Boiling point of 2-methylpyridine is less than that of 3-methylpyridine? [closed]








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



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> 2-methylpyridine boils at 129°C while 3-methylpyridine boils at 15° higher than 2-methylpyridine
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And the reason that is given is:



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> This lowering of boiling point is explained in terms of hindrance to association offered by the groups in the vicinity of the nitrogen atom
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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 ?




Two things:


1. The nitrogen in pyridine doesn't hydrogen bond with saturated carbons. It's more likely that you'd get pi-pi stacking, or interactions between the lone pair and the aromatic ring of a second molecule. Having a methyl group near to the lone pair disrupts the latter interactions.
2. If the nitrogen in pyridine *did* form interactions with the methyl group, 2-methylpyridine would still boil at a lower temperature. Internal interactions don't need to be broken for a compound to pass into the gas phase.


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