Elimination Reaction Overview and Traits

Introduction:

The elimination reactions of alkyl halides (R-X) is taught along side the substitution reaction.  The reaction involves the loss of the leaving group (X) and a hydrogen (H) and the formation of a carbon-carbon double bond (C=C).

This reaction is confusing for two reasons:  1) the reactants are the same as the substitution reactions (Nuc/base + R-X) but they give different products and 2) the reaction involves a more complex bond making and breaking mechanism that involves two adjacent carbons (alpha and beta).

Mechanisms:

There are two general substitution reactions (shown below) are E2 and E1. Pay close attention to the number of steps and order of steps in a mechanism. Think about which bonds get formed and broken in each step.

E2 Mechanism

Comments:  The key step is the base grabbing the proton on the beta carbon.  Therefore, 1) a strong base is required, 2) the proton is sticking out and so even a bulky base is okay, and 3) the beta-proton must be anti– (on the opposite side) to the leaving group (X). 

E1 Mechanism

E1:  This is a two-step mechanism.  The R-X bond first breaks.  This step is slow.  Then the nucleophile forms a bond to the carbocation intermediate (R⁺).

Comments:  The key step is the formation of the positivelycharged R+ intermediate.  So, R will be 3° or 2° to form more stable carbocation intermediates.  The carbocation intermediate is flat.  So, the base can grab either beta-carbon proton.  The base is usually weak (and neutral and protic) because it is not involved in the key step.

Summary of key traits: