Introduction
Many carbon compounds are invaluable to us. But here we shall study the properties of two commercially important compounds – ethanol and ethanoic acid.
Properties of Ethanol
Ethanol is a liquid at room temperature (refer to Table 4.1 for the melting and boiling points of ethanol).
Ethanol is commonly called alcohol and is the active ingredient of all alcoholic drinks. In addition, because it is a good solvent, it is also used in medicines such as tincture iodine, cough syrups, and many tonics. Ethanol is also soluble in water in all proportions. Consumption of small quantities of dilute ethanol causes drunkenness. Even though this practice is condemned, it is a socially widespread practice. However, the intake of even a small quantity of pure ethanol (called absolute alcohol) can be lethal. Also, long-term consumption of alcohol leads to many health problems.
Reactions of Ethanol
i. Reaction with sodium –
Activity 4.6
Teacher’s demonstration –
Drop a small piece of sodium, about the size of a couple of grains of rice, into ethanol (absolute alcohol).
What do you observe?
How will you test the gas evolved?
\(\begin{align} & 2Na+2C{{H}_{3}}C{{H}_{2}}OH\to 2C{{H}_{3}}C{{H}_{2}}{{O}^{-}}N{{a}^{+}}+ {{H}_{2}} \\ & \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,(Sodiumethoxide) \\ \end{align}\)
Alcohols react with sodium leading to the evolution of hydrogen. With ethanol, the other product is sodium ethoxide. Can you recall which other substances produce hydrogen on reacting with metals?
ii. Reaction to give unsaturated hydrocarbon:
Heating ethanol at 443 K with excess concentrated sulphuric acid results in the dehydration of ethanol to give ethene –
\(C{{H}_{3}}-C{{H}_{2}}OH\xrightarrow[{{H}_{2}}S{{O}_{4}}]{Hot\,\,Conc.}C{{H}_{2}}=C{{H}_{2}}+{{H}_{2}}O\)
The concentrated sulphuric acid can be regarded as a dehydrating agent which removes water from ethanol.
How do alcohols affect living beings?
When large quantities of ethanol are consumed, it tends to slow metabolic processes and to depress the central nervous system. This results in lack of coordination, mental confusion, drowsiness, lowering of the normal inhibitions, and finally stupor. The individual may feel relaxed without realising that his sense of judgement, sense of timing, and muscular coordination have been seriously impaired.
Unlike ethanol, intake of methanol in very small quantities can cause death. Methanol is oxidised to methanal in the liver. Methanal reacts rapidly with the components of cells. It coagulates the protoplasm, in much the same way an egg is coagulated by cooking. Methanol also affects the optic nerve, causing blindness.
Ethanol is an important industrial solvent. To prevent the misuse of ethanol produced for industrial use, it is made unfit for drinking by adding poisonous substances like methanol to it. Dyes are also added to colour the alcohol blue so that it can be identified easily. This is called denatured alcohol.
Alcohol as a fuel
Sugarcane plants are one of the most efficient convertors of sunlight into chemical energy. Sugarcane juice can be used to prepare molasses which is fermented to give alcohol (ethanol). Some countries now use alcohol as an additive in petrol since it is a cleaner fuel which gives rise to only carbon dioxide and water on burning in sufficient air (oxygen).
Ethanoic acid
Ethanoic acid is commonly called acetic acid and belongs to a group of acids called carboxylic acids. 5-8% solution of acetic acid in water is called vinegar and is used widely as a preservative in pickles. The melting point of pure ethanoic acid is 290 K and hence it often freezes during winter in cold climates. This gave rise to its name glacial acetic acid.
The group of organic compounds called carboxylic acids is obviously characterised by their acidic nature. However, unlike mineral acids like HCl, which are completely ionised, carboxylic acids are weak acids.
Activity 4.7
Compare the pH of dilute acetic acid and dilute hydrochloric acid using both litmus paper and universal indicator.
Are both acids indicated by the litmus test?
Does the universal indicator show them as equally strong acids?
Reactions of ethanoic acid:
Esterification reaction:
Activity 4.8
Take 1 mL ethanol (absolute alcohol) and 1 mL glacial acetic acid along with a few drops of concentrated sulphuric acid in a test tube.
Warm in a water-bath for at least five minutes as shown in Fig. 4.11.
Pour into a beaker containing 20-50 mL of water and smell the resulting mixture.
Figure 4.11 Formation of ester
Esters are most commonly formed by the reaction of an acid and an alcohol. Ethanoic acid reacts with absolute ethanol in the presence of an acid catalyst to give an ester –
Generally, esters are sweet-smelling substances. These are used in making perfumes and as flavouring agents. On treating with sodium hydroxide, which is an alkali, the ester is converted back to alcohol and sodium salt of carboxylic acid. This reaction is known as saponification because it is used in the preparation of soap. Soaps are sodium or potassium salts of
long-chain carboxylic acid.
\(C{{H}_{3}}COO{{C}_{2}}{{H}_{5}}\xrightarrow{NaOH}{{C}_{2}}{{H}_{5}}OH+C{{H}_{3}}COONa\)
Reaction with a base:
Like mineral acids, ethanoic acid reacts with a base such as sodium hydroxide to give a salt (sodium ethanoate or commonly called sodium acetate) and water:
\(NaOH+C{{H}_{3}}COOH\to C{{H}_{3}}COONa+{{H}_{2}}O\)
How does ethanoic acid react with carbonates and hydrogencarbonates?
Let us perform an activity to find out.
Activity 4.9
Set up the apparatus as shown in Chapter 2, Activity 2.5. (Once we shall see the activity and in place of Dilute Hydrochloric Acid use Dilute Ethanoic Acid)
Take a spatula full of sodium carbonate in a test tube and add 2 mL of dilute ethanoic acid.
What do you observe?
Pass the gas produced through freshly prepared lime-water. What do you observe?
Can the gas produced by the reaction between ethanoic acid and sodium carbonate be identified by this test?
Repeat this Activity with sodium hydrogencarbonate instead of sodium carbonate.
Reaction with carbonates and hydrogencarbonates: Ethanoic acid reacts with carbonates and hydrogencarbonates to give rise to salt, carbon dioxide and water. The salt produced is commonly called sodium acetate.
\(2C{{H}_{3}}COOH+N{{a}_{2}}C{{O}_{3}}\to 2C{{H}_{3}}COONa+{{H}_{2}}O+C{{O}_{2}}\)
\(C{{H}_{3}}COOH+NaHC{{O}_{3}}\to C{{H}_{3}}COONa+{{H}_{2}}O+C{{O}_{2}}\)