The time has come. You are past the introductory chapters of your organic chemistry class and now must dive into one of the hard parts: synthesis problems. These types of questions can be intimidating at first because they rely on your knowledge of a variety of reactions and can be like little puzzles. However, there is no need to be scared. By following these steps below, synthesis problems will become your new best friend in no time.
Step 1: Take a deep breath.
Syntheses can be scary at first. When you look at the problem, you see a complicated starting material and product, without much information. Step back, take a breath, and repeat these words “I can do this.”. The first step of these problems is making sure you don’t let them get to your head, so you have a clear mind.
Step 2: Count your carbons
Count the number of carbons in the starting material and product. If the number is the same, great. If not, think about reactions that add carbons or remove carbons (as needed). There are only a few reactions that do carbon-carbon bond breaking and forming, so this is a great place to start.
Step 3: Label functional groups
If you have, grab a pack of colored pens or markers. For each functional group you observe in the starting material, circle it in a color and label its name and position. Change your color for each functional group. Then do the same for the final product.
Step 4: Compare your functional groups
Look at each functional group you circled in the starting material and see if its identity or position changed or stayed the same in the product. Write out a list of the changes.
Step 5: Use your knowledge of reactions
For each change between the starting material and product, think about the reaction you have learned that does that change. On the list you made in step 3, write out the reagents that are needed to make the change to the new functional group (Hint: a functional group change may require 2 reactions). Sometimes, the change is not due to a new functional group, but due to its new position. In that case, think about which bonds are broken and formed, and brainstorm reactions that do that.
Step 6: Think about reaction order
Think back to any restrictions you know about the reactions you have in your list from Steps 3 and 4. For example, is there one that must go first because it can’t react with a different functional group later? Try to order the reactions based on your best guess. Remember: trial and error is expected here and there may be more than one correct answer or order to get to the product!
Step 7: Rinse and Repeat
After you have your proposed synthesis in order, walk through it by drawing out which molecule is formed after each step. You may realize that one reaction doesn’t get you the product you want due to a rearrangement or a side reaction. Go back to step 3 and repeat the process.
The bottom line: learning to do synthesis problems is like learning a new language. It can be challenging and intimidating at first, but the more you practice the easier it gets. And the 7 steps above will always lead you in the right direction.