Geometrical isomerism

Geometrical isomerism in alkenes


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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-butene 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, however, 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. As we know that isomers that have the same constitution but differ in the arrangement of their atoms in space are classified as stereoisomers. cis-2-Butene and trans-2-butene are stereoisomers, and the terms “cis” and “trans” specify the configuration of the double bond.
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.
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.
In principle, cis-2-butene and trans-2-butene may be interconverted by rotation about the C-2oeC-3 double bond. However, unlike rotation about the C-2±C-3 single bond in butane, which is quite fast, interconversion of the stereoisomeric 2-butenes does not occur under normal  circumstances. It is sometimes said that rotation about a carbon– carbon double bond is restricted, but this is an understatement. Conventional laboratory sources of heat do not provide enough thermal energy for rotation about the double bond in alkenes to take place. Rotation about a double bond requires the p orbitals of C-2 and C-3 to be twisted from their stable parallel alignment— in effect, the component of the double bond must be broken at the transition state. 
When the groups on either end of a double bond are the same or are structurally similar to each other, it is a simple matter to describe the configuration of the double bond as cis or trans. Oleic acid, for  example, a material that can be obtained from olive oil, has a cis double bond. Cinnamaldehyde, responsible for the characteristic odor of cinnamon, has a trans double bond.
The terms “cis” and “trans” are ambiguous, however, when it is not obvious which substituent on one carbon is “similar” or “analogous” to a reference substituent on the other. Fortunately, a completely unambiguous system for specifying double bond stereochemistry has been developed based on an atomic number criterion for ranking substituents on the doubly bonded carbons. When atoms of higher atomic number are on the same side of the double bond, we say that the doule bond has the Z configuration, where Z stands for the German word zusammen, meaning “together.” When atoms of higher atomic number are on opposite sides of the double bond, we say that the configuration is E. The symbol E stands for the German word entgegen, meaning “opposite.”

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