Question

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

Answer 1

cis–trans isomerism in hex-2-ene:

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Structure of hex-2-ene:

CH₃–CH = CH–CH₂–CH₂–CH₃

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(a) cis-hex-2-ene:

In the cis isomer, the two alkyl groups attached to the double-bonded carbon atoms are on the same side.

   CH3      CH2CH2CH3
    \        /
     C == C
    /        \
   H          H

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(b) trans-hex-2-ene:

In the trans isomer, the two alkyl groups are on opposite sides of the double bond.

   CH3          H
    \          /
     C == C
    /          \
   H        CH2CH2CH3

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Which isomer has higher boiling point?

cis-hex-2-ene has a higher boiling point than trans-hex-2-ene.

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Reason:

• The cis isomer has an uneven charge distribution due to its bent structure.
• This results in a net dipole moment, leading to stronger intermolecular attractions.
• The trans isomer is more symmetrical, has a nearly zero dipole moment, and therefore weaker intermolecular forces.

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Conclusion:

Because of stronger intermolecular attractions, cis-hex-2-ene has a higher boiling point than trans-hex-2-ene.