When Is the Mechanism El or

Given a particular starting material and base, how do we know whether a reaction occurs by the El or E2 mechanism  

Because the rate of both the El and E2 reactions increases as the number of R groups on the carbon with the leaving group increases, you cannot use the identity of the alkyl halide to decide which elimination mechanism occurs. This makes determining the mechanisms for substitution and elimination very different processes. 

Favored by strong, negatively charged bases, especially OH and OR. The reaction occurs with 1 2°, and 3° alkyl halides. Order of reactivity  

Much less useful because a mixture of Sn1 and E1 products usually results. 

This mechanism does not occur with 1 ° RX because they form highly unstable 1° carbocations. 

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