Organic Chemistry

Section 10.1 How Alkyl Halides React 361

Chapter 12, the substitution and elimination reactions of compounds with leaving
groups that are harder to displace.
Substitution reactions are important in organic chemistry because they make it pos-
sible to convert readily available alkyl halides into a wide variety of other compounds.
Substitution reactions are also important in the cells of plants and animals. Because
cells exist in predominantly aqueous environments and alkyl halides are insoluble in
water, biological systems use compounds in which the group that is replaced is more
polar than a halogen and therefore more soluble in water. The reactions of some of
these biological compounds are discussed in this chapter.

10.1 How Alkyl Halides React

A halogen is more electronegative than carbon. Consequently, the two atoms do not
share their bonding electrons equally. Because the more electronegative halogen has a
larger share of the electrons, it has a partial negative charge and the carbon to which it
is bonded has a partial positive charge.

It is the polar carbon–halogen bond that causes alkyl halides to undergo substitution
and elimination reactions. There are two important mechanisms for the substitution
reaction:

1. A nucleophile is attracted to the partially positively charged carbon (an elec-
   trophile). As the nucleophile approaches the carbon and forms a new bond, the
   carbon–halogen bond breaks heterolytically (the halogen takes both of the bond-
   ing electrons).

Nu + C X Nu + X−

− δ+δ−

C

substitution

product

RCH 2 X X F, Cl, Br, I

δ+δ−

=

SURVIVAL COMPOUNDS

Several marine organisms, including sponges,

corals, and algae, synthesize organohalides (halo-

gen-containing organic compounds) that they use to deter

predators. For example, red algae synthesize a toxic, foul-

tasting organohalide that keeps predators from eating them.

One predator, however, that is not deterred is a mollusk called

a sea hare. After consuming red algae, a sea hare converts the

original organohalide into a structurally similar compound it

uses for its own defense. Unlike other mollusks, a sea hare does

not have a shell. Its method of defense is to surround itself with

a slimy material that contains the organohalide, thereby protect-

ing itself from carnivorous fish.

synthesized by red algae

CCH

Br

Cl

O

synthesized by the sea hare

a sea hare

C

C

H

HO

Br

H

Cl

HO O