Compare Boiling Point of Isomers

Let's check out this question.

isomerism and boiling point question

We are required to determine the pair of isomers with identical boiling points.

Let's consider each pair of isomers.

Option A - Optical Isomers or Enantiomers

isomerism and boiling point optical isomers same boiling point

In A Level Chemistry Syllabus only optical isomers have identical boiling points.

So the answer to this question is option A.

The only difference in physical property between optical isomers is the direction of rotation of plane polarised light, or their optical activity.

Option B - Cis Trans Isomers or Geometric Isomers

isomerism and boiling point cis isomer higher boiling point

The bulky groups for cis isomers are on the same side of the alkene.

This makes the cis isomer less symmetrical and the electron cloud distribution is more unequal.

Therefore the cis isomer is more polar than the trans isomer and has a slight permanent dipole - permanent dipole attraction between molecules.

Thus the cis isomer has a higher boiling point than the trans isomer.

Interestingly the cis isomer has a lower melting point than the trans isomer, even though it is more polar.

The less symmetrical cis isomer is less efficiently packed in the solid state.

Thus the cis isomers are farther apart in the solid state, attraction between molecules are weaker, hence melting point is lower.

Option C & D - Chain Isomers

isomerism and boiling point less branched alcohol higher boiling point

isomerism and boiling point less branched alkane higher boiling point

For both options the difference is in the extent of branching of carbon chain.

For the unbranched or straight chain isomer, its surface area will be larger for interaction between molecules.

Hence intermolecular attraction is stronger and boiling point is higher for the unbranched isomer.

The branched isomer will have a smaller surface area for molecular interaction which results in lower boiling point.

Topic: Introduction to Organic Chemistry, Organic Chemistry, A Level Chemistry, Singapore

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