The Foundations of Chemistry

One of the commercially most important addition reactions of the alkenes forms poly-

mers.This reaction will be discussed in Section 27-19.

The alkynes contain two pi bonds, both of which are sources of electrons, and they are

more reactive than the alkenes. The most common reaction of the alkynes is addition

across the triple bond. The reactions with hydrogen and with bromine are typical.

Other unsaturated bonds can also undergo addition reactions. Probably the most

O

B

important example is the carbonyl group, XCX. Because of the availability of unshared

pairs of electrons on the oxygen atom, the products can undergo a wide variety of subse-

quent reactions. For example, HCN adds to the CUO bond of acetone.

This reaction is a key early step in the production of the transparent plastic known as

Plexiglas or Lucite.

ELIMINATION REACTIONS

An elimination reactioninvolves a decreasein the number of atoms or groups

attached to carbon. The degree of unsaturation increases.

An elimination reaction, dehydrohalogenation,can occur for chloro-, bromo- and

iodoalkanes. In such a reaction, the halogen, X, from one C atom and a hydrogen from

an adjacent C atom are eliminated. A double bond between two carbon atoms is formed;

the molecule becomes more unsaturated.The net reaction is the transformation of an alkyl

halide (or haloalkane) into an alkene. Dehydrohalogenation reactions usually require a

strong base such as sodium hydroxide, NaOH.

H H

HH

H H NaOH CC H 2 O NaBr

H

H

C

Br

bromoethane ethene

(ethylene)

H

C 

27-18

NaOH(aq)

O

CH 3 CH 3 CH 3 C CH 3

OH

CN

C HCN

HH

H H

HH

CC CC

H 2

HH

H Br

Br H

CC CC

Br 2

1,2-dibromoethene

1090 CHAPTER 27: Organic Chemistry I: Formulas, Names, and Properties