MP Board · Class 12 · Chemistry · Alcohols, Phenols and EthersExplain the preparation of phenol from Cumene with complete chemical equations and the mechanism involved. Also, discuss why phenols are more acidic than alcohols.

Preparation of Phenol from Cumene (Isopropylbenzene)

\nCumene is an important industrial starting material for the synthesis of phenol and acetone. The process involves the catalytic oxidation of cumene followed by acid-catalyzed cleavage.

Step 1: Oxidation of Cumene\nCumene is oxidized by passing air or oxygen through it in the presence of a cobalt catalyst at elevated temperatures. This reaction involves free radical oxidation, leading to the insertion of an oxygen molecule into the tertiary carbon-hydrogen bond of the isopropyl group, forming cumene hydroperoxide.

$$\text{C}_6\text{H}_5\text{-CH(CH}_3)_2 + \text{O}_2 \xrightarrow{\text{Catalyst}} \text{C}_6\text{H}_5\text{-C(CH}_3)_2\text{-O-OH} (\text{Cumene hydroperoxide})$$

Step 2: Acid Cleavage of Cumene Hydroperoxide\nCumene hydroperoxide is treated with dilute acid (such as dilute sulfuric acid) to undergo rearrangement and cleavage. The peroxy oxygen is protonated, followed by the migration of the phenyl group from the tertiary carbon to the adjacent oxygen atom, yielding a carbocation intermediate. Subsequent nucleophilic attack by water and cleavage yields phenol and acetone as co-products.

$$\text{C}_6\text{H}_5\text{-C(CH}_3)_2\text{-O-OH} \xrightarrow{\text{H}^+} \text{C}_6\text{H}_5\text{OH} (\text{Phenol}) + \text{CH}_3\text{COCH}_3 (\text{Acetone})$$

Why Phenols are More Acidic than Alcohols

\nThe higher acidity of phenols compared to aliphatic alcohols can be explained by examining the stability of their respective conjugate bases (alkoxide ions versus phenoxide ions).

• Resonance Stabilization of Phenoxide Ion: When a phenol loses a proton, it forms a phenoxide ion ($\text{C}_6\text{H}_5\text{O}^-$). The negative charge on the oxygen atom is delocalized into the aromatic benzene ring through resonance. There are several contributing canonical structures for the phenoxide ion that spread the negative charge over the ortho and para positions of the ring. This extensive delocalization significantly stabilizes the phenoxide ion.
• Lack of Resonance in Alkoxide Ion: In contrast, when an alcohol loses a proton, it forms an alkoxide ion ($\text{RO}^-$). The negative charge is localized entirely on the oxygen atom, with no resonance stabilization available.
• Inductive Effect of $sp^2$ Hybridized Carbon: The carbon atom attached to the hydroxyl group in phenol is $sp^2$ hybridized, which is more electronegative than the $sp^3$ hybridized carbon in alcohols. This helps in polarizing the $\text{O-H}$ bond and stabilizing the resulting negative charge. \nBecause the phenoxide ion is much more stable than the alkoxide ion, the equilibrium for proton loss lies further to the right for phenol, making it a stronger acid.