Mechanisms Made Easy – The Reaction Scheme
Our focus will now shift from our nucleophile/electrophile pair (Mechanisms Made Easy Part 1) to the overall reaction scheme. The nice thing about mechanism questions is that you always have the road map right in front of you – where to start and where to finish. Here are a couple tips to keep in mind:
- Number all your carbon structures
- See what has changed from the starting structure to the final product
- Find where the starting structures “fit” into the final product
Let’s look at an example to demonstrate these tips:
Numbering the Structures
Start by numbering the carbons in your starting structure, and number any carbon structures above and/or below the arrow.
Fitting the Structures Together
In this case, we can see that the two carbon molecules have simply been put together. Next, see what has changed and where your starting compounds might fit into your final structure. This will give you a reference point to “overlay” with your final structure.
If we flip the two-carbon aldehyde (acetaldehyde), it fits perfectly on the left side of our final structure. Therefore, the three-carbon aldehyde (propanal) must be what is left over. Now, we easily see how the two aldehydes were put together.
If it helps, you can rotate the aldehyde and write in the hydrogen that was part of this initial aldehyde.
Changing the order of reagents
A common question I get is “what if you changed the order?” Well, then this would be the reaction scheme:
Therefore, we need to line the structures up this way.
You will notice that in each example, one of the aldehydes is essentially “unchanged”, meaning that you can take the structure and place it on top of the final product.
Coming up Next…
Make sure to check out Mechanisms Made Easy-part 2.2. We will be looking at a couple more complicated examples.