Functional Classes II, Reactions - Organic Chemistry Concepts: An EFL Approach - page 107

FIGURE 7.5
Markovnikov product distribution.
FIGURE 7.6
Mechanism of electrophilic addition to an unsymmetrical alkene.
The mechanism in Figure 7.6 explains the formation of two different prod-
ucts. There are two possible carbocation intermediates which can be formed
by the initial reaction with the electrophile at either end of the alkene C ] C.
Because the alkene is not symmetrical, these carbocations are not the same
and lead to different products. Because they have different structures, the car-
bocations do not have equal energy. Based on this, Markovnikov's rule can
be restated:
Electrophilic addition always occurs through the more stable, lower
energy carbocation intermediate.
To predict the product distribution from these reactions, we need to only apply
the Chapter 5 concepts which control the relative stability of carbocations.
Electrophilic addition is the main reaction of most alkene chemistry. Many dif-
ferent two-part reagents can be added across the alkene bond. Some common
examples are listed in Table 7.1 . As the table shows, these reactions are prepara-
tions of other functional groups.