Organic Chemistry

428 CHAPTER 11 Elimination Reactions of Alkyl Halides • Competition Between Substitution and Elimination

The vinylic halide intermediates in the preceding reactions are relatively unreactive.

Consequently, a very strong base, such as is needed for the second elimination.

If a weaker base, such as is used at room temperature, the reaction will stop at

the vinylic halide and no alkyne will be formed.

Because a vicinal dihalide is formed from the reaction of an alkene with or

you have just learned how to convert a double bond into a triple bond.

PROBLEM 25

Why isn’t a cumulated diene formed in the preceding reaction?

11.11 Intermolecular Versus Intramolecular Reactions

A molecule with two functional groups is called a bifunctional molecule. If the two

functional groups are able to react with each other, two kinds of reactions can occur. If

we take as an example a molecule whose two functional groups are a nucleophile and

a leaving group, the nucleophile of one molecule can displace the leaving group of a

second molecule of the compound. Such a reaction is called an intermolecular reac-

tion. Interis Latin for “between,”so an intermolecular reactiontakes place between

two molecules. If the product of each intermolecular reaction subsequently reacts with

another bifunctional molecule, a polymer will be formed (Chapter 28).

Alternatively, the nucleophile of a molecule can displace the leaving group of the

same molecule, thereby forming a cyclic compound. Such a reaction is called an in-

tramolecular reaction. Intrais Latin for “within,”so an intramolecular reactiontakes

place within a single molecule.

Which reaction is more likely to occur—an intermolecular reaction or an intramol-

ecular reaction? The answer depends on the concentrationof the bifunctional mole-

cule and the size of the ringthat will be formed in the intramolecular reaction.

The intramolecular reaction has an advantage: The reacting groups are tethered to-

gether, so they don’t have to diffuse through the solvent to find a group with which to

react. Therefore, a low concentration of reactant favors an intramolecular reaction be-

cause the two functional groups have a better chance of finding one another if they are

in the same molecule. A high concentration of reactant helps compensate for the ad-

vantage gained by tethering.

How much of an advantage an intramolecular reaction has over an intermolecular

reaction depends on the size of the ring that is formed—that is, on the length of the

tether. If the intramolecular reaction forms a five- or six-membered ring, it will be

highly favored over the intermolecular reaction because five- and six-membered rings

are stable and, therefore, easily formed.

an intramolecular reaction

O

Br CH 2 (CH 2 )nCH 2 O

−

(CH 2 )n

H 2 CCH 2 + Br−

Br 2 Cl 2 ,

HO-,

• NH
  2 ,

−NH 2 −NH 2

BrBr Br

CH 3 CHCHCH 3 CH 3 CH CCH 3

Br 2

CH 3 CH CHCH (^3) CH 2 Cl 2
2-butene 2-butyne
CH 3 C CCH 3 ++2 NH 3 2 Br−
an intermolecular reaction
BrCH 2 (CH 2 )nCH 2 OBrCH 2 (CH 2 )nCH 2 O
−−
BrCH 2 (CH 2 )nCH 2 OCH 2 (CH 2 )nCH 2 O− + Br−
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