Organic Chemistry - "Change", Part 15: Introduction to First Order (E1) and Second Order (E2) Elimination Reactions

 Greetings,

Another very important reaction type is elimination. As the name suggests, this reaction involves the elimination of hydrogen and a "leaving group" across a single bond to form an alkene. A typical leaving group is Bromine, which leaves the molecule as Bromine ion. The E1 and E2 labels stand for "elimination - 1st order reaction rate" and "elimination - 2nd order reaction rate", respectively. The different order reaction rates are based on the mechanisms of the two reaction types. Those two mechanisms are called unimolecular(UM) and bimolecular(BM) and the elimination reaction types that are associated with them are just as you might think; E1 and E2, respectively. UM indicates that the E1 reaction rate depends on only one entity (the substrate) and BM means that the E2 reaction rate depends on two items (the substrate and a base). The mechanisms are discussed below.

Elimination Mechanisms and Their Relationship to 1st and 2nd Order Rates of Reaction

E1 Reaction - The E1 reaction type relies on the instability of a bromine-containing alkane. In other words, how readily the bromine can leave the molecule. If we consider a 2-degree (tertiary) bromine alkane, we find that the bromine can leave the alkane chain without the aid of another reactant, however, that happens fairly slowly.  The fact that the bromine leaves slowly, causes that 1st step, of the E1 Reaction, to be the rate-determining-step: This means that the E1 reaction rate only depends on the initial bromine-containing molecule (the substrate), even though there is a second step involving another compound.  The second step is the removal of hydrogen from an adjacent carbon atom, which happens when a dilute base, such as ethoxide ion, is in the vicinity. The rate law for E1 is reaction rate constant times substrate molar concentration: R = kr x [substrate].

E2 Reaction - This reaction is much more common than E1 because it is initiated by a wider range of conditions, such as substrate molarity, type of base and base concentration. A base is involved with the start of this reaction, which is the formation of a concerted intermediate entity (the activated complex). Because the reaction begins with a combination of base and substrate at the same time, the E2 reaction rate depends on both substrate and base concentrations. As a result, the rate law for E2 includes both substrate and base: R = kr  x [base] x [substrate]. It should also be mentioned that, unlike the E1 reaction, E2 tends to be in competition with another reaction, namely SN2 (bimolecular nucleophilic substitution). This means that the E2 products are a mixture of an alkene and a substituted alkane (from the competing SN2 reaction).

Graphical Summary of E1 and E2 Reactions - The figure, below, shows the difference between the reactions by depicting the mechanisms of both E1 and E2, on a molecular level.

That's all for this post. As always, thank you for reading!