol (CH₃OH) can be represented by the following structural  formulas. Such  structural representations are called complete structural formulas.

These structural formulas can be further abbreviated by omitting some or all of the dashes representing covalent bonds and by indicating the number of identical groups attached to an atom by a subscript. The resulting expression of the compound is called a condensed structural formula. Thus, ethane, ethene, ethyne and methanol can be written as:

CH₃CH₃        H₂C=CH₂       HC  \(% MathType!MTEF!2!1!+- % feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeyyyIOlaaa!37BF! \equiv \)CH          CH₃OH

Ethane          Ethene      Ethyne       Methanol

Similarly, CH₃CH₂CH₂CH₂CH₂CH₂CH₂CH₃ can be further condensed to CH₃(CH₂)6CH₃. For further simplification, organic chemists use  another way  of representing  the structures, in which only lines are used. In this bond-line structural representation of organic compounds, carbon and hydrogen atoms are not shown and the lines representing carbon-carbon bonds are drawn in a zig-zag fashion. The only atoms specifically written are oxygen, chlorine, nitrogen etc. The terminals denote methyl (–CH₃) groups (unless indicated otherwise by a functional group), while the line junctions denote carbon atoms bonded to appropriate number of hydrogens required to satisfy the valency of the carbon atoms. Some of the examples are represented as follows:

1. 3-Methyloctane can be represented in various forms as:

\(% MathType!MTEF!2!1!+- % feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaam4qaiaadI % eadaWgaaWcbaGaaG4maaqabaGccaWGdbGaamisamaaBaaaleaacaaI % YaaabeaakmaaxababaGaam4qaiaadIeaaSqaamaaxababaGaaiiFaa % adbaGaam4qaiaadIeadaWgaaqaaiaaiodaaeqaaaqabaaaleqaaOGa % am4qaiaadIeadaWgaaWcbaGaaGOmaaqabaGccaWGdbGaamisamaaBa % aaleaacaaIYaaabeaakiaadoeacaWGibWaaSbaaSqaaiaaikdaaeqa % aOGaam4qaiaadIeadaWgaaWcbaGaaGOmaaqabaGccaWGdbGaamisam % aaBaaaleaacaaIZaaabeaaaaa!4D2F! C{H_3}C{H_2}\mathop {CH}\limits_{\mathop |\limits_{C{H_3}} } C{H_2}C{H_2}C{H_2}C{H_2}C{H_3}\)

(b) 

(c)

(2) Various ways of representing 2-bromo butane are:

(a) CH₃CHBrCH₂CH₃ 

(b) 

(c) 

In cyclic compounds, the bond-line formulas may be given as follows:

PROBLEM 4

Expand each of the following condensed formulas into their complete structural formulas.

(a) CH₃CH₂COCH₂CH₃

(b) CH₃CH=CH(CH₂)₃CH₃

SOLUTION

(a)

(b)

PROBLEM 5

For each of the following compounds, write a condensed formula and also their bond-line formula.

(a) HOCH₂CH₂CH₂CH(CH₃)CH(CH₃)CH₃

(b) 

SOLUTION

Condensed formula:

(a) HO(CH₂)₃CH(CH₃)CH(CH₃)₂

(b) HOCH(CN)₂

Bond-line formula :

(a)

(b)

PROBLEM 6 

Expand each of the following bond - line formulas to show all the atoms including carbon and hydrogen.

(a) 

(b)

(c)

(d)

SOLUTION

 (a)

 (b)

(c)

(d)

Three-Dimensional Representation of Organic Molecules

The three-dimensional (3-D) structure of organic molecules can be represented on paper by using certain conventions. For example, by using solid  and dashed  wedge formula, the 3-D image of a molecule from a two-dimensional picture can be perceived. In these formulas the solid-wedge is used to indicate a bond projecting out of the plane of paper, towards the observer. The dashed-wedge is used to depict the bond projecting out of the plane of the paper and away from the observer. Wedges are shown in such a way that the broad end of the wedge is towards the observer. The bonds lying in plane of the paper are depicted by using a normal line (—). 3-D representation of methane molecule on paper has been shown in Fig. 12.1.

Fig. 12.1 Wedge-and-dash representation of CH₄.

CLASSIFICATION OF ORGANIC COMPOUNDS

The existing large number of organic compounds and their ever -increasing numbers has made it necessary to classify them on the basis of their structures. Organic compounds are broadly classified as follows:

1. Acyclic or open chain compounds

These compounds are also called as aliphatic compounds and consist of straight or branched chain compounds, for example:

2. Cyclic or closed chain or ring compounds

(a) Alicyclic compounds

Alicyclic (aliphatic cyclic) compounds contain carbon atoms joined in the form of a ring (homocyclic).

Cyclopropane  Cyclohexane     Cyclohexene

Sometimes atoms other than  carbon are also present in the ring (heterocylic). Tetrahydrofuran given below is an example of this type of compound:

Tetrahydrofuran

These exhibit some of the properties similar to those of aliphatic compounds.

(b) Aromatic compounds

Aromatic compounds are special types of compounds. You will learn about these compounds in detail in Unit 13. These include benzene and other related ring compounds (benzenoid). Like alicyclic compounds, aromatic comounds may also have hetero atom in the ring. Such compounds are called hetrocyclic aromatic compounds. Some of the examples of various types of aromatic compounds are:

Benzenoid aromatic compounds

Benzene                Aniline                Naphthalene

Non-benzenoid compound

Tropone

Heterocyclic aromatic compounds

      Furan               Thiophene                  Pyridine

Organic compounds can also be classified on the basis of functional groups, into families or homologous series.

Functional Group

The functional group is an atom or a group of atoms joined to the carbon chain which is responsible for the characteristic chemical properties of the organic compounds. The examples are hydroxyl group (–OH), aldehyde group (–CHO) and carboxylic acid group (–COOH) etc.

Homologous Series

A group or a series of organic compounds each containing a characteristic functional group forms a homologous series and the members of the series are called homologues. The members of a homologous series can be represented by general molecular formula and the successive members differ from each other in molecular formula by a –CH₂ unit. There are a number of homologous series of organic compounds. Some of these are alkanes, alkenes, alkynes, haloalkanes, alkanols, alkanals, alkanones, alkanoic acids, amines etc. It is also possible that a compound contains two or more identical or different functional groups. This gives rise to polyfunctional compounds.

NOMENCLATURE OF ORGANIC COMPOUNDS

Organic chemistry deals with millions of compounds. In order to clearly identify them, a systematic method of naming has been developed and is known as the IUPAC (International Union of Pure and Applied Chemistry) system of nomenclature. In this systematic nomenclature, the names are correlated with the structure such that the reader or listener can deduce the structure from the name.

Before the IUPAC system of nomenclature, however, organic compounds were assigned names based on their origin or certain properties. For instance, citric acid is named 

so because it is found in citrus fruits and the acid found in red ant is named formic acid since the Latin word for ant is formica. These names are traditional and are considered as trivial or common names. Some common names are followed even today. For example, Buckminsterfullerene is a common name given to the newly discovered C₆₀ cluster (a form of carbon) noting its structural similarity to the geodesic domes popularised by the famous architect R. Buckminster Fuller. Common names are useful and in many cases indispensable, particularly when the alternative systematic names are lengthy and complicated. Common names of some organic compounds are given in Table 12.1.

Table 12.1 Common or Trivial Names of Some Organic Compounds

The IUPAC System of Nomenclature

A systematic name of an organic compound is generally derived by identifying the parent hydrocarbon and the functional group(s) attached to it. See the example given below.

By further using prefixes and suffixes, the parent name can be modified to obtain the actual name. Compounds containing carbon and hydrogen only are called hydrocarbons. A hydrocarbon is termed saturated if it contains only carbon-carbon single bonds. The IUPAC name for a homologous series of such compounds is alkane. Paraffin (Latin: little affinity) was the earlier name given to these compounds. Unsaturated hydrocarbons are those, which contain at least one carbon- carbon double or triple bond.

IUPAC Nomenclature of Alkanes

Straight chain hydrocarbons: The names of such compounds are based on their chain structure, and end with suffix ‘-ane’ and carry a prefix indicating the number of carbon atoms present in the chain (except from CH₄ to C₄H₁₀, where the prefixes are derived from trivial names). The IUPAC names of some straight chain saturated hydrocarbons are given in Table 12.2. The alkanes in Table 12.2 differ from each other by merely the number of -CH₂  groups in the chain. They are homologues of alkane series.

Table 12.2 IUPAC Names of Some Unbranched Saturated Hydrocarbons

Branched chain hydrocarbons: In a branched chain compound small chains of carbon atoms are attached at one or more carbon atoms of the parent chain. The small carbon chains (branches) are called alkyl groups. For example:

In order to name such compounds, the names of alkyl groups are prefixed to the name of parent alkane. 

An alkyl group is derived from a saturated hydrocarbon by removing a hydrogen atom from carbon. Thus, CH₄ becomes -CH₃ and is called methyl group. An alkyl group is named by substituting ‘yl’ for ‘ane’ in the corresponding alkane. Some alkyl groups are listed in Table 12.3.

Table 12.3 Some Alkyl Groups

Abbreviations are used for some alkyl groups. For example, methyl is abbreviated as Me, ethyl as Et, propyl as Pr and butyl as Bu. The alkyl groups can be branched also. Thus, propyl and butyl groups can have branched structures as shown below.

Common branched groups have specific trivial names. For example, the propyl groups can either be n-propyl group or isopropyl group. The branched butyl groups are called sec- butyl, isobutyl and tert-butyl group. We also encounter the structural unit,–CH₂C(CH₃)₃, which is called neopentyl group.

Nomenclature of branched chain alkanes:

We encounter a number of branched chain alkanes. The rules for naming them are given below.

1. First of all, the longest carbon chain in the molecule is identified. In the example 

(a) given below, the longest chain has nine carbons and it is considered as the parent or root chain. Selection of parent chain as shown in (II) is not correct because it has only eight carbons.

2. The carbon atoms of the parent chain are numbered to identify the parent alkane and to locate the positions of the carbon atoms at which branching takes place due to the substitution of alkyl group in place of hydrogen atoms. The numbering is done in such a way that the branched carbon atoms get the lowest possible numbers.

Thus, the numbering in the above example should be from left to right (branching at carbon atoms 2 and 6) and not from right to left (giving numbers 4 and 8 to the carbon atoms at which branches are attached).

3. The names of alkyl groups attached as a branch are then prefixed to the name of the parent alkane and position of the substituents is indicated by the appropriate numbers. If different alkyl groups are present, they are listed in alphabetical order. Thus, name for the compound shown above is: 6-ethyl-2- methylnonane. [Note: the numbers are separated from the groups by hyphens and there is no break between methyl and nonane.]

4. If two or more identical substituent groups are present then the numbers are separated by commas. The names of identical substituents are not repeated, instead prefixes such as di (for 2), tri (for 3), tetra (for 4), penta (for 5), hexa (for 6) etc. are used. While writing the name of the substituents in alphabetical order, these prefixes, however, are not considered. Thus, the following compounds are named as:

5. If the two substituents are found in equivalent positions, the lower number is given to the one coming first in the alphabetical listing. Thus, the following compound is 3-ethyl-6-methyloctane and not 6-ethyl-3-methyloctane.

6. The branched alkyl groups can be named by following the above mentioned procedures. However, the carbon atom of the branch that attaches to the root alkane is numbered 1 as exemplified below.

The name of such branched chain alkyl group is placed in parenthesis while naming the compound. While writing the trivial names of substituents’ in alphabetical order, the prefixes iso- and neo- are considered to be the part of the fundamental name of alkyl group. The prefixes sec- and tert- are not considered to be the part of the fundamental name. The use of iso and related common prefixes for naming alkyl groups is also allowed by the IUPAC nomenclature as long as these are not further substituted. In multi- substituted compounds, the following rules may aso be remembered:

1. If there happens to be two chains of equal size, then that chain is to be selected which contains more number of side chains.

2. After selection of the chain, numbering is to be done from the end closer to the substituent.

Cyclic Compounds: A saturated monocyclic compound is named by prefixing ‘cyclo’ to the corresponding straight chain alkane. If side chains are present, then the rules given above are applied. Names of some cyclic compounds are given below.

PROBLEM 7

Structures and IUPAC names of some hydrocarbons are given below. Explain why the names given in the parentheses are incorrect.

SOLUTION

(a) Lowest locant number, 2,5,6 is lower than 3,5,7, (b) substituents are in equivalent position; lower number is given to the one that comes first in the name according to alphabetical order.

Nomenclature of Organic Compounds having Functional Group(s)

A functional group, as defined earlier, is an atom or a group of atoms bonded together in a unique manner which is usually the site of 

chemical reactivity in an organic molecule. Compounds having the same functional group undergo similar reactions. For example, CH₃OH, CH₃CH₂OH, and (CH₃)₂CHOH — all having -OH functional group liberate hydrogen on reaction with sodium metal. The presence of functional groups enables systematisation of organic compounds into different classes. Examples of some functional groups with their prefixes and suffixes along with some examples of organic compounds possessing these are given in Table 12.4.

First of all, the functional group present in the molecule is identified which determines the choice of appropriate suffix. The longest chain of carbon atoms containing the functional group is numbered in such a way that the functional group is attached at the carbon atom possessing lowest possible number in the chain. By using the suffix as given in Table 12.4, the name of the compound is arrived at.

Table 12.4 Some Functional Groups and Classes of Organic Compounds

In the case of polyfunctional compounds, one of the functional groups is chosen as the principal functional group and the compound is then named on that basis. The remaining functional groups, which are subordinate functional groups, are named as substituents using the appropriate prefixes. The choice of principal functional group is made on the basis of order of preference. The order of decreasing priority for some functional groups is:

-COOH, –SO₃H, -COOR (R=alkyl group), COCl, -CONH₂, -CN,-HC=O, >C=O, -OH, -NH₂, >C=C<, -C \(% MathType!MTEF!2!1!+- % feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeyyyIOlaaa!37BF! \equiv \) C- .

The –R, C₆H₅-, halogens (F, Cl, Br, I), –NO₂, alkoxy (–OR) etc. are always prefix substituents. Thus, a compound containing both an alcohol and a keto group is named as hydroxyalkanone since the keto group is preferred to the hydroxyl group.

For example, HOCH₂ (CH₂)₃ CH₂ COCH₃ will be named as 7-hydroxyheptan-2-one and not as 2-oxoheptan -7-ol. Similarly, BrCH₂CH=CH₂ is named as 3-bromoprop-1-ene and not 1- bromoprop-2-ene.

If more than one functional group of the same type are present, their number is indicated by adding di, tri, etc. before the class suffix. In such cases the full name of the parent alkane is written before the class suffix.

For example CH₂(OH)CH₂(OH) is named as ethane–1,2–diol. However, the ending – ne of the parent alkane is dropped in the case of

compounds having more than one double or triple bond; for example, CH₂=CH-CH=CH₂ is named as buta–1,3–diene.

PROBLEM 8

Write the IUPAC names of the compounds i-iv from their given structures.

SOLUTION

1. The functional group present is an alcohol (OH). Hence the suffix is ‘-ol’.

2. The longest chain containing -OH has eight carbon atoms. Hence the corresponding saturated hydrocarbon is octane.

3. The -OH is on carbon atom 3. In addition, a methyl group is attached at 6^th carbon.

Hence, the systematic name of this compound is 6-Methyloctan-3-ol.

SOLUTION

The functional group present is ketone (>C=O), hence suffix ‘-one’. Presence of two keto groups is indicated by ‘di’, hence suffix becomes ‘dione’. The two keto groups are at carbons 2 and 4. The longest chain contains 6 carbon atoms, hence, parent hydrocarbon is hexane. Thus, the systematic name is Hexane -2,4-dione.

SOLUTION

Here, two functional groups namely ketone and carboxylic acid are present. The principal functional group is the carboxylic acid group; hence the parent chain will be suffixed with ‘oic’ acid. Numbering of the chain starts from carbon of – COOH functional group. The keto group in the chain at carbon 5 is indicated by ‘oxo’. The longest chain including the principal functional group has 6 carbon atoms; hence the parent hydrocarbon is hexane. The compound is, therefore, named as 5-Oxohexanoic acid.

SOLUTION

The two C=C functional groups are present at carbon atoms 1 and 3, while the C\(% MathType!MTEF!2!1!+- % feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeyyyIOlaaa!37BF! \equiv \)C functional group is present at carbon 5. These groups are indicated by suffixes ‘diene’ and ‘yne’ respectively. The longest chain containing the functional groups has 6 carbon atoms; hence the parent hydrocarbon is hexane. The name of compound, therefore, is Hexa-1,3- dien-5-yne.

PROBLEM 9

Derive the structure of (i) 2-Chlorohexane, (ii) Pent-4-en-2-ol, (iii) 3- Nitrocyclohexene, (iv) Cyclohex-2-en-1-ol, (v) 6-Hydroxy- heptanal.

SOLUTION

(1) ‘hexane’ indicates the presence of 6 carbon atoms in the chain. The functional group chloro is present at carbon 2. Hence, the structure of the compound is CH₃CH₂CH₂CH₂CH(Cl)CH₃.

(2) ‘pent’ indicates that parent hydrocarbon contains 5 carbon atoms in the chain. ‘en’ and ‘ol’ correspond to the functional groups C=C and -OH at carbon atoms 4 and 2 respectively. Thus, the structure is CH₂=CHCH₂CH (OH)CH₃.

(3) Six membered ring containing a carbon-carbon double bond is implied by cyclohexene, which is numbered as shown in (I). The prefix 3-nitro means that a nitro group is present on C-3. Thus, complete structural formula of the compound is (II). Double bond is suffixed functional group whereas NO₂ is prefixed functional group therefore double bond gets preference over –NO₂ group:

(4) ‘1-ol’ means that a -OH group is present at C-1. OH is suffixed functional group and gets preference over C=C bond. Thus the structure is as shown in (II):

(5) ‘heptanal’ indicates the compound to be an aldehyde containing 7 carbon atoms in the parent chain. The ‘6-hydroxy’ indicates that -OH group is present at carbon 6. Thus, the structural formula of  the compound is:

CH₃CH(OH)CH₂CH₂CH₂CH₂CHO. Carbon atom of –CHO group is included while numbering the carbon chain.

Nomenclature of Substituted Benzene Compounds

For IUPAC nomenclature of substituted benzene compounds, the substituent is placed as prefix to the word benzene as shown in the following examples. However, common names (written in bracket below) of many substituted benzene compounds are also universally used.

Methylbenzene   Methoxybenzene  Aminobenzene

(Toluene)                   (Anisole)            (Aniline)

Nitrobenzene            Bromobenzene

1,2-Dibromo- benzene 1,3-Dibromo- benzene 1,4-Dibromo- benzene

In the trivial system of nomenclature the terms ortho (o), meta (m) and para (p) are used as prefixes to indicate the relative positions 1,2;1,3 and 1,4 respectively. Thus, 1,3-dibromobenzene (b) is named as m-dibromobenzene (meta is abbreviated as m-) and the other isomers of dibromobenzene 1,2-(a) and 1,4-(c), are named as ortho (or just o-) and para (or just p-)-dibromobenzene, respectively.

For tri - or higher substituted benzene derivatives, these prefixes cannot be used and the compounds are named by identifying substituent positions on the ring by following the lowest locant rule. In some cases, common name of benzene derivatives is taken as the base compound.

Substituent of the base compound is assigned number1 and then the direction of numbering is chosen such that the next substituent gets the lowest number. The substituents appear in the name in alphabetical order. Some examples are given below.

If benzene ring is disubstituted, the position of substituents is defined by numbering the carbon atoms of the ring such that the  substituents are located at the lowest numbers possible.For example, the compound(b) is named as 1,3-dibromobenzene and not as 1,5-dibromobenzene.

1,2-Dibromo- benzene 1,3-Dibromo- benzene 1,4-Dibromo- benzene

In the trivial system of nomenclature the terms ortho (o), meta (m) and para (p) are used as prefixes to indicate the relative positions 1,2;1,3 and 1,4 respectively. Thus, 1,3-dibromobenzene (b) is named as m-dibromobenzene (meta is abbreviated as m-) and the other isomers of dibromobenzene 1,2-(a) and 1,4-(c), are named as ortho (or just o-) and para (or just p-)-dibromobenzene, respectively.

For tri - or higher substituted benzene derivatives, these prefixes cannot be used and the compounds are named by identifying substituent positions on the ring by following the lowest locant rule. In some cases, common name of benzene derivatives is taken as the base compound.

Substituent of the base compound is assigned number1 and then the direction of numbering is chosen such that the next substituent gets the lowest number. The substituents appear in the name in alphabetical order. Some examples are given below.

1-Chloro-2,4-dinitrobenzene (not 4-chloro,1,3-dinitrobenzene)

2- Chloro-1-methyl-4-nitrobenzene (not 4-methyl-5-chloro-nitrobenzene)

2-Chloro-4-methylanisole 4-Ethyl-2-methylaniline

When a benzene ring is attached to an alkane with a functional group, it is considered as substituent, instead of a parent. The name for benzene as substituent is phenyl (C₆H₅-, also abbreviated as Ph).

PROBLEM 10

Write the structural formula of:

(a) o-Ethylanisole, (b) p-Nitroaniline,

(c) 2,3 - Dibromo -1 - phenylpentane,

(d) 4-Ethyl-1-fluoro-2-nitrobenzene.

SOLUTION

(a) 

(b) 

(c) 

(d) 

​​​​​​ISOMERISM

The phenomenon of existence of two or more compounds possessing the same molecular formula but different properties is known as

isomerism. Such compounds are called as isomers. The following flow chart shows different types of isomerism.

Structural Isomerism

Compounds having the same molecular formula but different structures (manners in which atoms are linked) are classified as structural isomers. Some typical examples of different types of structural isomerism are given below:

(1) Chain isomerism: When two or more compounds have similar molecular formula but different carbon skeletons, these are referred to as chain isomers and the phenomenon is termed as chain isomerism. For example, C₅H₁₂ represents three compounds:

(2) Position isomerism: When two or more compounds differ in the position of substituent atom or functional group on the carbon skeleton, they are called position isomers and this phenomenon is termed as position isomerism. For example, the molecular formula C₃H₈O represents two alcohols:

(3) Functional group isomerism: Two or more compounds having the same molecular formula but different functional groups are called functional isomers and this phenomenon is termed as functional group isomerism. For example, the molecular formula C₃H₆O represents an aldehyde and a ketone:

(4) Metamerism: It arises due to different alkyl chains on either side of the functional group in the  molecule. For  example, C₄H₁₀O represents methoxypropane (CH₃OC₃H₇) and ethoxyethane (C₂H₅OC₂H₅).

Stereoisomerism

The compounds that have the same constitution and sequence of covalent bonds but differ in relative positions of their atoms or groups in space are called stereoisomers. This special type of isomerism is called as stereoisomerism and can be classified as geometrical and optical isomerism.

FUNDAMENTAL CONCEPTS IN ORGANIC REACTION MECHANISM

In an organic reaction, the organic molecule (also referred as a substrate) reacts with an appropriate attacking reagent and leads to the formation of one or more intermediate(s) and finally product(s) The general reaction is depicted as follows :

Substrate is that reactant which supplies carbon to the new bond and the other reactant is called reagent. If both the reactants supply carbon to the new bond then choice is arbitrary and in that case the molecule on which attention is focused is called substrate. In such a reaction a covalent bond between two carbon atoms or a carbon and some other atom is broken and a new bond is formed. A sequential account of each step describing details of electron movement, energetics during bond cleavage and bond formation, and the rates of transformation of reactants into products (kinetics) is referred to as reaction mechanism. The knowledge of reaction mechanism helps in understanding the reactivity of organic compounds and in planning strategy for their synthesis.

In the following sections, we shall learn some of the principles that explain how these reactions take place.

Fission of a Covalent Bond

A covalent bond can get cleaved either by : (i) heterolytic cleavage, or by (ii) homolytic cleavage. 

In heterolytic cleavage, the bond breaks in such a fashion that the shared pair of electrons remains with one of the fragments.

After heterolysis, one atom has a sextet electronic structure and a positive cha ge and the other, a valence octet with at least one lone pair and a negative charge. Thus, heterolytic cleavage of bromomethane will give \(% MathType!MTEF!2!1!+- % feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaCbiaeaaca % WGdbaaleqabaGaey4kaScaaOGaamisamaaBaaaleaacaaIZaaabeaa % aaa!39A9! \mathop C\limits^ + {H_3}\) and Br^- as shown below.

A species having a carbon atom possessing sextext of electrons and a positive charge is called a carbocation (earlier called carbonium ion \(% MathType!MTEF!2!1!+- % feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaCbiaeaaca % WGdbaaleqabaGaey4kaScaaOGaamisamaaBaaaleaacaaIZaaabeaa % aaa!39A9! \mathop C\limits^ + {H_3}\)

ion is known as a methyl cation or methyl carbonium ion. Carbocations are classified as primary, secondary or tertiary depending on whether one, two or three carbons are directly attached to the positively charged carbon. Some other examples of carbocations are: \(% MathType!MTEF!2!1!+- % feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaam4qaiaadI % eadaWgaaWcbaGaaG4maaqabaGcdaWfGaqaaiaadoeaaSqabeaacqGH % RaWkaaGccaWGibWaaSbaaSqaaiaaikdaaeqaaaaa!3C30! C{H_3}\mathop C\limits^ + {H_2}\) (ethyl cation, a primary carbocation), (CH₃)₂\(% MathType!MTEF!2!1!+- % feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaCbiaeaaca % WGdbaaleqabaGaey4kaScaaaaa!37E9! \mathop C\limits^ + \)H (isopropyl cation, a secondary carbocation), and (CH₃)₃C (tert-butyl cation, a tertiary carbocation). Carbocations are highly unstable and reactive species. Alkyl groups directly attached to the positively charged carbon stabilise the carbocations due to inductive and hyperconjugation effects, which you will be studying in the sections 12.7.5 and 12.7.9. The observed order of carbocation stability is: \(% MathType!MTEF!2!1!+- % feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaCbiaeaaca % WGdbaaleqabaGaey4kaScaaOGaamisamaaBaaaleaacaaIZaaabeaa % aaa!39A9! \mathop C\limits^ + {H_3}\)  \(% MathType!MTEF!2!1!+- % feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeyipaWJaam % 4qaiaadIeadaWgaaWcbaGaaG4maaqabaGcdaWfGaqaaiaadoeaaSqa % beaacqGHRaWkaaGccaWGibWaaSbaaSqaaiaaikdaaeqaaOGaeyipaW % ZaaeWaaeaacaWGdbGaamisamaaBaaaleaacaaIZaaabeaaaOGaayjk % aiaawMcaamaaBaaaleaacaaIYaaabeaakmaaxacabaGaam4qaaWcbe % qaaiabgUcaRaaakiaadIeacqGH8aapdaqadaqaaiaadoeacaWGibWa % aSbaaSqaaiaaiodaaeqaaaGccaGLOaGaayzkaaWaaSbaaSqaaiaaio % daaeqaaOWaaCbiaeaacaWGdbaaleqabaGaey4kaScaaOGaaiOlaaaa % !4EC6! < C{H_3}\mathop C\limits^ + {H_2} < {\left( {C{H_3}} \right)_2}\mathop C\limits^ + H < {\left( {C{H_3}} \right)_3}\mathop C\limits^ + \) These carbocations have trigonal planar shape with positively  charged  carbon  being  sp² hybridised. Thus, the shape of \(% MathType!MTEF!2!1!+- % feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaCbiaeaaca % WGdbaaleqabaGaey4kaScaaOGaamisamaaBaaaleaacaaIZaaabeaa % aaa!39A9! \mathop C\limits^ + {H_3}\) may be considered as being derived from the overlap of three equivalent C(sp²) hybridised orbitals with 1s orbital of each of the three hydrogen atoms. Each bond may be represented as C(sp²)–H(1s) sigma bond. The remaining carbon orbital is perpendicular to the molecular plane and contains no electrons. [Fig. 12.3(a)].

Fig. 12.3 (a) Shape of methyl carbocation

The heterolytic cleavage can also give a species in which carbon gets the shared pair of electrons. For example, when group Z attached to the carbon leaves without 

electron pair, the methyl anion \(% MathType!MTEF!2!1!+- % feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaeWaaeaaca % WGibWaaSbaaSqaaiaaiodaaeqaaOGaam4qamaaxacabaGaaiOoaaWc % beqaaiabgkHiTaaaaOGaayjkaiaawMcaaaaa!3C05! \left( {{H_3}C\mathop :\limits^ - } \right)\) formed. Such a carbon species carrying a negative charge on carbon atom is called carbanion. Carbon in carbanion is generally sp³ hybridised and its structure is distorted tetrahedron as shown in Fig. 12.3(b).

Fig. 12.3 (b) Shape of methyl carbanion

Carbanions are also unstable and reactive species. The organic reactions which proceed through heterolytic bond cleavage are called ionic or heteropolar or just polar reactions.

In homolytic cleavage, one of the electrons of the shared pair in a covalent bond goes with each of the bonded atoms. Thus, in homolytic cleavage, the movement of a single electron takes place instead of an electron pair. The single electron movement is shown by ‘half- headed’ (fish hook : )curved arrow. Such cleavage results in the formation of neutral species (atom or group) which contains an unpaired electron. These species are called free radicals. Like carbocations and carbanions, free radicals are also very reactive. A homolytic cleavage can be shown as:

Alkyl free radical

Alkyl radicals are classified as primary, secondary, or tertiary. Alkyl radical stability increases as we proceed from primary to tertiary:

Methyl  Ethyl             Isopropyl        Tert-buty

  free       free                  free                    free

radical    radical            radical               radical

Organic reactions, which proceed by homolytic fission are called free radical or homopolar or nonpolar reactions.

Substrate and Reagent

Ions are generally not formed in the reactions of organic compounds. Molecules as such participate in the reaction. It is convenient to name one reagent as substrate and other as reagent. In general, a molecule whose carbon is involved in new bond formation is called substrate and the other one is called reagent. When carbon-carbon bond is formed, the choice of naming the reactants as substrate and reagent is arbitrary and depends on molecule under observation. Example:

(1) CH₂ = CH₂ + Br₂  \(\to\) CH₂ Br – CH₂Br

 Substrate      Reagent                  Product

Nucleophiles and Electrophiles

Reagents attack the reactive site of the substrate. The reactive site may be electron deficient portion of the molecule (a positive reactive site) e.g., an atom with incomplete electron shell or the positive end of the dipole in the molecule. If the attacking species is electron rich, it attacks these sites. If attacking species is electron deficient, the reactive site for it is that part of the substrate molecule which can supply electrons, e.g., \(\pi\)electrons in a double bond.

A reagent that brings an electron pair to the reactive site is called a nucleophile (Nu:) i.e., nucleus seeking and the reaction is then called nucleophilic. A reagent that takes away an electron pair from reactive site is called electrophile (E⁺) i.e., electron seeking and the reaction is called electrophilic.

During a polar organic reaction, a nucleophile attacks an electrophilic centre of the substrate which is that specific atom or part of the substrate which is electron deficient. Similarly, the electrophiles attack at nucleophilic centre, which is the electron rich centre of the substrate. Thus, the electrophiles receive electron pair from the substrate when the two undergo bonding interaction. A curved-arrow notation is used to show the movement of an electron pair from the nucleophile to the electrophile. Some examples of nucleophiles are the negatively charged ions with lone pair of electrons such as hydroxide (HO^– ), cyanide (NC^–) ions and carbanions (R C:^–). Neutral molecules such as \(% MathType!MTEF!2!1!+- % feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamisamaaBa % aaleaacaaIYaaabeaakiaad+eacaGG6aGaaiilaiaadkfadaWgaaWc % baGaaG4maaqabaGccaWGobGaaiOoaiaacYcacaWGsbWaaSbaaSqaai % aaikdaaeqaaOGaamOtaiaadIeaaaa!416B! {H_2}O:,{R_3}N:,{R_2}NH\) etc., can also act as nucleophiles due to the presence of lone pair of electrons. Examples of electrophiles include carbocations \(% MathType!MTEF!2!1!+- % feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaeWaaeaada % WfGaqaaiaadoeaaSqabeaacqGHRaWkaaGccaWGibWaaSbaaSqaaiaa % iodaaeqaaaGccaGLOaGaayzkaaaaaa!3B3C! \left( {\mathop C\limits^ + {H_3}} \right)\) and neutral molecules having functional groups like carbonyl group (>C=O) or alkyl halides (R3C-X, where X is a halogen atom). The carbon atom in carbocations has sextet configuration; hence, it is electron deficient and can receive a pair of electrons from the nucleophiles. In neutral molecules such as alkyl halides, due to the polarity of the C-X bond a partial positive charge is generated on the carbon atom and hence the carbon atom becomes an electrophilic centre at which a nucleophile can attack.

PROBLEM 11

Using curved-arrow notation, show the formation of reactive intermediates when the following covalent bonds undergo heterolytic cleavage.

(a) CH₃–SCH₃, (b) CH₃–CN, (c) CH₃–Cu

SOLUTION

PROBLEM 12

Giving justification, categorise the following molecules/ions as nucleophile or electrophile:

\(% MathType!MTEF!2!1!+- % feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamisaiaado % fadaahaaWcbeqaaiabgkHiTaaakiaacYcacaWGcbGaamOramaaBaaa % leaacaaIZaaabeaakiaacYcacaWGdbWaaSbaaSqaaiaaikdaaeqaaO % GaamisamaaBaaaleaacaaI1aaabeaakiaad+eadaahaaWcbeqaaiab % gkHiTaaakiaacYcadaqadaqaaiaadoeacaWGibWaaSbaaSqaaiaaio % daaeqaaaGccaGLOaGaayzkaaWaaSbaaSqaaiaaiodaaeqaaOGaamOt % aiaacQdacaGGSaGaam4qamaaxacabaGaamiBaaWcbeqaaiabgUcaRa % aakiaacYcacaWGdbGaamisamaaBaaaleaacaaIZaaabeaakiabgkHi % TmaaxacabaGaam4qaaWcbeqaaiabgUcaRaaakiabg2da9iaad+eaca % GGSaGaamisamaaBaaaleaacaaIYaaabeaakiaad6eadaWfGaqaaiaa % cQdaaSqabeaacqGHsislaaGccaGGSaWaaCbiaeaacaWGobaaleqaba % Gaey4kaScaaOGaam4tamaaBaaaleaacaaIYaaabeaaaaa!5EAB! H{S^ - },B{F_3},{C_2}{H_5}{O^ - },{\left( {C{H_3}} \right)_3}N:,C\mathop l\limits^ + ,C{H_3} - \mathop C\limits^ + = O,{H_2}N\mathop :\limits^ - ,\mathop N\limits^ + {O_2}\)

SOLUTION

Nucleophiles : \(% MathType!MTEF!2!1!+- % feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamisaiaado % fadaahaaWcbeqaaiabgkHiTaaakiaacYcacaWGdbWaaSbaaSqaaiaa % ikdaaeqaaOGaamisamaaBaaaleaacaaI1aaabeaakiaad+eadaahaa % WcbeqaaiabgkHiTaaakiaacYcadaqadaqaaiaadoeacaWGibWaaSba % aSqaaiaaiodaaeqaaaGccaGLOaGaayzkaaWaaSbaaSqaaiaaiodaae % qaaOGaamOtaiaacQdacaGGSaGaamisamaaBaaaleaacaaIYaaabeaa % kiaad6eadaWfGaqaaiaacQdaaSqabeaacqGHsislaaaaaa!4B5E! H{S^ - },{C_2}{H_5}{O^ - },{\left( {C{H_3}} \right)_3}N:,{H_2}N\mathop :\limits^ - \)

These species have unshared pair of electrons, which can be donated and shared with an electrophile.

Electrophiles : \(% MathType!MTEF!2!1!+- % feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamOqaiaadA % eadaWgaaWcbaGaaG4maaqabaGccaGGSaGaam4qamaaxacabaGaamiB % aaWcbeqaaiabgUcaRaaakiaac6cacaWGdbGaamisamaaBaaaleaaca % aIZaaabeaakiabgkHiTmaaxacabaGaam4qaaWcbeqaaiabgUcaRaaa % kiabg2da9iaad+eacaGGSaWaaCbiaeaacaWGobaaleqabaGaey4kaS % caaOGaam4tamaaBaaaleaacaaIYaaabeaaaaa!488B! B{F_3},C\mathop l\limits^ + .C{H_3} - \mathop C\limits^ + = O,\mathop N\limits^ + {O_2}\).

Reactive sites have only six valence electrons; can accept electron pair from a nucleophile.

PROBLEM 13

Identify electrophilic centre in the following: CH₃CH=O, CH₃CN, CH₃I.

SOLUTION

Among \(% MathType!MTEF!2!1!+- % feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaam4qaiaadI % eadaWgaaWcbaGaaG4maaqabaGccaWGibWaaCbiaeaacaWGdbaaleqa % baGaaiOkaaaakiabg2da9iaad+eacaGGSaGaaGPaVlaaykW7caaMc8 % UaamisamaaBaaaleaacaaIZaaabeaakiaadoeadaWfGaqaaiaadoea % aSqabeaacaGGQaaaaOGaeyyyIORaamOtaaaa!492C! C{H_3}H\mathop C\limits^* = O,\,\,\,{H_3}C\mathop C\limits^* \equiv N\) and \(% MathType!MTEF!2!1!+- % feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamisamaaBa % aaleaacaaIZaaabeaakmaaxacabaGaam4qaaWcbeqaaiaacQcaaaGc % cqGHsislcaWGjbaaaa!3B3A! {H_3}\mathop C\limits^* - I\), the starred carbon atoms are electrophilic centers as they will have partial positive charge due to polarity of the bond.

shows how changes in bonding occur due to electronic redistribution during the reaction. To show the change in position of a pair of electrons, curved arrow starts from the point from where an electron pair is shifted and it ends at a location to which the pair of electron may move.

Presentation of shifting of electron pair is given below :

(1)   from \(\pi\) bond to adjacent bond position

(2)      from \(\pi\) bond to adjacent atom

(3)  from atom to adjacent bond position

Movement of single electron is indicated by a single barbed ‘fish hooks’ (i.e. half headed curved arrow). For example, in transfer of hydroxide ion giving ethanol and in the dissociation of chloromethane, the movement of electron using curved arrows can be depicted as follows:

Electron Displacement Effects in Covalent Bonds

The electron displacement in an organic molecule may take place either in the ground state under the influence of an atom or a substituent group or in the presence of an appropriate attacking reagent. The electron displacements due to the influence of an atom or a substituent group present in the molecule cause permanent polarlisation of the bond. Inductive effect and resonance effects are examples of this type of electron displacements. Temporary electron displacement effects are seen in a molecule when a reagent approaches to attack it. This type of electron displacement is called electromeric effect or polarisability effect. In the following sections we will learn about these types of electronic displacements.

Inductive Effect

When a covalent bond is formed between atoms of different electronegativity, the electron density is more towards the more electronegative atom of the bond. Such a shift of electron density results in a polar covalent bond. Bond polarity leads to various electronic effects in organic compounds.

Let us consider cholorethane (CH₃ CH₂ Cl) in which the C–Cl bond is a polar covalent bond. It is polarised in such a way that the carbon-1 gains some positive charge (\(% MathType!MTEF!2!1!+- % feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeqiTdq2aaW % baaSqabeaacqGHRaWkaaaaaa!38AA! {\delta ^ + }\)) and the chlorine some negative charge (\(% MathType!MTEF!2!1!+- % feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeqiTdq2aaW % baaSqabeaacqGHRaWkaaaaaa!38AA! {\delta ^ - }\)). The fractional electronic charges on the two atoms in a polar covalent bond are denoted by symbol \(% MathType!MTEF!2!1!+- % feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeqiTdqgaaa!379B! \delta \)(delta) and the shift of electron density is shown by an arrow that points from \(% MathType!MTEF!2!1!+- % feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeqiTdq2aaW % baaSqabeaacqGHRaWkaaaaaa!38AA! {\delta ^ + }\) to \(% MathType!MTEF!2!1!+- % feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeqiTdq2aaW % baaSqabeaacqGHRaWkaaaaaa!38AA! {\delta ^ - }\) end of the polar bond.

In turn carbon-1, which has developed partial positive charge (\(% MathType!MTEF!2!1!+- % feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeqiTdq2aaW % baaSqabeaacqGHRaWkaaaaaa!38AA! {\delta ^ + }\)) draws some electron density towards it from the adjacent C-C bond. Consequently, some positive charge (\(% MathType!MTEF!2!1!+- % feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeqiTdqgaaa!379B! \delta \)\(% MathType!MTEF!2!1!+- % feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeqiTdq2aaW % baaSqabeaacqGHRaWkaaaaaa!38AA! {\delta ^ + }\) ) develops on carbon-2 also, where \(% MathType!MTEF!2!1!+- % feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeqiTdqgaaa!379B! \delta \)\(% MathType!MTEF!2!1!+- % feaagKart1ev2aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn % hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr % 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9 % vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x % fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeqiTdqgaaa!379B! \delta \) symbolises relatively smaller positive charge as compared to that on carbon – 1. In other words, the polar C – Cl bond induces polarity in the adjacent bonds. Such polarisation of \(\sigma\)- bond caused by the polarisation of adjacent \(\sigma\)-bond is referred to as the inductive effect. This effect is passed on to the subsequent bonds also but the effect decreases rapidly as the number of intervening bonds increases and becomes vanishingly small after three bonds. The inductive effect is related to the ability of substituent(s) to either withdraw or donate electron density to the attached carbon atom. 

Based on this ability, the substitutents can be classified as electron-withdrawing or electron donating groups relative to hydrogen. Halogens and many other groups such as nitro (- NO₂), cyano (- CN), carboxy (- COOH), ester (COOR), aryloxy (-OAr, e.g. – OC₆H₅), etc. are electron- withdrawing groups. On the other hand, the alkyl groups like methyl (–CH₃) and ethyl (–CH₂ –CH₃ ) are usually considered as electron donating groups.

PROBLEM 14

Which bond is more polar in the following pairs of molecules: (a) H₃C-H, H₃C-Br (b) H₃C-NH₂, H₃C-OH (c) H₃C-OH, H₃C-SH 

SOLUTION

(a) C–Br, since Br is more electronegative than H, (b) C–O, (c) C–O

PROBLEM 15

In which C–C bond of CH₃CH₂CH₂Br, the inductive effect is expected to be the least?

SOLUTION

Magnitude of inductive effect diminishes as the number of intervening bonds increases. Hence, the effect is least in the bond between carbon-3 and hydrogen.

Resonance Structure

There are many organic molecules whose behaviour cannot be explained by a single Lewis structure. An example is that of benzene. Its cyclic structure containing alternating C–C single and C=C double bonds shown is inadequate for explaining its characteristic properties.

As per the above representation, benzene should exhibit two different bond lengths, due to C–C single and C=C double bonds. However, as determined experimentally benzene has a uniform C–C bond distances of 139 pm, a value intermediate between the C–C single(154 pm) and C=C double (134 pm) bonds. Thus the structure of benzene cannot be represented adequately by the above structure. Further, benzene can be represented equally well by the energetically identical structures I and II.