Читать книгу The Handy Chemistry Answer Book - Justin P. Lomont - Страница 35

We actually just looked at an example of Markovnikov’s rule! This rule states that when you add a protic acid (like HBr) to an alkene, the proton ends up attached to the carbon with fewer alkyl substituents, while the conjugate base ends up bonded to the carbon with more alkyl substituents. This is because of the stability of the carbocation intermediate that is formed (remember, more substituted means more stable).

What is an elimination reaction?

If an addition reaction adds two groups to a molecule, then an elimination reaction takes them away. The textbook example again involves carbon-carbon double bonds, but this time we’re making that double bond.

A molecule of base (hydroxide ion, ⁻OH) causes the elimination of HBr. In this case, the process happens in one step, as shown.

Are there uni- and bimolecular elimination reaction mechanisms, like there were for substitution reactions?

Yes! And what do you think controls whether an elimination reaction happens in one step or two? Right—the stability of the carbocation. In the previous example, if bromide ion had dissociated first, a primary carbocation would have formed:

This is a much more difficult reaction than the bimolecular process where the elimination of HBr takes place in a single step.

But if an alkyl halide can form a stable carbocation, the unimolecular elimination reaction is faster. It’s referred to as “unimolecular” because the slow step has only one molecule in the transition state, just like substitution reactions.