To shift or not to shift - that is the question!

November 27, 2018
Flash PointsSarah Wegwerth

An interactive guide is available here

It’s that time of the year, the final stretch of first semester. To help prepare for finals, we’ve asked faculty what the most difficult concept is for students in Organic I and put together some practice problems. The topic: hydride and alkyl shifts.

Download Mechanisms from Google Play Store or the App Store

As I reflect on my time as a student, I remember mechanisms that incorporated a carbocation rearrangement to be particularly challenging. If you’re struggling with this concept as a student too, let’s take some time to talk through how to master this concept.

First and foremost, it is important to review carbocation stability.

Remember:

  1. Electron donating substituents (such as alkyl groups) stabilize carbocations, so more substituted carbocations are more stable
  1. Resonance delocalization also stabilizes carbocations

The goal with performing a hydride/alkyl shift is to form a more stable carbocation. When looking at a carbocation, get in the habit of looking at the neighboring carbons and see if moving either a hydrogen or an alkyl group would result in a more stable carbocation. We will be using the intermediate of the Addition 17 puzzle in the Mechanisms app as our guide.

Tips for evaluating if a carbocation will rearrange:

  1. Draw out the implicit hydrogens on the carbons next to the carbocation. You can check this this by tapping on the carbons in the Mechanisms app.


  1. Draw out the new carbocation(s) and compare with the original. Many students complain this takes too long BUT you are much less likely to make a mistake if you draw it out. You can use the Mechanisms app to test out your hypotheses about stability

Practice this process using the first five questions on the worksheet, each of them have a corresponding puzzle in the Mechanisms app. Eventually with enough practice you may be able to do this evaluation in your head. But start by drawing things out and running some tests in the app.

Now for the tricky part, catching when a carbocation rearrangement happens during a mechanism. The key to catching these is getting in the habit of seeing a carbocation and evaluating if a shift from a neighboring carbon will result in a more stable carbocation. Practice, practice, practice so that this becomes instinctive!

Let’s take this one step further, catching when a rearrangement occurs for “predict-the-product” questions. This means you need to know which reactions go through a carbocation in the mechanism. That is why it is so important to learn, practice, and know the mechanisms for the reactions you’ve covered!

If you can quickly recognize a specific reaction goes through a carbocation, then it is a signal to you that you will need to evaluate if a rearrangement will occur. For example, if you are asked to predict the product of an SN1 reaction, pause and examine the carbocation. It is highly recommended that to do this you draw out the mechanisms, or at a minimum the carbocation, then decide if a shift is favorable. Again, this takes a few seconds extra time to draw out another structure BUT the trade off is you are less likely to make a mistake! You can try Substitution 17 puzzle in Mechanisms as practice.

I can’t emphasize enough how knowing the mechanism will help you reason through problems. However, if you rely on rote memorization of the steps of the mechanisms you will miss the carbocation rearrangements. In fact, these shifts are so tricky because they only happen sometimes and are an additional layer to the mechanisms involving a carbocation intermediate. There are many recommended puzzles in Mechanisms to help you master this added layer at the end of the worksheet.

As you study for your final, work on understanding each step of a mechanism. This will help you develop your mechanistic reasoning and will help you greatly as you get into more complex mechanisms during second semester. After all, mechanisms are used by research organic chemists to understand how the reaction occurs and or reason why an unexpected product was formed during a reaction.

We produced the guide for you to use! You can download the pdf here or view the online version! Please feel free to use it in your studies and share with your classmates! Good luck on finals this semester!

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