Mechanisms Made Easy – More Complicated Examples
I know your confidence is much higher now in terms of mastering organic chemistry mechanisms. Way to go!!
So…what is next?
Last time (Mechanisms Made Easy Part 2.1), we began to look at how to put our structures together. Let’s take a look at a few more examples that are slightly more complicated…
Fitting the structures together
This reaction is known as the Robinson annulation. You can see that the complexity has increased dramatically; however, the same rules apply.
This time, we will skip the numbering and go straight to seeing how the structures fit together. If you look at the two carbon structures, you will see that the starting cyclohexanone (red structure) must fit here (on the “bottom”):We know this because the cyclohexanone ring does not have any substituents. Therefore, the unsaturated ketone (blue structure) must be the other six-membered ring.
Changing the starting structures
Let’s change the example to the following:Here are two possible ways to put the structures together:In this case, we do not have any real defining features to help us out. Therefore, the approach to this question will be slightly different. You will use electronegativity to assign partial charges to each atom.
Start with the oxygen atom and move to the other atoms, making sure to alternate charges.With all the charges, it is a bit overwhelming and hard to see. Let’s simplify by only showing the areas we are interesting in:
The partial charges reveal that the structures on the left are the only ones where the charges line up (negative – positive).
Make sure to pay attention to any substituents, such as the methyl group in the above example. These groups can be useful “landmarks” or clues to help you find the correct way to fit everything together.
It is time to start drawing arrows!
In Mechanisms Made Easy Part 3, you and I will be going over how to draw arrows. We will go over where the arrows should start and finish.