Question

Arrange the following compounds in order of their property as indicated

(i) Acetaldehyde,Acetone, di-tert-butyl ketone, Methyl tert-butyl ketonereactivity towards HCN 

(ii) CH₃CH₂CHBrCOOH, CH₃CHBrCH₂COOH, (CH₃) 2CHCOOH, CH₃CH₂CH₂COOH acid strength

(iii) Benzoic acid, 4‐Nitrobenzoic acid, 3,4‐Dinitrobenzoic acid, 4‐ Methoxybenzoic acid ( acid strength)

Answer 1

(i) Aldehydes are more reactive towards nucleophilic addition across the >C=O due to steric and electronic reasons. 

Sterically the presence of two relatively large substituents inketones hinders the approach of nucleophile to carbonyl carbon than in aldehydes having only one such substituent. –

Electronically , the presence of two alkyl groups reduces the electrophilicity of the carbonyl carbon in ketones.

(ii) (CH₃)₂CHCOOH < CH₃CH₂CH₂COOH < CH₃CHBrCH₂COOH < CH₃CH₂CHBrCOOH

Electron withdrawing groups like –Br increases the acidity of carboxylic aids by stabilizing the conjugate base through delocalisation of negative charge by negative inductive effect. The closer the electron withdrawing group to the –COOH group, greater is the stabilising effect.

Electron donating groups decrease the acidity by destabilizing the conjugate base.greater the number of –CH₃ groups, greater the destabilizing effect and lower the acidity.

(iii) 4- Methoxybenzoic acid < Benzoic acid < 4-Nitrobenzoic acid < 3,4- Dinitrobenzoic acid 

Benzoic acid is a stronger acid than aliphatic carboxylic acid due to stabilization of the conjugate base due to resonance. - 

Presence of electron withdrawing group-NO₂ on the phenyl ring of aromatic carboxylic acid increases their acidity while electron donating groups -OCH₃ decreases their acidity.