Organic Chemistry

258 CHAPTER 6 Reactions of Alkynes • Introduction to Multistep Synthesis

6.12 Commercial Use of Ethyne

Most of the ethyne produced commercially is used as a starting material for polymers

that we encounter daily, such as vinyl flooring, plastic piping, Teflon, and acrylics.

Polymers are large molecules that are made by linking together many small molecules.

The small molecules used to make polymers are called monomers. The mechanisms

by which monomers are converted into polymers are discussed in Chapter 28.

Poly(vinyl chloride), the polymer produced from the polymerization of vinyl chloride,

is known as PVC. Straight-chain poly(vinyl chloride) is hard and rather brittle; branched

poly(vinyl chloride) is the soft elastic vinyl commonly used both as a substitute for leather

and in the manufacture of such things as garbage bags and shower curtains (Section 28.2).

Poly(acrylonitrile) looks like wool when it is made into fibers. It is marketed under the

trade names Orlon®(DuPont), Creslan®(Sterling Fibers), and Acrilan®(Monsanto).

a monomer

a polymer

acrylonitrile

poly(acrylonitrile)

Orlon

po ymerization

HC CH + H 2 CCH 2 CH

CN

HCN CHCN

n

vinyl chloride

a monomer

poly(vinyl chloride)

PVC

a polymer

polymerization

HC CH + H 2 CCH 2 CH

Cl

HCl CHCl

n

ETHYNE CHEMISTRY

OR THE FORWARD PASS?

Father Julius Arthur Nieuwland (1878–1936) did much of

the early work on the polymerization of ethyne. He was born in Belgium

and settled with his parents in South Bend, Indiana, two years later. He

became a priest and a professor of botany and chemistry at the Universi-

ty of Notre Dame, where Knute Rockne—the inventor of the forward

pass—worked for him as a research assistant. Rockne also taught chem-

istry at Notre Dame, but when he received an offer to coach the football

team, he switched fields, in spite of Father Nieuwland’s attempts to con-

vince him to continue his work as a scientist.

Knute Rockne in his

uniform during the

year he was captain of

the Notre Dame

football team.

Summary

Alkynesare hydrocarbons that contain a carbon–carbon

triple bond. A triple bond can be thought of as a cylinder of

electrons wrapped around the bond. The functional group

suffix of an alkyne is “yne.”A terminal alkynehas the

triple bond at the end of the chain; an internal alkynehas

the triple bond located elsewhere along the chain. Internal

alkynes, with two alkyl substituents bonded to the spcar-

bons, are more stable than terminal alkynes. We now have

seen that alkyl groups stabilize alkenes, alkynes, carbo-

cations,andalkyl radicals.

An alkyne is lessreactive than an alkene because a

vinylic cationis less stable than a similarly substituted

s

alkyl cation. Like alkenes, alkynes undergo electrophilic

addition reactions. The same reagents that add to alkenes

add to alkynes. Electrophilic addition to a terminal

alkyne is regioselective; in all electrophilic addition reac-

tions to terminal alkynes, the electrophileadds to the sp

carbon that is bonded to the hydrogen because the inter-

mediate formed—a secondary vinylic cation—is more

stable than a primary vinylic cation. If excess reagent is

available, alkynes can undergo a second addition reaction

with hydrogen halides and halogens because the product

of the first reaction is an alkene. An alkyl peroxide has

the same effect on the addition of HBr to an alkyne that it

BRUI06-238_262r4 24-03-2003 11:47 AM Page 258