Another P3 question from the H2 'A' level 2008 paper :
(Or something close to it, according to my students)
List and explain the following alkyl halides (halogenoalkanes)
according to their boiling points :
Alkyl bromide, alkyl iodide, alkyl chloride.
Solution :
There are two opposing factors, which you have to describe
and explain in the exam to score full marks.
Electronegativity and hence strength of permanent dipole-dipole
interactions would point to (highest boiling point) alkyl chloride,
alkyl bromide, alkyl iodide (lowest boiling point).
However, no. of electrons and hence strength of induced
dipole-dipole van der Waals interactions would point to (highest
boiling point) alkyl iodide, alkyl bromide, alkyl chloride (lowest
boiling point).
As it turns out, based on experimental evidence, van der Waals
interactions trumps/owns/pwnz/outweighs permanent dipole-dipole
interactions (specifically in THIS context of alkyl
iodide versus alkyl bromide versus alkyl chloride;
because we're talking about a lot of electrons here).
Hence, the correct answer is (highest boiling point) alkyl
iodide, alkyl bromide, alkyl chloride (lowest boiling point).
