Question

Write the structural formula of benzaldehyde. Write chemical equations of the reaction of benzaldehyde with (A) NH$_2$NH$_2$, (B) Tollen’s reagent and (C) NaOH.

Hint:

Benzaldehyde lacks α-H, hence undergoes Cannizzaro reaction instead of aldol condensation.

Answer 1

Step 1: Structural formula.
Benzaldehyde has formula C\(_6\)H\(_5\)CHO.

Step 2: Reaction with NH\(_2\)NH\(_2\).
Benzaldehyde reacts with hydrazine to form benzaldehyde hydrazone: \[ C_6H_5CHO + NH_2NH_2 \rightarrow C_6H_5CH=NNH_2 + H_2O \] Step 3: Reaction with Tollen’s reagent.
Tollen’s reagent oxidises the –CHO group to –COOH, producing benzoic acid and metallic silver (silver mirror): \[ C_6H_5CHO + 2[Ag(NH_3)_2]^+ + 3OH^- \rightarrow C_6H_5COO^- + 2Ag \downarrow + 4NH_3 + 2H_2O \] Step 4: Reaction with NaOH (Cannizzaro reaction).
Benzaldehyde (without α-H) undergoes disproportionation in presence of conc. NaOH: \[ 2C_6H_5CHO + NaOH \rightarrow C_6H_5CH_2OH + C_6H_5COONa \] Step 5: Conclusion.
Thus, benzaldehyde reacts with hydrazine (condensation), Tollen’s reagent (oxidation), and NaOH (Cannizzaro reaction).