Question

The boiling points of the alkyl halides with the same alkyl group decrease in the given order: $RI \gt RBr \gt RCl \gt RF$. Explain the above statement.

Hint:

Larger halogen atom = Greater polarizability = Stronger van der Waals forces = Higher boiling point.

Answer 1

Step 1: Concept
Intermolecular forces governing physical properties of haloalkanes.

Step 2: Meaning
Boiling point depends on the magnitude of van der Waals (London dispersion) forces between molecules.

Step 3: Analysis
As we move down the halogen group from Fluorine to Iodine,
the atomic size and mass of the halogen atom increase significantly.
A larger atomic size provides a larger surface area and higher electron cloud polarizability.
This leads to much stronger van der Waals forces of attraction between $RI$ molecules compared to $RF$ molecules.

Step 4: Conclusion
Stronger intermolecular forces require more thermal energy to break, hence the boiling point is highest for alkyl iodides and lowest for alkyl fluorides.

Final Answer: As the size and mass of the halogen atom increase from F to I, the magnitude of van der Waals forces of attraction increases due to larger surface area and higher polarizability. Consequently, the boiling point increases in the order $RF \lt RCl \lt RBr \lt RI$.