Organic Chemistry

176 CHAPTER 4 Reactions of Alkenes

Summary

Alkenesundergo electrophilic addition reactions. Each re-
action starts with the addition of an electrophile to one of the
carbons and concludes with the addition of a nucleophile
to the other carbon. In all electrophilic addition reactions,
the electrophileadds to the carbon bonded to the greater
number of hydrogens. Markovnikov’s rulestates that the
hydrogen adds to the carbon of the alkene bonded to the
greater number of hydrogens. While all addition reactions
add the electrophile to the carbon bonded to the greater
number of hydrogens, they all do not follow Markovnikov’s
rule, because hydrogen is not always the electrophile.
Hydroboration–oxidationand the addition of HBrin the
presence of a peroxideare anti-Markovnikov additions.
The addition of hydrogen halides and the acid-catalyzed
addition of water and alcohols form carbocation interme-
diates. Hyperconjugationcauses tertiary carbocationsto
be more stable than secondary carbocations, which are
more stable than primary carbocations. A carbocation
will rearrange if it becomes more stable as a result of the
rearrangement. Carbocation rearrangements occur by
1,2-hydride shifts,1,2-methyl shifts, and ring expansion.
HBr in the presence of a peroxide forms a radical
intermediate. Radical intermediates do not rearrange.
Hydroboration–oxidationis a concerted reactionand
does not form an intermediate.
Oxymercuration,alkoxymercuration, and the addition
of and form cyclic intermediates. Oxymercuration
and alkoxymercuration are followed by a reduction reac-
tion. Reductionincreases the number of C H bonds or
decreases the number of C O, C N, or C X bonds
(where X denotes a halogen). Hydroborationis followed
by an oxidation reaction. Oxidationdecreases the number
of C H bonds or increases the number of C O, C N,
or C¬X bonds (where, again, X denotes a halogen).

¬ ¬ ¬

¬ ¬ ¬

¬

Br 2 Cl 2

sp^2

sp^2

sp^2

sp^2

sp^2

The Hammond postulate states that a transition

stateis more similar in structure to the species to which it

is more similar in energy. Thus, the more stable product

will have the more stable transition state and will lead to

the major product of the reaction. Regioselectivityis the

preferential formation of one constitutional isomerover

another.

In heterolytic bond cleavage, a bond breaks such that

both electrons in the bond stay with one of the atoms; in

homolytic bond cleavage, a bond breaks such that each of

the atoms retains one of the bonding electrons. An alkyl

peroxide is a radical initiatorbecause it creates radicals.

Radical addition reactions are chain reactions with

initiation,propagation, and termination steps. Radicals

are stabilized by electron-donating alkyl groups. Thus, a

tertiary alkyl radicalis more stable than a secondary

alkyl radical, which is more stable than a primary alkyl

radical. A peroxidereverses the order of addition of H and

Br because it causes instead of to be the elec-

trophile. The peroxide effectis observed only for the addi-

tion of HBr.

The addition of to a reaction is called hydrogena-

tion. The heat of hydrogenationis the heat released in a

hydrogenation reaction. The greaterthe stabilityof a com-

pound, the loweris its energyand the smalleris its heat of

hydrogenation. The more alkyl substituents bonded to the

carbons of an alkene, the greater is its stability. Hence,

carbocations, alkyl radicals, and alkenes are all stabilized

by alkyl substituents. Trans alkenesare more stable than

cis alkenesbecause of steric strain.

Electrophilic addition reactions of alkenes lead to the

synthesisof alkyl halides,vicinal dihalides,halohydrins,

alcohols,ethers, and alkanes.

sp^2

H 2

Br–, H+,

Summary of Reactions

As you review the reactions of alkenes, keep in mind the feature that is common to all of them: The first step of each reaction is the ad-
dition of an electrophile to the carbon of the alkene that is bonded to the greater number of hydrogens.

1. Electrophilic addition reactions

a.Addition of hydrogen halides ( is the electrophile; Section 4.1)

b.Addition of hydrogen bromide in the presence of a peroxide ( is the electrophile; Section 4.10)

RCH CH 2 HBr RCH 2 CH 2 Br

peroxide

+

Br–

RCH CH 2 + HX RCHCH 3

X

HX = HF, HCl, HBr, HI

H+

sp^2