London Forces

[Robert Zellmer]

I've had students in office hours who often ask what are London Forces.
This is in chapter 11.  People use different terms for this attractive 
force.
You may see London Forces or Dispersion Forces or London Dispersion Forces.
I call them London Forces and use the abbreviation LF.  LF are due to
the instantaneous (temporary) dipole moments created due to the
motion of the electrons.

There's also Dipole-Dipole attractive forces, which I abbreviate as DD.
These occur between polar molecules (molecules with permanent dipole
moments).

ALL molecular substances experience LF.

Non-polar substances have ONLY LF.

Polar molecules experience both LF and DD between them.

How about Hydrogen bonding (HB)?  This AF is in addition to
LF and DD so a molecule which can form HB experiences
LF, DD and HB.  That means the molecules of the substance
are attracted to each other through LF, DD and HB.

What is a HB and when can it form?

HB occurs between molecules which have a H atom directly
attached (covalently bonded) to a N, O or F atom and there
are lone pairs of electrons on a N, O or F.  This AF occurs
between a H attached to N, O or F and the lone pairs on
one of those three atoms in another molecule.

H atoms attached to other atoms (such as C, S, P, Cl, etc.)
do not form H bonds.  The lone pair electrons on other atoms
(such as S, P, Cl, etc.) do not participate in HB.

So HF, H2O, NH3, CH3OH, CH3NH2, (CH3)2NH are examples
of molecules which can form H bonds as pure substances
(between molecules) and in solution (to the solvent molecules -
ch 13).  The HB involving N is the weakest because it is the
least electronegative of the three atoms (N, O, F).

By the way, if a molecule can form HB does that mean it will always
have stronger AF than molecules which can not?  NO!  Large molecules
will have strong LF and overall those LF may be stronger than the
HB AF between a molecule which can form HB.

For instance, candle wax (paraffin) is essentially comprised of very
large nonpolar hydrocarbon molecules.  This means the AF present
between the molecules are LF.  Candles are solids at room temp.

Polar H2O has LF, DD and HB.  However, it is a liquid at room temp.
This means the AF between H2O molecules are weaker than those
between the molecules in candles.  The hydrocarbon molecules in
a candle have such large LF that the AF are stronger than the LF, DD
and HB between H2O molecules.

LF increase rapidly with the size of the molecule and large molecules
with only LF can have very strong AF overall.  Even in large molecules
which have polar groups and can form HB the overriding AF is often
the large LF due to the molecules large size.

Take for instance the example of the long-chain alcohol,

     CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH2-OH .

While this molecule does have a polar end, which can also form HB, the
nonpolar part of the molecule is so large the LF are the most impt
AF when thinking about the properties of this molecule, such as
boiling point or solubility.  This molecule behaves more like a nonpolar
molecule, especially when you are considering it's solubility, and is more
soluble in hexane, C6H14 (nonpolar) than in water (polar).

Finally, you may see in my notes the acronym, IAF.  This stands for
INTERmolecular Attractive Forces.  This is the attractive force BETWEEN
molecules (such as LF, DD and HB) rather than within a single molecule
(the covalent bonds holding the atoms together, which are often referred
to as INTRAmolecular).

The IAF between molecules, which are responsible for properties such
as b.p., m.p., heat of vaporization, viscosity, etc., are much weaker than
the attractive forces between the atoms in a molecule (the covalent bonds).
It takes a lot less energy to separate two water molecules when water
boils (breaking the LF, DD and HB) than it does to decompose the water
molecule into H2 and O2.

Dr. Zellmer