It’s not often that it comes up, but when it does, the temperature is an important clue.
One important thing to know: heat favors elimination reactions. Why? It’d take a paragraph or two to answer that, so I’ll refer you to here. Condensed version below.*
The bottom line here is that if you’ve asked yourself about the substrate, the nucleophile, and the solvent and still don’t have an answer about SN1/SN2/E1/E2, then look at the temperature. If there’s heat, it’s likely an elimination reaction. If heat isn’t applied, it’s likely substitution.
That’s it. Let’s have a last look at the examples we’ve been looking at.
So example 3, using H2SO4 and heat, is an elimination reaction (E1)
Example 4, using CH3OH and no heat, is a substitution reaction (SN1).
Not sure where to put this, so I’ll put it here: another important clue to the E1 is the use of H2SO4. The conjugate base of H2SO4, HSO4(-) is a poor nucleophile and does not commonly participate in SN1 reactions. Something to watch out for.
Tomorrow, I’ll talk about exceptions to this 4-question process.
Thanks for reading! James
P.S. *short version. As compared to substitution, elimination leads to the formation of an extra species in solution, which increases entropy. Entropy for elimination reactions (delta S) is therefore higher than that for substitution reactions. This means that as you heat the reaction, the TdeltaS term in the gibbs equation will start to dominate at high temperatures, making delta G more negative and therefore favoring the reaction. This video might help.