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CBSE 12th Standard Chemistry Subject Alcohols , Phenols and Ethers HOT Questions 3 Mark Questions With Solution 2021

Published on: 28/05/2021

QB365 Provides the HOT Question Papers for Class 12 Chemistry, and also provide the detail solution for each and every HOT Questions. HOT Questions will help to get more idea about question pattern in every exams and also will help to get more marks in Exams

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Std: 12th Standard CBSE - Chemistry

chapter: - HOT Questions

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Questions:

Generic 3

3 x 3 = 9

The treatment of alkyl chlorides with aq KOH leads to the formation of alcohols but in presence of alcoholic KOH, alkenes are the major products. Explain.

a. R—Cl is hydrolysed to R—OH slowly but the reaction is rapid if a catalytic amount of KI is added to the reaction mixture.
b. What is formed if cylcopentanone is reduced with H2 /Pt . Give equation for the reaction.

Identify X,Y and Z
\(C3H7OH+ConcH2SO4\overset { 430-450k }{ \longrightarrow } X\overset { Br_{ 2 } }{ \longrightarrow } Y\overset { Excess }{ \longrightarrow } Z\)
3 Alc KOH

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Answers Key:

Generic 3

3 x 3 = 9

In aq. solution, KOH is almost completely ionised to give OH^- ions which being a strong nucleophile brings about a substitution reaction to form alcohols. Further in aq. solution, OH^- ions are highly solvated (hydrated).
This solution reduces the basic character of OH^- ions which fail to abstract a hydrogen from the -carbon of the alkyl halide to form an alkene.
However an alcoholic solution of KOH contains alkoxide (RO^-) ions which being a much stronger base than OH^- ions preferentially abstracts a hydrogen from the carbon of the alkyl halide to form alkene.

Iodide ion is a powerful nucleophile and hence reacts rapidly with RCl to form RI.
KI ——— K⁺ + I^-; R — Cl + I^- ——— R — I + Cl^-
Further I^- is a better leaving group than CI^- ion, therefore, RI is more rapidly hydrolysed than RCl to form ROH.
HO^- + R — I ——— R — OH + I^-