Question

Arrange the following compounds in increasing order of their property as indicated:
(i) Acetaldehyde, Acetone, Di-tert-butyl ketone, Methyl tert-butyl ketone (reactivity towards HCN)
(ii) $C{H}_{3}C{H}_{2}CH\left(Br\right)COOH,C{H}_{3}CH\left(Br\right)C{H}_{2}COOH,(C{H}_3{)}_{2}CHCOOH,C{H}_{3}C{H}_{2}C{H}_{2}COOH$ (acid strength)
(iii) Benzoic acid, 4-Nitrobenzoic acid, 3,4-Dinitrobenzoic acid,4-Methoxybenzoic acid (acid strength)

Answer 1

(i) With increase in +I effect of alkyl groups or increase in the steric hindrance (to the nucleophilic attack by cyanide ion at carbonyl carbon), the reactivity towards HCN addition decreases. Hence, the increasing order of the reactivity towards HCN is Di-tert-butyl ketone > Methyl tert-butyl ketone > Acetone > Acetaldehyde.

(ii) The acid strength of carboxylic acid decreases due to +I effect and increases due to -I effect. Isoprpopyl group has higher +I effect than n propyl group. Hence, 2-methyl propanoic acid is weaker acid than butanoic acid.
With increase in distance, -I effect decreases. Hence, the acid strength of 2-bromobutanoic acid is higher than the acid strength of 3-bromobutanoic acid.
Hence, the increasing order of acid strength is
$(C{H}_3{)}_{2}CHCOOH<C{H}_{3}C{H}_{2}C{H}_{2}COOH<C{H}_{3}CH(Br)C{H}_{2}COOH$ $<C{H}_{3}C{H}_{2}CH\left(Br\right)COOH$.

(iii) Acid strength decreases due to presence of electron donating groups. Hence, 4-methoxybenzoic acid has lower acid strength than benzoic acid.
Acid strength increases due to presence of electron withdrawing group. Hence, the acid strength of benzoic acid is lower than the acid strength of 4-nitrobenzoic acid and 3,4-dinitrobenzoic acid. The acid strength of 3,4-ditrobenzoic acid is higher than the acid strength of 4-nitrobenzoic acid due to presence of second nitro group in the meta position to carboxylic group.

The increasing order of acid strength is as follows:
$4-methoxybenzoicacid<benzoicacid<4-nitrobenzoicacid<3,4-dinitrobenzoicacid$