Reaction Profiles & Hammond's Postulate in Organic Chemistry

What is a reaction profile?

A plot of energy changes as a function of time, reflecting free energy levels during a chemical reaction.

What happens during a chemical reaction?

There is an input and output of energy and work, involving the breaking of old bonds and formation of new ones.

What is a transition state?

A state reached every time a bond is broken, characterized by maximum levels of free energy.

Can transition states be isolated or studied?

No, transition states are not stable and cannot be isolated.

What are reaction intermediates?

Species found at minimum energy levels that can be isolated and studied.

What does Hammond's Postulate state?

In a reaction mechanism, related species that are close in energy resemble each other structurally.

In an exothermic reaction, when does the transition state occur?

Early in the reaction, with the reactants being closer in energy and structure.

In an endothermic reaction, when does the transition state occur?

Late in the reaction, with the products being closer in energy and structure.

What characterizes a mildly exothermic or endothermic reaction?

The transition state occurs about midway through the reaction, resembling both reactants and products.

What is the significance of the structure of the transition state?

It resembles the structure of the closest stable species in terms of energy.

What does the transition state resemble in an exothermic reaction?

The structure resembles the reactants more than the products.

What does the transition state resemble in an endothermic reaction?

The structure resembles the products more than the reactants.

What is the transition state in the first propagation step of the chlorination of methane?

It resembles the methyl free radical and HCl, with the C-H bond almost broken.

What is the transition state in the second propagation step of the chlorination of methane?

It resembles the methyl free radical and chlorine molecule, with the C-Cl bond barely formed.

What is the relationship between bond breaking and transition states?

Every time a bond is broken, a transition state is reached.

What does a reaction profile illustrate?

It illustrates how free energy levels change as a reaction proceeds.

What type of reactions tend to have early transition states?

Many exothermic reactions.

What type of reactions tend to have late transition states?

Many endothermic reactions.

How does the transition state relate to the stability of species?

The transition state is unstable and cannot be isolated, while reaction intermediates can be.

What is the implication of knowing the structure of reaction intermediates?

It allows us to infer the structure of the transition state from surrounding stable species.

What is the main focus of Hammond's Postulate?

The structural resemblance of species that are close in energy during a reaction.

What happens to energy levels during a chemical reaction?

Energy levels change as bonds are broken and formed.

What is the role of energy in chemical reactions?

Energy is involved in the breaking of bonds and the formation of new ones.