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Chemistry · Glossary

What is Catalytic cycle?

Also known as: precatalyst · turnover number · turnover frequency

Definition 20.11 University Chemistry — Year 2 · Chapter 20 — Organometallic Catalysis: Elementary Steps and Cycles

A catalytic cycle is a closed sequence of elementary steps that consumes the reagents, releases the products and regenerates its first complex. The compound added to the reaction, which turns into the first complex of the cycle, is the precatalyst. The turnover number (TON) is the amount of product formed per amount of catalyst; the turnover frequency (TOF) is the turnover number per unit time.

The hydrogenation of an alkene with Wilkinson’s catalyst, simplified (phosphine and solvent exchanges omitted). The four steps sum to RCH=CH2 + H2 -> RCH2CH3; the rhodium complexes cancel.
The hydrogenation of an alkene with Wilkinson’s catalyst, simplified (phosphine and solvent exchanges omitted). The four steps sum to RCH=CHX2+HX2→RCHX2CHX3\ce{RCH=CH2 + H2 -> RCH2CH3}; the rhodium complexes cancel.
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