Elements of Mechanisms

Mapping

Compare the structures of reactants and products. Consider the nature of any reagents or solvents that are specified. Answer questions like these:

What is the net reaction? Classified by overall structural changes, only four types of organic reactions exist:

substitution: one atom or group replaces another;
addition: two molecules or molecular fragments stick together;
elimination: a small fragment is lost from a larger molecule;
reorganization: the bonding pattern changes, but no atoms are added or lost.

Which bonds are broken and made?

Is the carbon skeleton of a reactant altered?

What is the reaction map? That is, which atoms of the products correspond to which atoms of the reactants, reagents, or solvents?

Are reactant atoms missing from products? Have "extra" atoms appeared? Where did they go? where did they come from?

What is the nature of the reagents and with what kinds of reactions are they typically associated. Are they acids? Bases? Oxidizing agents? Radical initiators?

Answer the questions of the previous bullet point for the solvent. Is it nucleophilic? Electrophilic? Hydrogen bonding? Aprotic? All these can give clues as to the type of reaction to expect.

Is a reaction temperature indicated? Remember that high temperatures or irradiation usually are required to initiate homolytic processes.

Write down your answers to these questions!

I work mechanism problems with a pile of scratch paper, transferring the result neatly to the working paper only when I believe I've solved the "mystery.

Here are some reactions to practice on. Don't try to write mechanisms, yet. Just answer the questions above.

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