isomerism in C2H4compounds

In the homologous series of alkanes, isomerism first appears at the C, level, two compounds of formula C4H,, being known. These are structural isomers:

There are in all six isomers of C₄H₈. Some are structural isomers and some stereoisomers (see Section 2.6B). Their boiling points and general physical properties are similar to those of butane and 2-methylpropane. Four of these compounds react quickly with bromine; one reacts slowly, and one not at all. The latter two compounds must be methylcyclopropane and cyclobutane, respectively (Section 3.4D), and these compounds are cycloalkanes, not alkenes. Note that the 2-butene structure is the only one that can exist in

two different configurational arrangements. The other two isomers, 1-butene and 2-methylpropene, have at least one carbon atom of the double bond with identical groups attached to it. Thus, a rotation about the double bond, even if it could occur, would produce an identical arrangement.

It is worth reviewing once again the meanings of the terms structure, conjiguration, and conformation (Sections2.2 and 2.6B). Of the six known compounds of formula C4H8, there are five different structures. These are cyclobutane, rnethylcyclopropane, 1-butene, 2-butene, and 2-methylpropene. One of these structures, z-butene, has two different stable configurations or spatial arrangements. All of these substances have many different possible conformations because rotation can occur to at least some degree about their single bonds. Putting it another way, the C4H8 compounds illustrate structural isomerism, geometrical isomerism, and conformational variation. Structural and geometrical isomers (but not conformational isomers), because of their stability to interconversion and their somewhat different physical constants, can be separated by physical techniques such as fractional distillation or, better, by chromatography (Section 7-1).

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