Newsgroups: alt.drugs
From: [an 58264] at [anon.penet.fi] (Dalamar)
Date: Wed, 18 May 1994 22:48:44 UTC
Subject: CHEMISTRY: Polarity.
Hi,
After reading some of the posts on polarity and the differences between a
polar and non-polar solvent, I sat down and tried to explain it in simple
terms. I soon realised this isn't as easy as it sounds so the resulting text
is quite a bit longer than i planned and strays now and again to introduce new
concepts. Post your thoughts if you can make it through it all. If you find it
ok and want to see more then i will go back and explain bonding, shapes of
molecules, acids and bases and nomenclature, as well as the principle
methods of purification etc. I feel that at least some background knowledge when
doing a reaction can be greatly rewarding, especially when something goes
wrong and you have to figure out what !
Solvents and Polarity
You will need : A copy of the periodic table !
Look at the symbols for the elements in your periodic table. When written down
like this each of the symbols you see can be taken to represent an ATOM of
that element. Each atom is capable of joining or 'bonding' to other atoms to
form _molecules_. You can think of the atoms as having a certain number of
arms which it uses to form links. The proper term for the number of bonds an
atom can form is its _valency_. For example hydrogen has a valency of 1 and thus
can form only one bond, either to an atom of a different element or to another
H atom. An example here might help....
Element: Hydrogen
Symbol: H
Valency: 1
Element: Oxygen
Symbol: O
Valency: 2
Now, most atoms don't like to be isolated and will take every opportunity to
use up their valency and form links with other atoms, up to the maximum allowed
for that element - in our above examples this is a mere one for hydrogen and
two for oxygen. If you have a cylinder of 'hydrogen' gas, what is actually
in the cylinder are millions of molecules of hydrogen made up of two atoms of
hydrogen linked by a chemical bond, or arm in arm if you like. Each atom of
H can only form one bond so we end up with the molecules made up of pairs of
atoms. Hydrogen is thus a _diatomic_ molecule.
This can be represented by H-H and written in shorthand form as H2 (the 2 should
be subscript).
When we turn to oxygen we see that its atoms are capable of forming two
bonds. It has a valency of two and just like hydrogen it can bond to itself or
to atoms of other elements. Combining H and O we can envisage :
1. H-O-H
2. H-O-O-H
The first is probably the most familiar to you - H2O or water! The second is
known as hydrogen peroxide. Examine the structures and see how each H atom
forms only one bond and each O atom forms only two.
Carbon has a valency of four, that is to say it can form 4 bonds to other
atoms, be they other carbon atoms, oxygen atoms or hydrogen atoms etc.
Carbon has a special ability - It can CATENTATE its atoms. This term means
that carbon atoms are capable of linking together to form long chains containing
many carbon atoms, numbering from 2 to thousands.
Just using carbon, hydrogen and oxygen as our building blocks it is possible
to link them together in different numbers and different ways which create
hundreds of thousands of _compounds_. Some may have similair properties to each
other - some radically different.
Let's take an example - something you are all familiar with - alcohol.
The alcohol found in beers etc is known as ethanol and has the molecular formula
C2H6O. Its _structural formula_, which shows you which atoms are connected
to which and in which order, is :
H H
| |
H-C-C-O-H
| |
H H
ETHANOL - STRUCTURAL FORMULA
Counting the number of bonds which each atom forms you can see each carbon
forms 4, oxygen 2, and all hydrogens 1. Another compound comprising only
C, H and O is dimethyl ether:
H H
| |
H-C-O-C-H
| |
H H
Notice there is no O-H bond in this compound, instead the oxygen binds to 2
carbon atoms. Dimethyl ether has different properties to ethanol - it is
very volatile compared to ethanol and drinking it probably wouldn't do you
much good ! However..count the number of atoms of each element present in both
ethanol and methanol ie look at the above molecular structures and write down
the molecular formula again from this. Then compare them. They are the same !
How can this be ? It should be obvious from this example that a MOLECULAR
FORMULA only shows the number of each kind of atom contained in a molecule
of our compound. It does NOT tell us anything about the ORDER in which the
atoms are connected. As the complexity of the m.formula increases so does the
number of possible structures we can write for it. Each different structure
is known as a _structural isomer_, in effect they are all structural isomers
of each other as each shares the same molecular formula.
The bonds which we represent as lines actually comprise of pairs of electrons.
You can think of the pair of electrons as a sort of glue - they sit between the
two 'bound' atoms and hold them together. The two atoms effectively share the
pair of electrons but, as usual when it comes to sharing, sometimes this is not
always equally. The elements on the far right of the periodic table have a real
love for electrons and pull them hard when competing for them. Fluorine, the
very top right element is the T.Rex of the elements in these regards. This
property of 'electron pulling' is known as _electronegativity_ and it can be
measured. A common scale used is the Pauling scale. Here fluorine is assigned
the highest value of 4, with hydrogen around the 2.1 mark. Anything with an
electronegativity of less than 2.1 doesnt have much pulling power and in fact
these atoms are sometimes known as _electropositive_ because they compete so
poorly for electrons. The lower the value the more 'electropositive' the atom
ie electrons are given up more easily.
Across rows in the table electronegativity increases from element to element.
So oxygen for example is more electronegative than nitrogen, but fluorine in
turn is more electronegative than oxygen. Down the colums of the table
electronegativity falls, so that chlorine is less electronegative than fluorine
and bromine in turn is less electronegative than chlorine. Accompanied by this
decrease in electronegativity down the group is an increase in the size of the
atoms.
When two atoms of the same electronegativity are bound to each other, as in
H2 or F2 then each atom competes equally for the electron pair and thus an
equal share is achieved ie you can visualise the pair as sitting exactly half
way between the atoms. However, with atoms of differing electronegativity the
pair of electrons is pulled closer to the _more electronegative atom_. The
more electronegative atom thus aquires a slight negative charge (electrons are
negatively charged) and the less electronegative atom aquires a slight positive
charge. To distinguish these slight charges from full blown ones the greek
symbol delta is used eg d+ represents a partial positive charge.
Such a bond is said to be _polarised_ ie the negative charge is polarised in
the direction of the more EN atom. A few examples will illustrate this point.
1. H-F
The fluorine is the more EN of the two and thus gets the bigger share of
the electrons and aquires a partial negative charge d-. The H on the other hand
is left with a slight deficiency of electrons and thus aquires a d+ charge.
2. H-O-H
Here oxygen is the more EN of the two atom types present. Thus each H has a
partial positive charge and the oxygen a partial negative one.
The above explained how bonds between atoms can be _polar_, but what about the
molecule as a whole ?? Why is CCl4 (carbon tetrachloride) non polar as a whole
although each C-Cl bond is indeed polarised with partial negative charge on Cl ?
The answer to this question is that the molecule is not flat. Each pair of
electrons which constitute the 4 single bonds will repel each other as they
are areas of negative charge. This repulsion is much like trying to put the
two north pole ends of a magnet together - they repel because they are the same
pole, however a north pole will attract a south pole, just like a positive
charge will attract a negative one. The electron pairs will thus get as far
away as possible in order to minimise the repulsion they mutually feel. To
do this each bond points itself into the corners of a regular tetrahedron, the
Cl-C-Cl bond angle is around 109 degrees, not 90 degrees as the written down
structure suggests. In this arrangement repulsive forces are minimised. The
resulting symmetry of the molecule results in a sort of overall cancellation
of polarisation. This effect is easier to see with a diatomic molecule such as
BeCl2 (beryllium chloride). This molecule is linear ie the Cl-Be-Cl bond angle
is 180 degrees. Each bond is polarised in the direction of the Cl which thus
aquires a partial negative charge d-. The Be atom is thus left partially
postitively charged. If we represent the direction of the polarisation as an
arrow, and the length of the arrow represents the _magnitude_ of the effect
then in BeCl2 we get two arrows of _equal_ length pointing in opposite
directions. These arrows are referred to as vectors and have the property of
being additive. A vector can be used to represent any property which has both
magnitude and direction. Properties which only have magnitude are known as
scalars. The effect of having two vectors of equal length (magnitude) pointing
in exactly opposite directions is overall cancellation. It's like a tug of war
between two teams that are exactly equal in strength - they are pulling in
opposite directions so the rope stays where it is. However if one team is
stronger then the rope will move in their direction, the force on the rope now
being positive in their direction and equal to the difference in the two teams
strength. In BeCl2 we have such a case :
Cl-Be-Cl
<~~~~~~ ~~~~~~>
Polarisation is equal but in
opposite directions. Overall effect is
total cancellation and the molecule as a
whole is non-polar.
In CCl4 the 4 polarisation vectors point to the corners of a regular tetrahedron
and are equal in length (magnitude), the overall effect is cancellation and zero
net polarisation. Compare this to chloroform for example where the dominating
EN of oxygen polarises the O-H bond more than the C-O bond. Here the vectors
are not equal and the molecule as a whole is polar.
Compare this to CHCl3 (chloroform) where one of the Cls has been replaced by
an atom of much less EN. The polarisation vectors due to C-Cl are equal in
magnitude but not direction - remember they point to the corners of a
tetrahedron. The C-H bond polarisation vector is much smaller and because
carbon is more EN than hydrogen the vector points in the opposite direction
towards the centre face of the tetrahedron which touches all 3 Cl atoms. Trying
to draw this in 2D doesn't give the proper picture but it gives you an idea.
Cl
|
<~~~~~~~~~~~ Cl-C-H <~~~ small vector along CH bond.
Overall vector due to |
C-Cl polarisations Cl
<~~~~~~~~~~~~~~~~ Total vector for entire molecule.
So chloroform is a more polar solvent than CCl4.
Similar arguments, based on symmetry and polarity of bonds, can be applied to
other molecules to determine their overall polarity.
Dalamar.
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