You are working on an organic synthesis and need to change a solvent to one that has a higher boiling point. Which would you expect to have the higher boiling point, p-dichlorobenzene (3) or o-dichlorobenzene (4)?

PLAN When two compounds have different dipole moments but are otherwise very similar, the molecules with the larger dipole moment interact more strongly. Therefore, assign the higher boiling point to the more strongly polar compound. To decide whether a molecule is polar, determine from the molecular structures whether the dipole moments of the bonds cancel each other, as explained in Topic 2E.

What should you assume? Assume that the polarity of a C—H group is much less than that of the a C—Cl group and therefore has little effect on the molecular polarity and can be ignored.