Altering the mechanism of a reaction so that the activation
energy is lower is the second major way to speed up the
reaction. A catalyst can do this by participating in the
activated complex for the rate-limiting step, even though the
catalyst itself is neither a reactant nor a product in the
overall stoichiometric equation. A good example of catalysis is
provided by the effect of I2 on the rate of isomerization of
cis-2-butene. You will recall from the section on rate equations
that the rate law for the catalyzed reaction involves the
concentration of I2 raised to the one-half power. This implies
that half a molecule of I2 (that is, an I atom) is involved in
the activated complex, which probably has the structure Since
there is no double bond between the two central C atoms, one end
of the activated complex can readily twist around the other. The
currently accepted mechanism for this catalyzed reaction
involves three steps: ½ I2 I I + cis-C4H8 → C4H8I‡ → trans-C4H8
+ I I ½ I2 The first and last steps have co