Commercial Alcohols
1. Methanol
Methanol is a colourless liquid with boiling point 337 K. it is highly poisonous. Commercially it is prepared by catalytic hydrogenation of carbon monoxide. Water gas (which is equimolar mixture of CO and H2) is mixed with half of its volume with hydrogen gas and is compressed to 200 atmospheric pressure. The compressed gaseous mixture is then passed over a catalyst heated at 573 K. the catalyst consists of the oxides of copper, zinc and chromium.
Methanol is also known as wood-spirit because earlier it was prepared by destructive distillation of wood.
Uses of methanol
1. In the manufacture of formaldehyde and dimethyl terephalate.
2. As aviation fuel.
3. Denaturant for ethyl alcohol.
4. Dehydrator for natural gas.
5. As automobile antifreeze.
6. Solvent for nitrocellulose, resin, dyes, plastics, paints and varnishes.
2. Ethanol
Ethanol is a colourless liquid with boiling point 351.3 K. it is prepared on large scale by hydration of ethane and fermentation of carbohydrates. It is also known as grain alcohol as it is manufactured from starchy grains.
Fermentation of carbohydrates yields ethanol containing water. This solution on fractional distillation forms an azeotrope containing 95 percent ethanol and 5 percent water. Thus, by simple fractional distillation we can get, at the most, a solution containing 95 percent ethanol. This 95 percent ethanol is known as rectified spirit.
Absolute alcohol is 100 percent ethanol. As already mentioned, simple fractional distillation yields a constant boilingazeotropic mixture containing 95 percent ethanol and 5 percent water. Absolute alcohol can be obtained by adding small amount of benzene to the rectified spirit and then distilling. The first fraction is an azeotropic mixture of water, ethanol and benzene (b.p. 337.8 K). This removes water completely. The second fraction is an azeotropic mixture of benzene and ethanol (b.p. 341.2 K). This removes benzene completely. Finally, pure ethanol or absolute alcohol distills over at 351.3 K.
Ethyl alcohol in the form of alcoholic beverages is used for drinking. The alcohol used for drinking is heavily taxed in the form of excise duty. 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 percent 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.
Used of ethanol
1. In the manufacture of dyes, synthetic drugs and detergents.
2. In explosives, surface coatings and cosmetics.
3. As a beverage.
4. As antifreeze.
5. As a solvent for resins, fats, oils, fatty acids and hydrocarbons.
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