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Phenol

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Phenol

chemical compound

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External Websites
  • GOV.UK – Phenol

  • Chemistry LibreTexts Library – Phenol

  • National Center for Biotechnology Information – PubChem – Phenol

Phenol, any of a family of

organic compounds

characterized by a hydroxyl (―OH) group attached to a

carbon

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

carbolic acid

.

methane molecule
Read More on This Topic
chemical compound: Alcohols and phenols
An oxygen atom normally forms two σ bonds with other atoms; the water molecule, H2O, is the simplest and most common example….

Phenols are similar to

alcohols

but form stronger hydrogen bonds. Thus, they are more soluble in

water

than are alcohols and have higher

boiling points

. Phenols occur either as colourless

liquids

or white

solids

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

antiseptic

. In 1865 the British surgeon

Joseph Lister

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

antioxidant

in foods.

In industry, phenol is used as a starting material to make

plastics

,

explosives

such as

picric acid

, and

drugs

such as

aspirin

. The common phenol

hydroquinone

is the component of photographic developer that reduces exposed silver bromide crystals to black metallic silver. Other substituted phenols are used in the

dye

industry to make intensely coloured

azo dyes

. Mixtures of phenols (especially the

cresols

) are used as components in wood preservatives such as

creosote

.

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Natural sources of phenols

Phenols are common in nature; examples include

tyrosine

, one of the standard

amino acids

found in most

proteins

;

epinephrine

(adrenaline), a stimulant

hormone

produced by the adrenal medulla;

serotonin

, a

neurotransmitter

in the brain; and urushiol, an irritant secreted by

poison ivy

to prevent animals from eating its leaves. Many of the more complex phenols used as flavourings and aromas are obtained from

essential oils

of plants. For example,

vanillin

, the principal flavouring in

vanilla

, is isolated from vanilla beans, and

methyl salicylate

, which has a characteristic minty taste and odour, is isolated from

wintergreen

. Other phenols obtained from plants include thymol, isolated from

thyme

, and eugenol, isolated from

cloves

.

Phenol. Chemical Compounds. Structural formulas for some phenols: phenol, (hydroxybenzen), n-hexylresorcinol, BHT, tyrosine, epinephrine (adrenalin), serotonin, urushiol, vanillin, methyl salicylate.

Phenol, the

cresols

(methylphenols), and other simple alkylated phenols can be obtained from the

distillation

of

coal tar

or crude

petroleum

.

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.

Phenol. Chemical Compounds. Structures of some phenolic compounds: hydroquinone, resorcinol, meta-cresol, eugenol, and thymol.

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.

Phenol. Chemical Compounds. Systematic names for some phenols: 2,4,6-trinitrophenol (picric acid), 2-bromophenol (ortho-bromophenol), and 4-hydroxybenzoic acid (para-hydroxybenzoic acid)

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.

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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.

Phenol. Chemical Compounds. The Dow process of converting benzene to chlorobenzene. 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.

Phenol. Chemical Compounds. Oxidation of isopropylbenzene. Benzene is converted to isopropylbenzene by treatment with propylene and an acidic catalyst. Oxidation yields a hydroperoxide which undergoes acid-catalyzed rearrangement to phenol and acetone.

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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

.

Phenol. Chemical Compounds. Diazotization of an arrylamine to give a diazonium salt, which hydrolyzes to a phenol.

Reactions of phenols

Much of the

chemistry

of phenols is like that of

alcohols

. For example, phenols react with acids to give

esters

, and phenoxide ions (ArO) can be good

nucleophiles

in Williamson

ether

synthesis.

Phenol. Chemical Compounds. Phenols react with acids to give esters, and phenoxide ions can be good nucleophiles in Williamson ether synthesis.

Acidity

of phenols

Although phenols are often considered simply as aromatic alcohols, they do have somewhat different properties. The most obvious difference is the

enhanced

acidity of phenols. Phenols are not as acidic as

carboxylic acids

, 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).

Phenol. Chemical Compounds. Most phenols are completely deprotonated by sodium hydroxide.

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Oxidation

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

quinones

. In the presence of

oxygen

in the air, many phenols slowly oxidize to give dark mixtures containing quinones.

Phenol. Chemical Compounds. In the presence of oxygen in the air, many phenols slowly oxidize to give dark mixtures contianing quinones.

Hydroquinone

(1,4-benzenediol) is a particularly easy compound to oxidize, because it has two hydroxyl groups in the proper relationship to give up

hydrogen

atoms to form a

quinone

. Hydroquinone is used in developing photographic film by reducing activated (exposed to

light

) silver bromide (AgBr) to black metallic silver (Ag↓). Unexposed grains of silver bromide react more slowly than the exposed grains.

Phenol. Chemical Compounds. Hydroquinone (1,4-benzenediol) is a particularly easy compound to oxidize, because it has two hydroxyl groups in the proper relationship to give up hydrogen atoms to form a quinone.

Electrophilic aromatic substitution

Phenols are highly reactive toward electrophilic aromatic substitution, because the nonbonding

electrons

on oxygen stabilize the intermediate

cation

. 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.

Phenol. Chemical Compounds. Phenols are highly reactive toward electrophilic aromatic substitution because the nonbonding electrons on oxygen stabilize the intermediate cation.

Picric acid

(2,4,6-trinitrophenol) is an important

explosive

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

aromatic compound

. 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.

Phenol. Chemical Compounds. Creation of picric acid by the addition of three nitro groups to a phenol.

Phenoxide ions, generated by treating a phenol with sodium hydroxide, are so strongly activated that they undergo electrophilic aromatic substitution even with very weak

electrophiles

such as

carbon dioxide

(CO2). This reaction is used commercially to make

salicylic acid

for conversion to

aspirin

and methyl salicylate.

Phenol. Chemical Compounds. Phenoxide ions are generated by treating a phenol with sodium hydroxide and undergo electrophilic aromatic substitution even with weak electrophiles such as CO2. The reaction is used to make salicylic acid.

Formation of phenol-formaldehyde resins

Phenolic

resins

account for a large portion of phenol production. Under the trade name

Bakelite

, a

phenol-formaldehyde resin

was one of the earliest

plastics

, invented by American industrial chemist

Leo Baekeland

and patented in 1909. Phenol-formaldehyde resins are inexpensive, heat-resistant, and waterproof, though somewhat brittle. The

polymerization

of phenol with

formaldehyde

involves electrophilic aromatic substitution at the ortho and para positions of phenol (probably somewhat randomly), followed by cross-linking of the polymeric chains.

Phenol. Chemical Compounds. Polymerization of phenol with formaldehyde involves electrophilic aromatic substitution at the ortho and para positions of phenol, followed by cross-linking of the polymeric chains.

Leroy G. Wade

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