Right. Now, we are into Module III where we
start looking at Organic Chemistry.
And so, I would like to introduce you to some
of the basic functional groups that you will
encounter over the course of this particular
module. In this lecture, we are going to be
looking at alkenes and alkynes. And here’s
a bit of nomenclature revision for you dealing
with alkenes in particular.
If a compound contains a double bond, such
as those shown here on the board, the name
of it is given by the suffix “ene”. A
number prefix is used to denote the position
of this unsaturation. And indeed, whenever
you see an alkane molecule with a double bond
in it, it automatically becomes an alkene
and is considered, therefore, unsaturated.
Alkanes are, by their very definition, saturated.
But, from a synthetic chemistry perspective,
since all we tend to do with alkanes is set
fire to them, it really isn’t that important
that we cover them as part of this module.
So, for example, if we look at the compound…
the first compound with the terminal alkene
i.e. the double bond in the terminal position,
we can see that that is called 1-butene. This
is because we have a double bond and it is
in the first position. Where we have an internal
alkene, such as that shown in the middle,
we call that a 2-butene. And where we have
a multiple double bonds, such as in the case
of our ‘diene’ compound, we would refer
to it as,, in this case as a 1,3-butadiene.
Note, the origins of the ‘but’ part comes
from the fact that it is four carbon atoms
in length. And if you recall, the way it goes
is methyl, ethyl, propyl, butyl, pentyl, hexyl,
heptyl and octyl, as we go up from the number
of carbons 1 through 8.
Okay. So, that’s just a bit of nomenclature
revision. The question is what do we use them
for? What do we do? Well, typically alkenes,
in laboratories, undergo Electrophilic Addition
reactions. Addition reactions are characterised
in the example… example equation showed
above where we have a double bonded carbon
to carbon and we have the addition of our
component X and Z over that double bond.
Let’s take a real world example which is
the addition of a hydrogen halide over an
alkene double bond. And this is where we introduce
the concept of nucleophile and electrophile.
An electrophile is something which has an
affinity for electrons and a nucleophile is
something that has an affinity for nucleophiles,
which is positive. If we look, for example,
at the double bond of our alkene and we see
that the pi bond serves as a nucleophile,
it will try to attack, or rather try to transfer
its electrons to something which is partially
If you go back as far as Module I where we
looked at dipoles such as that with hydrogen
bromide or hydrobromic acid, you see we have
a partial positive charge on a hydrobromic
acid given as delta+ and a partial negative charge
on our bromide given as delta-.
What happens, in this case, is that electrons
attack the hydrogen and generate a carbocation
intermediate, which is shown as the middle
structure. That, then, reacts with our now
negatively charged bromide and what you’ve
done is add hydrogen bromide over the double
bond. So, let’s actually look at that in
a bit more detail.