| Ethene Bromonium Ion (6-31G*) |
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Bromonium ions seem unusual at first, but in fact they closely resemble the hydrogen-bridged structures we have already seen for primary carbocations. They also look very much like the protonated oxiranes we saw earlier..
When a bromonium ion is formed from a symmetrical alkene (one with the same substituents on each end) it also will be symmetrical:
| Ethene Bromonium Ion (6-31G*) |
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Reaction with Br- or other nucleophile should therefore take place equally at both ends.
| LUMO of Ethene Bromonium Ion (6-31G*) |
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Notice that, just like the LUMO of a simple alkyl halide, it is antibonding between the carbon and the halogen. Notice also the symmetry; the orbital is exactly the same at the rear of both carbons of the bromonium ion, giving rise to equal reactivity at both ends.
If the bromonium ion is formed from an unsymmetrical alkene, it also will be unsymmetrical. The bond from bromine to the more substituted carbon will be longer, corresponding to more positive charge, or a larger LUMO contribution, on that carbon.
Here we have added a couple of CH3 groups to one end of the symmetrical bromonium ion:
| Unsymmetrical Bromonium Ion (6-31G*) |
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The longer bond to the substituted carbon is evident. Now look at the LUMO:
| LUMO of Unsymmetrical Bromonium Ion (6-31G*) |
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The reason for attack of the nucleophile at the more substituted carbon is clear: that's where the LUMO is.
Here are the structures of the two bromonium ions in Jmol windows:
| The Symmetrical Bromonium Ion
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The Unsymmetrical Bromonium Ion
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