chemical reactions in which a single molecule undergoes transformation in the elementary event. Monomolecular reactions include the numerous decomposition reactions of complex molecules and isomerizations. For example, the decomposition of ethyl chloride
C2H5Cl→C2H4 + HCl
and the isomerization of methyl isonitrile to acetonitrile
are monomolecular reactions.
The rate of monomolecular reactions (in ideal gaseous mixtures and liquid solutions) is described by a first-order kinetic equation r = kc, where r is the rate of reaction, c is the concentration of the initial material, and k is the rate constant (which depends on the temperature according to the Arrhenius equation). At a constant volume, r = −dc/dt, where t is time and c = c0e−kt(C0 is the value for c at t = 0).
At reduced pressures, the values of r for gaseous-phase monomolecular reactions are smaller than those expected from the first-order kinetic equation because of an insufficient rate of intermolecular energy transfer as a result of a reduced frequency of molecular collisions.