The Foundations of Chemistry

EXAMPLE 27-8 Cycloalkenes

Write the skeletal representation of 3-methylcyclohexene.

Plan

We draw the ring of the specified size with one double bond in the ring. We number the ring

so the double bond is between atoms 1 and 2. Then we add the designated substituents at the

indicated positions.

Solution

We number the six-membered ring so that the double bond is between atoms 1 and 2.

A methyl group is attached at carbon 3; the correct structure is

Remember that each intersection of two lines represents a carbon atom; there are enough H

atoms at each C atom to make a total of four bonds to carbon.

You should now work Exercise 21.

ALKYNES

The alkynes,or acetylenic hydrocarbons, contain one or more carbon–carbon triple

bonds, XCmCX. The noncyclic alkynes with one triple bond per molecule have the

general formula CnH 2 n 2. The bonding in all alkynes is similar to that in acetylene (Section

27-4

1 3

64

5

2 CH 3

13

64

5

2

105 8CHAPTER 27: Organic Chemistry I: Formulas, Names, and Properties

Problem-Solving Tip:How to Draw Skeletal Representations

The rules for drawing skeletal representations follow.

1.A carbon atom is assumed to be at each intersection of two lines and at the end of

each line. All carbon–carbon bonds are shown as lines. A carbon atom might be shown

for clarity.

2.Because carbon has four bonds, we mentally supply hydrogens to give each carbon

its four bonds.

3.Atoms other than hydrogen and carbon are shown. The following skeletal represen-

tation is that of 3-chloro-4-ethyl-cis-3-heptene.

and its formula is C 9 H 17 Cl.

Cl