Important Questions Class 12th BOARD

IMPORTANT QUESTIONS with SOLUTIONS

1.What is meant by enantiomers?

Ans: Enantiomers are those optical isomers which are non-superimposable mirror images of each other.   (1)

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2. Arrange the following in an increasing order of basic strength in water: (1)

C₆H₅NH₂, (C₂H₅)₂NH, (C₂H₅)₃N and NH₃

Ans: Out of amines, aromatic amine C₆H₅NH₂ is the weakest base followed by NH₃. For aliphatic amines, the basic strength is a function of inductive effect, salvation effect and steric effect of the alkyl group. Thus the increasing order of basic strength is  C₆H₅NH₂< NH₃ < (C₂H₅)₃N < (C₂H₅)₂NH ( in aqueous solution) (1)

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3. Write the equations involved in the following reactions(2)

(i) Reimer-Tiemann reaction

(ii) Kolbe’s reaction

(i) Reimer-Tiemann reaction   (1)

(i) Kolbe’s reaction (1)

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4. (a)Describe mechanism of alcohols reacting both as nucleophiles and as electrophiles in their reactions.(2)

(a)  Give a chemical test to distinguish between phenol and benzoic acid. (1)

Ans: (a)

Alcohols as nucleophiles: The bond between O-H is broken when alcohols react as nucleophiles.  (1)

Alcohols as electrophiles: The bond between C-O is broken when they react as electrophiles. Protonated alcohols react in this manner. (1)

(b) FeCl₃ test : Phenol gives a violet colour with neutral FeCl₃ but benzoic acid gives buff coloured precipitate of ferric benzoate.

C₆H₅OH + FeCl₃ ---> violet color            (1/2)
(Phenol)

3 C₆H₅COOH + FeCl₃ ---> (C₆H₅COO)₃Fe +3 HCl

Benzoic acid                                            Ferric benzoate ( Buff coloured ppt.)              (1/2)

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