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Leaving Groups and Nucleophiles

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The Leaving Group

The leaving group is a component of every substitution and elimination reaction discussed in this SparkNote. As such, it makes sense to learn the characteristics of a good leaving group.

In any substitution or elimination reaction, electrons from a nucleophile, carbon-hydrogen bond, or the solvent break a carbon-leaving group bond. Here the leaving group is abbreviated as "LG."

As you can see, the leaving group is aptly named; it is the group that leaves.

There is a bit of terminology dealing with the leaving group important to substitution and elimination. The α -carbon is the carbon atom bonded to the leaving group. β -carbons are attached to the α -carbon. The hydrogens attac hed to the β -carbon are called β -hydrogens. This terminology is vitally important for our discussion of substitution and elimination reactions.

Ranking Leaving Groups

Let's define a good leaving group as one that leaves easily. Then the effectiveness of a leaving group increases with the group's energetic stability after it has left. Thus a weak base is a better leaving group than a strong base. Likewise, a m olecule that is neutral after leaving is generally a better leaving group than one that is negatively charged after leaving.

Halides and the tosyl group (-OTs) are examples of commonly used leaving groups. In general, if the group is relatively stable after leaving the molecule with the C-LG bond's electrons, it's a good candidate for a leaving group.

The Nucleophile

The nucleophile is a key part of every substitution reaction. In these reactions, it is the group that "substitutes" for the leaving group. A nucleophile has a lone pair of electrons that makes up the molecule's business end. A polarizable nucleophile contribute more negative charge from its lone pair and has more punch than its non-polarizable fellow. On the same token, good nucleophiles tend to be negatively charged, but can also be neutral.