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Kinetic control 2026-10-20
Under kinetic control, the observed product distribution reflects competing formation rates rather than an equilibrated distribution. Heating may speed access to a lower-Gibbs free energy product, but does not itself prove that the equilibrium favors it.
Past exam of the chemistry B course of the University of Cambridge 2025 Paper 2 B6 b ii Solution 2026-10-20
The N-bound isomer is the thermodynamically favored form; heating accelerates access to it. Formation of retains the original metal–oxygen attachment and is a plausible kinetic control outcome. Rearranging that attachment requires overcoming an activation free energy, especially at a substitutionally inert low-spin cobalt(III) center. Raising temperature speeds the linkage isomerization, allowing the more stable N-bound nitro ligand to accumulate. Its stronger ligand field and different covalent bonding help rationalize the stability, but a larger field splitting alone is not a complete calculation of the isomerization Gibbs free energy. The classical crystallographic comparison establishes the different attachments and thermodynamic direction.[ref]
Nitrito-Nitro Linkage Isomerization in the Solid State. 2. A Comparative Study of the Structures of Nitrito- and Nitropentaamminecobalt(III) Dichloride by Ingmar Grenthe and Ellika Nordin. 1979, pp. 1869–1874, structural comparison and solid-state isomerization
C2 is favoured by kinetic control; C1 is favoured by thermodynamic control. Starting from the central reactant well, the path towards C2 has the smaller activation barrier, so it normally forms faster at low temperature or short reaction time, assuming comparable prefactors. C1 lies lower in energy and is the more stable product on the drawn energy scale.
Raising temperature can make the larger barrier accessible and permit equilibration through reverse reactions. If products interconvert and entropy contributions do not reverse the energy ordering, the equilibrium mixture then favours C1. It would be incorrect to say that high temperature alone guarantees C1: an irreversible reaction may remain kinetically controlled, and the figure does not provide the full temperature-dependent free energies. The useful distinction is rate selection before equilibration versus relative free energy after equilibration.
Thermodynamic control 2026-10-20
Product selection governed by equilibrium free energies after reversible interconversion, in contrast to kinetic control before equilibration.