Q.Explain geometrical isomerism in 2-butene.

2-Butene exists as cis- and trans- geometrical isomers because the C=C double bond prevents free rotation, fixing the two methyl groups either on the same side (cis) or opposite sides (trans).

Geometrical (cis-trans) isomerism arises in alkenes when each of the two double-bonded carbon atoms carries two DIFFERENT groups, and rotation about the C=C bond is restricted (because rotating would require breaking the pi bond). In 2-butene, CH3-CH=CH-CH3, each double-bond carbon is attached to one -CH3 group and one -H atom, which are different from each other on each carbon — satisfying the condition for geometrical isomerism.

Because the double bond is rigid (no free rotation), two distinct spatial arrangements are possible and do not interconvert at ordinary temperatures:

• cis-2-butene: the two -CH3 groups (the higher-priority/bulkier groups) lie on the SAME side of the double bond, with the two -H atoms on the other side.

• trans-2-butene: the two -CH3 groups lie on OPPOSITE sides of the double bond (diagonally across from each other), with the -H atoms also on opposite sides.

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