1

### AIPMT 2011 Mains

Consider the reactions.

The mechanisms of reactions (i) and (ii) are respectively
A
SN1   and  SN2
B
SN1   and  SN1
C
SN2   and  SN2
D
SN2   and  SN1

## Explanation

These reactions are purely SN2 reactions as in reaction (i) and (ii) there is no rearrangement takes place (rearrangement occurs in SN1 mechanism). Simple substitution of nucleophile takes place.
2

### AIPMT 2010 Prelims

The correct order of increasing reactivity of

C$-$X bond towards nucleophile in the following compounds is
A
I < II < IV < III
B
II < III < I < IV
C
IV < III < I < II
D
III < II < I < IV

## Explanation

The order of reactivity is dependent on the stability of the intermediate carbocation formed by cleavage of C—X bond. The 3° carbocation formed in III will be more stable than its 2° homologue (IV) which in turn will be more stable than arenium ion (I). Moreover, the aryl halide has a double bond character in the C—X bond which makes the cleavage more difficult. On other hand II will be more reactive than I due to presence of electron withdrawing —NO2 group. C—X bond becomes weak and undergoes nucleophilic substitution reaction

So Correct order is I < II < IV < III.
3

### AIPMT 2010 Mains

In the following reaction

the product 'X' is
A
C6H5CH2OCH2C6H5
B
C6H5CH2OH
C
C6H5CH3
D
C6H5CH2CH2C6H5

## Explanation

4

### AIPMT 2010 Prelims

Which one is most reactive towards SN1 reaction?
A
C6H5CH(C6H5)Br
B
C6H5CH(CH3)Br
C
C6H5C(CH3)(C6H5)Br
D
C6H5CH2Br

## Explanation

SN1 reactions involve the formation of carbocations, hence higher the stability of intermediate carbocation, more will be reactivity of the parent alkyl halide. The tertiary carbocation formed from (c) is stabilized by two phenyl groups and one methyl group, hence most stable.