The Foundations of Chemistry

8-14). Triply bonded carbon atoms are sp-hybridized. The triply bonded atoms and their
adjacent atoms lie on a straight line (Figure 27-10).
Alkynes are named like the alkenes except that the suffix -yneis added to the charac-
teristic root. The first member of the series is commonly called acetylene. Its molecular
formula is C 2 H 2. It is thermodynamically unstable, decomposing explosively to C(s) and
H 2 (g) at high pressures. It may be converted into ethene (also known as ethylene) and
then to ethane by the addition of hydrogen. The condensed formulas for several small
alkynes are

Figure 27-10 Models of acetylene,

HXCmCXH.

27-4 Alkynes 1059

CH CH CH 3 C CH

ethyne

(acetylene)

propyne 1-butyne

CH 3 CH 2 C CH

CH 3 C C CH 3 CH 3 C

CH 3

CH CH

3-methyl-1-butyne

2-butyne

The triple bond takes positional precedence over substituents on the carbon chain. It is
assigned the lowest possible number in naming.

Summary of IUPAC Rules for Naming Alkynes

Alkynes are named like the alkenes except for the following two points.

1.The suffix -yneis added to the characteristic root.

2.Because the linear arrangement about the triple bond does not lead to geometric

isomerism, the prefixes cis- and trans- are not used.

EXAMPLE 27-9 Alkynes

Write the structural formula of 5,5-dimethyl-2-heptyne.

Plan

The structure is based on a seven-carbon chain with a triple bond beginning at carbon 2. We
add methyl groups at the positions indicated.

Solution

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CXCmCXCXCXCXC

Two methyl groups are attached to carbon 5, and sufficient hydrogens are added to complete
the bonding at each C atom.

The carbon chain is numbered to give the triple bond the lowest possible numbers even though
that results in higher position numbers for the methyl substituents.

You should now work Exercise 28.

CH 3 C

CH 3

CH 3

CCH 2 CCH 2 CH 3

172 34 56