Question

Arrange the following compounds in the increasing order of their property as indicated:
i. Acetaldehyde, acetone, di-tert-butyl ketone, methyl tert-butyl ketone (reactivity towards HCN).
ii. `CH_(3)CH_(2)CH(Br)COOH, CH_(3)CH(Br)CH_(2)COOH, (CH_(3))_(2)CHCOOH, CH_(3)CH_(2)CH_(2)COOH` (acidic strength).
iii. Benzoic acid, 4-nitrobenzoic acid, 3,4-dinitro-benzoic acid, 4-methoxybenzoic acid (acidic strength).

Answer 1

(i) Cyanohydrin derivatives are formed as a result of the reaction in which the nucleophile`(CN^(-) ion)` attacks the carbon atom of the carbonyl group. The order of reactivity
decreases with increase in+1 effect of the alkyl group.
decreases with increase in steric hindrance due to the size as well as number of the alkyl groups.
In the light of the above information, the decreasing order of reactivity is :
(ii) We know that alkyl group with +1 effect decreases the acidic strength. The +1 effect of isopropyl group is more than that of n-propyl group. Similarly, bromine (Br) with -I effect increases the acidic strength. Closer its position in the carbon atom chain w.r.t., carboxyl (COOH) group, more will be its-I effect and stronger will be the acid. In the light of this, the increasing order of acidic strength is : `(CH_(3))_(2)CHCOOHltCH_(3)CH_(2)CH_(2)COOHltCH_(3)CH(Br)CH_(2)COOHltCH_(3)CH_(2)CH(Br)COOH`
(iii) We have learnt that the electron donating group `(OCH_(3))` decreases the acidic strength of the benzoic acid. At the same time, the electron withdrawing group `(NO_(2))` increases the same. Keeping this in mind, the increasing order of acidic strength is :