. Give a possible explanation for each one of the following


(a) Give a possible explanation for each one of the following:

(i) There are two —NH2 groups in semicarbazide. However, only one such group is involved in the formation of semicarbazones.

(ii) Cyclohexanone forms cyanohydrin in good yield but 2, 4, 6-trimethylcyclohexanone does not

(b) An organic compound with molecular formula C9H10O forms 2, 4-DNP derivative, reduces Tollen’s reagent and undergoes Cannizzaro’s reaction. On vigorous oxidation it gives 1,2-benzene-di-carboxylic acid. Identify the compound.

                                                                  OR

(a) Give chemical tests to distinguish between

(i) Phenol and Benzoic acid

(ii) Benzophenone and Acetophenone

(b) Write the structures of the main products of following reactions:

Anhydrous

Best Answer

 semicarbazide. However

Although semicarbazide has two —NH2 group but one which is directly attached to C—O is involved in the resonance as shown above. Consequently, electron density on this —NH2 group decreases and hence it does not act as a nucleophile. On the other hand, the lone pair of electrons on the other —NH2 group is not involved in resonance and hence is available for nucleophilic attack on the C—O group of aldehydes and ketones

(ii) Due to presence of three methyl groups, the nucleophilic attack by CN ion does not occur due to steric hindrance in 2, 4, 6-trimethyl cylcohexanone. As there is no such steric hindrance in cyclohexanone so nucleophilic attack by the CN ion occurs readily and hence cyclohexanone cyanohydrin is obtained in good yield.

molecular formula, Phenol and Benzoic acid

(ii) Acetophenone being a methyl ketone when treated with NaOI(I2 /NaOH) gives yellow precipitate of iodoform while benzophenone does not give this test

Benzophenone and Acetophenone, structures

 

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