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:

isomerism in C2H4compounds

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

isomerism in C2H4compounds

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).



Frequently Asked Questions

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Ans: In the early days of organic chemistry, when it was found that the alkenes, but not the alkanes, readily undergo addition reactions with substances such as halogens, hydrogen halides, sulfuric acid, and oxidizing agents, the chemical affinity of alkanes was said to be " saturated" while that of the alkenes was said to be " unsaturated." Now, even though we recognize that no chemical entity (even the noble gases such as helium and xenon) can surely be classified as saturated, the description of alkanes and alkenes as saturated and unsaturated is still commonly used. view more..
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Ans: An important and interesting group of hydrocarbons, known as cycloalkanes, contain rings of carbon atoms linked together by single bonds. The simple unsubstituted cycloalkanes of the formula (CH,), make up a particularly important homologous series in which the chemical properties change in a much more striking way than do the properties of the open-chain hydrocarbons, CH3(CH2),-,CH3. view more..
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Ans: As a class, alkanes are singularly unreactive. The name saturated hydrocarbon (or " paraffin," which literally means " little affinity " [L. par(um), little, + afins, affinity1)arises because their chemical affinity for most common reagents may be regarded as saturated or satisfied. view more..
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Ans: In the homologous series of alkanes, isomerism first appears at the C, level, two compounds of formula C4H,, being known. These are structural isomers: view more..
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Ans: 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 view more..
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Ans: We have previously examined briefly two addition reactions of ethene, the first member of the homologous series of alkenes. These were addition of hydrogen, catalyzed by surfaces of finely divided metals such as nickel, and the addition of bromine. view more..




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