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.
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:
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–
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.