Addition of Halogens – Alkanes and Alkenes

00:00 The H+ will also add to the carbon that already has more hydrogens.

00:00 As we saw previously, it’s possible to carry out an addition reaction over a double bond.

00:05 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.

00:49 Halo, of course, belonging to the halogen which is involved in that addition reaction.

00:55 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.

01:22 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.

02:43 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 carbon. Now, I’d like to move on to Alkynes.

03:28 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.

03:45 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.