Electrophilic addition with bromine

This post is a slight correction to this old post. 

Electrophilic addition reactions are relatively straightforward. You go from an alkene to an alkane through this mechanism; as an example, I'll react ethene with HCl to form chloroethane:

You start off with the C=C double bond in ethene attacking the H in HCl, which itself is changing quite a bit. Cl is far more electronegative than H, so is pulling electrons towards itself, which causes the H-Cl bond to break. 

From here, you end up with a carbocation which the now nucleophilic Cl^- ion will attack, and you'll end up forming chloroethane.

That's all very A-Level stuff. In fact, the image I included above isn't really what happens - the H is sourced from a H₃O⁺ ion - but otherwise it's kind of similar. Besides, A-Level is kind of basic compared to uni, and so that's not my issue with the old blogpost.

My issue...is less an issue, and more the fact I learnt something recently. I gave the example of an ethene and chlorine reaction in the original post, which I said followed the exact mechanism I just described. But that isn't correct - the actual mechanism is slightly different.  

This is stage two of the alkene to dibromoalkane reaction pathway. A bromonium ion would form instead of a carbocation, and then a bromide ion would attack a carbon as follows. 

This whole process is indeed kind of strange, but you can isolate a bromonium ion, as with any intermediate, so this is very much what actually happens. 

Now you might next be asking "could we have a carbocation in this intermediate", and the answer is, in a way, yes. 

You can have resonance forms of the bromonium ion, and even though this one isn't as common as the previous one, it does expose something cool - it kind of explains why the bromide ion will react as it does. Now for a molecule like ethene with just one double bond, this seems extremely trivial, but as you end up looking at more complex molecules where you have carbocations of varying stabilities, and you consider the Markovnikoff rules, it starts to become very important.

Because remember - the Markovnikoff product is going to be the one with the more stable carbocation! That's the one where the negatively charged ion is going to attack, with the order of preference being tertiary > secondary > primary. 

Because you have greater σ donation from the methyl groups to the carbocation the more methyls you have. That helps stabilise the carbocation by delocalising its positive charge, because that carbocation hates being charged, that's why it lets so many nucleophiles attack it. Carbon really wants to have four bonds, after all, with a few notable exceptions where carbon can settle on, at most, a triple bond with a lone pair. 

Also importantly, note the carbocation will be sp² hybridised, and so will have an empty p orbital, uninvolved in bonding. You can end up with electrons being delocalised from C-H or C-C bonds and donated into the empty p orbital, which will again end up stabilising the carbocation.

That's hyperconjugation, which is a massive deal in reaction mechanisms with carbocation intermediates like nucleophilic substitution, which you'll learn at uni involves two different pathways - SN1 and SN2 - where a reaction is more likely to be SN1 if you have loads of bonded methyls, due to the resultant hyperconjugation.

So the old blogpost isn't wrong, it's just very misleading at uni level. But at A-Level, it's good enough, which is honestly fine by me. I don't remember doing electrophilic addition with ethene and chlorine in exams anyways, I just thought it was a cool-sounding example, and clearly it wasn't right but I also wrote that post in year 12.