Give a plausible explanation for the following.
There are two $N H_{2}$ groups in semicarbazide. However, only one such group is involved in the formation of semicarbazones.

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Solution

Semicarbazide undergoes resonance as given on the structure.
One of the anion group involves in resonance and hence the electron density on this $- N H_{2}$ group decreases and it does not act as a nucleophile. But the other group (attached to $- N H$ ) has a lone pair of electrons which are not involved in resonance and is available for the nucleophilic attack on carbonyl group tp product semicarbazones.

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Q. Assertion :There are two

{- N H}_{2}

groups in semicarbazide Reason: One group is involved in the formation of semicarbazones

Give plausible explanation for each of the following:
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These are two $- N H_{2}$ groups in semicarbazide. However, only one is involved in the formation of semicarbazones.

During the preparation of esters from a carboxylic acid and an alcohol in the presence of an acid catalyst, the water or the ester should be removed as soon as it is formed.

Q. (a) Give a possible explanation for each one of the following:
(i) There are two

- N H_{2}

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(ii) Cyclohexanone forms cyanohydrin in good yield but 2, 4, 6-trimethylcyclohexanone does not.
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Q. Consider the following statements
(i) Out of two

- N H_{2}

groups in semicarbazide only one is involved in formation of semicarbazones.
(ii) Cyclohexane froms cyanohydrin in good yield but

2, 2, 6 - t r i m e t h y l c y c l o h e x a n o n e

does not.
(iii) The

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in aldehydes and ketones are acidic.

Q. When semicarbazide reacts with a ketone(or aldehyde) to form semicarbazone. Only one nitrogen atom of semicarbazide acts as a nucleophile and attack the carbonyl carbon of the ketone. The product of the reaction consequently is

R_{2} C = N - N H - C O {N H}_{2}

rather than

R_{2} C = N C O N H - {N H}_{2}

. What factor account for the fact that two nitrogen atoms of semicarbazide are relatively non nucleophilic?

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Give a plausible explanation for the following.There are two NH2 groups in semicarbazide. However, only one such group is involved in the formation of semicarbazones.

Give a plausible explanation for the following.
There are two $N H_{2}$ groups in semicarbazide. However, only one such group is involved in the formation of semicarbazones.