Phenols

General characteristics of phenols

Phenols are hydroxyl derivatives of aromatic hydrocarbons. Their general formula is Ar-OH, where Ar is an aryl group. The hydroxyl group is bonded directly to the carbon atom in the benzene ring. The simplest compound classified as a phenol is hydroxybenzene, commonly known as phenol.

See the range of phenols from the PCC Group.

Phenols exhibit acidic properties (stronger than alcohols). In reactions with hydroxides they form salts called phenolates. They also undergo oxidation, resulting in the formation of quinones. The hydroxyl group in phenols is nucleophilic. It activates the aromatic ring and directs further substituents to the ortho- and para- positions. Phenols react easily with bromine. In the process of phenol bromination, a precipitate is formed – 2,4,6-tribromophenol (as many as three hydrogen atoms are substituted with bromine).

Phenols react with iron (III) compounds. During the reaction, the colour of the solution changes from orange to violet. This method is used to identify phenols.

Phenols can be produced in several ways:

• In the process of alkaline hydrolysis of chlorobenzene. This process must be carried out in high temperatures and at an increased pressure. By treating chlorobenzene with sodium hydroxide, sodium phenoxide is obtained. Then, it reacts with hydrochloric acid or carbonic acid to give a phenol.
• In the Cumene process. The phenol is obtained by the oxidation of cumene, or propan-2-ylbenzene, with oxygen, in an elevated temperature.
• From gas tar formed during the coking of hard coal.

Di- and trihydric phenols

In addition to monohydric phenols, there are also di- and trihydric phenols. They are called polyphenols. Their characteristic feature is that two and three hydroxyl groups are attached to their aromatic ring, respectively. Dihydric phenols are of greater importance in the chemical industry.

The simplest dihydric phenols:

• 1,2-dihydroxybenzene – commonly known as pyrocatechin. The hydroxyl groups are attached to the ring in the ortho position. The compound is used, e.g. as a component of developers in photography, a raw material for the production of medicines or as an antiseptic.
• 1,3-dihydroxybenzene – its common name is resorcinol. The hydroxyl groups are in the meta- position to each other. Resorcinol is applied mainly in the cosmetics industry, where it is used in the composition of anti-acne preparations (due to its bactericidal properties) and in preparations indicated for the treatment of discolourations.
• 1,4-dihydroxybenzene – commonly known as hydroquinone. In 1,4-dihydroxybenzene, the -OH groups are in the para- position to each other. It is commonly used in cosmetics (preparations indicated in the treatment of discolourations), the pharmaceutical and rubber industries.

Trihydric phenols include 1,2,3-trihydroxybenzene, 1,2,4-trihydroxybenzene and 1,3,5-trihydroxybenzene. Their common names are pyrogallol, hydroxyhydroquinone, and phloroglucin, respectively.

Like phenols, polyphenols react with sodium hydroxide to form phenolates (salts). The presence of more than one hydroxyl group in the aromatic ring increases their reactivity towards other chemical compounds.

Phenol

The best known compound from the group of phenols is hydroxybenzene or phenol. It is a colourless, crystalline solid with a low melting point. Under the influence of air and light it darkens over time. It must be noted that it is a highly poisonous compound. Direct contact causes burns and hard-to-heal wounds.

The solubility of phenol in water depends on its temperature. It usually dissolves poorly in cold water and very well in hot water. The properties of phenol indicate that it is a weak acid (even weaker than carbonic acid). This can be observed by introducing a phenol solution into, e.g., an aqueous solution of sodium hydroxide, which will have a raspberry colour in the presence of phenolphthalein. The reaction produces a salt, sodium phenolate, and the phenolphthalein becomes discoloured. Other phenols react similarly. In reactions with hydroxides, they form salts, called phenolates. The acidic properties of phenol result from the presence of the phenolic group. It facilitates the substitution of the hydrogen atom in the hydroxyl group with sodium (alcohols,  for example, do not react with NaOH). Phenolates can be easily decomposed by treating them with acids. The presence of a hydroxyl group makes the aromatic ring in the phenol molecule much more likely to react than, e.g., benzene. This results from the fact that the -OH group is an activating substituent that facilitates the substitution of hydrogen atoms in the ortho- and para- positions. This way, for example by chlorinating phenol, a mixture of o-chlorophenol and p-chlorophenol is formed.

Phenol is of great importance, primarily in the chemical industry, where it is a component for the production of intermediates and end-products. It is a substrate in the production of plastics, resins and explosives. In the pharmaceutical industry, phenol is used to synthesize some medications.