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CBSE 12th Standard Chemistry Subject Aldehydes , Ketones and Carboxylic Acids Ncert Exemplar 2 Mark Questions With Solution 2021

Published on: 25/05/2021

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Std: 12th Standard CBSE - Chemistry

chapter: - NCERT Exemplers

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Questions:

Generic 2

5 x 2 = 10

Ethyl benzene is generally prepared by acetylation of benzene followed by the reduction and not by direct alkylation. Think of a possible reason.

Can Gattermann-Koch reaction be considered simular to Friedel-Crafts acylation? Discuss

State reasons for the following :
(i) Monochloroethanoic acid has a higher pK_a value than dichloroethanoic acid.
(ii) Ethanoic acid is a weaker acid than benzoic acid.

Write chemical equations when :
(i) Acetone reacts with ethanol.
(ii) Ethanal reacts with HCN.
(iii) Acetone reacts with HCN.
(iv) Acetone reacts with NaHSO₃.
(v) Ethanal reacts with NaHSO₃

Explain the mechanism of a nucleophilic attack on the carbonyl group of an aldehyde or a ketone ?

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Answers Key:

Generic 2

5 x 2 = 10

Preparation of ethyl benzene from acylation of benzene yields acetophenone which on reduction gives ethyl benzene. The complete reactions can be shown as:

The direct alkylation cannot be polysubstitution product is formed. Due to disadvantage of polysubstitution, the Friedel-Crafts alkylation reaction is not used for the preparation of alkyl benzene. Instead of that, Friedel-Crafts acylation is used.

In Gattermann-Koch reaction, benzene or its derivative is treated with CO and HCI in the presence of anhydrous aluminium chloride to produce benzaldehyde. In this reaction, CO and HCI combine to form HCOCI, i.e. formyl chloride which gets substituted on benzene by the replacement of l-l-atom.

(i) It is because dichloroethanoate ion is more stable than mono-chloroethanoate ion due to two chlorine atoms which are electron withdrawing group that is why dichloroethanoic acid is stronger acid than monochloroethanoic acid.
(ii) It is because benzoate ion is stabilized doe to resonance, whereas CH₃COO- is destabilized due to (+ I) inductive effect.

Acetaldehyde reacts with C₂H₅ MgBr.