thermodynamic product

Meaning: When a reactant can form two different products, the thermodynamic product is the more stable one, with the lower Gibbs free energy. It dominates when the reaction is reversible and has enough time and energy to reach equilibrium, so the product ratio reflects stability rather than speed. The competing kinetic product forms faster because its activation energy is lower, but it is often less stable. Example: HBr adds to 1,3-butadiene to give mainly the 1,2-adduct at low temperature (kinetic) and mainly the more substituted 1,4-adduct at higher temperature (thermodynamic).

Equation: ΔG° = −RT ln Keq, where ΔG° is the standard free energy change (J/mol), R is 8.314 J/(mol·K), T is temperature (K), and Keq is the equilibrium constant (unitless). A more negative ΔG° means a larger Keq.

On the MCAT: Passages give an energy diagram with one reactant and two pathways. The lower barrier marks the kinetic product; the lower final energy marks the thermodynamic product. Questions ask which product predominates at low versus high temperature, or after a short versus long reaction time.

Common confusion: Students equate thermodynamic with faster. Speed depends on activation energy, while stability depends on ΔG. If the reaction is effectively irreversible, the kinetic product stays even when the mixture is heated, because it cannot convert to the more stable product.