Re: molecular weight vs polarity - boiling point

In article <428A2164.86C1953C@xxxxxxxxxxxxx>, Uncle Al <UncleAl0@xxxxxxxxxxxxx> wrote:

: Boiling point is independent of molecular weight.

This is so obviously untrue that it's probably not even worth pointing
out how stoopid it is. The boiling points of the straight-chain alkanes,
2-methylalkanes, 2,2-dimethylalkanes, and straight-chain perfluoroalkanes
(to name a few) are linear functions of the square root of the molecular
weight. That's hardly independent.

: Look up the bp for cyclohexane, benzene, and their perfluorinated analogues.

It's true that the molecular weight alone is only a good predictor of
boiling point for a series of compounds of similar molecular structure. That
doesn't mean that the boiling point is independent of molecular weight,
only that there are other factors besides molecular weight that affect
the boiling point. CF4 and n-hexane have very similar molecular weights,
but CF4 boils at a temperature some 200 degrees lower. So knowing one, I
cannot predict the boiling point of the other. But I can be fairly sure
that methane will boil at a lower temperature than n-hexane and that
perfluoro-n-hexane will boil at a higher temperature than carbon tetrafluoride.

-----
Richard Schultz schultr@xxxxxxxxxxxxxx
Department of Chemistry, Bar-Ilan University, Ramat-Gan, Israel
Opinions expressed are mine alone, and not those of Bar-Ilan University
-----
"You don't even have a clue about which clue you're missing."
.

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