Effect of Structureit is found that the SN 2 reaction rate falls as we pass along the series,
the drop being particularly marked as we go from the isobutyl to the
neopentyl halide, in which attack' from the back * by e.g.^0 OH on the
oc-carbon atom along the line of centres might be expected to be very
highly hindered. The main factor inhibiting S,v2 attack is however the
highly crowded transition state
Me
I
Me—C—Me
l-f | 8-- WJ C CI
H H
that would have to be formed. The drop in rate probably owes little
to the inductive effect of the increasing number of substituent methyl
groups owing to the interposition of a saturated carbon atom (the
jS-carbon) between them and the carbon atom (a-) to be attacked.
A very interesting example of the effect of structure on the reactivity
of a halide is provided by 1-brgmotriptycene (II): q
The bromine atom in this compound is virtually inert to nucleophiles.
SJV2 'attack from the back' is inhibited sterically by the cage-like
structure, and the formation of a transition state, in which the three
groups attached to the carbon atom attacked must pass through a
co-planar arrangement, is prevented as this atom is held rigidly in
position by the substituents attached to it. SN\ attack is also inhibited
because the carbonium ion that would be formed by ionisation is
unable to stabilise itself for, being unable to achieve coplanarity with
its substituents, charge-delocalisation with them (p. 84) cannot take
place.