Hammond Postulate

In-depth explanation of Hammond Postulate

The Hammond postulate states that the structure of a transition state resembles the nearest stable species in energy, whether that is a reactant, a product or an intermediate. In an exothermic step the transition state occurs early and looks like the reactants, since it lies close to them along the reaction coordinate. In an endothermic step the transition state occurs late and resembles the products. This principle allows qualitative predictions of reactivity and selectivity from the relative stabilities of intermediates and products, without calculating the transition state directly. It is widely used in organic chemistry to rationalize substituent effects and product distributions.

Going deeper: Hammond Postulate

Key ideas: Use it as a guide: stable intermediate means lower-lying transition state, so a faster step. For exothermic steps picture the transition state as early and similar to the reactants, for endothermic steps as late and similar to the products. It explains why the most stable radical or carbocation forms fastest and why more selective radical bromination occurs than chlorination, as bromination is endothermic.

Common mistakes and connections: Students apply it to the whole reaction rather than a single step, or claim it is a law that holds quantitatively. It is a qualitative guideline. Another mistake is to read an early transition state as one with low energy; early means resembling the reactants in structure. Associated with reaction coordinate diagrams, the stability of intermediates and kinetic control.

Visualization of Hammond Postulate

The top of the barrier lies closer to reactants in an exothermic step and closer to products in an endothermic step, resembling the nearer species. The interactive lesson shows this as a live animation or chart.

Worked example and use case

The formation of a tertiary carbocation has a lower barrier than that of a primary carbocation, because the transition state lies closer in energy to the more stable tertiary cation.

Where Hammond Postulate fits in Kinetics & Equilibrium

Hammond Postulate is topic 72 of 91 in Kinetics & Equilibrium.

Related topics

• Relationship of Rate Constants and KAdvanced
• Principle of Microscopic ReversibilityAdvanced
• Kinetic vs Thermodynamic ProductAdvanced
• Eyring EquationAdvanced
• Lindemann MechanismAdvanced
• Diffusion-Controlled ReactionsAdvanced

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