19. The Simple Compounds
                                   of Carbon
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      Unsaturated Hydrocarbons

All that would be needed to change cis-2-butene to trans-2-butene is to break the double bond momentarily, twist the molecule around, then reconnect the same bond. One cannot go from 2-butene to either of the other structural isomers by breaking, twisting, and then re-forming the same bond. Cis-trans isomerism is possible whenever two different groups are connected to each end of a double bond ("cis-" means "on the same side," and "trans-" means "across from").

All of the isomers of butene have similar chemical and physical properties, and the distinction between geometrical and structural isomers is less significant to their properties than is the actual shape of the molecule.

1-Butene has a "swivel' about the bond from the CH3CH2- to the C=C, and is a flexible molecule. It therefore packs clumsily into a solid crystalline lattice, and melts at a low -1850C. All of the other three isomers are rigid.

Trans-2-butene is a lumpy rod that packs well with its neighbors in the solid and hence does not melt until -1060C. The other two isomers are more irregular, but are similar in shape and melt at an intermediate temperature within 10C of each other.

Shape is less important in affecting boiling points because the molecules in a liquid already move freely past one another, so all four isomers boil within a 110C range.

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