Identify Stereogenic Elements and Consider Possible Stereochemical Outcomes.
This actually is part of creating a reactant-product map, but stereochemistry plays such an important role in defining a mechanism that we consider it separately.
- A mechanism must account in detail for any stereochemical changes that take place in the course of the reaction.
- Hence, we need to take special note of stereogenic elements (chiral centers, double bonds, and rings) in the reactants and products and determine whether they are affected by the bond making and breaking processes identified in the previous section.
- The Table below summarizes the possible situations:
| |
Stereogenic Element In |
|
| Case |
Reactant |
Product |
Outcome |
| 1 |
No |
No |
Irrelevant |
| 2 |
Yes |
No |
Stereochemistry Lost;
How? |
| 3 |
No |
Yes |
Stereochemistry Created;
How? Selectivity? |
| 4 |
Yes |
Yes |
Inversion? Retention?
How? Selectivity? |
Here are some simple examples to classify:
- Go back to the reactions of the previous section. Identify the stereogenic elements in each reactant and product. Sort the examples by case.
If the reactant or product structure as written does not specify a configuration for a particular element, then all possibilities must be considered.