A Guidebook to Mechanism in Organic Chemistry

Nucleophilic Substitution at a Saturated Carbon Atom

and further kinetic investigation shows a change in order of reaction,
and hence presumably of mechanism, as the series is traversed. Thus
methyl and ethyl bromide show second order kinetics, isopropyl
bromide shows a mixture of second and first order, with the former
predominating—the total rate here being a minimum for the series-
while t-butyl bromide exhibits first order kinetics.
The reason for the changeover is not far to seek: methyl groups
exert an inductive effect as already mentioned (p. IS), so that in
Me-»-CH 2 —Br the carbon atom attached to bromine will be more
negative than that in CH 3 —Br, it will tj|us be less susceptible to attack
by eOH and the SN 2 mechanism will be inhibited. This effect will be

more pronounced in ^e^CH—Br and SJV2 attack will be further

inhibited, but transmission of some of the negative charge accumulated

on carbon to the bromine atom will promote ionisation, i.e. attack on

the carbon atom by the SN\ mechanism will be facilitated. By the time

Me-»-C—Br is reached, Sjy2 attack is entirely inhibited, but that by

Me^.

Sjyl is now considerably promoted, a complete changeover in the

mechanistic type of the reaction has been effected and the overall rate

increases. The stability of the carbonium ion produced on going

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CH 3 ->Me'CH 2 ->Me 2 CH-»-Me 3 C will be increasingly promoted by

hyperconjugation

via the hydrogen atoms attached to the a-carbons, the above series of

ions having 0, 3, 6 and 9 such hydrogen atoms, respectively. Such

hyperconjugative stabilisation can only take place in a planar ion,

hence the need for a carbonium ion to be able to attain such a

planar state if it is to be formed at all (cf. p. 84). This increasing

stabilisation of the carbonium ion as the series is traversed will

naturally further promote the SN\ mechanism at the expense of SJV2.

There will also be an increasing steric resistance to S^2 as the series

is traversed. Not only will eOH find it more difficult to attack the

carbon atom attached to bromine as the former becomes more heavily

etc

CH 3