(b)(i) Account for the following:
(1) The alpha (α) hydrogens of aldehydes and ketones are acidic in nature.
(2) Oxidation of aldehydes is easier than that of ketones.
(ii) Arrange the following:
(1) decreasing reactivity towards nucleophilic addition: propanal, acetone, benzaldehyde
(2) increasing order of boiling point: propane, ethanol, dimethyl ether, propanal
(iii) Give a simple chemical test to distinguish between benzoic acid and benzaldehyde.
(1) The alpha (α) hydrogens of aldehydes and ketones are acidic in nature.
(2) Oxidation of aldehydes is easier than that of ketones.
(ii) Arrange the following:
(1) decreasing reactivity towards nucleophilic addition: propanal, acetone, benzaldehyde
(2) increasing order of boiling point: propane, ethanol, dimethyl ether, propanal
(iii) Give a simple chemical test to distinguish between benzoic acid and benzaldehyde.
Solution
(i)(1) Removal of an α-H gives an enolate whose negative charge delocalises onto the carbonyl oxygen — this resonance stabilisation makes the α-hydrogens acidic.
(2) An aldehyde carries an H on the carbonyl carbon which is abstracted easily in oxidation; a ketone must break a strong C–C bond — so aldehydes oxidise under far milder conditions.
(ii)(1) Decreasing nucleophilic-addition reactivity:
(one +I group beats ring conjugation; two alkyls deactivate most).
(2) Increasing boiling point:
(van der Waals < weak dipole < stronger dipole < hydrogen bonding).
(iii) NaHCO: benzoic acid → CO effervescence; benzaldehyde → none.
(2) An aldehyde carries an H on the carbonyl carbon which is abstracted easily in oxidation; a ketone must break a strong C–C bond — so aldehydes oxidise under far milder conditions.
(ii)(1) Decreasing nucleophilic-addition reactivity:
(one +I group beats ring conjugation; two alkyls deactivate most).
(2) Increasing boiling point:
(van der Waals < weak dipole < stronger dipole < hydrogen bonding).
(iii) NaHCO: benzoic acid → CO effervescence; benzaldehyde → none.
CBSE Class 12 Board · 2024 · Subjective