Question

How will you convert benzaldehyde into following compounds? More than one step be required.
(i) Benzophenone
(ii) Benzoic acid

Answer 1

Hint: Benzaldehyde is a compound having an aldehyde group attached to benzene. Benzophenone is a compound having two benzene groups connected with a carbonyl group. While benzoic acid is a compound having carboxyl groups attached to benzene. Benzene to benzoic acid is a simple step conversion while the other involves two to three steps.

Complete step by step answer:
(i) When aldehyde is oxidized, a carboxylic acid is obtained. Benzaldehyde is the simplest aromatic aldehyde. It has an almond-like odor. Benzoic acid is a colorless solid. The chemical formula of benzoic acid is ${{\text{C}}_6}{{\text{H}}_5}{\text{COOH}}$.
Thus benzaldehyde is converted to benzophenone in three steps:
Oxidation of benzaldehyde to benzoic acid.
Benzaldehyde is oxidized to benzoic acid in the presence of alkaline potassium permanganate. Since benzaldehyde possesses a benzene ring, it facilitates the electrophilic substitution reaction. The chemical equation for oxidation of benzaldehyde to benzoic acid is given below:
${{\text{C}}_6}{{\text{H}}_5}{\text{CHO}}\xrightarrow{{{\text{KMn}}{{\text{O}}_4}}}{{\text{C}}_6}{{\text{H}}_5}{\text{COOH}}$
Benzaldehyde Benzoic acid
Conversion of benzoic acid to benzene.
Benzoic acid is heated with soda lime which is a mixture of ${\text{NaOH}}$ and ${\text{CaO}}$, benzene is obtained. The reaction is given below:
${{\text{C}}_6}{{\text{H}}_5}{\text{COOH}}\xrightarrow[{{\text{CaO}}}]{{{\text{NaOH}}}}{{\text{C}}_6}{{\text{H}}_6}$
Conversion of benzene to benzophenone.
Benzene is reacted with benzoyl chloride, ${{\text{C}}_6}{{\text{H}}_5}{\text{COCl}}$ in the presence of ${\text{Al}}{{\text{Cl}}_3}$, benzophenone, ${{\text{C}}_6}{{\text{H}}_5}{\text{CO}}{{\text{C}}_6}{{\text{H}}_5}$ is obtained. The chemical equation is given below:
${{\text{C}}_6}{{\text{H}}_6} + {{\text{C}}_6}{{\text{H}}_5}{\text{COCl}}\xrightarrow{{{\text{Al}}{{\text{Cl}}_3}}}{{\text{C}}_6}{{\text{H}}_5}{\text{CO}}{{\text{C}}_6}{{\text{H}}_5}$
This reaction is called Friedel Crafts acylation.

Note:
Benzaldehyde can also be converted to benzoic acid by Cannizaro reaction. Cannizaro reaction is a type of disproportionation reaction. This involves disproportionation of an aldehyde which lacks $\alpha - $ hydrogen atom to salt of an acid and a primary alcohol. The reaction is given below:
$2$

$ + {\text{NaOH}} \to $

$ + $

$\xrightarrow{{{{\text{H}}^ + }}}$

Benzaldehyde Benzyl alcohol Sodium benzoate