Elimination ReactionsNote: Often the wordperiplanaris used instead ofcoplanar.Coplanarimplies precisely
180 degree separation and ”peri-”, from Greek for ”near”, implies near 180 degrees. Peri-
planar may actually be more accurate. In the case of the 1-chloro-3-isopropylcyclohexane
example, because of molecular forces, the chlorine atom is actually slightly less than 180
degrees from both the hydrogen and the isopropyl group, so in this case,periplanarmight
be a more correct term.
56.2.3 E1
Figure 163 E1 elimination of an alkyl halide by a base
TheE1mechanism begins with the dissociation of the leaving group from an alkyl, pro-
ducing a carbocation on the alkyl group and a leaving anion. This is the same way the
SN1 reaction begins, so the same thing that helps initiate that step inSN1 reactions, help
initiate the step inE1reactions. More specifically, secondary and tertiary carbocations
are preferred because they’re more stable than primary carbocations. The choice of solvent
is the same as SN1 as well; a polar protic solvent is preferred because the polar aspect
stabilizes the carbocation and the protic aspect stabilizes the anion.
What makes the difference between whether the reaction takes theSN1 orE1pathway then,
must depend on the second step; the action of the nucleophile. InSN1 reactions, a strong
nucleophile that’s a weak base is preferred. The nucleophile will then attack and bond to the
carbocation. InE1reactions, a strong nucleophile is still preferred. The difference is that a
strong nucleophile that’s also a strong base, causes the nucleophile to attack the hydrogen at
the β-carbon instead of the α-carbocation. The nucleophile/base then extracts the hydrogen
causing the bonding electrons to fall in and produce a pi bond with the carbocation.
Because the hydrogen and the leaving group are lost in two separate steps and the fact that
it has no requirements as to geometry, theE1mechanism more reliably produces products
that follow Zaitsev’s rule.