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Question
(a) Account for the following :
(i) Cl−CH2COOH is a stronger acid than CH3COOH.
(il) Carboxylic acids do not give reactions of carbonyl group.
(b) Write the chemical equations to illustrate the following name reactions :
(i) Rosenmund reduction
(ii) Cannizzaro's reaction
(c) Out of CH3CH2−CO−CH2−CH3 and CH3CH2−CH2−CO−CH3, which gives iodoform test?
(i) Cl−CH2COOH is a stronger acid than CH3COOH.
(il) Carboxylic acids do not give reactions of carbonyl group.
(b) Write the chemical equations to illustrate the following name reactions :
(i) Rosenmund reduction
(ii) Cannizzaro's reaction
(c) Out of CH3CH2−CO−CH2−CH3 and CH3CH2−CH2−CO−CH3, which gives iodoform test?
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Solution
(a) (i) Cl due to -I effect withdraws electron from O-H bond and decreases electron density in O-H bond. It facilitates the release of H+ by making O-H bond weaker. −CH3 group has +I effect. It increases electron density in O−H bond and this makes the release of H+ more difficult from acetic acid than chloroacetic acid. Thus, CI−CH2COOH is a stronger acid than CH3COOH.
(ii) Carboxylic acid is a resonance hybrid of following structures I and II.
Similarly, a carbonyl group of aldehydes and ketones is a resonance hybrid of following structures III and IV.
Due to structure IV, the carbonyl carbon of aldehydes and ketones is electrophilic. However due to contribution of structure (II), the electrophilic character of carboxyl carbon is reduced. As a result the nucleophilic addition reactions of aldehydes and ketones do not occur in carboxylic acids.
(b) (i) Rosenmund reduction :
CH3−O||C−Cl+H2Boiling xylene−−−−−−−−→Pd,BaSO4Acetyl chloride
CH3−O||C−H+HClAcetaldehyde
(ii) Cannizzaro's reaction:
2HCHOFormaldehyde+NaOH(50%)→CH3OHMethyl alcohol+HCOONaSodium fermate
CH3CH2CH2COCH3 gives iodoform test due to - CH3
group attached to -O||C-group.
CH3CH2CH2COCH3+3NaOINaOH/I2−−−−−−→CH3CH2CH2COONa+CHI3Iodoform+2NaOH
(ii) Carboxylic acid is a resonance hybrid of following structures I and II.
Similarly, a carbonyl group of aldehydes and ketones is a resonance hybrid of following structures III and IV.
Due to structure IV, the carbonyl carbon of aldehydes and ketones is electrophilic. However due to contribution of structure (II), the electrophilic character of carboxyl carbon is reduced. As a result the nucleophilic addition reactions of aldehydes and ketones do not occur in carboxylic acids.
(b) (i) Rosenmund reduction :
CH3−O||C−Cl+H2Boiling xylene−−−−−−−−→Pd,BaSO4Acetyl chloride
CH3−O||C−H+HClAcetaldehyde
(ii) Cannizzaro's reaction:
2HCHOFormaldehyde+NaOH(50%)→CH3OHMethyl alcohol+HCOONaSodium fermate
CH3CH2CH2COCH3 gives iodoform test due to - CH3
group attached to -O||C-group.
CH3CH2CH2COCH3+3NaOINaOH/I2−−−−−−→CH3CH2CH2COONa+CHI3Iodoform+2NaOH
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