Hammond Postulate

Hammond’s Postulate – Master Organic Chemistry

Hammond’s postulate (or Hammond–Leffler postulate) is a physical organic chemistry hypothesis that describes the geometric structure of an organic chemical reaction’s transition state.

The postulate, which George Hammond first proposed in 1955, states that if two states, such as a transition state and an unstable intermediate, occur consecutively during a reaction process and have nearly the same energy content, their interconversion will result in only a minor reorganisation of molecular structures.

As a result, a state’s geometric structure can be predicted by comparing its energy to the energy of species nearby along the reaction coordinate. For example, in an exothermic process, the transition state is closer to the reactants than to the products in terms of energy.

In an endothermic reaction, on the other hand, the transition state is closer to the products than to the reactants in terms of energy. As a result, the structure of the transition state would resemble the products more than the reactants, according to Hammond’s postulate.

Table of Contents

• The Postulate’s Interpretation
• Transition State Structure
• The Bell–Evans–Polanyi Principle and Kinetics
• Application of the postulate
• Frequently Asked Questions

The Postulate’s Interpretation

The postulate effectively states that the structure of a transition state is similar to that of the species with the lowest free energy. With the help of potential energy diagrams, this can be explained:

The energy of the transition state in example (a), which is an exothermic reaction, is closer to the reactant than the intermediate or product. As a result of the assumption, the transition state’s structure closely mimics the reactants.

In example (b), the transition state’s energy is not close to either the reactant or the product; therefore, neither is an acceptable structural model for the transition state.

Case (c) shows a potential diagram for an endothermic process in which the transition state should mirror that of the intermediate or product, according to the postulate.

Transition State Structure

S_N1 Reactions

The structure of the transition states of an S_N1 reaction can be investigated using Hammond’s postulate. In an S_N1 reaction, the dissociation of the leaving group is the first transition state. The carbocation stabilities generated by this dissociation are known to follow the order tertiary > secondary > primary > methyl.

In inorganic chemistry, the S_N1 reaction is known as the dissociative mechanism. A carbocation intermediate is formed during this reaction. It is most commonly encountered in the reactions of tertiary or secondary alkyl halides with secondary or tertiary alcohols when the circumstances are strongly acidic or strongly basic.

S_N2 Reactions

Bimolecular nucleophilic substitution (S_N2) reactions are concerted reactions in which the rate-limiting step involves both the nucleophile and the substrate. Because this reaction is coordinated, it occurs in a single step, with bonds being broken and new bonds being formed. As a result, it’s crucial to look at the transition state, which mirrors the concerted rate-limiting step, to understand this reaction.

E1 Reactions

The rate-determining step of an E1 reaction is unimolecular elimination, in which the removal of a single molecular species determines the rate. The activation energy is reduced when the carbocation intermediate is stabilised. The reaction will occur faster if the carbocation intermediate is more stable.

The more stable diastereomer is generated faster, according to Hammond’s postulate.

E2 Reactions

Bimolecular elimination reactions are one-step, concerted reactions in which the base and the substrate are involved in the rate-limiting step. An E2 process consists of a base taking a proton near the leaving group, driving the electrons down to form a double bond, and then forcing off the leaving group all in one coordinated step. It’s a 2nd order (bimolecular) elimination reaction because the rate law is based on the first order concentration of two reactants. Stereochemistry, leaving groups, and base strength are all factors that influence the rate-determining step.

The Bell–Evans–Polanyi Principle and Kinetics

Hammond’s postulate describes only the geometric structure of a chemical reaction. On the other hand, Hammond’s postulate provides information regarding the pace, kinetics, and activation energy of processes in an indirect manner. As a result, it gives a theoretical foundation for comprehending the Bell–Evans–Polanyi (BEP) principle, which describes the experimental finding that the enthalpy and rate of similar processes are frequently associated.

Consider the SN1 reaction to understand the relationship between Hammond’s postulate and the BEP principle. Although there are two transition states in an SN1 reaction (leaving group dissociation and then nucleophile assault), the leaving group dissociation is nearly always the rate-determining phase. As a result, the activation energy and, as a result, the reaction rate will be solely determined by the dissociation step.

Application of the postulate

The relationship between the reaction rate and the stability of the products can be understood using Hammond’s postulate. While the activation energy (commonly expressed in organic chemistry as ΔG‡ “delta G double dagger”) determines the rate of a reaction, the ultimate ratios of products in chemical equilibrium are determined solely by the standard free-energy change ΔG (“delta G”). The stability of the final products is directly proportional to their ratio at equilibrium.

Hammond’s postulate links the rate of a reaction process to the structural properties of the states that make up that process, stating that molecular reorganisations must be minimal in steps involving two states with similar energy levels. The structural comparison of the starting materials, products, and possibly “stable intermediates” resulted in the realisation that the most stable product is not always the one that is preferred in a reaction process.