Q. 26 B5.0( 1 Vote )

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Answer :

a) Etard reaction:


Toluene is oxidized to benzaldehyde with a solution of chromyl chloride (CrO2Cl2) in CS2 or CCl4. The brown chromium complexes thus, precipitated is separated and decomposed with dilute acids to give benzaldehyde.



The above reaction is called Etard reaction.


b) The increasing order of their reactivity towards nucleophilic addition reaction:


CH3—COCH3 < CH3CHO < HCHO


Explanation:


Aldehyde is more reactive than ketone towards reaction with HCN due to the steric and electronic reasons:


Sterically, the presence of two relatively large substituents in ketones hinders the approach of nucleophile to carbonyl carbon than in aldehydes having only one such substituent.


Electronically two alkyl groups reduce the positivity of the carbonyl carbon more effectively in ketones than in aldehydes.


Among HCHO and CH3CHO, HCHO is more reactive because there is no alkyl group which hinders the approach of nucleophile.


CH3—CO—CH3 (ketone)will be the less reactive.


c) pKa of chloroacetic acid is lower than pKa of acetic acid. This means, chloroacetic acid is a stronger acid than acetic acid because of the following two reasons:


Due to I-effect of Cl atom and +I-effect of CH3 group, the electron density in the O-H bond in chloroacetic acid is much lower than in acetic acid. As a result, O-H bond in chloroacetic acid is much weaker than in acetic acid and hence loses a proton more easily than acetic acid.


Due to the –I-effect of Cl, dispersal of the negative charge occurs in chloroacetate ion but due to the +I-effect of CH3 group, intensification of negative charge occurs in acetic acid. In other words, chloroacetate ion much more stable than acetate ion.


Note: +I-effect means pushing electrons and –I effect means pulling electrons.



d) Nitriles (CH3CH2CH=CH—CH2CN) can be selectively reduced by diisobutylaluminium hydride, [(CH3)2CH2CH2]2AlH, abbreviated as


(i—Bu)2AlH or DIBAL-Hto imines which upon hydrolysis give aldehydes. The reaction taking place is given below:


CH3CH2CH=CH—CH2CN CH3CH2CH=CH—CH2CHO


e) A : CH3CH2CHO B : CH3COCH3


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