Question

An organic compound (A) has characteristic odour. On treatment with $$NaOH$$, it forms compounds (B) and (C). Compound (B) has molecular formula $$C_7H_8 O$$ which on oxidation gives back (A). The compound (C) is a sodium salt of an acid. When (C) is treated with soda-lime, it yields an aromatic compound (D). Deduce the structures of (A), (B) , (C) and (D). Write the sequence of reaction involved.

Answer 1

(i) Since the M.F. of compound (B) is $$C_7H_8O$$, it can have any one of the following five possible structures.
$$\underset{Benzyl \, alcohol}{C_6H_5CH_2OH}$$ $$\underset{Anisole}{C_6H_5OCH_3}$$ $$\underset{o^-, m^- \, and \, p^- Cresol}{CH_3 - C_6H_4OH}$$
(ii) Since cresols on oxidation give complex products and anisole cannot be easily oxidised, therefore compound (B) most probably must be benzyl alcohol.
(iii) Since alcohols are easily oxidised and oxidation of compound (B) gives back compound (A) with characteristic odour, therefore, compound (B) must be benzyl alcohol and compound (A) may be benzaldehyde.
(iv) Now benzaldehyde (A) on treatment with $$NaOH$$ undergoes Cannizzaro reaction to give two compounds (B) and (C). Since (B) is benzyl alcohol, therefore, (C) must be sodium benzoate, $$C_6H_5COONa$$.
(v) Since sodium benzoate on treatment with soda-lime gives an aromatic compound (D), therefore compound (D) must be benzene.
(vi) The chemical equations of all the reactions described above are given below:
$$\underset{Benzaldehyde (A)}{\underset{M.F. \, C_7H_8O}{C_6H_5CHO}} \xrightarrow[(Cannizzaro \, reaction)]{NaOH} \underset{Benzyl \, alcohol \, (B)}{C_6H_5CH_2OH} + \underset{Sod. \, benzoate \, (C)}{C_6H_5COONa} \xrightarrow{Soda-lime, \Delta} \underset{Benzene (D)}{C_6H_6 + } Na_2CO_3$$
Thus, $$A = C_6H_5CHO, \, B = C_6H_5CH_2OH, \, C = C_6H_5COONa, \, D = C_6 H_6$$