Question

As I was reading about physical properties of pyridines and substituted pyridines, I came across this:

2-methylpyridine boils at 129°C while 3-methylpyridine boils at 15° higher than 2-methylpyridine
And the reason that is given is:
This lowering of boiling point is explained in terms of hindrance to association offered by the groups in the vicinity of the nitrogen atom
Assuming that the book is talking about hydrogen bonding as association, wouldn't 2-methylpyridine boil at higher temperature than 3-methylpyridine as the methyl group is closer to nitrogen atom in 2-methylpyridine and hence can form better hydrogen bonds than 3-methylpyridine ?

Answer 1

The boiling points of pyridines are largely determined by hydrogen bonding, which is influenced by steric hindrance. Despite initial assumptions, 2-methylpyridine has a lower boiling point than 3-methylpyridine, due to the hindrance to association near the nitrogen atom impacting hydrogen bond formation.

Step-by-step explanation:

In discussing the boiling points of pyridines and substituted pyridines, it's important to note that intermolecular forces (IMFs), particularly hydrogen bonding, play a crucial role. Generally, molecules with hydrogen atoms bonded to electronegative atoms such as O, N, and F tend to exhibit strong hydrogen bonding, leading to significantly higher boiling points.

However, the case of 2-methylpyridine and 3-methylpyridine reveals an interesting deviation. While one might assume that the methyl group being closer to the nitrogen in 2-methylpyridine would facilitate stronger hydrogen bonding and thus a higher boiling point, the boiling point is actually lower than that of 3-methylpyridine. The explanation for this unexpected observation lies in the hindrance to association caused by the groups in proximity to the nitrogen atom. This hindrance affects the strength of the hydrogen bonds that can be formed, and consequently, influences the boiling points.