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Phenol, any of a family of
characterized by a hydroxyl (―OH) group attached to a
atom that is part of an aromatic ring. Besides serving as the generic name for the entire family, the term phenol is also the specific name for its simplest member, monohydroxybenzene (C6H5OH), also known as benzenol, or
Phenols are similar to
but form stronger hydrogen bonds. Thus, they are more soluble in
than are alcohols and have higher
. Phenols occur either as colourless
at room temperature and may be highly toxic and caustic.
Phenols are widely used in household products and as intermediates for industrial synthesis. For example, phenol itself is used (in low concentrations) as a disinfectant in household cleaners and in mouthwash. Phenol may have been the first surgical
. In 1865 the British surgeon
used phenol as an antiseptic to sterilize his operating field. With phenol used in this manner, the mortality rate from surgical amputations fell from 45 to 15 percent in Lister’s ward. Phenol is quite toxic, however, and concentrated solutions cause severe but painless burns of the skin and mucous membranes. Less-toxic phenols, such as n-hexylresorcinol, have supplanted phenol itself in cough drops and other antiseptic applications. Butylated hydroxytoluene (BHT) has a much lower toxicity and is a common
In industry, phenol is used as a starting material to make
. The common phenol
is the component of photographic developer that reduces exposed silver bromide crystals to black metallic silver. Other substituted phenols are used in the
industry to make intensely coloured
. Mixtures of phenols (especially the
) are used as components in wood preservatives such as
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Natural sources of phenols
Phenols are common in nature; examples include
, one of the standard
found in most
(adrenaline), a stimulant
produced by the adrenal medulla;
in the brain; and urushiol, an irritant secreted by
to prevent animals from eating its leaves. Many of the more complex phenols used as flavourings and aromas are obtained from
of plants. For example,
, the principal flavouring in
, is isolated from vanilla beans, and
, which has a characteristic minty taste and odour, is isolated from
. Other phenols obtained from plants include thymol, isolated from
, and eugenol, isolated from
(methylphenols), and other simple alkylated phenols can be obtained from the
Nomenclature of phenols
were discovered and used long before chemists were able to determine their structures. Therefore, trivial names (i.e., vanillin,
, 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
. 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
; 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
and much higher
for phenols than for
with similar molecular weights. For example, phenol (molecular weight [MW] 94,
[bp] 182 °C [359.6 °F]) has a boiling point more than 70 degrees higher than that of
(C6H5CH3; MW 92, bp 111 °C [231.8 °F]).
The ability of phenols to form strong hydrogen bonds also
. 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
, it picks up enough water from the air to form liquid droplets.
Synthesis of phenols
Most of the phenol used today is produced from
, through either
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
at high temperatures to give a phenoxide salt, which is acidified to phenol.
Benzene is converted to isopropylbenzene (cumene) by treatment with
and an acidic
. Oxidation yields a hydroperoxide (
), which undergoes acid-catalyzed rearrangement to phenol and
. 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
on the molecule. A milder, more general reaction is the
(a derivative of
, C6H5NH2) to give a
, which hydrolyzes to a phenol. Most functional groups can survive this technique, as long as they are stable in the presence of dilute
Reactions of phenols
Much of the
of phenols is like that of
. For example, phenols react with acids to give
, and phenoxide ions (ArO−) can be good
Although phenols are often considered simply as aromatic alcohols, they do have somewhat different properties. The most obvious difference is the
acidity of phenols. Phenols are not as acidic as
, but they are much more acidic than aliphatic alcohols, and they are more acidic than water. Unlike simple alcohols, most phenols are completely deprotonated by sodium hydroxide (NaOH).
Like other alcohols, phenols undergo oxidation, but they give different types of products from those seen with aliphatic alcohols. For example, chromic acid oxidizes most phenols to conjugated 1,4-diketones called
. In the presence of
in the air, many phenols slowly oxidize to give dark mixtures containing quinones.
(1,4-benzenediol) is a particularly easy compound to oxidize, because it has two hydroxyl groups in the proper relationship to give up
atoms to form a
. Hydroquinone is used in developing photographic film by reducing activated (exposed to
) silver bromide (AgBr) to black metallic silver (Ag↓). Unexposed grains of silver bromide react more slowly than the exposed grains.
Electrophilic aromatic substitution
Phenols are highly reactive toward electrophilic aromatic substitution, because the nonbonding
on oxygen stabilize the intermediate
. This stabilization is most effective for attack at the ortho or para position of the ring; therefore, the hydroxyl group of a phenol is considered to be activating (i.e., its presence causes the aromatic ring to be more reactive than benzene) and ortho- or para-directing.
(2,4,6-trinitrophenol) is an important
that was used in
World War I
. An effective explosive needs a high proportion of oxidizing groups such as nitro groups. Nitro groups are strongly deactivating (i.e., make the aromatic ring less reactive), however, and it is often difficult to add a second or third nitro group to an
. Three nitro groups are more easily substituted onto phenol, because the strong activation of the hydroxyl group helps to counteract the deactivation of the first and second nitro groups.
Phenoxide ions, generated by treating a phenol with sodium hydroxide, are so strongly activated that they undergo electrophilic aromatic substitution even with very weak
(CO2). This reaction is used commercially to make
for conversion to
and methyl salicylate.
Formation of phenol-formaldehyde resins
account for a large portion of phenol production. Under the trade name
was one of the earliest
, invented by American industrial chemist
and patented in 1909. Phenol-formaldehyde resins are inexpensive, heat-resistant, and waterproof, though somewhat brittle. The
of phenol with
involves electrophilic aromatic substitution at the ortho and para positions of phenol (probably somewhat randomly), followed by cross-linking of the polymeric chains.
Leroy G. Wade
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