t-butanol vs. n-butanol melting vs. boiling points

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renai - 31 Jan 2007 09:51 GMT

Can someone explain why t-butanol has a higher melting point but lower
boiling point than n-butanol?

I know t-butanol is more soluble in water because the hydroxy group
is more localized in the molecule allowing for h-bonding between water
and t-butanol molecules. I assume this is why the melting point of
the t-butanol is higher than n-butanol's which is non-polar for the
majority of the length of the molecule. But then why doesn't this
logic hold for the boiling point as well, i.e. shouldn't t-butanol's
boiling point (like its melting point) also be higher than n-
butanol's?

n-butanol t-butanol
boling point 118 82
melting point -90 26 degrees
Celcius

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Andy Resnick - 31 Jan 2007 14:34 GMT

> Can someone explain why t-butanol has a higher melting point but lower
> boiling point than n-butanol?

Two reasons, assuming you are referring to n-butyl alcohol and t-butyl
alcohol- increasing the dipole moment increases the boiling point, and
as the molecule becomes more spherical or branched, the boiling point
decreases due to the decreased London dispersion forces between the
solute molecule and solvent. So n-butanol, being a linear chain, has a
larger dipole moment and is able to interact more strongly with solvent
(or other n-butenol molecules), and so the boiling point is higher.

For some reason, my book doesn't discuss melting points...

> I know t-butanol is more soluble in water because the hydroxy group
> is more localized in the molecule allowing for h-bonding between water
[quoted text clipped - 9 lines]
> melting point -90 26 degrees
> Celcius

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Andrew Resnick, Ph.D.
Department of Physiology and Biophysics
Case Western Reserve University

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Natural Science Forum / Physics / General Physics / January 2007

t-butanol vs. n-butanol melting vs. boiling points