Question

2-Butene exists as two different stereoisomers. what are they? why are they unique molecules?

Answer 1

Answer:-

2- Butene exists as two different geometrical isomers. They are cis- 2-butene and trans-2-butene. The cis isomer is one in which two methyl group (CH₃-) are on the same side of the carbon- carbon double bond whereas the trans isomer is that in which two methyl group (CH₃-) are on the opposite side of the carbon- carbon double bond. Consequently , this type of isomerism is often called as cis-trans isomerism. Geometrical isomers are those stereoisomers in which there is restriction of rotation about  carbon- carbon double bond, they have the same structural formula but different spatial arrangement of atoms or groups. Therefore both stereoisomer of 2- butene are unique in nature because they have the same structural formula but different spatial arrangement of methyl (CH₃-) groups.

The trans-2- butene isomer is more stable than the cis-2- butene because in cis-2- butene, the both bulky groups [(CH₃-) groups] are on the same side of the double bond . The steric repulsion between the both (CH₃-) groups makes the cis - isomer less stable than trans -isomer of 2-butene in which both groups [(CH₃-) groups] are far apart.

The geometrical isomers have different physical and chemical properties. they can be separated by fractional distillation, gas chromatography etc.