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7.4: Isomerism in Alkenes
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Organic Chemistry

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Isomerism in Alkenes
 
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7.4: Isomerism in Alkenes

Alkenes like 1-butene and 2-butene exhibit constitutional isomerism, as they differ in the position of the double bond. Further, 2-butene exhibits stereoisomerism and exists as two distinct compounds differing in spatial arrangement.

An isomer is called cis-2-butene when the methyl groups are on the same side of the double bond, and the other stereoisomer, in which methyl groups are on the opposite side of the double bond, is called trans-2-butene. The cis and trans stereoisomers are not interconvertible at room temperature because of the restricted rotation of the double bond.

Figure1left Figure1right
cis-2-Butene trans-2-Butene

For tri- or tetrasubstituted compounds, the E,Z nomenclature has been adopted over the cis and trans nomenclature. The E,Z nomenclature is based on the sequence rules. The carbon atoms across the double bond are envisioned separately, and the substituent with connecting atoms having higher atomic numbers is assigned the highest priority. If two substituents have the same atomic number, the first point of difference is decisive. When the high-priority substituents on both the carbon atoms of the double bond are on the same side, the alkene has a Z configuration, whereas when the high-priority substituents are on the opposite side of the double bond, the alkene is said to have an E configuration.

Figure2left Figure2right
Z Configuration E Configuration

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