As you were going through substitution and elimination puzzles, did you notice how similar the reagents were? Did you wonder why the products are so different? Here’s the thing, science is not always as cut and clear as people think. The reality is, often times substitution and elimination reactions are in competition with each other. How do we know? By looking at the products. Sometimes in your reaction mixture you will find multiple products are present. Usually though one is favored over the other and you can play around with conditions to alter the ratio of the products. How do you know what conditions to alter? To answer that question, you need to understand the mechanism and what drives each pathway.
Most of you will not be working in a lab though, and while the above is interesting (ok well at least in my opinion) you are more likely to be asked to predict the major product of the reaction. The first step is to identify the reagent and what it is good at doing. Is it a good nucleophile? A good base? In many cases this will help narrow down the possible pathways. Then look at the substrate (the molecule undergoing either substitution or elimination).Is the carbon bearing the leaving group primary, secondary, or tertiary? Usually this will provide the additional information you need to come to a conclusion.However, there are some cases where you will also need to consider the solvent and the temperature of the reaction.
To see this analysis process in action, watch the videos below. Then test your own mechanistic reasoning skills by predicting which pathway is most likely as you play through each of the recommended puzzles.