Functional Classes II, Reactions - Organic Chemistry Concepts: An EFL Approach - page 106

stepwise reaction mechanisms below. The reaction of symmetrical and unsym-
metrical alkenes is shown separately in Figures 7.4 and 7.5 .
These are our first detailed mechanisms, and they use curly arrows and reactive
intermediates. Because of this, some discussion of the steps is given.
The stepwise reaction starts with heterogenic bond making between the elec-
trophilic portion of the reagent and the pair of electrons of the π-bond. This
occurs at one carbon of the original double bond. This leaves the carbon atom
at the other end of the original double bond with only three ligands. This is the
intermediate carbocation with only six valence shell electrons. The reaction is
completed by the second heterogenic bond formation between this carbocation
and the negatively charged anion portion of the reagent.
FIGURE 7.4
Mechanism of electrophilic addition to a symmetrical alkene.
In Figure 7.4 , the mechanism shows it does not matter at which end of the origi-
nal double bond the first bond is formed. The carbocation intermediate is the
same in both cases and gives the same product. This result is because the alkene
is symmetrical. Now compare this result to the unsymmetrical alkene example
in Figure 7.6 .
If the alkene is not symmetrical, two different products are possible when an
unsymmetrical reagent is added. This is shown in Figure 7.5 . Studies of the prod-
ucts from many of these reactions led to a finding known as the Markovnikov
Rule. This can be stated simply as:
The addition of an unsymmetrical reagent to an unsymmetrical alkene
always occurs so that the electrophilic part of the reagent adds to the
end of the alkene which has the most hydrogen ligands.