CBSE Class 12 Chemistry Aldehyde Ketones And Acids Worksheet Set 03

Subjective Questions

Question. (i) What type of aldehydes undergo Cannizzaro reaction?
(ii) An aromatic organic compound ‘A’ with molecular formula \( C_8H_8O \) gives positive DNP and iodoform tests. It neither reduces Tollens’ reagent nor does it decolourise bromine water. Write the structure of ‘A’.
Answer: (i) Aldehydes having no \( \alpha \)-hydrogen. (ii) \( C_6H_5COCH_3 \).
Detailed Answer: ‘A’ gives positive DNP test. Therefore, it is an aldehyde or a ketone. Since it does not reduce Tollens’ reagent, ‘A’ must be a ketone. ‘A’ responds to iodoform test. Therefore, it should be a methyl ketone. The molecular formula of ‘A’ indicates high degree of unsaturation, yet it does not decolourise bromine water. This indicates the presence of unsaturation due to an aromatic ring. The molecular formula of ‘A’ indicates that it should be phenyl methyl ketone (acetophenone).
 

Question. (i) \( (CH_3)_3C–CHO \) does not undergo aldol condensation. Comment.
(ii) Out of \( CH_3CH_2COCH_2CH_3 \) and \( CH_3CH_2CH_2COCH_3 \), which gives iodoform test.
Answer: (i) No \( \alpha \)-H is present.
(ii) \( CH_3CH_2CH_2COCH_3 \) will give iodoform test as it has a terminal ketomethyl group.

Question. Which bond C–OH or CO–H of carboxylic acid is broken when
(i) Acid reacts with alcohol (ii) Acid reacts with Sodium
Answer: (i) C–OH (ii) CO–H

Question. (i) What type of aldehydes undergo Cannizzaro reaction ?
(ii) Arrange the following compounds in increasing order of their property as indicated :
(a) \( CH_3COCH_3 \), \( C_6H_5COCH_3 \), \( CH_3CHO \) (reactivity towards nucleophilic addition reaction)
(b) \( Cl – CH_2 – COOH \), \( F – CH_2 – COOH \), \( CH_3 – COOH \) (acidic character)
Answer: (i) Having no \( \alpha \)-hydrogen.
(ii) (a) \( C_6H_5COCH_3 < CH_3COCH_3 < CH_3CHO \).
(b) \( CH_3COOH < Cl – CH_2 – COOH < F – CH_2 – COOH \).

Question. Ketones are less reactive than aldehydes Why?
Answer: Ketones are less reactive than aldehydes due to following facts :
(i) Electron releasing effect: In ketones, the carbonyl carbon is attached to alkyl groups which are electron releasing in nature. These alkyl groups push electrons towards carbonyl carbon and therefore, decrease the magnitude of positive charge on it and make it less reactive toward nucleophilic attack.
(ii) Steric effect: In ketones, the presence of two bulky alkyl groups also hinders the approach of the nucleophile to the carbonyl carbon.

Question. Account for the following :
(a) Aromatic carboxylic acids do not undergo Friedel-Crafts reaction.
(b) pKa value of 4-nitrobenzoic acid is lower than that of benzoic acid.
Answer: (a) Because the carboxyl group is deactivating and the catalyst aluminium chloride (Lewis acid) gets bonded to the carboxyl group.
(b) Nitro group is an electron withdrawing group (-I effect) so it stabilizes the carboxylate anion and strengthens the acid. Greater is the acidic character, lower is the pKa value.