So, let’s have a look in a little bit more
detail as to the exact mechanism of electrophilic
attack on our alkene from a couple of slides
ago. Here, we have the pi bond which is an
electron rich. Bearing in mind the pi bond,
remember from Module I, contains two electrons.
The underlying sigma bond also contains two
electrons. So, that’s quite an electron
rich kind of area.
Remember also, when we talked about double
headed arrows and their significance. A double
headed arrow imparts that two electrons or
an electron pair are moved from one end to
the other where the actual end of the arrow
is. In this scenario, what’s happening is
that the electron ridge, alkene group, is
having its electrons transferred on to the
delta+ of the H+ or if it were in solution,
the H+ cation in the presence of the Br-
What happens there in the first place is we
have generated now a new sigma bond between
a hydrogen and the carbon on that alkene.
But, of course, charges conserved and where
we have moved the pair of electrons from that
bond, we must, by necessity, be creating a
positive charge on the other carbon. This
carbocation intermediate then reacts with
the bromide ion that results and you have
now the addition of hydrogen bromide over
that double bond.
This is a two step mechanism. The carbocation
happens to be the intermediate. Strong acids
add to a double bond, but weak acids, such
as ascetic acid and water, don’t. Water,
though, can add to a double bond in the presence
of an acid, strong acid.
Right. So, as we saw before, it’s possible
for hydrogen bromide to add over an alkene.
However, when we have more than two carbons
involved such as, as we see here in the case
of propene; of course, we have three carbons
in our alkene here, there are two possible
products depending on where the H+ or delta
H is attacked.
So, in the case of this propene molecule,
as we can see here, it’s possible for either
the hydrogen or the H+ to be added on to
either C-1 resulted… resulting in a carbocation
with a charge in the center. So, this is shown
at the top or at the bottom where, for example,
the hydrogen adds to the carbon in the center,
thus resulting in a carbocation which has
its positive charge on a terminal, CH2 group,
as you can see.
The bromide ion can then add either to the
center, in the case of the first situation,
or it can add to the terminal position, in
the case of the second. This gives us two
different regioisomers. Remember, we talked
about stereochemistry back in Module I. And
this is an example of how we can achieve,
from a synthetic perspective, two particular
regioisomers. So, two different products.