Elements of Mechanisms

Writing Mechanisms

Organic reaction mechanisms should be written as a sequence of elementary steps, or elementary processes, each of which represents the simplest possible way of writing a particular bond breaking or making.
Keep the following guidelines in mind to be sure each step is a valid elementary process:
1. Each step should involve no more than two molecules or ions; three-body collisions almost never happen.
2. Each step, except in a few rare pericyclics, should make or break no more than four bonds.
3. Each individual step must balance, not just atoms, but electrons and charges as well.
4. Every species you write should be either a given reactant or a product of an earlier step.
5. All intermediates must be plausible - either stable molecules or recognized transient intermediates. Implausible structures include the following:
  - More than 8 electrons on an element from the first full row of the periodic table;
  - Any atom more than 2 electrons short of an octet;
  - Two like charges on the same ion, except for metals;
  - Two unlike charges on the same ion, except for ylids (charges adjacent) and recognized zwitterions, such as those of amino acids;
  - More than two unpaired electrons.
6. Every step of every heterolytic reaction mechanism is either a Lewis acid-base reaction or a Bronsted-Lowery acid-base reaction.
Your drawings of reactants and curly arrows for each step should be explicit enough that a reader could generate the structure of the product from that information alone. The curly arrows supply a set of instructions for transforming reactant to products in each step.
1. The electrons to be shifted by each arrow or fish hook should be shown explicitly.
2. Each arrow or hook should have its tail located at the pair of electrons to be moved and its head pointing clearly to their new location.
3. Curly arrows and fish hooks move electrons only, NEVER atoms!
Return to Questions 1 - 15, and write complete mechanisms for all reactions.
Finally, let's put the entire package together at one time. For each of the following reactions, complete mapping and stereochemical analyses, draw stereoprojections as necessary, identify the fundamental mechanistic class, and write a carefully detailed mechanism. Since we have largely ignored pericyclic processes in the other problems, each of these reactions involves at least one pericyclic step.

Answers
References	Problems 1, 2	Problems 3, 4
Problems 5, 6	Problems 7, 8	Problems 9, 10
Problems 11, 12	Problems 13, 14	Problems 15, 19
Problems 20, 21	Problems 22,23

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