Question

Why does the elimination of hydrogen halide (a halogen acid) from an alkyl halide require a strong base?

Answer 1

It is useful to distinguish between a good nucleophile and a strong base. A good nucleophile targets an electrophilic center, whereas a strong base can easily acquire a proton from a proton source.

Targeting an electrophilic center is one of the first steps to substitution, while acquiring a proton is one of the first steps to elimination.

Therefore, you often see good nucleophiles in `S_N1` and `S_N2`, but you often see strong bases in `E1` and `E2`.

An example is tert-butoxide with heat on, let's say, a primary alkyl halide. This elimination may also be called `beta`-elimination, because it involves grabbing the proton on the `beta`-carbon.

Tert-butoxide is a strong base (more alkyl groups tends to increase basicity, not to mention it is negatively charged) but a terrible nucleophile (due to the noticeable steric hindrance).

Therefore, the reaction is slow, and heating the reaction helps to both speed it up and minimize the likelihood of substitution ever occurring.

Lastly, acquiring the proton at an antiperiplanar conformation allows for the optimal elimination of the proton and the halide.