Chemistry Class 12 NCERT Solutions: Chapter 12 Aldehydes Ketones and Carboxylic Acids Part 13

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Q: 18. Give plausible explanation for each of the following:

(i) Cyclohexanone forms cyanohydrin in good yield but 2, 2, 6 trimethylcyclohexanone does not.

(ii) There are two groups in semicarbazide. However, only one is involved in the formation of semicarbazones.

(iii) 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.

Answer:

(i) Cyclohexanones form cyanohydrins according to the following equation.

Q 18 i Cyclohexanones form Cyanohydrins

Q 18 I Cyclohexanones Form Cyanohydrins

In this case, the nucleophile can easily attack without any steric hindrance. However, in the case of 2, 2, 6 trimethylcyclohexanone, methyl groups at α-positions offer steric hindrances and as a result, cannot attack effectively.

Q 18 i 1 Trimethylcyclohexanone

Q 18 I 1 Trimethylcyclohexanone

For this reason, it does not form a cyanohydrin.

(ii) Semicarbazide undergoes resonance involving only one of the two groups, which is attached directly to the carbonyl-carbon atom.

Q 18 ii The Carbonyl-Carbon Atom

Q 18 II the Carbonyl-Carbon Atom

Therefore, the electron density on group involved in the resonance also decreases. As a result, it cannot act as a nucleophile. Since the other group is not involved in resonance; it can act as a nucleophile and can attack carbonyl-carbon atoms of aldehydes and ketones to produce semicarbazones.

(iii) Ester along with water is formed reversibly from a carboxylic acid and an alcohol in presence of an acid.

If either water or ester is not removed as soon as it is formed, then it reacts to give back the reactants as the reaction is reversible. Therefore, to shift the equilibrium in the forward direction i.e., to produce more ester, either of the two should be removed.

Q: 19. An organic compound contains carbon, hydrogen, and rest oxygen. The molecular mass of the compound is . It does not reduce Tollens’ reagent but forms an addition compound with sodium hydrogensulphite and give positive iodoform test. On vigorous oxidation, it gives ethanoic and propanoic acid. Write the possible structure of the compound.

Answer:

Thus, the ratio of the number of carbon, hydrogen, and oxygen atoms in the organic compound can be given as:

Therefore, the empirical formula of the compound is . Now, the empirical formula mass of the compound can be given as:

Molecular mass of the compound

Therefore, the molecular formula of the compound is given by .

Since the given compound does not reduce Tollen’s reagent, it is not an aldehyde. Again, the compound forms sodium hydrogen sulphate addition products and gives a positive iodoform test. Since the compound is not an aldehyde, it must be a methyl ketone.

The given compound also gives a mixture of ethanoic acid and propanoic acid.

Hence, the given compound is pentan−2−ol.

The given reactions can be explained by the following equations:

Q 19 The Mixture of Ethanoic Acid and Propanoic Acid

Q 19 the Mixture of Ethanoic Acid and Propanoic Acid

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