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Lecture 1

CHE 220 Lecture Notes - Lecture 1: Unpaired Electron, Pi Bond, Hyperconjugation


Department
Chemistry
Course Code
CHE 220
Professor
Andy Mitchell
Lecture
1

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The Golden Rules of Organic Chemistry
Your goal should be to understand, not memorize organic chemistry. The following 7 Golden
Rules should be learned at the beginning of this semester. These simple ideas explain a very
large number of things about the way organic molecules interact. Thus, understanding the 7
Golden Rules will allow you to develop an intuitive feel for organic chemistry, and things will
make sense! (Warning: this means you will start thinking like a chemist, but, of course, no one
needs to know if you don't want them to know.)
1. Atoms prefer filled valence shells. This rule explains why atoms make bonds, and the
type of bonds created. A corollary is that centers of electron density (bonds and lone pairs of
electrons) repel each other so they stay as far apart as possible. This latter rule, the basis for the
so-called VSEPR model, explains 3-dimensional molecular structure.
2. The most important question in chemistry is "Where are the electrons?"
The answer is that electrons are generally in higher amounts around the more electronegative
atoms (e.g. F, Cl, O, N). The electronegative atoms pull electron density away from the less
electronegative atoms (e.g. C, H) to which they are bonded. Thus, understanding
electronegativities provides a simple method of deciding which portions of a molecule have a
relatively high electron density, and which portions have a relatively low electron density.
3. Nature hates unpaired electrons. If a molecule must have an unpaired electron (a.k.a.
radical), it is better to have the unpaired electron distributed over as many atoms as possible
through resonance, inductive effects, and hyperconjugation.
4. Nature hates localized charges. If a molecule must have a charge, it is better to have
the charge distributed over as many atoms as possible through resonance, inductive effects, and
hyperconjugation. In addition, when given the choice, it is better to have more negative charge
on a more electronegative atom (e.g. O), and more positive charge on a less electronegative atom
(e.g. C).
5. Most reactions involve nucleophiles (molecules with a location of
particularly high electron density) attacking electrophiles (molecules with a
location of particularly low electron density). When in doubt, transfer a proton! Thus,
simply understanding where electrons are provides you with the best way of analyzing new
molecules so that you will be able to PREDICT how they will react.
6. Steric interactions (atoms bumping into each other) can prevent reactions
by keeping the reactive atoms away from each other.
7. Pi electrons prefer to be delocalized over as many adjacent sp2 hybridized
atoms (or sp hybridized atoms in some cases) as possible, and aromaticity is
the most stable form of pi electron delocalization. Pi electrons cannot delocalize onto
or through sp3 hybridized atoms since an sp3 atom has no 2p orbital available.
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