Functional Classes II, Reactions - Organic Chemistry Concepts: An EFL Approach - page 136

PROGRAM 20 Elimination
A Program 19 showed nucleophilic substitution at a saturated carbon atom. The
nucleophile attacked the saturated center with an electron pair and displaced a
suitable leaving group.
There is another reaction which can occur if the nucleophilic electron pair acts as a
base. Study again the reaction of an alkyl halide with hydroxide.
The overall reaction is elimination. It is a 1,2-elimination between adjacent centers.
This loss of a proton and a bromide causes an increase in the bond order between the
original carbon centers.
Can you rewrite the above reaction with a curly arrow description?
B The detailed equation should look like the following:
Like nucleophilic substitution, the elimination reaction can follow two different path-
ways. This is again controlled by the exact order of events. If the leaving group leaves
by ionization to give a carbocation, the mechanism is called E1. If the leaving group is
displaced in a single step when the base removes an adjacent proton, the mechanism
is called E2. This is the mechanism shown above.
Like substitution reactions, E1 shows that the rate-determining step is controlled only
by the concentration of substrate. In E2, the rate-determining step is controlled by both
the concentration of substrate and base.
Use the above example to draw the alternative E1 sequence.
C You should have drawn the following.