1
MCQ (Single Correct Answer)

NEET 2016 Phase 2

Consider the reaction,
CH3CH2CH2Br + NaCN $$ \to $$ CH3CH2CH2CN + NaBr

This reaction will be the fastest in
A
ethanol
B
methanol
C
N, N' -dimethylformamide (DMF)
D
Water

Explanation

The reaction,
CH3CH2CH2Br + NaCN $$ \to $$ CH3CH2CH2CN + NaBr

follows SN2 mechanism which is favoured by polar aprotic solvent i.e., N, N'–dimethylformamide (DMF).
2
MCQ (Single Correct Answer)

NEET 2016 Phase 1

Which of the following biphenyls is optically active ?
A
B
C
D

Explanation

Ortho substituted biphenyls are optically active as both the rings are not in one plane and their mirror images are non-superimposable.
3
MCQ (Single Correct Answer)

AIPMT 2015 Cancelled Paper

The reaction of C6H5CH $$=$$ CHCH3 with HBr produces
A
B
C
D

Explanation

4
MCQ (Single Correct Answer)

AIPMT 2015

In an SN1 reaction on chiral centers, there is
A
inversion more than retention leading to partial racemisation
B
100% retention
C
100% inversion
D
100% racemisation

Explanation

In case of optically active alkyl halides, SN1 reaction is accompanied by racemisation. The carbocation formed in the slow step being sp2 hybridised is planar and attack of nucleophile may take place from either side resulting in a mixture of products, one having the same configuration and other having inverted configuration.

The isomer corresponding to inversion is present in slight excess because SN1 also depends upon the degree of shielding of the front side of the reacting carbon.

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