The Foundations of Chemistry

UNSATURATED HYDROCARBONS

There are three classes of unsaturated hydrocarbons: (1) the alkenes and their cyclic coun-

terparts, the cycloalkenes; (2) the alkynes and the cycloalkynes; and (3) the aromatic

hydrocarbons.

ALKENES

The simplest alkenescontain one carbon–carbon double bond, CUC, per molecule. The

general formula for noncyclic alkenes is CnH 2 n. The simplest alkene is C 2 H 4 , ethene,

which is usually called by its common name, ethylene.

The bonding in ethylene was described in Section 8-13. The hybridization (sp^2 ) and

bonding at other double-bonded carbon atoms are similar. Both carbon atoms in C 2 H 4

are located at the centers of trigonal planes. Rotation about CUC double bonds does not

occur significantly at room temperature. Compounds that have the general formula

(XY)CUC(XY) therefore exist as a pair of cis–transisomers. Figure 27-9 shows the cis–

transisomers of 1,2-dichloroethene. The existence of compounds with different arrange-

ments of groups on the opposite sides of a bond with restricted rotation is called geometric

isomerism.This cis–transisomerism can occur across double bonds in alkenes and across

single bonds in rings.

Two shared electron pairs draw the atoms closer together than a single electron pair

does. Thus, carbon–carbon double bonds are shorter than CXC single bonds, 1.34 Å

versus 1.54 Å. The physical properties of the alkenes are similar to those of the alkanes,

but their chemical properties are quite different.

The root for the name of each alkene is derived from the alkane having the same

number of C atoms as the longest chain containing the double bond. In the trivial

(common) system of nomenclature, the suffix -yleneis added to the characteristic root. In

systematic (IUPAC) nomenclature, the suffix -eneis added to the characteristic root.

27-3

Four CXH 
bonds, one CXC 


bond (green, hatched), and one CXC

bond (tan, hatched) in the planar

C 2 H 4 molecule.

27-3 Alkenes 1053

Figure 27-9 Two isomers of 1,2-dichloroethene are possible because rotation about the

double bond is restricted. This is an example of geometricisomerism. A ball-and-stick model

and a space-filling model are shown for each isomer. (a) The cisisomer. (b) The trans

isomer.

(b)

C C

H Cl

Cl H

trans-1,2-dichloroethene

mp –50°C, bp 47.5°C

(a)

C C

Cl Cl

H H

cis-1,2-dichloroethene

mp –80.5°C, bp 60.3°C

one bond

H

H H

H

C σ C

σ

σσ

σ

π