X-Message-Number: 11479
Date: Sun, 28 Mar 1999 16:39:47 -0500
From:  (Will Ware)
Subject: Re: Help From Chemists Requested

Olaf Henny asks:
>     propylene glycol consists of a chain of 
>     carbon atoms
> Well, propylene glycol's formula is C3H8O2, hardly much of a chain of 
> carbon atoms, more like one of hydrogen atoms.
> Can anybody provide me with an explanation please?

There is a good reason for thinking of the carbons (and secondarily
the oxygens) as the fundamental chunks of the chain, having to do with
the bonding properties of carbon, oxygen, and hydrogen. You may
remember some of this stuff from school.

The spatial distribution of an atom's electrons is described by the
quantum mechanical wave equation, qualitatively similar to the
equation describing vibration in a taut string or a drum head. As with
the taut string, there are independent modes that can be superposed.
The electrons arrange themselves in shapes called orbitals. Orbitals
like to have two electrons in them, but if the atom doesn't have
enough electrons to fill an orbital, the orbital can share its
electron with a similarly deprived orbital in a neighboring atom.
That's a single bond. There are also double and triple bonds but if I
talk about them, this post will run on for pages and pages.

Hydrogen has only one electron, occupying a spherical orbit around the
nucleus. In order to be full, the orbital wants to share its electron
with another atom's incomplete orbital. So we can say that a hydrogen
atom wants to form one single bond. Carbon's outer shell of electrons
has a few different ways of arranging itself, but for the moment I'll
discuss only one (the sp3 hybridization) where there are four
incomplete orbitals, sticking out in the directions of a tetrahedron's
corners. (A tetrahedron is the simplest regular solid, a pyramid with
a triangular base.)

If you had four hydrogen atoms and an sp3-hybridized carbon, you could
stick a hydrogen on each of the carbon's four sp3 orbitals, and get a
stable tetrahedron-shaped molecule, methane. If you had only three
hydrogens, the carbon would be left with one unbonded orbital, so
you'd have a CH3 object, called a methyl group, ready to bond with
anything that came along looking to share an electron. If two hydrogens
were missing, you have a CH2 object (don't know the name) which could
form two single bonds. (It could also form one double bond but that's
a story for another time.)

Suppose you connect two methyl groups together: CH3-CH3. Now you have
ethane, with two carbons connected by a single bond, and hydrogens
occupying all the carbons' remaining available bonds. If you sight
down the carbon-carbon bond, you'll see that the hydrogens are 60
degrees out of sync between the two ends; this minimizes the repulsive
force between the hydrogens. To make propane, take a CH2 and put a CH3
on each of its available bonds, like this: CH3-CH2-CH3. If you feel
ambitious, make octane (the stuff in your car's fuel tank):
CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH3.

With these simple hydrocarbons, it's natural to think of the carbons
as forming a spine, with hydrogens almost as an afterthought. Moving
on to oxygen: it has two more electrons than carbon, so while there
are still four orbitals arranged tetrahedrally, two of the orbitals
already have a pair of electrons each and have no wish to bond with
other atoms' orbitals. So the oxygen has two single bonds (or one
double bond), and for fun we can stick hydrogens on them to make
water, which is shaped tetrahedrally.

How does ice form? There is an interesting electrostatic phenomenon
that occurs. You already know that the two filled orbitals of the
oxygen will be negatively charged, no surprises there. The hydrogen
nucleus turns out to have a very weak hold on its own electron, so
that electron spends most of its time close to the oxygen nucleus.
When you look at the hydrogens, what you see are little balls of
positive charge floating out in the breeze. So the tetrahedral water
molecule has two negatively charged corners, and two positively
charged corners, and the electrostatic attractions between water
molecules at low temperatures makes them line up in a very pretty
hexagonal arrangement, the ice crystal (structurally quite similar
to diamond, which is a bunch of singly-bonded sp3 carbons).

When a water molecule gets ripped apart for some reason, one of the
hydrogens separates from the oxygen, leaving an OH object, called a
hydroxyl, which will very happily join up with the next free
hydrogen atom it finds. The methoxyl group discussed in the 21CM
work looks like this: CH3-O-... and it's ready to find something to
occupy the oxygen's other bond.

My own conjecture about the success of the 21CM work is that the
normal glycol molecule with its hydroxyl group will participate with
difficulty in the formation of ice crystals, but the methoxyl group is
so different in the arrangement of electrical charges that it will
disrupt the ice crystal pattern almost completely. Very cool stuff.

We're nearing the limits of my chemistry knowledge. You gave the
formula C3H8O2 for propylene glycol. I could tell you the shape of the
molecule if I knew how the carbons and oxygens were bonded to one
another, I'd just add hydrogens to any leftover dangling bonds. The
"ene" ending is a clue that there might be double bonds involved, but
I'm not a good enough chemist to infer any more about the structure.
 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Will Ware		email: wware[at]world[dot]std[dot]com
PGP fp (new key 07/15/97) 67683AE2 173FE781 A0D99636 0EAE6117

Rate This Message: http://www.cryonet.org/cgi-bin/rate.cgi?msg=11479