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Preparation of Alkanols

Alkanols are prepared by the following general methods:

 

1. BY THE HYDROLYSIS OF HALOALKANES

Haloalkanes when boiled with aqueous solution of an alkali hydroxide or moist silver oxide furnish alkanols.

Primary alkyl halides give good yield of alcohols. However, tertiary alkyl halides, in this reaction give, mainly alkene due to dehydrohalogenation.

Secondary alkyl halides give a mixture of alkanol and alkene.

This method is not very useful for preparing alcohols because haloalkanes are themselves obtained from alcohols. However, the preparative utility of this method lies in the preparation of aromatic alcohols. For example,

2. BY HYDRATION OF ALKENES

Alkenes are obtained by cracking of petroleum. They are easily converted to alkanols by the addition of water in presence of tetraoxosulphate(VI) acid.

In case of unsymmetrical alkenes, the addition takes place according to Markownikov’s rule.

3. BY OXO PROCESS

Alkenes react with carbon monoxide and hydrogen in the presence of octacarbonyl dicobalt [Co(CO)4h as catalyst and at high temperature and pressure to yield aldehydes. Catalytic hydrogenation of aldehydes gives primary alkanols.

4. BY FERMENTATIO-N OF CARBOHYDRATES

Formation of ethanol by the. fermentation of sugar (obtained from mollases, grapes or beet) is one of the oldest method. Sucrose is first converted to glucose and fructose with an enzyme invertase.

The fermentation process is carried out under anaerobic conditions i.e., in the absence of air. Carbon(IV) oxide released during fermentation keeps the fermentation mixture out of contact of air. If the fermentation mixture gets. Exposed to air, the oxygen of air oxidizes ethanol to ethanoic acid which makes the mixture sour.

In Africa, as in many other parts of the world ethanol is obtained from starchy materials such as barley, millet, rice, maize, cassava, unripe banana and potatoes with enzymes diastase and maltase.

In the manufacture of ethanol by fermentation of cellulose, the cellulosic material is first hydrolysed to glucose by digesting it with dilute tetraoxosulphate(VI) acid and steam at a  Pressure of 6-7 atmospheres.

The excess acid is then neutralized with lime and the resulting solution is fermented with yeast. The enzyme zymase converts glucose to ethanol

 

Fermentation of carbohydrates yields ethanol containing water. This solution is concentrated by fractional distillation. By simple fractional distillation we can get, at the most, a solution containing 95 per cent ethanol. This 95 per cent ethanol is known as rectified spirit.

Ethanol in the form of alcoholic beverages is used for drinking. The alcohol used for drinking is heavily taxed. However, alcohol used for industrial purposes is not taxed. To prevent the misuse, the industrial alcohol is made unfit for drinking by addition of 5-10 per cent methanol. The ethanol denaturated by methanol (and a little of pyridine and acetone), is called methylated spirit.

Ethyl alcohol may also be mixed with petrol. A mixture of 20% ethanol and 80% gasoline is used in internal combustion engines. Since alcohol does not mix with petrol therefore, a third solvent benzene or ether or tetralin (tetra hydronaphthalene) is used as a solvent. Ethyl alcohol used for the purpose of generation of power in internal combustion engine is called power alcohol.