1

### AIPMT 2009

Which of the following reactions is an example of nucleophilic substitution reaction?
A
2RX + 2Na $\to$ R $-$ R + 2NaX
B
RX + H2 $\to$ RH + HX
C
RX + Mg $\to$ RMgX
D
RX + KOH $\to$ ROH + KX

## Explanation

In nucleophilic substitution, a nucleophile provides an electron pair to the substrate and the leaving group departs with an electron pair.
2

### AIPMT 2008

In a SN2 substitution reaction of the type
R $-$ Br + Cl$-$ $\buildrel {DMF} \over \longrightarrow$ R$-$Cl + Br$-$
which one of the following has the highest relative rate?
A
B
CH3CH2Br
C
CH3CH2CH2Br
D

## Explanation

SN2 mechanism is followed in case of primary and secondary halides i.e., SN2 reaction is favoured by small groups on the carbon atom attached to halogens so

CH3 – X > R – CH2 – X > R2CH – X > R3C – X
3

### AIPMT 2008

How many stereoisomers does this molecule have?

CH3CH$=$CHCH2CHBrCH3
A
8
B
2
C
4
D
6

## Explanation

Both geometrical isomerism (cis-trans form) and optical isomerism is possible in the given compound.

No. of optical isomer = 2n = 21 = 2
(where n = no. of asymmetric carbon)

Hence total no. of stereoisomers = 2 + 2 = 4
4

### AIPMT 2007

If there is no rotation of plane polarised light by a compound in a specific solvent, through to be chiral, it may mean than
A
the compound is certainly meso
B
there is no compound in the solvent
C
the compound may be a racemic mixture
D
the compound is certainly a chiral.

## Explanation

Compounds which do not show optical activity inspite of the presence of chiral carbon atoms are called meso-compounds. The absence of optical activity in these compound is due to the presence of a plane of symmetry in their molecules. e.g. meso-tartaric acid is optically inactive.