Industrial source of alcohol;
If an organic chemist were allowed to choose ten aliphatic compounds with which to be stranded on a desert island, he would almost certainly pick alcohols.
From them he could make nearly every other kind of aliphatic compound : alkenes, alkyl halides, ethers, aldehydes, ketones, acids, esters, and a host of others. From the alkyl halides, he could make Grignard reagents, and from the reaction between these and the aldehydes and ketones obtain more complicated alcohols and so on.
Our stranded chemist would use his alcohols not only as raw materials but frequently
as the solvents in which reactions are carried out and from which products are recrystallized.
For alcohols to be such important starting materials in aliphatic chemistry, they must be not only versatile in their reactions but also available in large amounts and at low prices. There are two principal ways to get the simple alcohols that are the backbone of aliphatic organic synthesis: by hydration of alkenes obtained from the cracking of petroleum, and by fermentation of carbohydrates. In addition to these two chief methods, there are some others that have more limited application.
(a) Hydration of alkenes. We have already seen that alkenes containing up to four or five carbon atoms can be separated from the mixture obtained from the cracking of petroleum. We have also seen that alkenes are readily converted into alcohols either by direct addition of water, or by addition of sulfuric acid followed by hydrolysis. By this process there canbe obtained only those alcohols whose formation is consistent with the application of Markovnikov s rule: for example, isopropyl but not w-propyl, sec-butyl but not w-butyl, terf-butyl but not isobutyl. Thus the only primary alcohol obtainable in this way is ethyl alcohol. (b) Fermentation of carbohydrates. Fermentation of sugars by yeast, the oldest synthetic chemical process used by man, is still of enormous importance for the preparation of ethyl alcohol and certain other alcohols. The sugars come from a variety of sources, mostly molasses from sugar cane, or starch obtained from various grains; the name "grain alcohol" has been given to ethyl alcohol for this reason.
When starch is the starting material, there is obtained, in addition to ethyl alcohol, a smaller amount offusel oil (German: Fusel, inferior liquor), a mixture of primary alcohols: mostly isopentyl alcohol with smaller amounts of w-propyl alcohol, isobutyl alcohol, and 2-methyl-l-butanol, known as active amyl alcohol (amyl pentyl). (a) Which amino acid gives which alcohol? (b) Although both amino acids are optically active, and the transformation processes are analogous, only one gives an alcohol that is optically active. Why is this?
Ethyl alcohol
Ethyl alcohol is not only the oldest synthetic organic chemical used by man, but it is also one of the most important.
In industry ethyl alcohol is widely used as a solvent for lacquers, varnishes, perfumes, and flavorings; as a medium for chemical reactions; and in recrystallizations.
In addition, it is an important raw material for synthesis; after we have learned more about the reactions of alcohols we can better appreciate the role played by the leading member of the family. For these industrial purposes ethyl alcohol is prepared both by hydration of ethylene and by fermentation of sugar from molasses (or sometimes starch); thus its ultimate source is petroleum, sugar cane, and various grains.
Ethyl alcohol is the alcohol of "alcoholic" beverages. For this purpose it is prepared by fermentation of sugar from a truly amazing variety of vegetable sources. The particular beverage obtained depends upon what is fermented (rye or corn, grapes or elderberries, cactus pulp or dandelions), how it is fermented (whether carbon dioxide is allowed to escape or is bottled up, for example), and what is done after fermentation (whether or not it is distilled). The special flavor of a beverage is not due to the ethyl alcohol but to other substances either characteristic of the particular source, or deliberately added.
Medically, ethyl alcohol is classified as a hypnotic (sleep producer); it is less toxic than other alcohols. (Methanol, for example, is quite poisonous: drinking it, breathing it for prolonged periods, or allowing it to remain long on the skin can lead to blindness or death.)
Because of its unique position as both a highly taxed beverage and an important industrial chemical, ethyl alcohol poses a special problem: it must be made available to tfie chemical industry in a form that is unfit to drink. This problem is solved by addition of a denaturant, a substance that makes it unpalatable or even poisonous. Two of the eighty-odd legal denaturants, for example, are methanol and high-test gasoline. When necessary, pure undenatured ethyl alcohol is available for chemical purposes, but its use is strictly controlled by the Federal Government.
Except for alcoholic beverages, nearly all the ethyl alcohol used is a mixture of 95% alcohol and 5% water, known simply as 95% alcohol. What is so special about the concentration of 95%? Whatever the method of preparation, ethyl alcohol is obtained first mixed with water; this mixture is then concentrated by fractional distillation. But it happens that the component of lowest boiling point is not ethyl alcohol (b.p. 78.3) but a binary azeotrope containing 95% alcohol and 5% water (b.p. 78.15). As an azeotrope, it of course gives a vapor of the same composition, and hence cannot be further concentrated by distillation no matter how efficient the fractionating column used. Pure ethyl alcohol is known as absolute alcohol. Although more expensive than 95% alcohol, it is available for use when specifically required. It is obtained by taking advantage of the existence of another azeotrope, this time a ternary one of b.p. 64.9: 7.5% water, 18.5% ethyl alcohol, and 74% benzene. For certain special purposes even the slight trace of water found in commercial absolute alcohol must be removed. This can be accomplished by treatment of the alcohol with metallic magnesium; water is converted into insoluble Mg(OH)2 , from which the alcohol is then distilled.