sn1/sn2/e1/e2 – Substrate

Having gone through the basics of the SN1, SN2, E1 , and E2 reactions, now it’s time to start putting it all together.

Here’s the question: If we’re given an alkyl halide, how do we know what’s going to happen?

One of the tricky things about this exercise is that we are dealing with multiple variables. How do we know which one is important? For instance, how do we know when to pay attention to the nucleophile, and when to pay attention to the solvent?

Instead of biting this all off at once, I think it’s good to have a system: a problem solving matrix that helps you untangle each of the individual components of the reactions, one by one.

Here’s a handy mental trick. It will be far more helpful to us to rule things out than to look for confirmation. You want to discard possibilities first, and then work from there.

So today, let’s just focus on one component as we examine 4 different reactions.

The most important component for understanding whether a  reaction is SN1/SN2/E1/E2 is the substrate.

Here’s the key question: Is the alkyl halide primary, secondary, or tertiary?

Recall that the “big barrier” for the SN2 is steric hindrance. Since steric hindrance is greatest for tertiary alkyl halides, you can rule out the SN2 if you encounter this situation. That’s the case for the third and fourth examples below. 

The “big barrier” for the SN1 and E1 is “carbocation stability”. Since primary carbocations are the least stable, you can safely rule out the SN1 and E1 for primary alkyl halides. That’s the case for the first example. 

Finally – the E2 doesn’t have a big barrier. So we can’t rule it out based on substrate.

Here’s what it looks like when we apply this rule to 4 different reactions. Note that in the first one, we’ve ruled out SN1/E1, and the third and fourth we’ve ruled out SN2. That’s all we can do for now.

Tomorrow, let’s look at the nucleophile!

Thanks for reading! James