# Nucleophiles and bases (elimination and substitution)

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)

• 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. – bon Nov 12 '15 at 22:10
• 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 – Beerhunter Nov 13 '15 at 0:16
• Show me the base in this reaction $\ce{Mg + (CH2Cl)2 = C2H4 + MgCl2}$ – permeakra Dec 13 '15 at 18:03

## 1 Answer

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.