Unsymmetrical bromonium ions open regioselectively

We ignored the issue of symmetry in the alkene when we discussed the bromination of alk-enes because even unsymmetrical alkenes give the same 1,2-dibromides, whichever way the 

But when a bromination is done in a nucleophilic solvent—water or methanol, for example—solvent molecules compete with the bromide to open the bromonium ion. As you know, alcohols are much worse nucleophiles than bromide but, because the concentration of solvent is so high (remember—the concentration of water in water is 55 M), the solvent gets there first most of the time. This is what happens when isobutene is treated with bromine in methanol. An ether is formed by attack of methanol only at the more substituted end of the bromonium ion. When a functional group can react in more than one position, the choice is known as the regioselectivity of the reaction. We will return to the concept of regioselectivity in Chapter 24. 

Methanol is attacking the bromonium ion where it is most hindered, so there must be some effect at work more powerful than steric hindrance. One way of looking at this is to reconsider our assumption that bromonium ion opening is an 5 2 process, ffere, it hardly looks 5 2. We have a tertiary centre, so naturally you expect 5 1/ via the cation below. But we have already said that cations like this can be stabilized by formation of the three-membered bromonium ion and, if we let this happen, we have to attack the bromonium ion, which gets us back to where we started an 5 2 mechanism  

The nucleophile now has a choice does it attack the more accessible, primary end of the bromonium ion, or does it attack the more charged end with the weaker C—Br bond Here, the latter is clearly the faster reaction. The transition state has considerable positive charge on carbon and is known as a loose 8, 2 transition state. 

Interactive mechanism for regioselective addition to unsymmetrical alkenes 

The products of bromination in water are called bromohydrins. They can be treated with base, which deprotonates the alcohol. A rapid intramolecular Sjyj2 reaction follows bromide is expelled as a leaving group and an epoxide is formed. This can be a useful alternative synthesis of epoxides avoiding peroxy-acids. 

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