Category: Elimination Reactions

Some classes of compounds like acetates, xanthates and amine oxides undergo unimolecular reaction via a cyclic transition state, within which an intramolecular proton transfer is accompanied by elimination to form a new carbon–carbon double bond. These reactions are carried out in a gas phase under high temperature. The cyclic transition state exhibits syn-elimination, that is, the hydrogen atom…

We can also compare these with the other elimination reactions by thinking of the relative timing of proton removal and leaving group departure. E1 is at one end of the scale: the leaving group goes first, and proton removal follows in a second step. In E2 reactions, the two events happen at the same time:…

It is worthwhile comparing the E1cB reaction with some others with which you are familiar. For a start, you may have noticed that an E1cB reaction is the reverse of the conjugate addition reaction. Conjugated carbonyl compounds were the starting materials, now they are the products–but both reactions go through a stabilized anion intermediate. E1cB…

The rate-determining elimination step in an E1cB reaction is unimolecular, so you might imagine it would have a first-order rate equation. But, in fact, the rate is also dependent on the concentration of the base. This is because the unimolecular elimination involves a species–the anion–whose concentration is itself determined by the concentration of the base…

This is an elimination catalyzed by a strong base (KOH), so it looks like an E2 reaction. But the leaving group is hydroxide, which we categorically stated could not be a leaving group in E2 eliminations. The key to what is going on is the carbonyl group. We know that the negative charges are stabilized…

The elimination of HX from a vinyl halide produces an alkyne. Very strong bases, such as NaNH2 and LDA are generally used. There are three possible pathways for elimination reactions at sp2 hybridized atoms. The product alkyne has no stereochemical features. The E1 route is unlikely because the vinyl cation (sp2-hybridized) is very unstable, and energetic considerations…

Here are two deceptively similar elimination reactions. The leaving group changes and the reaction conditions are very different, but the overall process is elimination of HX to produce one of two alkenes. In the first example, acid-catalyzed elimination of water from a tertiary alcohol produces a trisubstituted alkene. Elimination of HCl from the corresponding tertiary…

The stereospecificity of the reactions is very good evidence that E2 reactions proceed through an anti-periplanar transition state. We know with which diastereoisomer we started, and which alkene we get, so there is no question over the course of the reaction. More evidence comes from the reactions of substituted cyclohexanes. Generally, all cyclohexanes are most stable…

It has been observed that alkyl halides tend to proceed through E2 mechanism and protonated alcohols through E1. The vast majority of the two classes of eliminations use one of these two types of starting materials. Since the leaving group is involved in the rate-determining step of both, E1 and E2, in general, any good leaving…