Is it safe to say that all elimination reactions (E1 and E2) involve bases (since in elimination reactions, a hydrogen is being removed, and thus something that accepts a $\ce{H+}$ is called a base)

Whereas, In substitution reactions, bases are not involved.

To summarise:

  1. substitution reactions- can involve bases/nucleophiles/both

  2. elimination reactions- must involve a base, but the base can also be a nucleophile (i.e. elimination reactions cannot occur if you only have a nucleophile)

  • 4
    $\begingroup$ All nucleophiles are bases and all bases are nucleophiles. It's just a question of how good they are at being a base versus a nucleophile. $\endgroup$
    – bon
    Commented Nov 12, 2015 at 22:10
  • $\begingroup$ The term base usually has a definition that I don't think a thiol or a phenyl group would meet, but both of which can act as nucleophiles $\endgroup$
    – Beerhunter
    Commented Nov 13, 2015 at 0:16
  • $\begingroup$ Show me the base in this reaction $\ce{Mg + (CH2Cl)2 = C2H4 + MgCl2}$ $\endgroup$
    – permeakra
    Commented Dec 13, 2015 at 18:03

1 Answer 1


There are two possible definitions of base:

  • Substance which accepts a proton throughout the course of the reaction (catalyticly or stoichiometricly). This is basically the Brønsted definition. To accept a proton, the molecule in question needs a lone pair whose energy level and orientation is in a good range for a proton to attach (e.g: ammonia: good; phosphane: bad). We could also extend the definition Lewis-style.

  • Substance which generally may accept a proton in reactions. This is still basically the Brønsted definition only without the requirement of it actually happening. The molecule needs a lone pair whose energy level and orientation is in a good range for a proton to attach even if it doesn’t happen. We could also extend it to a Lewis-style definition.

Applying the first definition, nucleophilic substitution reactions usually do not require bases. However, the molecules that attack are generally bases: A nucleophile generally needs an accessable lone pair, as would a base.

Consider the case of an alcoholate. If it is methanolate, it is small and can attack nucleophilicly. However, we also know that it can act as a base to be protonated to generate methanol. Depending on the substrate, it would choose to do substitution or elimination reactions.

If the alcoholate is tert-butanolate, we have a large molecule. Technically, it still could attack as a nucleophile. However, it is usually too big to attack and rather abstracts protons to allow elimination reactions.

You cannot really draw a line. Both are bases, both could be nucleophiles.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.