SN2 kinetics and geometry

First, if you would like to see the MO pictures that I used in class today, check out last year’s post on back side attack & HOMO-LUMO overlap (Oct 13, 2008).

There are also a couple of points that I want to add concerning reaction rates (kinetics):

Carefully read section 9.3, Reaction Rates. I couldn’t find a convenient way to fit this material into lecture, but it is vitally important. Now for some specific points:

#1 – We usually compare competing reactions by comparing their rate constants.
Loudon says this is legitimate when the competing reactions are both
performed under “standard conditions” and he’s right. It is also a fair comparison when
the rate laws for the competing reactions are the same, i.e., they
differ only in the numerical values of their respective rate constants. When this condition is satisfied,
we don’t have to be limited to standard conditions.

#2 – The lifetime of a species is inversely related to the rate
constant for its disappearance. A large rate constant (for
disappearing) means a short life. This also means that the “time to
completion” for a chemical reaction is inversely related to the rate
constant for the reaction. A large rate constant means a short time to

#3 – The ratio between two rate constants is exponentially
related to the difference in their activation energies. See equations
9.22. Also compare data in different columns of Table 9.2. Reactions
that have “time to completion” of 12 minutes, 20 hours, and 83 days
have nearly the same free energies of activation. The exponential
relationship makes this possible.

#4 – (see
section 9.4B) Acid-base reactions happen much, much faster than SN2
reactions (and much, much faster than just about any other kind of
organic reaction). A mechanistic hypothesis can’t be valid if it
requires favorable acid-base reactions to “wait” for other reactions.

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