Hammond's postulate

Hammond's postulate says a transition state resembles the stable species—reactant, intermediate, or product—closest to it in energy. It helps predict transition-state structure from reaction energetics.

Bell–Evans–Polanyi principle: An empirical relationship linking reaction activation energies to reaction enthalpies within related reaction series. It extends the energy-based reasoning used to connect reaction progress with activation barriers.
Transition state: A high-energy configuration at a local maximum along a reaction pathway. The postulate makes its structural resemblance the central prediction.
George S. Hammond: An American physical organic chemist who formulated Hammond's postulate in 1955. He stated the principle that links transition-state resemblance to relative energy.
Bell's principle of least nuclear motion: A principle proposing that reaction pathways tend to involve the least nuclear displacement compatible with bond changes. It predicts transition-state geometry from nuclear motion rather than energetic proximity.
Carbocation rearrangement: A shift of a hydrogen atom or carbon group within a carbocation, producing a more stable carbocation. The postulate helps explain why developing carbocation character favors rearrangement during a reaction.
Activation energy: The minimum energy difference between reactants and a reaction's transition state. Its relation to reaction energy determines which species the transition state is expected to resemble.
Physical organic chemistry: The study of how molecular structure and physical properties govern organic reaction mechanisms and rates. Hammond's postulate arose within this field's study of reaction mechanisms.
Marcus theory: A theory describing electron-transfer rates through the energetics of reactant and product states and their reorganization. It gives a specialized quantitative account of electron-transfer barriers beyond Hammond's qualitative resemblance rule.
Kinetic isotope effect: A change in reaction rate caused by replacing an atom with one of its isotopes. Isotope effects can reveal which bonds are changing in a transition state inferred using the postulate.
Reaction coordinate: A parameter describing progress along a chemical reaction pathway. The postulate concerns structures at different positions along this pathway.

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