Attractive Forces: London Forces, Dipole-Dipole, Hydrogen Bonding, etc.

[robert zellmer]

I got the following question from someone today about the AF e-mail I 
sent earlier
today.

"How could we tell what attractive forces were stronger in two different 
molecules
if one has hydrogen bonding but the other is bigger? In other words, 
which aspect,
size or hydrogen bonding, should we consider first when 
determining which molecule
has the stronger intermolecular force"

Here's my answer:

There's no hard and fast rule for this.  For instance, lets look at the
following molecules

             H2O        C8H18        CCl4
MW       18            114            154
b.p.      100            125            77    (all in degC)

As I pointed out in class when discussing the Clausius-Clapeyron Eqn.
and the associated graph, H2O has a bigger slope than CCl4 and thus
a higher H_vap.  It has a higher b.p.  This means it has stronger
attractive forces (AF) than CCl4, even though CCl4 is much larger.
The LF for CCl4 are much larger than the LF between H2O molecules.
However, H2O is polar and can form HB between H2O molecules as
well.  Water's higher b.p. is mainly due to the HB between H2O molecules.

What about octane (C8H18, more specifically, n-octane the straight-chain
isomer)?  It's nonpolar and has only LF, like CCl4.  It's smaller in 
size than
CCl4.  Yet, it has a higher b.p., indicating stronger AF (LF) between the
molecules in the liquid state.  Why?  CCl4 has a rather spherical charge
distribution while C8H18 is long and cylindrical.  That means C8H18 has
more points of contact (more surface contact) with another C8H18. This
maximizes the LF.  The CCl4 molecules have less surface contact and
can't maximize the LF which it would have based on its size if it wasn't
spherical.

In fact the C8H18 is big enough that it's AF are stronger than the AF
between the H2O molecules, even though water has DD and HB in
addition to it's small LF.  n-Heptane (C7H16) which is only slightly
smaller than octane has a b.p. of 98 degC, slightly below that of H2O.

There in lies the problem.  There's no hard and fast rule for how much
larger a molecule has to be than H2O to have stronger AF than H2O.
Remember, shape also plays a rule.  Even so, water has a remarkably
high b.p. relative to its size due to HB.  Nonpolar molecules have to be
"significantly" larger (The MW of 118 for octane is 6.3 times bigger
than 18 for H2O).

Propanol (CH3CH2CH2-OH) and butanol (CH3CH2CH2-OH) have
LF, DD and HB, like H2O.  Propanol has a b.p. of 97 degC and butanol
has a b.p. of 118 degC.   Propanol, butanol and H2O all have LF, DD
and HB.  H2O is more polar and can form more HB per molecule.
This fact is enough to outweigh the extra size and larger LF of
C3H7-OH (which can form only up to 3 HB per molecule).  However,
adding another CH2 group to get butanol (C4H9-OH) makes it
big enough so its overall AF consisting of DD (weaker than H2O) and
HB (fewer and weaker than H2O) and larger LF are enough to outweigh
the stronger and more frequent HB in H2O.  Butanol's molar mass is
about 4 times larger than that of water.

Furthermore, keep in mind, the strength of a single HB between two
molecules increases in the following order, N < O < F.  This is because
F is the most electronegative of the three atoms and a single HB between
two HF molecules is stronger than a single HB between two H2O molecules.
Water has a higher b.p. than HF because water can form more HB per
molecules (up to 4 for H2O compared to up to 2 for HF).  More HB AF have
to be broken to separate the H2O molecules than to separate HF molecules
so more energy has to be added to separate (boil) the H2O molecules.
So the number of HB a molecule can form plays a role.  NH3 is similar to
HF in that it forms only up to 2 HB per molecule (HF has 3 lone pair 
electrons
and 1 H and NH3 has 1 lone pair electrons and 3 H atoms, H2O has a
balance of 2 H atoms and 2 lone pairs).

Generally, when we give questions about AF and how they affect properties
of different substances we try to give molecules which are relatively close
in size or very different in size.  Certainly if I gave you H2O and C20H42
you would expect C20H42 to have much stronger AF than H2O even though
the former has only LF.  It's really large.  A molecule that large is 
most likely
to be a solid at room temp (in fact, this is icosane or eicosane and the 
m.p.
of its straight-chain isomer is ~ 37 degC and it's b.p. is 343 degC).

I hope this answers the question.  Perhaps not as satisfying as it would be
if there was a hard and fast cutoff but that's the way it is sometimes when
you're speaking in generalities.  We have to often speak generally because
there's a whole lot of different substances and sometimes exceptions to
these general statements.  The general statements help and when we find
those interesting exceptions we try to explain them.  Like the fact ice 
(solid
H2O) floats on liquid water.  That's very unusual.

Dr. Zellmer



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