preface to fifteenth edition

ORGANIC COMPOUNDS 1.43
FIGURE 1.9
isomer.
Two isomers of 1,4-dimethylcyclohexane. (a) Trans isomer; (b) cis
FIGURE 1.10 Two isomers of decahydronaphthalene, or bicyclo[4.4.0]decane.
(a) Trans isomer; (b) cis isomer.
The bicyclic compound decahydronaphthalene, or bicyclo[4.4.0]decane, has two fused six-membered
rings. It exists in cis and trans forms (see Fig. 1.10), as determined by the configurations at
the bridgehead carbon atoms. Both cis- and trans-decahydronaphthalene can be constructed with
two chair conformations.
1.1.4.2 Geometrical Isomerism. Rotation about a carbon-carbon double bond is restricted because
of interaction between the p orbitals which make up the pi bond. Isomerism due to such
restricted rotation about a bond is known as geometric isomerism. Parallel overlap of the p orbitals
of each carbon atom of the double bond forms the molecular orbital of the pi bond. The relatively
large barrier to rotation about the pi bond is estimated to be nearly
1
63 kcal · mol
1
(263 kJ · mol ).
When two different substituents are attached to each carbon atom of the double bond, cis-trans
isomers can exist. In the case of cis-2-butene (Fig. 1.11a), both methyl groups are on the same side
of the double bond. The other isomer has the methyl groups on opposite sides and is designated as
trans-2-butene (Fig. 1.11b). Their physical properties are quite different. Geometric isomerism can
also exist in ring systems; examples were cited in the previous discussion on conformational isomers.
For compounds containing only double-bonded atoms, the reference plane contains the doublebonded
atoms and is perpendicular to the plane containing these atoms and those directly attached
to them. It is customary to draw the formulas so that the reference plane is perpendicular to that of