Engineering Chemistry S Datta

MECHANISM OF ORGANIC REACTIONS 181
Second step is attack by electrophile SO 3
and the formation of a transition state.
: O:
:
S
O:
:
O:
: :
+
T.S
So, sulphonation of benzene is an electrophilic substitution.
Q. 40. What are the electrophiles for (i) halogenation and (ii) alkylation
reactions of benzene?
Ans. (i) For halogenation viz. chlorination, the reagents are anhydrous FeCl 3
and Cl 2
.
Electrophile is thus generated.
Cl 2
+ FeCl 3
FeCl
–
4
+ Cl +
So, Cl + acts as an electrophile during chlorination of benzene.
(ii) For alkylation the reagents are anhydrous AlCl 3
and RCl. Electrophile is thus
generated.
RCl + AlCl 3
AlCl
–
4
+ R +
So, R + acts as an electrophile during alkylation i.e., in Friedel–Craft’s reaction.
Q. 41. Mention an organic reaction where rearrangement takes place.
Ans. Pinacol-pinacolone rearrangement (example of 1, 2-alkyl shift)
: O:
:
S
H
– O
::
:
O:
: :
O
S
OH
O
CH 3
CH 3
H + C
CH 3
CH 3
CH 3
CH 3 C C
OH OH
Pinacol
CH 3
CH 3
C
OH
+
OH 2
–HO 2
CH 3
C
CH 3
C CH 3
CH 3
C
CH 3
C CH 3
CH 3 CH 3
CH 3 C C CH 3
OH
: OH
:
CH 3
: OH
+
CH 3
–H +
C
CH 3
C CH 3
CH 3
O CH 3
Pinacolone