7.10 The Hammond Postulate

The Hammond Postulate and Electrophilic Addition to Alkenes

The Hammond Postulate gives you a way to predict what a transition state looks like, even though you can never directly observe one. This matters because the structure and energy of the transition state control how fast a reaction goes. For electrophilic addition to alkenes, the postulate explains why more substituted alkenes react faster: they form more stable carbocations, which lowers the energy barrier.

Hammond Postulate and Transition States

A transition state sits at the energy maximum along a reaction coordinate. You can't isolate it or take a snapshot of it, so how do you figure out what it looks like? The Hammond Postulate answers this:

This leads to two key scenarios:

• Exothermic step: The transition state is closer in energy to the reactants, so it resembles the reactants in structure. This is called an early transition state.
• Endothermic step: The transition state is closer in energy to the products, so it resembles the products in structure. This is called a late transition state.

The practical payoff is that you can use the stability of reactants or products to reason about the transition state, even though you can't observe it directly.

Application to Alkene Protonation

When an alkene reacts with an electrophile like $H^+$ (the first step of electrophilic addition), the transition state involves partial formation of a new C–H bond and partial development of positive charge on carbon. The key question is: how much does this transition state look like the carbocation product versus the alkene reactant?

The Hammond Postulate predicts that the answer depends on carbocation stability:

• More stable carbocation forming (e.g., tertiary): The carbocation product is relatively low in energy, so the transition state is also lower in energy and more product-like. That means:
  • The C–H bond is more fully formed
  • The positive charge on carbon is more fully developed
• Less stable carbocation forming (e.g., primary): The carbocation product is higher in energy, pushing the transition state higher as well and making it more reactant-like. That means:
  • The C–H bond is less formed
  • The positive charge on carbon is less developed

In both cases, you're using the stability of the product of that step (the carbocation) to infer what the transition state looks like.

Carbocation Stability and Reaction Rates

The rate of electrophilic addition depends on the activation energy of the rate-determining step, which is typically carbocation formation. The Hammond Postulate connects carbocation stability directly to reaction rate:

1. A more substituted alkene (like 2-methylpropene) forms a more stable carbocation (tertiary).
2. The more stable carbocation means a lower-energy transition state.
3. A lower-energy transition state means a smaller activation energy ($E_a$).
4. A smaller $E_a$ means a faster reaction.

The reverse logic applies to less substituted alkenes. Ethylene, for example, would need to form a primary carbocation, which is highly unstable. The transition state energy is correspondingly high, making the reaction much slower.

Reactivity order for electrophilic addition:

$\text{tetrasubstituted} > \text{trisubstituted} > \text{disubstituted} > \text{monosubstituted} > \text{unsubstituted}$

This trend follows directly from carbocation stability: tertiary > secondary > primary.

Energetics and Potential Energy Diagrams

A potential energy diagram for electrophilic addition plots free energy on the y-axis against reaction progress on the x-axis. For the protonation step, the diagram shows:

• The reactants (alkene + electrophile) at the starting energy
• The transition state at the energy maximum, representing the activation energy barrier
• The carbocation intermediate at a local energy minimum after the barrier

Comparing two alkenes on the same diagram makes the Hammond Postulate visual. The alkene that forms the more stable carbocation has a lower transition state peak and a smaller $E_a$. The alkene forming the less stable carbocation has a higher peak and larger $E_a$. Both reactions may be exothermic overall (after nucleophilic attack in the second step), but the kinetics of the first step are what determine relative rates.