The H+ will also add to the carbon that
already has more hydrogens.
As we saw previously, it’s possible to carry
out an addition reaction over a double bond.
It takes place not just in the presence of
a hydrobromic acid or derivative where you
have a dipole. But, it can also take place
in the presence of a molecule which consists
of two atoms which are the same.
As we see in the top example, bromine itself
is red in color. In the presence of an alkene,
it’s converted into a dibromoalkane, which
in itself is colorless. If bromine is added
in the presence of water, the so called bromine
water experiment, it is decolorised. The bromine
water being orange in color reacts directly
with the alkene in an addition reaction resulting
in the formation of a colorless halohydrin.
Halo, of course, belonging to the halogen
which is involved in that addition reaction.
In this case, it would be a bromohydrin.
Markovnikov’s rules are very important and
it relates, again, to which is the most stable
carbocation that’s formed in the process
of an electrophilic addition reaction. In
short, an electrophilic addition of an unsymmetrical
reagent to an unsymmetrical double bond proceeds
in such a way to involve the most stable carbocation.
We saw this in the case when we reacted propene
with hydrobromic acid. In this scenario, what
we are doing is we are reacting hydrobromic
acid and bromine to give us the addition reaction
over carbons 1 and 2.
The stability of carbocations, as I indicated
before, in the case of the reaction with hydrobromic
acid, is predicated on the number of alkyl
groups which donate electrons into the carbocation
in question. The more alkyl groups you have
on a particular carbon, the more stable it
will be. And so, the order of stability goes
tertiary, secondary, primary and methyl.
In addition, as we’ll see here, it is possible
to add water over an alkene double bond. As
I said before, water itself does not react
with alkenes. However, in the presence of
an acid, it is possible to add water over
the double bond and create an alcohol, as
indicated there. Note in this particular case
though, the H+, the acid, the proton is
not actually consumed in the reaction, rather
it is catalysed.
The reason we can say it’s a catalyst is
because it’s regenerated, if you look at
the product of this reaction on the board.
So, if sulfuric acid is dissolved in water,
as we said, it is completely ionised because
it is a strong acid. It is also possible to
add, as I said before, not just things like
a bromine molecule (Br2) or chlorine (Cl2),
but it is also possible to add hydrogen molecule
which is H2. In the presence of a metal
catalyst, such as nickel or platinum, it’s
possible to carry out the same reaction that
we showed you before with the bromine. In
this case, one hydrogen is added on to one
carbon and the other is added on to the neighboring
Now, I’d like to move on to Alkynes.
This is some revision about nomenclature.
The suffix for an alkyne is “yne” Y-N-E
and the number of where it is located is given
as the prefix.
So, let us take a look at this particular
example. If a compound contains a triple bond,
its end names with “yne” and the number
prefix is used to denote the position of this
unsaturation - 5-methyl-1-hexyne.
Note the importance of this. What we are saying
effectively in this scenario is that we are
having a substitution reaction which is furthest,
in this case the CH3, a substitution pattern,
furthest from where the triple bond is. This
means that the triple bond takes priority
in numbering. So, where the triple bond is,
the terminal carbon has the number 1; the
carbon along 2, 3, 4, 5. Hence, 5-methyl-hexyne.