Open-Ended Solvent Choice

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. 

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