Stereoselectivity in E1
Monday--Nov 19, 2007 Filed in: 233
There have been several
questions about the stereoselectivity of an E1
elimination mechanism. Two arguments can be made.
A kinetic
argument compares the relative energies of
the carbocation intermediates leading to each
product. The carbocation intermediate on the left is
more stable than the carbocation intermediate on the
right due to reduced steric strain; therefore the
trans alkene is favored. Because this reaction is
reversible a thermodynamic
argument can
also be made and is actually more correct; although
the outcome is the same in this instance. Thus, the
trans alkene product is favored because it is more
stable (lower in energy) than the cis alkene. Trans
is more stable than cis for the same reason already
mentioned, less steric strain. Caution: thermodynamic
arguments (comparing relative stabilities of
products)
can only be made when reactions are reversible
(equilibrium). This reaction is reversible and
explains why distillation was necessary to remove the
alkene products and shift the equilibrium toward
products (LeChatleir principle).
