Questions

Assertion (A) & Reason (B) MCQ

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4 questions · timed · auto-graded

Question 11 Mark
Assertion $(A): (CH_3)_3 C-O-CH_3$ gives $(CH_3)_3 C-I$ and $CH_3OH$ on treatment with $HI.$
Reason $(R):$ The reaction occurs by $S_N1$ mechanism.
Answer
$(CH_3)_3-C-O-CH_3$ is an ether with two different alkyl groups, of which $(CH_3)_3-C-I,$ tertiary alkyl group, on with hydrogen halide $(HI)$ forms a tertiary halide.
This occurs as the reaction is an $S_N1$ reaction. The reaction involves the formation of a stable carbocation. If the ether has a primary alkyl group, then the reaction follows the $S_N2$ mechanism.
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Question 31 Mark
Assertion (A): A solution of sucrose in water is dextrorotatory but on hydrolysis in presence of little hydrochloric acid it becomes laevorotatory.
Reason (R): Sucrose on hydrolysis gives unequal amounts of glucose and fructose as a result of which change in sign of rotation is observed.
Answer
(c) A is true but R is false.
Explanation: A is true but R is false.
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Question 41 Mark
Assertion (A): Di-tert-butyl ether cannot be prepared by Williamson's synthesis.
Reason (R): tert-Butyl bromide on treatment with sodium lert-butoxide preferentially undergoes elimination to form isobutylene and rert-butyl alcohol.
Answer
(a) Both A and R are true and R is the correct explanation of A.
Explanation: Williamson's synthesis is not applicable to tert. alkyl halides because alkoxide ions being both powerful nucleophiles and bases would bring dehydro-halogenation of the tert. alkyl halides to form alkenes preferentially.
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Assertion (A) & Reason (B) MCQ - Chemistry STD 12 Science Questions - Vidyadip