Is it E1, E2, SN1, SN2??
This is such a common question, not only for students but on exams too. How the heck do you tell the difference between an E1, E2, SN1, SN2 reaction? Check out the chart below to start.
The factors that will decide:
1) Do you have a strong nucleophile? If you do, it would favor an SN2 reaction. If it mediocre, it will favor an SN1 reaction. What is a strong nucleophile? Check out these blog posts on strong nucleophiles and strong electrophiles.
2) Does your nucleophile double as a base? If yes, it is going to favor elimination (E1/E2) over substitution (SN1/SN2).
3) How good is your leaving group? If it is awesome, it is more likely to be a carbocation intermediate, ie E1 or SN1 reaction. If the leaving group is only OK, that means it has to be forced off and is more likely to be a concerted reaction mechanism like SN2 or E2.
4) What is your solvent? Polar protic solvents will stabilize a carbocation better, therefore promote an E1 or SN1 reaction. Polar aprotic solvents favor SN2 and E2.
5) What kind of substrate do you have? If your starting material is a tertiary substrate, you are definitely E1 or SN1. If it is a primary substrate, you are definitely SN2 or E2. If it is a secondary substrate, it could go any one of the ways.
Let’s look at an example.
This is an easier example, but let’s start with it. Here is the most important thing to see: The product has OTf substituted, NOT eliminated. Just by looking at the product, we know it has to be an SN1 or SN2 reaction NOT an E1 or E2 reaction. Therefore, when we look at the different factors below, we are going to ignore E1 and E2.
1) Nucleophile: Cl is good but not great. Mediocre Nu = SN1
2) Basic: NaCl is not basic. No base = SN1/SN2
3) Leaving group: OTf is a dynamite leaving group. Awesome LG = SN1
4) Solvent: tBuOH is a polar protic solvent = SN1
5) Substrate: It’s tertiary at the leaving group = SN1
All of the factors point to an SN1 reaction, therefore I feel comfortable saying it is an SN1 reaction.
How about this one:
This is still clearly a substitution, but it’s on a secondary substrate, so it could go either SN1 or SN2. Here are the factors:
1) Nucleophile: CN is a great nucleophile. Great Nu = SN2
2) Basic: NaCN is not basic. No base = SN1/SN2, but we already knew that.
3) Leaving group: Cl is a decent leaving group. Decent LG = SN2
4) Solvent: acetone is a polar aprotic solvent = SN2
5) Substrate: It’s secondary at the leaving group = SN1 OR SN2
Almost all of the factors point to an SN2 reaction, with the notable exception of the type of substrate. I still feel comfortable saying it is an SN1 reaction.
What do you think? What is the most difficult substitution/elimination problem you have seen on an exam or in class?