Draw the cis and trans structures of hex-2-ene.


Draw the cis and trans structures of hex-2-ene. Which isomer will have higher b.p. and why?


Hex-2-ene is represented as:


Geometrical isomers of hex-2-ene are:

The dipole moment of cis-compound is a sum of the dipole moments of $\mathrm{C}-\mathrm{CH}_{3}$ and $\mathrm{C}-\mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}$ bonds acting in the same direction.

The dipole moment of trans-compound is the resultant of the dipole moments of $\mathrm{C}-\mathrm{CH}_{3}$ and $\mathrm{C}-\mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}$ bonds acting in opposite directions.

Hence, cis-isomer is more polar than trans-isomer. The higher the polarity, the greater is the intermolecular dipole-dipole interaction and the higher will be the boiling point. Hence, cis-isomer will have a higher boiling point than trans-isomer.

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