cis and trans isomers
By convention, the configuration of complex alkenes is taken to correspond to the configuration of the longest continuous chain as it passes through the double bond. Thus the following compound is 4-ethyl-3-methyl-trans-3- heptene, despite the fact that two identical groups are cis with respect to each
other, because the longest continuous chain is trans as it passes through the double bond.
The trans isomers of the simple alkenes are usually more stable than the corresponding cis isomers. The methyl groups in trans-2-butene are far apart; in cis-2-butene, they are much closer to one another. Scale models, which reflect the sizes of the methyl groups, indicate some interference between the methyl groups of the cis isomer. The cis alkenes with large groups have very considerable repulsive interactions (steric hindrance) between the substituents, and are much less stable than the corresponding trans isomers (see Figure 4.1).
The generally greater stability of trans over cis isomers (see, however, Section 2-6B) is reflected in their lower heats of combustion. Table 4.1 compares the heats of combustion and the boiling and melting points of some cis and trans isomers. The data also reveal that trans isomers tend to have higher melting points and lower boiling points than cis isomers. Although the differences are not large, they may be of some help in assigning configurations. When electron-withdrawing groups such as halogens are attached to the
double bond, the dipole moments of cis and trans isomers are different (Section 2-6B), allowing an assignment of configuration to be made. Infrared spectroscopy (Section 7.4) is also useful for distinguishing cis and trans isomers.
Occasionally, a chemical method, ring closure, can be used to determine the configuration of cis-trans isomers. In general, cis isomers can undergo ring closure much more readily than the corresponding trans isomers because it is not possible to prepare a five- or six-membered ring compound with a trans double bond in the ring. The kind of difference which is observed is well illustrated by maleic acid, which has a cis double bond and, on heating to 1 50°, loses water to give maleic anhydride. The corresponding trans isomer, fumaric acid, does not give an anhydride at 150". In fact, fumaric anhydride, which would have a trans double bond in a five-membered ring, has never been prepared. Clearly, of this pair, maleic acid has the cis configuration and
fumaric acid the trans configuration.
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