Level 3 – Case 33
222
F
NO 2
NO 2
NO 2
NO 2
F NH
Me
NH 2
NO 2
HN
NO 2
Me
NO 2
F
NO 2
NO 2
NO 2
F NH
NO 2
HN
NO 2
NH 2
Me
rds
rds
HF
k -1
k k
k 2
k k + k 3
k k [o-toluidine]
+ HF
+
k 1
fast
k 1
+
Me
Me
Me
Scheme 33.6
In Summary
The influence of steric effects on the S N Ar mechanism has been studied by means
of leaving group fluorine kinetic isotope effects. The reaction of DNFB with oand p-toluidines shows a change in the rate-determining step depending on the
steric hindrance of the amine employed.
Additional Comments
To obtain Eq 33.1 we have applied the steady-state approximation to the intermediate 2. If 2 achieves a steady-state, then we can write:
0
=
]
[2][ArNH
+
]
H
[DNFB][ArNH H
[2]
+
[2]
=
d
d[2]
2
3
2
1
1
2
k 3
k 1
k
k 2
t
(33.3)
Hence,
]
]
[
2
3
1
2
2
1
=
[ArNH
+
+
]
H
[DNFB][ArNH H
k 3
k
k 2
k 1
2 2
(33.4)
If we consider that
]
[2][ArNH
[2]
d
d[product]
2
3
2
=
k 3
k
t
v
(33.5)
the rate expression will be: