Chapter 22, Problem 30A

Interpretation Introduction

Interpretation:

The reason for increase in the boiling points of alkyl halides on moving down the column in the periodic table needs to be explained.

Concept introduction:

The boiling point of an organic compound depends on the structure of the compound and its molecular mass. Intermolecular forces in organic compounds also play an important role in determining the boiling point.

Explanation of Solution

The three important factors on which the boiling point of an organic compound depends are as follows:

1. Intermolecular forces such as hydrogen bonding, ionic, dipole-dipole and Van der Waals forces.
2. If number of carbon atoms in an organic compound increases, boiling point also increases.
3. If in an organic compound branching increases, boiling point will decrease.

On moving down the column of halides, molecular mass of halide increases thus, boiling point increases. Also, the Van der Waal dispersion forces become stronger if molecules get longer there are more number of electrons. The number of electrons increases on moving from fluorine to iodine. Also, the carbon-halogen bonds are polar in nature thus; halogen atoms pull the electron pair more strongly due to higher electronegtaivity than carbon atom.

This results in the attractions between the dipoles. If bonds get less polar, the size of these dipole-dipole attractions will be less. This happens on moving from fluorine to iodine. Therefore, boiling point increases.