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Answer the following.
Write the chemical equations to illustrate the following name reactions: (1) Wolff-Kishner…
(i)[3.0]
Write the chemical equations to illustrate the following name reactions:
(1) Wolff-Kishner reduction.
(2) Aldol condensation.
(3) Cannizzaro reaction.
(ii)[2.0]
Account for the following:
(1) $\text{CH}_3\text{CHO}$ is more reactive with $\text{HCN}$ than $\text{CH}_3\text{COCH}_3$.
(2) Carboxylic acids are stronger acids than phenol.
Answer
Answer (i)
AI(1) Wolff-Kishner reduction: a carbonyl compound with hydrazine, then KOH in ethylene glycol at high temperature, gives the alkane.
$\mathrm{R_2C=O + NH_2NH_2 \rightarrow R_2C=NNH_2 \xrightarrow{KOH/glycol,\ \Delta} R_2CH_2 + N_2}$
(2) Aldol condensation: $\mathrm{2CH_3CHO \xrightarrow{dil.\ NaOH} CH_3CH(OH)CH_2CHO \xrightarrow{\Delta} CH_3CH=CHCHO + H_2O}$
(3) Cannizzaro reaction: $\mathrm{2HCHO + conc.\ NaOH \rightarrow CH_3OH + HCOONa}$
Answer (ii)
AI(1) Because $\mathrm{CH_3CHO}$ has only one electron-donating alkyl group and less steric hindrance, its carbonyl carbon is more electrophilic than in $\mathrm{CH_3COCH_3}$, which has two.
(2) Because the carboxylate ion is stabilised by resonance with two equivalent structures with the charge shared on both oxygens, whereas the phenoxide ion has the charge less effectively delocalised.
From ISC 2018 Specimen Chemistry Paper 1, question 18(b).