Determining SN1 Versus SN2 Reactions
Substitution reactions may seem like a difficult concept to understand. However, as long as you follow these guidelines, you will be successful. When you first see a substitution reaction, you should answer these questions.
I will explain the questions in detail and then I will show you a practice problem.
Is the carbon bonded to the leaving group primary, secondary, or tertiary?
Observe the carbon bonded to the leaving group. To figure out if it is primary, secondary, or tertiary, look at the carbon bonded to the leaving group and count how many carbons are attached to it:
If 1 carbon is attached, we have a primary carbon.
If 2 carbons are attached, we have a secondary carbon.
If 3 carbons are attached, we have a tertiary carbon.
Primary carbons can only be SN2 substitutions. Tertiary carbons can only be SN1. Secondary, benzyllic, or allylic carbons can be either SN1 or SN2.
Here are examples of the types of carbons to look for.
2. Is the nucleophile strong or weak?
Strong nucleophiles have negative charges but exceptions to this rule are halogens with negative charges and resonance stabilized negative charges. Strong nucleophiles indicate SN2 reactions while weak nucleophiles indicate SN1 reactions.
Strong nucleophile examples are CN-, OR-, OH-, RS-, NR2-, R-.
Weak nucleophile examples are RCO2–, HOR, H2O, HSR, HNR2, I–, Br–, or Cl–
Is this reaction SN1 or SN2?
Now that we have gone through the steps to answer a SN1 versus SN2 question, we will now go over a practice problem and answer the questions listed.
Is the carbon bonded to the leaving group primary, secondary, or tertiary? Looking at the leaving group Br, we see that the carbon bonded to the leaving group is a benzylic carbon. This means that this reaction can either be SN1 or SN2, so let's move on to the next question.
Is the nucleophile strong or weak? The nucleophile is CN- in the reaction. CN- is a strong nucleophile which indicates a SN2 reaction.