This is important in the physical and chemical
characteristics of this class of compound.
Physically, there are hydrogen bonds that
can be formed. These are a special type of
dipole-dipole interaction. And what I’ve
shown here is an example of four alcohol molecules
all close together. Because you have a partial
positive charge on the hydrogen and a partial
negative charge on the oxygen, it’s possible
to form a dipole-dipole bond between hydrogens
and oxygens on neighbouring molecules. These
are a particular type of dipole-dipole interaction
known as hydrogen bonds. And whenever you
see a hydrogen attached to an oxygen like
that, you should be thinking about a possibility
for hydrogen bonding not just by itself with
another molecules of the same, but with other
molecules as well.
Hydrogen bonds are a special type of attractive
interaction that exist between an electronegative
atom and a hydrogen atom bonded to each other.
It has only 5-10% strength of the comparative
covalent bond, but it has a profound effect
on the boiling point and solubility of the
The cells of living things are made up of
a vast variety of different molecules, but
two important classes, which I’d like to
bring to your attention now, are nucleic acids
found in DNA and proteins.
Parts of these very, very large molecules
are involved in hydrogen bondings with other
parts of the same molecules. And this is what
gives them their unique structural characteristics
in many cases. Not just the strong covalent
bonds, but indeed the shape of the molecules
is informed largely by the intermolecular
interactions which, in this case, are manifest
in an intramolecular fashion.
This is very important in establishing their
structure and their properties, particularly
when you’re looking at things like proteins;
the structure is essential for its biological
Let’s have a quick look at hydrogen bonding
as it occurs in DNA: deoxyribonucleic acid.
The double helical structure of DNA is due
largely to hydrogen bonding between the base
pairs. Here, we have an example of so-called
duplex or Watson-Crick base pairing, after
the people who have discovered this phenomenon.
Here you can see the hydrogen bonding is possible
between a hydrogen and a nitrogen. Note that
as before, when we were talking about electrophilic
species, nitrogen and oxygen and also sulphur
fall into that category. And so, hydrogen
bonding, as you can see, between the oxygen
and the delta positive of the hydrogen directly
attached to the nitrogen at the top facilitates
the interaction of guanine and cysteine, the
pairing of the bases.
You’ll also see, in the case of adenine
and thiamine, the other two bases found in
DNA, that hydrogen bond interactions take
place there as well. And it’s this binding
together that holds the complementary bases
aside from each other on either strand.
Let’s have a quick look at hydrogen bonds
If we look, for example, at hydrogen bonds,
they play an important role in the three-dimensional
secondary structures of proteins. The primary
structure of a protein is given by one amino-acid
attached to the next which is attached to
the next in a covalent polyamide fashion.
But, the reality is that the complex nature
of the proteins is down to, in many cases,
the three-dimensional structure which is informed
not just by the pH, but also by the interactions
of amino-acid with neighbouring amino-acids
via hydrogen bonding.
Bearing in mind, if you look for example at
an amide bond, which we’ll do a little later
on, it’s possible for it to act both as
a hydrogen bond donor and also a hydrogen
Hydrogen bonds form between a backbone oxygen
and amide hydrogens to form the more frequently
either alpha-helix or the beta-sheet secondary
structure of the proteins, as shown here.
It also, as I said, strongly influences the
physical characteristics of this homologous
series. Let us compare the corresponding alkane
to the alcohol.
As you can see, in row one, we have one carbon.
In the alkane form, this is methane. The boiling
point of methane is -161 degrees. However,
the corresponding alcohol, with only one carbon
in it, actually has a boiling point of around
If you actually compare, more energy is needed
to break the hydrogen bonds between molecules
of alcohols, so the boiling points are higher
than the compare... the alkane in comparison,
which has a similar molecular weight.
If we go down all the way down to number 5,
where we are looking at the equivalent pentanol
on the right hand... left hand side, you’ll
see it has a boiling point of 136 degrees.
And this compares to 35 degrees in the case
of the pentanol on the other side.
As we said before, the difference between
alkanes and alcohols is related, to a large
extent, to their ability to hydrogen bond,
not just with each other, as we’ve shown,
and therefore, to increase their boiling point,
but also with water. So, for relatively low
molecular-weight alcohols such as methanol
and also the favourite ethanol, solubility
in water can also be achieved.
Water-miscible alcohols include methanol,
ethanol and propanol. Butanol is partially
soluble at around 8% by weight and pentanol
slightly less. The larger the alcohol chain,
the more it will define the degree of solubility
of a given alcohol in water.
Again, as I showed you, the water solubility
is predicated on the ability of an OH dipole
to interact with another OH dipole.
Now, let’s have a quick look at the reactivity.
You may recall this style of diagram from
when we were talking about haloalkanes, where
we’re trying to find which is the delta
negative and which is the delta positive species
in this case.
As we know that oxygen is more electronegative
than carbon, we can be reasonably confident
that that bond, that R-O bond, is polarised.
Oxygen bears a partial negative charge and
the alkyl carbon, shown there as R, bears
a partial positive charge. And the reactivity
depends on the structure of the alcohol group.
The properties, in terms of the physical properties
observed, are determined by the presence of
the OH group. But, in either cases, as we
saw in the case of the haloalkanes, substitution
or elimination reactions may be the result.
So, let’s have a look at the formation of
alkyl halides from alcohols.