Question

Give reason : NH2CONHNH2 has two NH2 groups but only one undergoes condensation with carbonyl group of aldehydes or ketones​

Answer 1

Answer:

(i)Out of two -NH2 groups in semicarbazide, one is a part of amide functional group. In this, the lone pair on nitrogen atom is involved in the resonance with carbonyl group. Hence, this -NH2 group cannot act as a nucleophile. Hence, it is not involved in the formation of semicarbazones.

(ii) In cyclohexanone, the carbonyl carbon is not hindered. Hence, the nucleophile CN-ion can easily attack the carbonyl carbon. However in 2,2,6-trimethyl cyclohexanone, the carbonyl carbon is sterically hindered due to presence of methyl groups. Hence, the nucleophile CN- ion cannot easily attack the carbonyl carbon.

(iii) This is because of simply the difference between the electronegativities of carbon and double bonded oxygen and as you know oxygen the second most electronegative element because of which partial positive and partial negative charges appears and carbon and oxygen respectively because of which dissociation of Alpha hydrogen becomes easier (as we know the acidic character is how easily a particular compound loses H ion). Thus, it show acidic character

Answer 2

(i) Out of two -NH2 groups in semicarbazide, one is a part of amide functional group. In this, the lone pair on nitrogen atom is involved in the resonance with carbonyl group. Hence, this -NH2 group cannot act as a nucleophile. Hence, it is not involved in the formation of semicarbazones.

(ii) In cyclohexanone, the carbonyl carbon is not hindered. Hence, the nucleophile CN-ion can easily attack the carbonyl carbon. However in 2,2,6-trimethyl cyclohexanone, the carbonyl carbon is sterically hindered due to presence of methyl groups. Hence, the nucleophile CN- ion cannot easily attack the carbonyl carbon.

(iii) This is because of simply the difference between the electronegativities of carbon and double bonded oxygen and as you know oxygen the second most electronegative element because of which partial positive and partial negative charges appears and carbon and oxygen respectively because of which dissociation of Alpha hydrogen becomes easier (as we know the acidic character is how easily a particular compound loses H ion). Thus, it show acidic character

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