Key Reactions for Semester 2: A Review for your Final Exam

April 19, 2019
Julia Winter

The second semester of organic chemistry is chock-full of mechanisms, most of which have a carbonyl or C=O group as a central feature. We want students who use our Mechanisms app to understand the principles behind each of the moves in a mechanism: Why does the move happen? What is the driving force?  

Reading the textbook, practicing in the app, can get you only so far – the big step happens when you can explain every single step in terms of reactivity. When you do that, you move beyond memorizing procedures and can use mechanisms to solve new problems.

We want you to get to that point, and hopefully, before your final exam!

So, grab a bunch of paper or find a big whiteboard, because in order to reach this epiphany in mechanistic-reasoning, you are going to need to practice! Physically drawing out the steps helps with your understanding.Our research has shown that the Mechanisms app is another tool in your learning toolbox, so have it available to help with drawing both the structures and the arrows of organic chemistry mechanisms.

We added closed captions to the videos in the app, so you can see and hear the words as you review the mechanisms. The three reactions chosen below come from the free section of Mechanisms.

Imine formation

Questions:  

1)     Why is the acid content kept low? And which step requires an acid?

2)     What are other resonance structures you can draw as the reaction proceeds?

3)     What functional group transformation occurs if the imine is treated with a reducing agent? (Think about this transformation to practice for synthesis questions.)

Other puzzles to try: Essentials – Carbonyl 5 (an enamine formation), Carbonyl 18 (another imine), Aldehyde and Ketone 6.

Esterification

Questions:

1)     What does the initial protonation step accomplish?

2)     Why does the -OH not leave when the methanol attacks, i.e. from the C=O?

3)     What does it mean to be a good leaving group?

Other puzzles to try: Essentials – Carbonyl 6 (hydration of acetyl chloride), Carbonyl 7 (base hydrolysis of an ester), and in the Carboxylic Acid section – 3, 4, 11, and 12 (all related reactions)

Aldol reaction

Questions:

1)     Why is the alpha proton acidic?

2)     Why is the hydroxide ion a catalyst in this reaction?

3)     What does in mean to be “driven by stabilization”in the last step?

Other puzzles to try: Essentials – Carbonyl 11 and 12 (both aldol reactions), and Carbonyl 14 (alpha-chlorination of a ketone)

A favorite reaction from my own final exam.

Predict the product and draw the mechanism.

Editor

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