Chemistry Reference
In-Depth Information
Scheme 1.4 Illustration of homolytic cleavage.
Scheme 1.5 Illustration of heterolytic cleavage.
bonds can be broken. In general, there are three ways in which this process can be initiated.
As shown in Scheme 1.4, the first is simple separation of a single bond where one electron
from the bond resides on one atom and the other electron resides on the other atom. This
type of bond cleavage is known as homolytic cleavage because the electron density is
equally shared between the separate fragments and no charged species are generated. It
is this process that leads to free radical mediated reactions.
Unlike homolytic cleavage, heterolytic cleavage (Scheme 1.5) of a chemical bond
results in one species retaining both electrons from the bond and one species retaining
no electrons from the bond. Generally, this also results in the formation of ionic species
where the fragment retaining the electrons from the bond becomes negatively charged
while the other fragment becomes positively charged. These charged species then
become available to participate in ion-based transformations governed by the electronic
nature of reactants or adjacent functional groups.
Having introduced homolytic cleavage and heterolytic cleavage as the first two ways in
which bonds are broken at the initiation of organic reactions, attention must be drawn to the
Scheme 1.6 Illustration of a concerted reaction (Cope rearrangement).
Scheme 1.7 Illustration of arrow pushing applied to the Cope rearrangement.
