Hi Madison,

This is a little challenging to go over text, but I will do my best.

For this question, I focused on where the position of the S-COA is and counted carbons from it. It doesn't matter which S-COA is removed to make compound 2, since either or you would end up with the same carbon that is two carbons away from the S-COA being labelled.

You see from reaction 1 that 2 carbons from the sulfur, we find a labelled carbon. This corresponds to carbon 2 in the question. I found it helpful to keep an eye on that carbon. This helped me to remove answer choices A and B.

Lets look at the differences in the answers now. There is a difference in either carbon 3 or carbon 4 being labelled for answers C or D. If you were really short on time, getting to this far is pretty good, as you now have a 50:50 chance of getting it right, so you could take a guess and move on. If you got a bit more time, you can definitely scrutinize the question a bit more.

Since carbon 3 would be a carbon right next to carbon 2, the starting materials would need to have added onto one another in a way that the labelled carbons were right next to each other. Keeping your eyes on the oxygens helps to keep a track of what carbons are moving where. For the two molecules to add together to give the structure we see in compound 2, we would have to have appended the labelled carbon from acetyl CoA onto the carbonyl (unlabelled) of acetoacetyl CoA, generating a "hanging" CH3 group which is labelled. I knew this because I was keeping an eye on the oxygens and their positions with the carbons, and the only way it made sense for the two compounds to come together, would be through that method. Since the labelled carbon is not directly attached to C2, I know that therefore, C is not the correct answer, and D is the right answer.

Hope that helps. Let me know if you are still confused.

Molly