Dienes and MO Theory
By James Ashenhurst
What To Expect In Organic Chemistry 2
Last updated: March 26th, 2019
A large part of organic chemistry 1 is devoted to laying the foundations: introducing structural concepts such as bonding, geometry, stereochemistry, conformations, resonance, and steric effects, while introducing concepts in chemical reactivity such as nucleophilicity, electrophilicity, acidity, basicity and so on.
While there are several important new concepts introduced in Org 2 (especially in the context of pi-bonding) you will find that it largely builds on the foundations of Org 1 and assumes that you fully understand these concepts. The focus in Org 2 will be much more on reactions.
If I had to name an overall theme for Org 2, it would be : “the chemistry of pi bonds.” Understanding resonance will be very important!
Here is a brief introduction to some of the highlights of Org 2. By the end of the course, the answers to the following questions hopefully won’t seem as mysterious.
Why is it that diene A is more stable than diene B? How is it that replacing a single substituent on an alkene can drastically affect the reactivity? For example, why is it that alkene C does not react with electrophiles such as alkyl halides, but alkene
D does. And alkene E does not react with nucleophiles, but alkene F does. How do we explain this?
2. Thermodynamic and kinetic control.
In the addition of HBr to an isolated alkene such as, say, 1-butene, there is only one product possible (not counting stereoisomers). However, if we add HBr to butadiene, we can get two products, A and B. At low temperature, we obtain A as the dominant product, but at high temperature, B is dominant. Why is this?
3. Cycloaddition reactions.
When the diene (cyclopentadiene) is added to ethene, no reaction occurs. But when we use the middle alkene instead, there is a rapid reaction to provide a new cyclic compound. What is different about this alkene that allows this reaction to occur? And why will this alkene react readily with cyclopentadiene, but when it is added to ethene, nothing happens?
Normal alkenes (like cyclohexene) give trans dibromides when treated with Br2. But the cyclic triene in the middle (benzene) doesn’t react with bromine at all (unless you add a catalyst, and even then you get a different product!) Why the difference in reactivity? And why is this behavior similar for molecules like furan, pyrrole, and pyridine, but a molecule like cyclooctatetraene (bottom) will also add Br2?
5. Carbonyl chemistry
Grignard reagents react readily with aldehydes and ketones, but not amides. What causes the difference in reactivity?
When an alcohol is added to a carboxylic acid, the flask has less mojo than a Shaker meetinghouse. But when just a drop of acid is added, they readily combine to form an ester. Why is this?
Your body is made up of proteins, sugars, and fats. What do these molecules look like? What are their properties? How can we make them in the laboratory, and potentially modify their structures?
It’s vitally important to: make sure you have a firm handle on the key concepts and reactions from organic 1, because it will be assumed that you know it all.
The biggest challenge will be: getting a handle on the large number of reactions to learn, especially in carbonyl chemistry.
It will make your life easier if… you look for underlying trends and themes and see that underneath what appears to be tremendous variety, there are really only a handful of mechanisms.
The most important skill the course will provide: being able to design and plan multiple-step syntheses of relatively complicated molecules. Furthermore, the course will give you a much greater understanding of the chemistry of biologically important molecules such as amino acids, lipids, and sugars. It will be a lot of work, but you’ll see the world around you in a different way.
Edit – fixed some typos. Thanks to Prasanna for pointing them out.