Core Concepts
In this article you will be able to understand the Hammond Postulate, after reading this, you will understand its importance in organic chemistry!
Related Topics
- Markovnikov’s Rule
- Nucleophilic Substitution Reaction
- Electrophilic Aromatic Substitution
- What is a Leaving Group?
- Understanding E1 vs E2 Reactions
Hammond Postulate
According to the postulate, which George Hammond first put forth in 1955, if two states, such as a transition state and an unstable intermediate, happen concurrently during a reaction process and have nearly the same amount of energy, their interconversion will only cause a slight reorganization of molecular structures. As a result, by comparing a state’s energy to the energy of species close along the reaction coordinate, its geometric structure can be predicted. For instance, in an exothermic process, the transition state is, in terms of energy, closer to the reactants than to the products.
To summarize, Hammond’s postulate, states that depending on which is closest in energy, the transition state of a reaction resembles either the reactants or the products.
Hammond’s Postulate for Exergonic Reactions
The transition state is more energetically near to the reactants in an exergonic process. Therefore, it can be assumed that the transition state’s structure is more similar to the reactants than the products. We depict the hypothetical exergonic reaction between reactant substances A and B producing the product AB. According to the Hammond postulate, A and B are two isolated species in the reactants, and the distance between them in the transition state would be rather considerable.
Hammond’s Postulate for Endergonic Reactions
The transition state is more energetically similar to the product in an endergonic reaction. Therefore, one can assume that the structure of the transition state is more similar to the products than to the reactants. The hypothetical endergonic reaction involves the reaction of the reactant components C and D to produce the result CD. According to the Hammond postulate, C and D would be quite close to one another in the transition state, mimicking the products where C and D are chemically bound to form a single product called CD.
Transition State Structures on the Hammond Postulate
SN1 Reaction
Firstly, with the help of Hammond’s postulate, the structure of the transition states in an SN1 reaction may be examined. The leaving group’s dissociation is the first transition state in an SN1 reaction. This dissociation is known to produce carbocation stabilities in the following sequence: tertiary > secondary > primary > methyl.
SN2 Reaction
Secondly, in bimolecular nucleophilic substitution reactions, both the nucleophile and the substrate actively participate in the rate-limiting phase, constituting a concerted process. The coordinated nature of this reaction allows for the breaking of bonds and the creation of new ones in a single step. In order to comprehend this reaction, it is essential to examine the transition state, which replicates the concerted rate-limiting step.
E1 Reaction
Unimolecular elimination, in which the removal of a single molecular species dictates the rate, is the rate-determining step of an E1 process. Additionally, stabilizing the carbocation intermediate results in a decrease in the activation energy. If the intermediate carbocation is more stable, the reaction will proceed more quickly. Hammond’s hypothesis states that the faster generation of the more stable diastereomer.
E2 Reaction
Concurrently, concerted, one-step reactions involving both the base and the substrate constitute bimolecular elimination reactions, with the rate-limiting step encompassing them. An E2 process entails the coordinated steps of a base accepting a proton close to the leaving group, driving the electrons down to form a double bond, and finally forcing off the leaving group. Because the rate law is dependent on the first order concentration of two reactants, it is a second order elimination reaction. Furthermore, the rate-deciding step is influenced by stereochemistry, leaving groups, and base strength.
Hammond Postulate Applications
Hammond’s postulate asserts that steps involving two states with equal energy levels must minimize molecular reorganizations. This postulate links the rate of a reaction process to the structural characteristics of the states that make up that process. As discovered through the structural comparison of the starting materials, products, and potentially “stable intermediates,” researchers have found that the favored product in a reaction process is not always the most stable one.
Hammond Postulate Practice Problems
Problem 1
In an exothermic reaction, the transition state is more energetically similar to which of the following:
a) Reactants
b) Products
c) Neither
Problem 2
Consider an endergonic reaction in which the transition state is closer in energy to the products. According to Hammond’s Postulate, what inference can you make about the distance between these species in the transition state if two isolated species constitute the reactants?
Problem 3
In an SN1 reaction, the leaving group’s dissociation is the first transition state. Arrange the following carbocations in order of stability based on Hammond’s Postulate: a) Primary
b) Tertiary
c) Secondary c) Methyl
Problem 4
In an E2 reaction, the rate-deciding step involves both the base and the substrate. How does stereochemistry influence the rate of this reaction according to Hammond’s Postulate?
Problem 5
Hammond’s Postulate suggests that the structural characteristics of the states involved link to the rate of a reaction process. If two states have nearly the same energy and undergo a reaction concurrently, what can you conclude about the degree of molecular reorganization in these states?
Hammond Postulate Practice Problem Solutions
Answer 1
Reactants
Answer 2
Rather considerable
Answer 3
Tertiary > secondary > primary > methyl
Answer 4
Stereochemistry can influence the rate of the E2 reaction by affecting the rate-deciding step.
Answer 5
The degree of molecular reorganization is minimal when two states have nearly the same energy.
Further Reading
If you liked the topic, we recommend you to read the following article!