Organic Chemistry

A cyclic bromonium ion

W

e have seen that an alkene such as 2-butene undergoes an
electrophilic addition reactionwith HBr (Section 3.6).
The first step of the reaction is a relatively slow addition of
the electrophilic proton to the nucleophilic alkene to form a carbocation
intermediate. In the second step, the positively charged carbocation inter-
mediate (an electrophile) reacts rapidly with the negatively charged bromide
ion (a nucleophile).

In this chapter, we will look at a wide variety of alkene reactions. You will see that
some of the reactions form carbocation intermediates like the one formed when HBr
reacts with an alkene, some form other kinds of intermediates, and some don’t form an
intermediate at all. At first, the reactions covered in this chapter might appear to be
quite different, but you will see that they all occur by similar mechanisms. So as you
study each reaction, notice the feature that all alkene reactions have in common:The
relatively loosely held electrons of the carbon–carbon double bond are attracted to
an electrophile. Thus, each reaction starts with the addition of an electrophile to one
of the carbons of the alkene and concludes with the addition of a nucleophile to the
other carbon. The end result is that the bond breaks and the carbons form
new bonds with the electrophile and the nucleophile.

CC C

YZ

++Y+ Z− C

the double bond is composed

of a σ bond and a π bond

electrophile nucleophile

the π bond has broken and

new σ bonds have formed

s

sp^2 p sp^2

sp^2

p

CC++H

slow

H

a carbocation

intermediate

C+ C

fast

Br Br−

Br H

C C

4

Reactions of Alkenes

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