Question

Consider the following statements
(i) Out of two – $N{H_2}$ groups in semicarbazide only one is involved in formation of semicarbazones.
(ii) Cyclohexane forms cyanohydrin in good yield but $2,2,6$−trimethylcyclohexanone does not.
(iii) The $\alpha $−hydrogens in aldehydes and ketones are acidic.
A. I and iii
B. ii and iii
C. I and ii
D. i, ii and iii

Answer 1

Hint:The lone pair on nitrogen atoms is involved in the resonance with the carbonyl group, thus one of them cannot act as a nucleophile. Nucleophile ions cannot easily attack the carbonyl carbon.

Complete step-by-step answer:
The following information given in the question can be stated by:
(i) Out of two -$N{H_2}$ groups in semicarbazide, actually one is a part of amide functional group. In this we can say, the lone pair on the given nitrogen atom is involved in the resonance behaviour with the carbonyl group. Hence, this $N{H_2}$ group cannot act like a nucleophile. Therefore, it is not the one which is involved in the formation of semicarbazones.
(ii) In cyclohexanone, the carbonyl carbon is not actually hindered. Thus, the nucleophile $C{N^ - }$- ion is easily able to attack the carbonyl carbon. Although in $2,2,6$-trimethyl cyclohexanone, the carbonyl carbon is hindered because of the existence of methyl groups. Thus, the nucleophile $C{N^ - }$ion cannot easily attack the carbonyl carbon.
(iii) This is just because of simply the difference b/w the electronegativities of carbon and double bonded oxygen and as we all know oxygen is the second most electronegative element as of which partial positive and partial negative charges both appears and carbon and oxygen respectively due to which dissociation of Alpha hydrogen gets easier as we all know the acidic character is how easily any compound loses H ion. Hence, it shows an acidic character.

Hence, Option B) ii and iii is correct.

Note:Hint should be clearly observed and the options should also be compared, answers can be determined using the hint and options. As nucleophile ions cannot easily attack the carbonyl carbon and also the $\alpha $−hydrogens in aldehydes and ketones are acidic in nature.