Writing Mechanisms

Understanding the mechanisms by which organic reactions occur is one of two keys to organizing reaction information (the other is functional group identification). Simply trying to memorize all of the reactions is the road to disaster; with 35 million organic compounds out there, each reaction will look new and different unless you recognize the underlying categories into which functional groups and mechanism types place the reactions.

You must try very hard not to regard reaction mechanisms as a kind of magic, with no rational foundation, and attempt simply to memorize all of the steps. This is a dreadful mistake.

Reaction mechanisms are the most logical and easy-to-understand part of organic chemistry, if one only takes the time to think about what is happening in each step.

Here is the key idea: every individual step of every mechanism is either a Bronsted or a Lewis acid-base reaction!

As you study the reaction mechanisms presented in class and in the text, you should do so with pencil or pen in hand, and actually write out the steps for a specific set of reactants and products.

Try to identify the role played by each species in each step: which species is the acid, which the base, and so on. Here are some more things to keep in mind.
1. Each step should involve no more than two molecules or ions, and should make and break no more than four bonds.
2. Each species you write should be either
  - a given reactant, or
  - a product of an earlier step.
3. Each step must balance with respect to
  - atoms;
  - electrons; and
  - charges.
4. Your drawings of reactants and curly arrows for each step should be explicit enough that a reader could supply the product if it were missing. That is, the curly arrows supply a set of instructions for transforming reactants to products in each step.
5. The electrons to be shifted by each curly arrow should be shown explicitly, and should be either an unshared pair or a bond, lying at the tail of the arrow. The head of the arrow points to the location of a new bond or the new location of an unshared pair. [Here is a page on using curly arrows!]
6. All species intermediate between the reactants and products must be plausible - either stable molecules or recognized transient intermediates. Practically this limits the possibilities to:
  - conjugate acids or bases of reactants or products, or previously produced neutral intermediates;
  - Lewis acid-base complexes;
  - carbocations, carbanions, free radicals, or carbenes;
  - neutral organic molecules.
7. The following are characteristics that identify implausible intermediates:
  - more than 8 electrons on a first row element;
  - a first row element more than two electrons short of an octet;
  - two positive or two negative charges on the same organic molecule;
  - opposite charges on the same molecule, except in well-recognized cases;
  - two or more unpaired electrons on an organic molecule.
The overall reaction should match, both in outcome and in the roles played by the various species, one of the ten types that we will study this year:
1. Radical Substitution
2. Radical Addition
3. S_N2
4. S_N1 - E1
5. E2
6. Electrophilic Addition
7. Electrophilic Substitution
8. Pericyclic (cyclic transition states; the Diels-Alder reaction)
9. Nucleophilic Addition
10. Nucleophilic Acyl Substitution
(Click on the red button to display the generic mechanism for that kind of process.)

This page last modified 3:00 PM on Friday November 20th, 2009.
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