It’s a shame to see smart students lose points on an exam when they actually know the answer.
There’s an easily preventable mistake I often see students making when drawing answers to mechanism problems.
Today I’ll talk about a technique that can help you avoid making simple mistakes.Freelance Teacher Steven calls it the “Redraw and Modify” technique.
I just call it “Draw the Ugly Version First”.
Here’s the bottom line. Once you figure out where the arrows go…. ONLY do the things which the arrows tell you to do.
It sounds easy, but is surprisingly hard! But every arrow, once drawn (and drawn properly), gives specific instructions about the bonds that form, the bonds that break, and how the charges change.
Some students make the changes that the arrows tell them to do, and they say that they just don’t “feel” right. Unfortunately, feelings don’t come into this. Arrows are dictatorial!
One type of reaction I find students have a hard time with is reactions that form or break rings. This spooks them. So let’s use this as a perfect occasion to bust out “Redraw and Modify”.
Let’s take a look. Here, we’re adding NaOH to a ketone. This deprotonates one of the alpha carbons, forming an enolate. Next, the enolate is going to attack that carbon several bonds away.
Let’s number the carbons and draw the arrows first (I don’t care if the numbering isn’t IUPAC-approved or anything… it’s just to keep track of each individual atom).
Then, redraw the starting material on the other side of the reaction arrow, complete with curved arrows. We’re going to use the arrows to help us draw the product.
Look at arrow A. What’s it telling us to do?
- Form a bond between C7 and C2
- The charge on C7 will become more positive by 1 (from -1 to zero)
Now draw a line between C7 and C2. It doesn’t have to look nice. It’s just to show you that these are connected.
Let’s look at Arrow B.
Arrow B shows the tail at the C-O Pi bond, and the pair of electrons moving to the oxygen. So what it’s telling us to do is…
- Break the pi bond between C and O
- the charge on the O will become more negative by one (from zero to -1)
Now that we’ve formed and broken the bonds, we can make it look pretty. Count the atoms – we’ve formed a six-membered ring. So draw a hexagon. Then – slowly – atom by atom, “translate” your ugly drawing into the pretty drawing.
I like this becauase it’s methodical. If you follow the method, you’ll get the results. It helps to avoid errors from drawing products improperly, which can sometimes account for ~20% of the lost points on exams. It could easily make the difference between a B and an A.
Thanks for reading! James