Question

For the same alkyl group, an alkyl bromide has a higher boiling point than alkyl fluoride because:

Options

• Iodide has more electrons

• The magnitude of Van der Waal forces

• Iodide is bigger in size

• All of these

Answer 1

All of these

Explanation:

• Van der Waal's forces increase with the increase in molecular mass and boiling point increases with Van der Waal's forces. The main factor affecting the boiling point is the molecular weight, as can be pictured from the noble gases boiling point in the periodic table (the noble gases are those which have the weakest interactions due to their complete outer shell). But note that if you want to consider all the effects altogether, you must use Kelvin instead of Celsius. So going from 44°C to 88°C is not twice as much but a mere 14% increase.
• The other factor affecting the boiling point is the nature and strength of intermolecular interactions, usually in the order: ionic > hydrogen bond > dipole-dipole > Van der Waal's.
• In the comparison of R-F vs R-Br boiling points, molecular weight is definitely in favor of higher boiling points for R-Br, at least for small molecules. It is true that the C-F bond is much more ionized than C-Br so it will create stronger dipole-dipole interactions but not sufficient to compensate for the effect of the mass increase.