SN1 vs SN2. E1, E2, SN1, or SN2? [with printable chart]

SN1 vs SN2. Is it SN1, SN2, E1, E2?

This is such a common question, not only for students but on exams too.  How do you tell if it is SN1 vs. SN2? 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 SN1 vs SN2 and whether it is SN1, SN2, E1, E2:

1) Do you have a strong nucleophile?  If you do, it will favor an SN2 reaction in the SN1 vs SN2 fight.  If it is a mediocre nucleophile, it will favor an SN1 reaction.  This is because of the two mechanisms.  In the SN1, we have an open position (carbocation), so any old nucleophile can just waltz in and form a bond.  In the SN2, we are pushing off the leaving group, which requires a stronger nucleophile.  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).  Bases want to take protons, which leads to elimination.

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.  This is because a protic solvent is more likely to stabilize a carbocation intermediate and therefore promote the E1/SN1 pathway.

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.

The SN1 vs. SN2 video:

Here are two videos were are particularly fond of that coach you through the way to decide which of these things it is.

Some Examples:

Is it SN1, SN2, E1, E2?

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.

  • Nucleophile: Cl is good but not great. Mediocre Nu = SN1.
  • Basic: NaCl is not basic.  No base = SN1/SN2.
  • Leaving group: OTf is a dynamite leaving group.  Awesome LG = SN1.
  • Solvent: tBuOH is a polar protic solvent = SN1.
  • 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.

organic chemistry help

How about this one:

SN1 vs SN2 reaction

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:

  • Nucleophile: CN is a great nucleophile.  Great Nu = SN2
  • Basic: NaCN is not basic.  No base = SN1/SN2, but we already knew that.
  • Leaving group: Cl is a decent leaving group. Decent LG = SN2
  • Solvent: acetone is a polar aprotic solvent = SN2
  • 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?

Reference: E1 E2 SN1 SN2 chart 

Dr. Michael Pa got a bachelors degree in chemistry from Binghamton University, a masters degree in organic chemistry from the University of Arizona and a Ph.D. in organic chemistry from the University of Arizona. His research focus was on novel pain killers which were more potent than morphine but designed to have fewer side effects. There may even be a patent or two that came out of it. Prior to all of this, he was a chemist at Procter and Gamble. After all of that, he (briefly) worked as a post-doctoral assistant at Syracuse University, working on novel organic light-emitting diodes (OLEDs). In between, he did NOT compete at the 1996 Olympics, make the Atlanta Braves opening day roster, or become the head coach of the Indiana Pacers, as he had intended. #fail During this entire time, he always loved helping students, especially if they were struggling with organic chemistry. In 2006, Dr. Pa founded in order to make learning organic chemistry fast and easy. 14 years and about 60,000 students later we are still helping students to learn organic chemistry one reaction at a time at