Chemical Properties of Phenols: Meaning, Members, Structure

Chemical Properties of Phenol: Phenols are formed by replacing one hydrogen atom from an aromatic hydrocarbon (aliphatic in case of alcohols) with a \({\rm{ – OH}}\) group. Phenol is a hydroxyl derivative of benzene, wherein a \({\rm{ – OH}}\) group is attached to the benzene ring. Depending upon the number of \({\rm{OH}}\) groups attached to the ring, the phenols are classified as mono or dihydroxy or polyhydroxy derivatives. The name ‘phenol’ is given to the simplest derivative, and the group members are named as derivatives of phenol. For example, o-cresol, with \( – {\rm{C}}{{\rm{H}}_3}\) group and one \({\rm{ – OH}}\), is called \(2\)-methyl phenol.

Common Members of Phenol Family and their Structures

Some common and significant members of the phenol family include:

Structure of Phenol

Phenol has a ring structure, and therefore, the carbon atom in the ring (with alternate double bonds) is \({\rm{s}}{{\rm{p}}^2}\) hybridized. The \({\rm{ – OH}}\) group is attached to the \({\rm{s}}{{\rm{p}}^2}\) hybridized carbon atom present in the aromatic ring. Hence, the \({\rm{C – O}}\) bond is formed when an overlapping of \({\rm{s}}{{\rm{p}}^3}\) orbital of oxygen and the \({\rm{s}}{{\rm{p}}^2}\) hybridized orbital of carbon in the aromatic ring takes place.

Hence, compared to alcohol (methanol, for example), the \({\rm{C – O}}\) bond length in phenols is slightly less (\({\rm{136}}\,{\rm{pm}}\)). The reason behind this is that:

a. There is a partial double bond character between \({\rm{C – O}}\) in phenols due to the conjugation of the unshared electron pairs around oxygen with the aromatic ring structure.
b. The \({\rm{s}}{{\rm{p}}^2}\) hybridized state of the \({\rm{‘C’}}\) atom in the ring, to which the oxygen of the \({\rm{OH}}\) group is attached.

The lone pair of electrons on the \({\rm{ – OH}}\) group of phenol is in resonance with the ring, thereby always in a state of delocalization.

Physical Properties of Phenol

The structure of phenols consists of two parts:

a. The aromatic ring or the aryl group
b. The functional group \(\left( {{\rm{OH}}} \right)\) or the hydroxyl group

The properties of phenols are, however, chiefly due to the hydroxyl group. In phenols, the presence of substituents in the ring can alter or modify their properties slightly or considerably depending upon the type and number of substituents present.

Some significant physical properties of phenols are:

I. Physical state: Phenols are colourless liquids or solids. However, they mostly turn reddish-brown in the atmosphere due to oxidation.
II. Boiling point: Boiling points of phenols increase with an increase in the number of carbon atoms due to the enhancement of van der Waals forces. Also, the hydroxyl group is in an intermolecular hydrogen bonding and, therefore, exists as associated molecules, thereby increasing the boiling point of phenols.
III. Solubility in water: Due to the ability to form hydrogen bonding, phenols are readily soluble in water. However, with the addition of other hydrophobic groups in the ring, the solubility decreases.

Study The s-block Elements

Chemical Properties of Phenolic Compounds

Again here, the chemical reactions of phenols can be classified under two headings – one involving the cleavage of the \({\rm{O – H}}\) bond and the other involving the cleavage of the \({\rm{C – O}}\) bond.

I. Reactions involving \({\rm{O – H}}\) bond cleavage
a. With metals: Phenols reacts with metals such as \({\rm{Na}},\,{\rm{K}}\) and \({\rm{Al}}\), etc., to form phenoxide with the release of hydrogen gas.

Apart from active metals, phenol gives sodium phenoxide on reaction with aqueous \({\rm{NaOH}}\), too, releasing a water molecule.

The above reaction shows that phenols are acidic in nature. Phenols are Bronsted acids and, therefore, donate a proton to a stronger base like \({\rm{NaOH}}\).

Acidity of phenols is due to the release of \({{\rm{H}}^ + }\) ions from the hydroxyl group. The reason behind the release of a proton is that the \({\rm{OH}}\) group is involved in resonance, and therefore, the oxygen gets a partial positive charge. This enables the \({{\rm{H}}^ + }\) ion to move out easily, thereby making phenols a Bronsted acid. Also, phenols are stronger acids than their counterparts, alcohols, due to the fact that the phenoxide ion (formed after the release of a proton) is stabilized by resonance. This makes the removal of \({{\rm{H}}^ + }\) ions easier in phenols.

b. Formation of Esters (Esterification)

Phenols react with carboxylic acids, acid anhydrides and acid chlorides to form esters. The esterification reaction is carried out in the presence of sulphuric acid. The reaction is reversible. The water molecule formed is immediately removed to facilitate the completion of the reaction.

Esterification reaction with acid chloride takes place in the presence of pyridine in order to neutralize the \({\rm{HCl}}\) formed during the reaction.

c. Acetylation

Reaction of phenols with an acid anhydride and the introduction of acetyl \(\left( {{\rm{C}}{{\rm{H}}_3}{\rm{CO}} – } \right)\) are called acetylation, and the product obtained is acetylsalicylic acid or aspirin.

II. Electrophilic Aromatic Substitution

The \({\rm{ – OH}}\) group in the phenol activates the benzene ring and pushes it towards electrophilic substitution. Also, the substituents or the incoming groups are directed by the \({\rm{OH}}\) group towards the ortho and para positions of the ring. The reason behind this is because the resonance effect of the ring structure makes these positions electron-rich.

a. Nitration Reaction

Addition of an \( – {\rm{N}}{{\rm{O}}_2}\) group to the ring is called nitration. Phenol reacts with dilute nitric acid at \(298\;{\rm{K}}\) (low temperature) yields ortho and para nitrophenols.

The ortho and para nitrophenols thus obtained can be separated using steam distillation. The intramolecular hydrogen bonding in ortho nitrophenol makes it steam volatile, while para nitrophenol is less volatile because it is bound in intermolecular hydrogen bonding, as shown:

Phenol yields \({\rm{2,}}\,{\rm{4,}}\,{\rm{6}}\)- trinitrophenol or picric acid when treated with concentrated nitric acid. The yield is poor. Picric acid is a strong acid than phenol because of the three electron-withdrawing nitro-groups in the ring.

b. Halogenation Reaction

Phenols react with bromine to yield different products under different experimental conditions.

It reacts with bromine in low polarity solvents, such as chloroform or \({\rm{C}}{{\rm{S}}_2}\), at low temperatures to form mono bromophenols. The reason is that the non-polar or low-polar solvents can only activate the ring in the \(1\) and \(4\) positions. Hence, mono-substituted products are only formed.

The presence of a highly activating group, \({\rm{ – OH}}\) in phenol, makes polarisation of bromine molecule easier.

Phenol forms \({\rm{2,}}\,{\rm{4,}}\,{\rm{6}}\)- tribromophenol on reaction with bromine water. The product is formed as a white precipitate.

c. Kolbe’s Reaction

Phenol reacts with sodium hydroxide and carbon dioxide to form sodium salicylate, which then acidifies to yield \(2\)-hydroxy benzoic acid or salicylic acid. The phenoxide ion formed with \({\rm{NaOH}}\) has higher reactivity than phenol towards aromatic, electrophilic substitution, and therefore, acidifies to produce salicylic acid.

d. Reimer-Tiemann Reaction

Phenol, on reaction with chloroform \(\left( {{\rm{CHC}}{{\rm{l}}_3}} \right)\), in the presence of \({\rm{NaOH}}\), forms an aryl aldehyde. A \({\rm{ – CHO}}\) group is introduced into the ring. This reaction is known as the Reimer-Tiemann reaction. The electrophile formed in this reaction \(\left( {:{\rm{CC}}{{\rm{l}}_2}} \right)\) is called dichlorocarbene. An intermediate, substituted benzal chloride is formed, which is then hydrolyzed into salicylaldehyde, the product, in the presence of alkali.

e. Reaction with Zinc Dust

Phenol, on heating with zinc dust, gets converted into benzene. The reaction is as follows:

f. Oxidation of Phenol

Phenol, on oxidation with chromic acid, forms a conjugated diketone called benzoquinone. In the presence of air, the oxidation reaction of phenols yields a dark-coloured mixture containing quinones.

Uses of Phenol

Phenols and their derivatives are used in several applications, and as:

1. Disinfectants
2. Antiseptics in soaps
3. Making of drugs in the pharmaceutical industry
4. Preparation of azo-dyes
5. As preservatives for inks
6. Manufacturing of picric acid

Summary

Phenols have an \({\rm{ – OH}}\) group attached to the benzene ring, which exhibits some characteristic physical and chemical properties. They resemble in their chemical and physical properties to alcohol because of the presence of the \({\rm{OH}}\) group. However, they have their unique chemical properties due to the ring structure and resonance, such as electrophilic substitution reactions. They are used as disinfectants, antiseptics in soaps, preservatives for inks, etc.

FAQs on Chemical Properties of Phenols

Q.1. What are the significant physical and chemical properties of phenol?
Ans: Phenols are colourless liquids or solids and turn dark brownish colour when exposed to the atmosphere due to oxidation. Phenols undergo specific reactions owning to the ring structure, called electrophilic substitution reactions, wherein the substituents are diverted to the electron-rich ortho or para positions.

Q.2. What is the most important chemical property of phenol?
Ans: Electrophilic substitution reactions and the formation of phenoxides with active metals are some important chemical properties of phenols. The reactions are as given below:

Electrophilic substitution: Nitration

Reaction with active metals: \({\rm{Na}}\)

Q.3. What is the structure of phenol?
Ans: Phenol has a ring structure (benzene) followed by an \({\rm{ – OH}}\) group attached to it, as shown below:

Q.4. What is the \({\rm{pH}}\) of phenol?
Ans: Phenols are weak acids, and therefore, the \({\rm{pH}}\) will be in a higher range than those of mineral acids.

Q.5. What is the colour of phenol?
Ans: Phenols are colourless liquids or solids. But they turn slightly brownish due to their exposure to the atmosphere.

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