Substitution vs Elimination Reactions: The Key Differences

Alkyl halide reactions typically follow one of four pathways: S_N1, S_N2, E1, or E2. When completing synthesis problems or writing out reaction mechanisms, choosing the right pathway for the reaction can be tricky. That's why it is important that you have the best organic chemistry book available. Otherwise, if you were unfortunate enough to be cursed with a sub par textbook, I will try my best to explain the differences between nucleophilic substitution and beta elimination with this hub.

The first, most important step to choosing the right reaction pathway is taking into account both the reactants and the solvents involved.  So check whether your solvent is protic or aprotic and polar or nonpolar.  The type of solvent involved has a very big effect on the reaction, so it's important to check.  If you have any confusion about what type of solvent you're using, check out this solvent list from the University of Maine.

Next you will want to check the properties of your replacement group.  Is it a strong base or a strong nucleophile?  Strong bases have preference towards elimination reactions while strong nucleophiles favor substitution.

Last, you'll want to check the molecules themselves.  Primary and secondary alkyl halides typically favor fast S_N2 or E2 reactions, while the more strained tertiary alkyl halides favor E1 or S_N1.

So what you need to take home from this table is, that if it's a strong base (such as H₂N-, R-O-, or OH-) the reaction will most likely be elimination. If the reactant is a good nucleophile, but a poor base then substitution is a safe bet.

Protic, highly polar solvents favor S_N1 and E1, while aprotic, polar solvents will be found in S_N2 reactions. Following these simple tips will get you through the vast majority of nucleophilic substitution and beta elimination reactions.