JEE Chemistry Alcohols Phenols and Ethers Notes Set 05

Phenols: Definition

Phenols have an OH group attached directly to an aromatic ring, with phenol (C6H5OH) as the parent member. Cresols, catechol, resorcinol, quinol and naphthols are common examples. Compounds such as benzyl alcohol, in which the OH is on a side-chain carbon, are not phenols; they behave like aliphatic alcohols.

Preparation of Phenols

1. From chlorobenzene: heating with aqueous NaOH at high temperature and pressure gives sodium phenoxide, which is acidified to phenol (Dow process).
2. From benzenesulphonic acid: fusion of its sodium salt with NaOH gives sodium phenoxide, then acidification gives phenol.
3. From cumene (the industrial method): benzene and propene with H3PO4 give cumene (electrophilic substitution). Air oxidation of cumene gives cumene hydroperoxide by a free-radical reaction. Acid then rearranges the hydroperoxide (a phenyl shift to oxygen) and hydrolysis gives phenol and acetone.
4. From benzenediazonium chloride: warming the aqueous solution gives phenol with loss of N2.
5. From benzene directly with hydrogen peroxide in fluorosulphonic acid.
6. From the Grignard reagent: \( C_6H_5MgBr \) reacts with oxygen to give \( C_6H_5OMgBr \), which on hydrolysis gives phenol.

\[ \ce{C6H5Cl + NaOH ->[623 K, 300 atm] C6H5ONa ->[H+] C6H5OH} \]
\[ \ce{C6H5N2+Cl- + H2O ->[heat] C6H5OH + N2 + HCl} \]
\[ \ce{C6H5CH(CH3)2 ->[O2 (air)] C6H5C(CH3)2OOH ->[H+ / H2O] C6H5OH + (CH3)2CO} \]

Acidic Nature of Phenol

Phenol is a weak acid. It turns neutral ferric chloride solution violet but does not affect litmus, and it dissolves in NaOH and reacts with sodium to give sodium phenoxide. It does not react with Na2CO3 or NaHCO3, which is a way to separate phenols from carboxylic acids.
Phenol is more acidic than alcohols and water for two reasons. The carbon bearing the OH is sp2 hybridised and more electronegative, so the O-H bond is more polar. More importantly, the phenoxide ion is stabilised by delocalisation of its negative charge into the ring (five resonating structures), whereas an alkoxide ion has no such stabilisation.

\[ \ce{C6H5OH + NaOH -> C6H5O-Na+ + H2O} \]
\[ \ce{2C6H5OH + 2Na -> 2C6H5O-Na+ + H2} \]

Reactions of the OH Group

1. Ester formation: acetylation and benzoylation of phenol (with acetyl chloride, acetic anhydride or benzoyl chloride in the presence of alkali or pyridine) is called the Schotten-Baumann reaction.
2. Ether formation: sodium phenoxide with an alkyl halide or alkyl sulphate (Williamson synthesis) gives an alkyl aryl ether such as anisole.
3. With PCl5: gives chlorobenzene in poor yield; the main product is triphenyl phosphate.
4. With zinc dust: phenol is reduced to benzene (\( C_6H_5OH + Zn \rightarrow C_6H_6 + ZnO \)).
5. Oxidation: air slowly darkens phenol, and a chromic acid oxidation gives p-benzoquinone.
6. Condensation with phthalic anhydride: two molecules of phenol with phthalic anhydride and concentrated H2SO4 give phenolphthalein, an acid-base indicator (a Friedel-Crafts acylation). With resorcinol the product is fluorescein.
7. With ammonia: heating phenol with NH3 over alumina at about 673 K under high pressure gives aniline.

Electrophilic Substitution in the Ring

The OH group is strongly activating and o-, p-directing (+M effect), so phenol undergoes substitution much more readily than benzene, often without a Lewis acid catalyst.

Picric acid is made in better yield by first sulphonating phenol to phenol-2,4-disulphonic acid and then treating this with concentrated nitric acid, because direct nitration gives a poor yield (the nitro group is deactivating and phenol is oxidised).

Reimer-Tiemann and Kolbe Reactions

Reimer-Tiemann reaction: chloroform and aqueous alkali generate dichlorocarbene, \( :CCl_2 \), a neutral electrophile, which attacks the highly reactive phenoxide ring. The ortho position is attacked mainly because it is the more activated site and because the product is stabilised by chelation (intramolecular hydrogen bonding).
Kolbe's reaction: carbon dioxide acts as the electrophile toward sodium phenoxide, giving mainly salicylic acid. The small Na+ ion stabilises the ortho transition state. With potassium phenoxide, the larger K+ ion does not do this, and the para acid (p-hydroxybenzoic acid) becomes the major product. Salicylic acid is the starting material for several medicines, such as aspirin.

\[ \ce{C6H5OH + CHCl3 + 3NaOH -> o-HOC6H4CHO + 3NaCl + 2H2O} \]
\[ \ce{C6H5ONa + CO2 ->[400 K, 4-7 atm] o-HOC6H4COONa ->[H+] o-HOC6H4COOH} \]