Elimination reactions

Just as there are S_N2 and S_N1 reactions, there are two types of elimination reactions: The E1 and the E2 reaction. And, just like you might expect, the difference is that an E2 reaction proceeds by second-order kinetics and involves a single step:

whereas an E1 reaction proceeds by first-order kinetics and is a two-step reaction.

In the scheme at the right, the H and X being eliminated are in an anti position with respect to each other. This preference can be explained in terms of molecular orbitals: the LUMO in bromoethane has some density on just this hydrogen. You can check this using the editor. Construct the molecule (or 2-bromobutane, or axial bromocyclohexane) starting from methane or cyclohexane, check the orbital data box, and finally select the LUMO at a 0.15 contour level.

An example of the E2-elimination is the dehydrohalogenation reaction that can occur in alkyl halides. Let's look at one such alkyl halide: 2-bromobutane (below).

Several different eliminations can occur in this molecule: a base could pull out a

(on/off) proton at C-1, while simultaneously the bromide departs and a double bond is formed between C-1 and C-2.
Or, a base could pull out a

proton at C-3 while the bromide also leaves the molecule, which results in a double bond between C-2 and C-3.
To be accurate, there is one more possible reaction, because the

other hydrogen at C3 can also be abstracted, if we

(click repeatedly) rotate it to the required anti position (with respect to the bromine) first.

Do you already see, by looking at the carbon skeleton, that we get different products in the latter two cases?

In the next part, one of these three possible eliminations is visualised.