Phenol | Definition, Structure, Uses, & Facts

Nomenclature of phenols

Many phenolic compounds were discovered and used long before chemists were able to determine their structures. Therefore, trivial names (i.e., vanillin, salicylic acid, pyrocatechol, resorcinol, cresol, hydroquinone, and eugenol) are often used for the most common phenolic compounds.

Systematic names are more useful, however, because a systematic name specifies the actual structure of the compound. If the hydroxyl group is the principal functional group of a phenol, the compound can be named as a substituted phenol, with carbon atom 1 bearing the hydroxyl group. For example, the systematic name for thymol is 5-methyl-2-isopropylphenol. Phenols with only one other substituent can be named using either the appropriate numbers or the ortho (1,2), meta (1,3), and para (1,4) system. Compounds with other principal functional groups can be named with the hydroxyl group as a hydroxy substituent. For example, the systematic name for vanillin is 4-hydroxy-3-methoxybenzaldehyde.

Physical properties of phenols

Similar to alcohols, phenols have hydroxyl groups that can participate in intermolecular hydrogen bonding; in fact, phenols tend to form stronger hydrogen bonds than alcohols. (See chemical bonding: Intermolecular forces for more information about hydrogen bonding.) Hydrogen bonding results in higher melting points and much higher boiling points for phenols than for hydrocarbons with similar molecular weights. For example, phenol (molecular weight [MW] 94, boiling point [bp] 182 °C [359.6 °F]) has a boiling point more than 70 degrees higher than that of toluene (C6H5CH3; MW 92, bp 111 °C [231.8 °F]).

The ability of phenols to form strong hydrogen bonds also enhances their solubility in water. Phenol dissolves to give a 9.3 percent solution in water, compared with a 3.6 percent solution for cyclohexanol in water. The association between water and phenol is unusually strong; when crystalline phenol is left out in a humid environment, it picks up enough water from the air to form liquid droplets.

Synthesis of phenols

Most of the phenol used today is produced from benzene, through either hydrolysis of chlorobenzene or oxidation of isopropylbenzene (cumene).

Hydrolysis of chlorobenzene (the

Dow process)

Benzene is easily converted to chlorobenzene by a variety of methods, one of which is the Dow process. Chlorobenzene is hydrolyzed by a strong base at high temperatures to give a phenoxide salt, which is acidified to phenol.

Oxidation of

isopropylbenzene

Benzene is converted to isopropylbenzene (cumene) by treatment with propylene and an acidic catalyst. Oxidation yields a hydroperoxide (cumene hydroperoxide), which undergoes acid-catalyzed rearrangement to phenol and acetone. Although this process seems more complicated than the Dow process, it is advantageous because it produces two valuable industrial products: phenol and acetone.

General synthesis of phenols

To make more-complicated phenolic compounds, a more general synthesis is needed. The cumene hydroperoxide reaction is fairly specific to phenol itself. The Dow process is somewhat more general, but the stringent conditions required often lead to low yields, and they may destroy any other functional groups on the molecule. A milder, more general reaction is the diazotization of an arylamine (a derivative of aniline, C6H5NH2) to give a diazonium salt, which hydrolyzes to a phenol. Most functional groups can survive this technique, as long as they are stable in the presence of dilute acid.