Question

Write the structural formula of benzaldehyde. Write chemical equations of the reactions of benzaldehyde with (i) NH$_2$NH$_2$
(ii) Tollen's reagent
(iii) HNO$_3$ and H$_2$SO$_4$
(iv) NaOH.

Hint:

Benzaldehyde (\(\mathrm{Ph{-}CHO}\)) has no \(\alpha\)-H, so it does not enolize and undergoes Cannizzaro with base; \(-CHO\) is meta-directing in electrophilic substitution; carbonyls form hydrazones with \(\mathrm{NH_2NH_2}\); Tollen's reagent oxidizes aldehydes to acids/benzoates with a silver mirror.

Answer 1

Structural formula of benzaldehyde:
\[ \mathrm{C_6H_5CHO \ (Ph{-}CHO)} \]
(i) With hydrazine (formation of hydrazone):
\[ \mathrm{C_6H_5CHO + NH_2NH_2 \;\rightarrow\; C_6H_5CH{=}NNH_2 + H_2O} \]
(ii) With Tollen's reagent (silver mirror test; oxidation to benzoate/benzoic acid):
\[ \mathrm{C_6H_5CHO + 2\,[Ag(NH_3)_2]^+ + 3\,OH^- \;\rightarrow\; C_6H_5COO^- + 2\,Ag \downarrow + 4\,NH_3 + 2\,H_2O} \]
(On acidification \(\rightarrow\) \(\mathrm{C_6H_5COOH}\)).
(iii) Nitration (HNO$_3$/H$_2$SO$_4$; meta directing):
\[ \mathrm{C_6H_5CHO \xrightarrow{HNO_3/H_2SO_4} m{-}O_2N{-}C_6H_4{-}CHO + H_2O} \]
(Product: \(\mathrm{m}\)-nitrobenzaldehyde).
(iv) With NaOH (Cannizzaro reaction; no \(\alpha\)-H):
\[ \mathrm{2\,C_6H_5CHO + NaOH \;\rightarrow\; C_6H_5CH_2OH + C_6H_5COONa} \] (On acidification of benzoate \(\rightarrow\) \(\mathrm{C_6H_5COOH}\)).