Organic Chemistry

Acetylene , the common name for the smallest alkyne, may be a famil-

iar word because of the oxyacetylene torch used in welding. Acetylene is supplied to

the torch from one high-pressure gas tank, and oxygen is supplied from another. Burn-

ing acetylene produces a high-temperature flame capable of melting and vaporizing

iron and steel.

PROBLEM 1

What is the molecular formula for a cyclic alkyne with 14 carbons and two triple bonds?

6.1 Nomenclature of Alkynes

The systematic name of an alkyne is obtained by replacing the “ane”ending of the

alkane name with “yne.”Analogous to the way compounds with other functional

groups are named, the longest continuous chain containing the carbon–carbon triple

bond is numbered in the direction that gives the alkyne functional group suffix as low

a number as possible. If the triple bond is at the end of the chain, the alkyne is classi-

fied as a terminal alkyne. Alkynes with triple bonds located elsewhere along the

chain are called internal alkynes. For example, 1-butyne is a terminal alkyne, where-

as 2-pentyne is an internal alkyne.

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In common nomenclature, alkynes are named as substituted acetylenes. The com-

mon name is obtained by citing the names of the alkyl groups, in alphabetical order,

that have replaced the hydrogens of acetylene. Acetylene is an unfortunate common

name for the smallest alkyne because its “ene”ending is characteristic of a double

bond rather than a triple bond.

If the same number for the alkyne functional group suffix is obtained counting from

either direction along the carbon chain, the correct systematic name is the one that

contains the lowest substituent number. If the compound contains more than one sub-

stituent, the substituents are listed in alphabetical order.

The triple-bond-containing propargyl group is used in common nomenclature. It is

analogous to the double-bond-containing allyl group that you saw in Section 3.2.

PROBLEM 2

Draw the structure for each of the following compounds.

a. 1-chloro-3-hexyne d. propargyl chloride

b. cyclooctyne e. 4,4-dimethyl-1-pentyne

c. isopropylacetylene f. dimethylacetylene

propargyl group

HC CCH 2

allyl group

H 2 C CHCH 2

propargyl bromide

HC CCH 2 Br

allyl alcohol

H 2 C CHCH 2 OH

3-bromo-2-chloro-4-octyne

not 6-bromo-7-chloro-4-octyne

because 2 < 6

CCH 2 CH 2 CH 3

ClBr

CH 3 CHCHC

1234567 8

1-bromo-5-methyl-3-hexyne

not 6-bromo-2-methyl-3-hexyne

because 1 < 2

CCH 2 CH 2 Br

CH 3

CH 3 CHC

6 5 4 32 1

(HC‚CH)

1-hexyne

a terminal alkyne

3-hexyne

an internal alkyne

Section 6.1 Nomenclature of Alkynes 239

3-D Molecules:

1-Hexyne; 3-Hexyne

A substituent receives the lowest

possible number only if there is no

functional group suffix or if the same

number for the functional group suffix

is obtained in both directions.

ethyne

acetylene

Systematic:

Common:

HC CH

1-butyne

ethylacetylene

a terminal alkyne

CH 3 CH 2 C CH

4321

2-pentyne

ethylmethylacetylene

an internal alkyne

CH 3 C CCH 2 CH 3

1234 5

4-methyl-2-hexyne

sec-butylmethyl-

acetylene

CCH 3

CH 2 CH 3

CH 3 CHC

5 6

4 3 21

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