Follow-up Question Regarding Attractive Forces: London Forces, Dipole-Dipole, Hydrogen Bonding, etc.

[Zellmer, Robert]

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

"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)

If you look at the Clausius-Clapeyron Eqn. in the book and my notes and
the associated graph, H2O has a bigger slope than CCl4 and thus a higher
H_vap (proportional to slope).  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.
Plus, see the paragraph below about the important role of the shape of
CCl4 on its AF.

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 (MW)
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 with lots of points of contact 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, as discussed above, 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
molecule (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
C20H42 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 there's 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.  It's due to the fact the
water molecules actually move further apart as they form the many HB
between the H2O molecules (each H2O in the interior of ice has 4 HB
between it and 4 other H2O molecules).

Dr. Zellmer
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