Substitution and Elimination Reaction Mechanisms
56.2.2 E2
Reactivity Due to Structure of E2RCH 2 X > R 2 CHX >> R 3 CXTheE2mechanism is concerted and higly stereospecific, because it can occur only when
the H and the leaving group X are in an anti-coplanar position. That is, in a Newman
projection, the H and X must be 180°, or in the anti-configuration. This behaviour stems
from the best overlap of the 2porbitals of the adjacent carbons when the pi bond has to
be formed. If the H and the leaving group cannot be brought into this position due to the
structure of the molecule, theE2mechanism will not take place.
Figure 161 Mechanism of E2 elimination. Note the anti-coplanarity of the
X-C-C-H atoms
Therefore, only molecules having accessible H-X anti-coplanar conformations can react via
this route. Furthermore, theE2mechanism will operate contrary to Zaitsev’s rule if the
only anti-coplanar hydrogen from the leaving group results in the least stable alkene. A
good example of how this can happen is by looking at how cyclohexane and cyclohexene
derivatives might operate inE2conditions.
Figure 162 E2with preferential elimination
Let’s look at the example above. The reactant we’re using is 1-chloro-2-
isopropylcyclohexane. The drawing at the top left is one conformation and the drawing
below is after a ring flip. In the center are Newman projections of both conformations and
the drawings on the right, the products.
If we assume we’re treating the 1-chloro-2-isopropylcyclohexane with a strong base, for
example CH 3 CH 2 O- (ethanolate), the mechanism that dominates isE2. There are 3
hydrogens off of the carbons adjacent to our chlorinated carbon. The red and the green
ones are two of them. The third would be hard to show but is attached to the same carbon
as the red hydrogen, angled a little down from the plane and towards the viewer. The red
hydrogen is the only hydrogen that’s 180° from the chlorine atom, so it’s the only one eligible
for theE2mechanism. Because of this, the product is going to be only 3-isopropylcylcohex-
1-ene. Notice how this is contrary to Zaitsev’s rule which says the most substituted alkene
is preferred. By his rule, 1-isopropylcyclohexene should be our primary product, as that
would leave the most substituted alkene. However it simply can’t be produced because of
the steric hindrance.
The images below shows the molecule after a ring flip. In this conformation, no product is
possible. As you can see from the Newman projection, there are no hydrogens 180° from
the chlorine atom.
So it’s important, when considering theE2mechanism, to understand the geometry of the
molecule. Sometimes the geometry can be used to your advantage to preferentially get a
single product. Other times it will prevent you from getting the product you want, and
you’ll need to consider a different mechanism to get your product.