Organic Chemistry

H

C X

H H

H

C X

H H

δ−

δ−

O

rganic compounds that have an

electronegative atom or group

bonded to an hybridized carbon

undergo substitution reactions and/or elimina-

tion reactions. In a substitution reaction, the electroneg-

ative atom or group is replaced by another atom or group. In an elimination reaction,

the electronegative atom or group is eliminated, along with a hydrogen from an adja-

cent carbon. The atom or group that is substitutedor eliminatedin these reactions is

called a leaving group.

This chapter focuses on the substitution reactions of alkyl halides—compounds in

which the leaving group is a halide ion ( or ). The nomenclature of

alkyl halides was discussed in Section 2.4.

In Chapter 11, we will discuss the elimination reactions of alkyl halides and the factors

that determine whether substitution or elimination will prevail when an alkyl halide

undergoes a reaction.

Alkyl halides are a good family of compounds with which to start our study of sub-

stitution and elimination reactions because they have relatively good leaving groups;

that is, the halide ions are easily displaced. We will then be prepared to discuss, in

alkyl halides

an alkyl fluoride

RF

an alkyl chloride

RCl

an alkyl bromide

RBr

an alkyl iodide

RI

F-, Cl-, Br-, I-

+

a substitution reaction

an elimination reaction

the leaving group

RCH 2 CH 2 X Y−

RCH 2 CH 2 Y + X−

RCH CH 2 + HY + X−

sp^3

360

10

Substitution Reactions

of Alkyl Halides