Alcohols, Phenols and Ethers
7.0 Tests for Alchols
7.0 Tests for Alchols
Alcohols dissolve in cold concentrated sulfuric acid. This property they share with alkenes, amines, practically all compounds containing oxygen, and easily sulfonated compounds. (Alcohols, like other oxygen-containing compounds, form oxonium salts, which dissolve in the highly polar sulfuric acid.)
Alcohols are not oxidized by cold, dilute, neutral permanganate (although primary and secondary alcohols are, of course, oxidized by permanganate under more vigorous conditions.). However, as we have seen, alcohols often contain impurities that are oxidized under these conditions, and so the permanganate test must be interpreted with caution
Alcohols do not decolourize bromine in carbon tetrachloride. This property serves to distinguish them from alkenes and alkynes.
Alcohols are further distinguished from alkenes and alkynes-and, indeed, from nearly every other kind of compound-by their oxidation by chromic anhydride, CrO3, in aqueous sulfuric acid: within two seconds, the clear orange solution turns blue-green and becomes opaque.
Tertiary alcohols do not give this test. Aldehydes do but are easily differentiated in other ways.
Reactions of alcohols with sodium metal, with the evolution of hydrogen gas, is of some use in characterization; a wet compound of any kind, of course, will do the same thing, until the water is used up.
The presence of the –OH group in a molecule is often indicated by the formation of an ester upon treatment with an acid chloride or anhydride. Some esters are sweet-smelling; others are solids and sharp melting points and can be derivatives in identifications. (If the molecular formulas of starting material and product are determined, it is possible to calculate how many –OH groups are present.
