We ignored the issue of symmetry in the alkene when we discussed the bromination of alkenes because even unsymmetrical alkenes give the same 1,2-dibromides, whichever way the bromide attacks the bromonium ion.

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 fi rst 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. 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 SN2 process. Here, it hardly looks SN2. We have a tertiary centre, so naturally you expect SN1, 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 SN2 mechanism!

The answer to the conundrum is that substitution reactions don’t always go by pure SN1 or pure SN2 mechanisms: sometimes the mechanism is somewhere in between. Perhaps the leaving group starts to leave, creating a partial positive charge at carbon, which is intercepted by the nucleophile. This provides a good explanation of what is going on here. The bromine begins to leave and a partial positive charge builds up at carbon. The departure of bromine can get to a more advanced state at the tertiary end than at the primary end because the sub stituents stabilize the build-up of positive charge. A more accurate representation of this bromonium ion is shown in the margin, with one C–Br bond longer than the other and more polarized than the other. 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 SN2 transition state.

The products of bromination in water are called bromohydrins. They can be treated with base, which deprotonates the alcohol. A rapid intramolecular SN2 reaction follows: bromide is expelled as a leaving group and an epoxide is formed. This can be a useful alternative synthe sis of epoxides avoiding peroxy-acids.

Rates of bromination of alkenes

The pattern you saw for epoxidation with peracids (more substituted alkenes react faster) is followed by bromination reactions too. The bromonium ion is a reactive intermediate, so the rate-determining step of the brominations is attack of bromine. The scale below shows the effect on the rate of reaction with bromine in methanol of increasing the number of alkyl substituents from none (ethylene) to four. Each additional alkene substituent produces an enormous increase in rate. The degree of branching (Me versus n-Bu versus t-Bu) within the substituents has a much smaller, negative effect (probably of steric origin) as does the geometry (E versus Z) and substitution pattern (1,1-disubstituted versus 1,2-disubstituted) of the alkene.

مواضيع ذات صلة

Enolization is catalysed by acids and bases

Evidence for the equilibration of carbonyl compounds with enols

Tautomerism: formation of enols by proton transfer

Hydration of alkynes

Breaking a double bond completely: periodate cleavage and ozonolysis

Adding two hydroxyl groups: dihydroxylation

Electrophilic addition to alkenes can produce stereoisomers

Hydration of alkynes

Breaking a double bond completely: periodate cleavage and ozonolysis

Adding two hydroxyl groups: dihydroxylation

Electrophilic addition to alkenes can produce stereoisomers

Electrophilic additions to alkenes can be stereospecific