The Hammond Postulate: Transition States, Energy Diagrams & Reaction Selectivity

Section	Topic
1	What is a Transition State? Revisiting the Reaction Coordinate
2	The Hammond Postulate: Statement and Logic
3	Energy Diagrams: Exothermic Steps and Early Transition States
4	Energy Diagrams: Endothermic Steps and Late Transition States
5	Application: Chlorination vs Bromination Selectivity
6	Application: Why More Stable Intermediates Form Faster (SN1, Markovnikov)
7	Hammond Across the Course: A Reference Map

Before the Hammond Postulate makes sense, you need a clear mental picture of what a transition state is and how it sits on an energy diagram.

The Reaction Coordinate Diagram

Every elementary reaction step can be plotted on a reaction coordinate diagram: energy on the y-axis, progress of bond breaking and forming on the x-axis. The x-axis is not time — it is how far along the chemical transformation has gone, from reactants on the left to products on the right.

• Reactants: starting point at a defined energy level on the left.
• Products: ending point on the right.
• Transition state (TS): the peak of the energy curve — the highest-energy point on the path. It is NOT a stable species. It cannot be isolated, stored, or detected directly.
• Activation energy (Ea): energy difference between reactants and the TS. Higher Ea = slower reaction.
• ΔH: energy difference between reactants and products. Negative = exothermic; positive = endothermic.

Transition State vs Intermediate — A Critical Distinction

Feature	Transition State (TS)	Intermediate
Energy position	Peak of energy curve	Valley between two peaks
Stability	Unstable; collapses instantly	Can persist briefly
Bonds	Partially formed / broken	Full bonds
Isolable?	No — never	Sometimes (in principle)
Notation	[A···B]‡	Written as normal structure

George Hammond published his postulate in 1955 to address a practical problem: transition states cannot be observed directly — so how can we reason about their structure and energy?

Plain-Language Version

The transition state of a reaction step resembles whichever species is closest to it in energy — the reactants (if the step is exothermic) or the products (if the step is endothermic).

The Logic

Think about the geometry of the energy curve. The TS sits at the peak. If the products are much lower in energy (strongly exothermic), the peak comes early on the x-axis and is close to the reactant energy level. If the products are much higher (strongly endothermic), the peak comes late and is close to the product energy level.

Step Thermodynamics	TS Position	TS Resembles
Strongly exothermic	Early	Reactants
Slightly exothermic	Slightly early	Mostly reactants
Thermoneutral	Middle	Both equally
Slightly endothermic	Slightly late	Mostly products
Strongly endothermic	Late	Products

Consider chlorine radical H-abstraction from an alkane — an exothermic step:

R–H + Cl• → R• + HCl    ΔH ≈ −2 to −5 kcal/mol

What This Means for Selectivity

Because little radical character has developed at the early TS, the stability difference between a 1°, 2°, or 3° radical barely registers in the TS energy. The energy curves for 1° and 3° abstraction are nearly identical — giving low selectivity (~5:1 per H for 3° vs 1°). Chlorination gives a statistical mixture of products.

Now contrast with bromine radical H-abstraction — an endothermic step:

R–H + Br• → R• + HBr    ΔH ≈ +5 to +18 kcal/mol

What This Means for Selectivity

Because the late TS closely resembles the product radical R•, the stability of R• directly controls the TS energy. The 3° radical is ~4–5 kcal/mol more stable than a 1° radical — and because the TS is late, this full difference appears in the TS energy gap. A 4 kcal/mol difference in Ea translates to ~1000-fold selectivity at room temperature via the Arrhenius equation. Bromination is essentially completely selective for the most substituted position (~1600:1 per H for 3° vs 1°).

Feature	Chlorination (Cl• + R–H)	Bromination (Br• + R–H)
ΔH of H-abstraction	~−2 to −5 kcal/mol (exothermic)	+5 to +18 kcal/mol (endothermic)
TS position	Early (reactant-like)	Late (product-like)
C–H breaking at TS	Minimal (<25%)	Substantial (>75%)
Radical character at TS	Small	Large
3° vs 1° ΔEa	~0.5–1 kcal/mol	~4–5 kcal/mol
3° vs 1° selectivity per H	~5:1	~1600:1
2° vs 1° selectivity per H	~4.5:1	~82:1
Practical outcome	Mixture of products	Highly predictable single major product

The Reactivity–Selectivity Principle

The Hammond Postulate is the mechanistic foundation for several rules you already know. In each case the logic is identical: an endothermic step has a late TS that closely resembles the intermediate, so the intermediate's stability controls the rate.

SN1 Reactions — Carbocation Formation

The rate-determining step of SN1 is ionization of the C–X bond to form a carbocation. This step is endothermic. By Hammond, the TS is late and resembles the carbocation. Therefore, the stability of the carbocation directly controls the activation energy.

• 3° carbocation is most stable → lowest TS energy → fastest ionization → SN1 works well.
• 2° carbocation is moderate → higher TS → slower, possible under some conditions.
• 1° carbocation is very unstable → very high TS → SN1 essentially never occurs at primary substrates.

Markovnikov's Rule — HX Addition to Alkenes

When HX adds to an unsymmetrical alkene, the step that forms the carbocation is endothermic. By Hammond, the TS is late and resembles the carbocation. The more stable carbocation forms faster.

• CH₂=CHCH₃ + H⁺ → CH₃⁺CHCH₃ (2°, more stable) → faster → Markovnikov product
• CH₂=CHCH₃ + H⁺ → ⁺CH₂CH₂CH₃ (1°, less stable) → slower → minor product

Markovnikov's rule is not an empirical coincidence — it is a direct consequence of endothermic carbocation formation having a late TS.

Carbocation Rearrangements

When a less-stable carbocation forms in an endothermic step, the activation energy for 1,2-hydride or 1,2-alkyl shifts to a more stable carbocation is low — because the TS for these shifts also resembles the more stable cation being formed. Rearrangements proceed in the direction of greater carbocation stability because that direction has the lower TS energy.

Reaction / Concept	How Hammond Applies
Radical halogenation: Br₂ vs Cl₂	Endothermic H-abstraction (Br•) → late TS → radical stability controls selectivity
SN1 substrate requirements	Endothermic ionization → late TS → stable carbocation = lower Ea = faster rate
Markovnikov's rule (HX + alkene)	Endothermic protonation → late TS → more stable carbocation forms faster
E1 and Zaitsev's rule	Endothermic carbocation formation → same logic as SN1
Carbocation rearrangements	Late TS for rearrangement steps → more stable cation = lower TS = faster shift
Allylic/benzylic radical selectivity	Endothermic H-abstraction by Br• → resonance-stabilized radical lowers TS dramatically
Acid-base: endothermic proton transfer	Late TS → stability of conjugate base controls activation energy and rate

Quick Recap

Concept	Key Point
Transition state	Energy peak; partial bonds; cannot be isolated; drawn as [A···B]‡
Intermediate	Energy valley; full bonds; can persist briefly; can sometimes be trapped
Exothermic step	Products lower; TS early; resembles reactants; intermediate stability ≈ invisible
Endothermic step	Products higher; TS late; resembles products; intermediate stability controls rate
Hammond Postulate	TS resembles the species closest to it in energy on the reaction coordinate
Reactivity–selectivity	More reactive reagent → lower Ea → earlier TS → less selective
Bromination selectivity	Endothermic H-abstraction → late TS → radical stability matters → ~1600:1
SN1/Markovnikov	Endothermic cation formation → stable cation = lower TS = faster = major product