Mechanism.
Step 1. Generation of nitronium ion (
+ NO 2 ), an electrophile
O
H NO 2
H OSO 3 H
O NO 2
H
H
+
+
+ NO 2 + H 2 O
Nitronium ion
+ HSO 4
−
..
..
..
Step 2. Formation of arenium ion complex
H
NO 2
+
+
+
Arenium ion
NO 2
Step 3. Loss of a proton from the arenium ion complex
H
NO 2
NO 2
O H
H
+
Nitrobenzene
+
+ H 3 O +
..
:
..
Sulphonation of benzene
Benzene reacts with fuming sulphuric acid at room temperature to give
benzenesulphonic acid. Fuming sulphuric acid contains added sulphur trioxide
(SO 3 ). Sulphonation of benzene can also be carried out with concentrated
H 2 SO 4 , but with slower speed. In both cases, SO 3 acts as an electrophile.
SO 3 H
conc. H 2 SO 4
25 o C
+ H 2 O
Benzenesulphonic acid
+ SO 3
Mechanism.
Step 1. Generation of electrophile (SO 3 )
2 H 2 SO 4
SO 3 + H 3 O + + HSO 4
−
Step 2. Formation of arenium ion complex
H
SO 3
O
S
O
O
+
+
Arenium ion
:
..
..
:
: ..
_
5.5 SUBSTITUTION REACTIONS
259