Isomerism in alkenes

Sample Solution (a) Recall when writing bond-line formulas for hydrocarbons that a carbon occurs at each end and at each bend in a carbon chain. The appropriate number of hydrogens are attached so that each carbon has four bonds. Thus the compound shown is 

The general molecular formula for an alkene is C H2 . Ethylene is C2H4 propene is C3H6. Counting the carbons and hydrogens of the compound shown (CgHie) reveals that it, too, corresponds to C H2 . 

Although ethylene is the only two-carbon alkene, and propene the only three-carbon alkene, there are four isomeric alkenes of molecular formula C4H8  

1-Butene has an unbranched carbon chain with a double bond between C-1 and C-2. It is a constitutional isomer of the other three. Similarly, 2-methylpropene, with a branched carbon chain, is a constitutional isomer of the other three. 

The pair of isomers designated cis- and trans-2-huttnt have the same constitution both have an unbranched carbon chain with a double bond connecting C-2 and C-3. They differ from each other in that the cis isomer has both of its methyl groups on the same side of the double bond, but the methyl groups in the trans isomer are on opposite sides of the double bond. Recall from Section 3.11 that isomers that have the same constitution but differ in the arrangement of their atoms in space are classified as stereoisomers. c/ -2-Butene and trans-l-hvXtnt are stereoisomers, and the terms cis and trans specify the configuration of the double bond. 

Stereoisomeric alkenes are sometimes referred to as geometric isomers. 

Cis-trans stereoisomerism in alkenes is not possible when one of the doubly bonded carbons bears two identical substituents. Thus, neither 1-butene nor 2-methylpropene can have stereoisomers. 

The activation energy for rotation about a typical carbon-carbon double bond is very high—on the order of 250 kJ/mol (about 60 kcal/mol). This quantity may be taken as a measure of the TT bond contribution to the total C=C bond strength of 605 kJ/mol (144.5 kcal/mol) in ethylene and compares closely with the value estimated by manipulation of thermochemical data on page 171. 

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Figure 6.4 The requirement for cis-trans isomerism in alkenes. Compounds that have one of their carbons bonded to two identical groups can t exist as cis-trans isomers. Only when both carbons are bonded to two different groups are cis-trans isomers possible.

Cis-trans isomerism in alkenes arises because the electronic structure of the carbon-carbon double bond makes bond rotation energetically unfavorable at normal temperatures. Were it to occur, rotation would break the pi part of the double bond by disrupting the sideways overlap of two parallel p orbitals (Figure 23.2). In fact, an energy input of 240 kj/mol is needed to cause bond rotation. 

STEREO-ISOMERISM IN ALKENES - GEOMETRIC OR cis-trans ISOMERISM... 

Skeletal, Positional, and Functional Isomerism in Alkenes and Alkynes... 

Another approach for a stereochemical index Q encoding information on cis-trans isomerism in alkenes was described by Estrada on including a corrected electron charge density calculated with the MOP AC version 6.0, the following... 

Constitutional isomerism in alkenes is more complicated than in alkanes. There are two ways in which alkenes can differ in connectivity—not only different carbon skeletons as in alkanes but also different locations of the double bond within any carbon skeleton. 

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