30.9 • A Summary of Rules for Pericyclic Reactions
How can you keep straight all the rules about pericyclic reactions? The summary in Table 30.1, Table 30.2, and Table 30.3 can be distilled into a mnemonic phrase that provides an easy way to predict the stereochemical outcome of any pericyclic reaction:
The Electrons Circle Around (TECA)
Thermal reactions with an Even number of electron
pairs are Conrotatory or Antarafacial.
A change either from thermal to photochemical or from an even to an odd number of electron pairs changes the outcome from conrotatory/antarafacial to disrotatory/suprafacial. A change from both thermal and even to photochemical and odd causes no change because two negatives make a positive.
These selection rules are summarized in Table 30.4; knowing them gives you the ability to predict the stereochemistry of literally thousands of pericyclic reactions.
|Electronic state||Electron pairs||Stereochemistry|
|Ground state (thermal)||Even number
|Excited state (photochemical)||Even number