Q. What is Tollens’ reagent? Write one usefulness of this reagent.
Ans. Ammonical silver nitrate (AgN03 + NH40H) solution is known as Tollens’ reagent. It is used to detect the presence of —CHO group in an organic compound.
Q What do you mean by Schiff’s base? Give an example.
Ans. Aldehydes and ketones react with primary aliphatic or aromatic amines to form azomethines or Schiff’s bases.
Q. Name the aldehyde which does not give Fehling’s solution test.
Ans. Benzaldehyde
Q. Which of the following compounds would undergo Cannizza reaction:
Benzaldehyde, Cyclohexanone, 2-Methylpentanal.
Ans. Benzaldehyde
Give reasons:
Q(i) Oxidation of aldehydes is easier than ketones.
(ii) CH2=CH-COOH is more acidic than CH3CH2-COOH.
Ans (i) As aldehydes contain H atom on the carbonyl group but ketones do not. Cleavage of C—H bond in aldehydes is easier than cleavage of C—C bond in ketones.
(ii) This is because in CH2=CH-COOH the carbonyl group attached to sp2 hybridised carbon atom which is more electronegative and makes release of H+ ion easy.
Q (i) Give reason:
(a) Benzoic acid is a stronger acid than acetic acid.
(b) Methanal is more reactive towards nucleophilic addition reaction than ethanal.
(ii) Give a simple chemical test to distinguish between propanal and propanone.
Ans (i) (a) This is because of greater electronegativity of sp2 hybridised carbonyl to which carboxyl carbon is attached in benzoic acid.
(b) The methyl group due to its +ve I effect reduces the positive charge on carbonyl carbon atom. Moreover it also hinders the approach of the nucleophile. Since in ethanal there is one methyl group while in methanal there is no methyl group on carbonyl carbon atom therefore methanal is more reactive towards nucleophilic addition reaction than ethanal.
(ii) Propanal being an aldehyde gives silver mirror with Tollens’ reagent while propanone being a ketone does not give this test.
Q. Give reasons:
(i) The a-hydrogen of aldehydes and ketones are acidic in nature.
(ii) Propanone is less reactive than ethanal towards addition of HCN.
(iii) Benzoic acid does not give Friedel-Crafts reaction.
Ans (i) The acidity of a-hydrogen atom of carbonyl carbon is due to the strong withdrawing effect of the carbonyl group and resonance stabilisation of the conjugate base.
(ii) This is due to steric and electronic reasons. Sterically, the presence of two methyl groups in propanone hinders more the approach of nucleophile to carbonyl carbon than in ethanal having one methyl group. Electronically two methyl groups reduce the positivity of the carbonyl carbon more effectively in propanone than in ethanal.
(iii) Benzoic acid does not give Friedel Craft reaction because:
(a) the carboxyl group is strongly deactivating.
(b) the catalyst AIC13 which is a lewis acid gets bonded to the carboxyl group strongly.