Todd:Chem3x11 ToddL4
Chem3x11 Lecture 4
Under construction Sat Apr 28.
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Key concepts
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Why do we Study Reaction Mechanisms?
A reaction mechanism is the detail of how/why one molecule becomes another. Why do we care? Three reasons:
1) Truth - even if a reaction is well-behaved, we're human being and like to understand the universe.
2) Complexity - in many cases reactions do unexpected things. Reactions never "don't work" - they either give back unreacted starting material, give the expected product, give something else, or give mixtures of things. Frequently we want to understand reaction mechanisms so we can understand complex reaction outcomes - the "Why did that happen?" question.
3) Prediction - if we understand reaction mechanisms we can start to make intelligent predictions about how to make new reactions work.
Stereochemistry is useful in the study of reaction mechanisms. If we compare the stereochemistry of starting materials with the stereochemistry in the products, we can deduce things that must have happened in a reaction mechanism even if we can't see those things directly. This forms one very useful part of the study of mechanisms (when coupled with things like spectroscopy and kinetics).
In this lecture we'll talk about the standard polar organic reactions that you've seen already.
The first thing to say, by way of introduction, is that we need to remember that reactions don't just happen as if by magic between the two ions. The nucleophile and electrophile have molecular orbitals on them that need to align for a reaction to occur. In the case of polar reactions an electron pair in a filled molecular orbital on the nucleophile needs to overlap with an empty molecular orbital on the electrophile. Good overlap (giving a strong bond) requires these orbitals be close in energy.
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