How would you account for the following?
i) Aldehydes are more reactive than ketones towards nucleophiles.
ii) The boiling points of aldehydes and ketones are lower than those of the corresponding acids.
iii) Aldehydes and ketones undergo a number of addition reactions.
i) Aldehydes are more reactive than ketones towards nucleophiles.
ii) The boiling points of aldehydes and ketones are lower than those of the corresponding acids.
iii) Aldehydes and ketones undergo a number of addition reactions.
Solution
(i) A ketone's two alkyl groups both donate (+I) and shield the carbonyl carbon; an aldehyde has one — its carbon is more positive and more open, hence more reactive to nucleophiles.
(ii) Carboxylic acids associate as hydrogen-bonded dimers (two O–H···O bridges), demanding much more energy to vaporise; aldehydes/ketones have only dipole attractions — lower b.p.
(iii) The C=O is a polarised double bond (C=O): the π-bond opens readily, the nucleophile bonding to C and the electrophile to O — so HCN, NaHSO, alcohols, amine derivatives all add across it.
(ii) Carboxylic acids associate as hydrogen-bonded dimers (two O–H···O bridges), demanding much more energy to vaporise; aldehydes/ketones have only dipole attractions — lower b.p.
(iii) The C=O is a polarised double bond (C=O): the π-bond opens readily, the nucleophile bonding to C and the electrophile to O — so HCN, NaHSO, alcohols, amine derivatives all add across it.
CBSE Class 12 Board · 2008 · Delhi · Subjective