Ethanol

Ethanol is the second member of the alcoholic series. Ethanol is commonly known as alcohol. It is a constituent of all alcoholic beverages namely beer, wine, whisky, some cough syrups, digestive syrups etc. In industries, alcohol is produced by the fermentation of sugar present in molasses. Molasses is a by-product of sugar industry in India. In our country, most of the ethanol is prepared from molasses.

Fermentation

The slow chemical change taking place in an organic compound by the action of enzymes leading to the formation of smaller molecules is called Fermentation. In our daily life, there are many instances of fermentation. For example, the change of milk into curd, souring of kneaded flour, etc., is due to fermentation. The fermentation of sugar is a process in which the sugar molecules are broken down into ethanol and carbon dioxide by the action of enzymes called invertase and zymase present in yeast.

Manufacture of Ethanol from Molasses

Molasses is a dark colored syrupy liquid left after the crystallization of sugar from the concentrated sugar cane juice. Molasses still contain about 30% sucrose which could not be separated by crystallization. Molasses is converted into ethanol by the following steps.

Dilution

Molasses is first diluted with water to bring down the concentration of sugar to about 8 to 10 percent.

Addition of Ammonium salts

Molasses usually contains enough nitrogenous matter to act as food for yeast during fermentation. If the nitrogen content of the molasses is poor, it may be fortified by the addition of ammonium sulphate or ammonium phosphate.

Addition of yeast

The solution from step (2) is collected in large ‘fermentation tanks’ and yeast is added to it. The mixture is maintained at about 303K for a few days. During this period, the enzymes invertase and zymase present in yeast bring about the conversion of sugar into ethanol. During this process, the liquor froths owing to the evolution of CO₂ which is recovered and used for preparing aerated drinks. The fermented liquid is technically called wash.

Distillation of wash

The fermented liquid containing 15 to 18 percent alcohol and the rest water is now subjected to fractional distillation. The main fraction drawn is an aqueous solution of ethanol which contains 95.6% ethanol and 4.4% water. This is called Rectified spirit. This mixture is then heated under reflux over quicklime for about 5 to 6 hours and then allowed to stand for 12 hours. On distillation of this mixture, pure alcohol (C₂H₅OH = 100%) is obtained. This is called Absolute alcohol.

Physical Properties of Ethanol

• Ethanol is a colorless liquid having a pleasant smell and a burning taste.
• It is a volatile liquid having a low boiling point of 78° C (351 K).
• It is miscible with water in all proportions.
• Ethanol does not contain any ions, as it is a covalent compound and has no effect on litmus paper.
• The boiling point of alcohols is, in general, much higher than the corresponding alkanes. This is because in alcohols there is intermolecular association of a large number of molecules due to Hydrogen bonding which is absent in alkanes.

Chemical Reactions of Ethanol

Reaction with oxygen or combustion

Ethanol is a highly inflammable liquid (it catches fire easily). It burns with a blue flame to form carbon dioxide and water.

Reaction with Sodium Metal

When a small piece of sodium metal is put into ethanol in a dry test tube, brisk effervescence is produced. When a burning splinter is brought near the mouth of the test tube, the gas burns with a ‘pop’ sound, which is a characteristic property of hydrogen gas. This shows that the gas produced by the action of sodium metal on ethanol is hydrogen. Ethanol reacts with sodium to produce sodium ethoxide and hydrogen.

Esterification

Ethanol reacts with acetic acid in the presence of conc. H2SO4 to form an ester, ethyl ethanoate and water. The ester formed has sweet smell and the reaction is known as esterification

Oxidation into Acetic Acid

Acidified K2Cr2O7 oxidizes ethanol to acetic acid.

Inflammability

An ethanol-water solution that contains 40% ABV (alcohol by volume) will catch fire if heated to about 26 °C (79 °F) and if an ignition source is applied to it. This is called its flash point. The flash point of pure ethanol is 16.60 °C (61.88 °F), less than average room temperature.

Applications of Ethanol

• Ethanol is used in the following:
• Manufacture of paints, varnishes, lacquers and medicines.
• In the preparation of organic compounds like ether, chloroform and iodoform.
• As an antiseptic to sterilize wounds and syringes in hospitals and dispensaries.
• In alcoholic drinks (beverages) like whisky, wine, beer and other liquors. Beer contains around 3 to 6% ethanol, whisky contains 30% ethanol and wine contains 8 to 10% ethanol.
• In spirit lamps as methylated spirit (contains ethanol mixed with a small amount of methanol and water).
• As power alcohol to generate power in internal combustion engines. Power alcohol is a mixture of 25% absolute alcohol and 75% petrol and it is a good fuel for motor cars. In the present days, due to scarcity of petrol and petrolatum products, power alcohol can be used as a substitute for petrol in motor cars which may also reduce pollution of air.

Special Note

The cooling sensation experienced after applying a cosmetic cold cream on one’s skin is the result of the evaporation of an alcohol (e.g., ethanol) contained in the cold cream. Formulators of skin products include ethanol to achieve a variety of benefits. For example, alcohol enhances the ability of the components in the cold cream to dissolve. For the consumer, the presence of alcohol eases the application of the cream on the skin, enhances the perfume quality of the mixture, and provides a cooling effect on the skin.

Why does the evaporation of an alcohol achieve a cooling effect? Evaporation is a process that requires heat (an endothermic process).