Organic Chain Length and Phase Changes
Date: Fall 2010
Why do longer chain organic molecules have a higher flash point
and a higher boiling point?
Molecules attract molecules next to them (these attractions are called
"intermolecular forces"), but the strength of these forces depends on
the size and type of the molecule. When these forces are weak, the
molecules are easier to pull apart. When these forces are strong
enough, they tend to keep the material together.
Counteracting these attractive forces is 'temperature'. At a molecular
level, temperature is a measure of random fluctuations of molecules --
sometimes you get a little bump in energy (a stronger fluctuation)
that is enough to cause a molecule to bump away from its neighbors.
If there are enough random fluctuations, the molecules will bounce
apart -- this is evaporation. If the attractions are strong enough,
the molecules will stay together as a liquid or a solid. Materials
that have strong intermolecular forces tend to stay together, and
therefore have higher boiling points.
With very small molecules, there are very few points that molecules
can bond to each other, so the overall forces are lower, and so these
tend to be gases at lower temperatures compared with larger molecules.
It takes less energy for one of these small molecules to be broken
away from another. With larger molecules, there are many points of
attraction, so more energy is needed.
Water is a weird example - it's small, but the very high attraction
due to hydrogen bonding makes for a much higher boiling point other
molecules of the same size. Most small molecules like methane and
sulfur dioxide are gases at room temperature and pressure.
Hope this helps,
There is only one intrinsic property that controls the boiling point of
a substance and that is how well the molecules of that substance attract
or hold on to each other in the liquid phase. This ability is controlled
by many factors, the strength of the intermolecular forces, how well the
molecule "fits" with one another, possible entanglements for longer
molecules, etc. For straight-chain hydrocarbons, CH4, CH3CH3, CH3CH2CH3,
etc. The intermolecular force -called London Forces- is due to the
momentary dipoles developed from the, if we think of electrons as in
motion, random motion of electrons and the formation of dipoles due to
electrons being randomly drawn to one side of the molecule. Thus, if all
the straight-chain hydrocarbons can only exhibit London Forces, then it
becomes a question of how much of these momentary dipoles the molecules can
make. Since LF is based entirely on the polarizability of the molecule, it
stands to reason that with more electrons dispersed throughout the molecule,
that the larger molecules will have more opportunity for polarization. Thus,
while the LF does not get stronger in larger molecules, it does get more
"plentiful". Thus, as the molecule gets longer, the boiling point goes
higher. With even higher length molecules, entanglements (such as string
being wrapped around each other) add to the attraction or "holding on" of
each molecule and the boiling point goes higher still.
As for the flash point, since it is the same C-C and C-H bonds that are
breaking in order for a molecule to "flash" or burn, then it is a question
of how much of the molecule is in a gaseous phase. The molecules in the
gaseous phase are the ones that burn first because molecules in that phase
are more in contact with the oxidizer (usually O2) and the spark. So this
goes back to the previous paragraph.
Greg (Roberto Gregorius)
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Update: June 2012