The E1cB elimination reaction is a type of elimination reaction which occurs under basic conditions, where the hydrogen to be removed is relatively acidic, while the leaving group (such as -OH or -OR) is a relatively poor one. Usually a moderate to strong base is present. E1cB is a two-step process, the first step of which may or may not be reversible. First, a base abstracts the relatively acidic proton to generate a stabilized anion. The lone pair of electrons on the anion then moves to the neighboring atom, thus expelling the leaving group and forming double or triple bond. The name of the mechanism - E1cB - stands for Elimination Unimolecular conjugate Base. Elimination refers to the fact that the mechanism is an elimination reaction and will lose two substituents. Unimolecular refers to the fact that the rate-determining step of this reaction only involves one molecular entity. Finally, conjugate base refers to the formation of the carbanion intermediate, which is the conjugate base of the starting material.
E1cB should be thought of as being on one end of a continuous spectrum, which includes the E1 mechanism at the opposite end and the E2 mechanism in the middle. The E1 mechanism usually has the opposite characteristics: the leaving group is a good one (like -OTs or -Br), while the hydrogen is not particularly acidic and a strong base is absent. Thus, in the E1 mechanism, the leaving group leaves first to generate a carbocation. Due to the presence of an empty p orbital after departure of the leaving group, the hydrogen on the neighboring carbon becomes much more acidic, allowing it to then be removed by the weak base in the second step. In an E2 reaction, the presence of a strong base and a good leaving group allows proton abstraction by the base and the departure of the leaving group to occur simultaneously, leading to a concerted transition state in a one-step process.
Mechanism
There are two main requirements to have a reaction proceed down an E1cB mechanistic pathway. The compound must have an acidic hydrogen on its β-carbon and a relatively poor leaving group on the α- carbon. The first step of an E1cB mechanism is the deprotonation of the β-carbon, resulting in the formation of an anionic transition state, such as a carbanion. The greater the stability of this transition state, the more the mechanism will favor an E1cB mechanism. This transition state can be stabilized through induction or delocalization of the electron lone pair through resonance. In general it can be claimed that an electron withdrawing group on the substrate, a strong base, a poor leaving group and a polar solvent triggers the E1cB mechanism. An example of an E1cB mechanism that has a stable transition state can be seen in the degradation of ethiofencarb - a carbamate insecticide that has a relatively short half-life in earth's atmosphere. Upon deprotonation of the amine, the resulting amide is relatively stable because it is conjugated with the neighboring carbonyl. In addition to containing an acidic hydrogen on the β-carbon, a relatively poor leaving group is also necessary. A bad leaving group is necessary because a good leaving group will leave before the ionization of the molecule. As a result, the compound will likely proceed through an E2 pathway. Some examples of compounds that contain poor leaving groups and can undergo the E1cB mechanism are alcohols and fluoroalkanes. It has also been suggested that the E1cB mechanism is more common among alkenes eliminating to alkynes than from an alkane to alkene. One possible explanation for this is that the sp2 hybridization creates slightly more acidic protons. Although this mechanism is not limited to carbon-based eliminations. It has been observed with other heteroatoms, such as nitrogen in the elimination of a phenol derivative from ethiofencarb.