Chapter 6, Problem 6.1VP

Interpretation Introduction

Interpretation & Concept Introduction:

Which of the four constitutional isomers has the highest boiling point?

Answer to Problem 6.1VP

Solution:

Image (a) – heptane has the highest boiling point.

Explanation of Solution

We are given four constitutional isomers of heptane $\left(C_7{H}_{16}\right)$.

None of the four isomers of heptane is polar. Therefore, the only intermolecular forces they experience are London dispersion forces. The strength of the dispersion force between molecules depends on the number of valence electrons and the surface area of the molecule. The number of valence electrons is the same for all four as the formulas are the same. So, in this case, the strength will depend only on the surface area of the molecules. Larger the surface area, larger the overlap between molecules and stronger the dispersion forces.

The carbon atoms of heptane in (a) form a single chain, which gives the molecule the overall cylindrical shape. This shape gives the molecule a larger surface area and more opportunity to interact with other heptane molecules than the other molecules of heptane.

Branching leads to the overall shape becoming more like a sphere. As the shape becomes more and more like a sphere, the surface area over which molecules can interact reduces. So they interact less strongly with the adjacent molecules than straight-chain heptane.

Therefore, the straight-chain heptane in image (a) has the strongest dispersion forces and hence the highest boiling point.

Conclusion

Boiling point of a chemical substance depends on the strength of the intermolecular forces present in its molecules (or atoms).