the terminal carbon has the number 1; the
carbon along 2, 3, 4, 5. Hence, 5-methyl-hexyne.
Hydrogen halides addition
We covered this briefly in the context of
alkenes. It is also possible to add, for example,
hydrobromic acid over a triple bond. If we
look at this equilibrium equation here, we
can see what’s happening. Either we get
protonation of one carbon or we get protonation
of the neighboring carbon. Note that in this
scenario, what we are trying to do is generate
the carbocation which is the most stable.
This means that on protonation of one of those
carbons with the electrons moving from the
triple bond to the H+, we generate the carbocation
which is most stable which is that, which
is the most substituted in the center.
We can see this because the compound which
is formed is the 2 bromoalkene, not the 1
bromoalkene. It’s also possible to add halogens
themselves over the tops of double bonds,
as shown in the lower part of the screen.
As you can see, it’s possible to, therefore,
generate a dibromoalkene which can further
be brominated again as an alkene to give
you a tetra bromoalkene, shown in the bottom
of the screen there.
It’s also possible to reduce an alkyne down
to an alkene. We’ve seen this before when
we reduced an alkene down to an alkane in
the presence of H2. But, the problem we’ve
seen, and we go back to Module I here, is
the stereochemistry because it is possible
to generate two geometrical isomers, the cis
and the trans alkene. Note the difference
between the two. Cis, which is the first one
we observe in the middle of the board, where
the two alkyl groups shown as R and R prime
are on the same side and of course, where
they are on in the trans position, where they
are on opposite sides of the double bond.
Now, were this a single bond, you could merely
get a rotation around the central sigma like
an orbital. But, of course, as double bonds,
you cannot get this rotation. And so, these
are geometrically, physically and chemically
distinct isomers. To achieve one or the other,
there are some basic reactions that you can
do. To achieve the cis, H2 in the presence
of nickel and platinum, and in the case of
achieving the trans, you will see that Lindlar’s
reagent or sodium dissolved in liquid ammonia
can achieve a trans geometrical isomer.
It should be stressed that cis and trans carry
with them other nomenclature. So, for example,
the cis relates to being together and in German,
you’ll see it sometimes referred to as zusammen
or Z. Trans, meaning apart, is sometimes
referred to in German by entgegen or E.
And so, as a consequence, it’s possible
to generate two geometric isomers using different
conditions. And this electivity for one geometric
isomer over the other becomes increasingly
important as we start to synthesise more complicated