James Richard Fromm
Only positional isomers are possible for alkanes, but an additional form of isomer is possible for alkenes and other similar molecules. The kind of isomerism which we have already studied is called STRUCTURAL ISOMERISM. The new type is called GEOMETRIC ISOMERISM. Geometric isomers are a result of the rigidity of a molecule, which prevents rotation of atoms about a bond. This rigidity is usually due to a ring structure or a double bond. Such rigidity is usually not present in singly bonded, open-chained molecules.
These two compounds can be separated physically and are separately identifiable, even though they have the same name, 2-butene. Most cases of geometric isomerism are a result of molecule rigidity which prevents rotation about a double bond. In those molecules in which the substituent groups are on the same side of the double bond, the name is preceded by the prefix cis-. In those molecules in which the groups are on opposite side of the double bond, the name is preceded by the prefix trans-.
A compound with methyl groups on the same (cis) side of the double bond is called cis-2-butene, while the compound with the two methyl groups across (trans) the double bond is called trans-2-butene. The other positional isomer of butene, 1-butene, has no geometric isomers because both methyl groups are on the same carbon. One geometric isomer cannot be converted into the other without breaking the double bond.
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