Question

If substitution of a nucleophile has happened, does that mean it is an E or an SN reaction?

Answer 1

`S_NX` literally stands for "substitution nucleophilic X-order". X can be second order, meaning `S_N2`, or first order, meaning `S_N1`. Clearly, that means `E2` stands for "elimination second order" and `E1` stands for "elimination first order". It's just like kinetics.

First order just means that the reaction depends on the concentration of only one reactant. In other words, one species is participating, like in a half-life reaction, a unimolecular reaction.

Second order means that the reaction depends on two of these "first-order" reactants---a bimolecular reaction, or one "second-order" reactant.

You can see how `S_N2` emanates these second-order qualities. One nucleophile attacks one electrophile. That's two first-order reactants reacting in a bimolecular reaction (redundancy was on purpose).

Order corresponds to how:

`r(t) = k[A]`

is first order in `[A]` because the exponent is `1`.

or

`r(t) = k[A]^2`

is second order in `[A]` because the exponent is `2`.