Q. 9

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

Answer :

Hex-2-ene is represented by the following structure:-

H3C — HC = CH — CH2 — CH3


The geometrical isomers of hex-2-ene are:



The dipole moment of cis-compound is actually the sum of the dipole moments of the C–CH3 and C–CH2CH3 bonds acting in together in the same direction.


The dipole moment of trans-compound is the resultant of the dipole moments of C–CH3 and C–CH2CH3 bonds acting in opposite directions.


Hence, cis-isomer is more polar than trans-isomer.


Now, the higher the polarity, the greater is the intermolecular dipole-dipole interaction in between and the higher will be the boiling point.


Hence, the cis-isomer will have a higher boiling point than trans-isomer.


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