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  • (a)    Account for the following :         (i)    Propanal is more reactive than propanone towards nucleophilic reagents.       

(a)    Account for the following :
        (i)    Propanal is more reactive than propanone towards nucleophilic reagents.
       (ii)    Electrophilic substitution in benzoic acid takes place at the meta position.
      (iii)    Carboxylic acids do not give characteristic reactions of the carbonyl group.
(b)    Give a simple chemical test to distinguish between the following pairs of compounds:
        (i)    Acetophenone and benzaldehyde.
       (ii)    Benzoic acid and ethylbenzoate.

 

 

 

 
 
 
 
 

Answers (1)

(a)

(i) Propanone is sterically more hindered than propanal due to presence of alkyl group on both sides of carbonyl carbon, making them less reactive towards nucleophilic attack both methyl groups have electron releasing tendency due to -I effect. These alkyl groups makes ketone less reactive by donating an electron to a carbonyl group. Hence, propanal is more reactive than propanone towards nucleophilic reagents.

(ii) -COOH is an electron withdrawing group which deactivates the benzene ring lowering the electron density at ortho and para position in comparison to meta position. Electrophiles easily attacks at meta position. Therefore, due to higher density at meta position electrophilic substitution takes place at meta position.

(iii) Carbonyl carbon present in ketones and aldehyde is more electrophilic than in carboxylic acids. This is due to lone pairs on oxygen atom attached to hydrogen atom in the -COOH group causing resonance thereby making the carbon atom less electrophilic. Thus, carboxylic acids do not give the characteristic reaction of a carbonyl group.

(b)

(i) Acetophenone and Benzophenone :
  Acetophenone gives yellow ppt of idoform with alkaline solution of iodine (iodoform test) and Benzophenone doe not give this test.

(ii) Benzoic acid and Ethyl benzoate :

Ethyl benzoate does not react with sodium hydrogen carbonate and benzoic acid gives effervescence of carbon dioxide gas upon reaction with sodium hydrogen carbonate. When ethyl benzoate is boiled with excess NaOH, it gives ethanol. Ethanol on heating with iodine gives yellow ppt of iodoform. This test is not given by benzoic acid.
 

 

Posted by

Satyajeet Kumar

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