The Woodward-Hoffmann Rules
Chemical transformations of the atomic structure of matter have been proceeding on Earth for billions of years. These transformations, or chemical reactions, play an integral part in our planet’s development. One of the prerequisites for life on Earth is that chemical reactions should be governed by natural laws. For thousands of years, human beings have actively utilised chemical reactions for mastering their environment – in preparing food and drink, in fashioning tools and clothes, and in combating disease, and so on. However, over time, the mass of empirical knowledge we gained as a result of our ever-growing utilisation grew so large that theoretical concepts had to be resorted to, so that a meaningful overall view was possible. Theories thus became necessary for the continued, systematic use of chemical reactions [1].
Given this importance of understanding chemical reactions and developing advanced theories to simplify and classify them, Dr. Roald Hoffmann’s work proves to be especially consequential. Essentially, his work helped scientists predict which chemical reactions, specifically organic ones, would be easy and which relatively difficult. In a fruitful collaboration with the venerated Nobel Laureate R.B. Woodward, he helped come up with a set of rules, known as the Woodward-Hoffmann rules, which predict whether a pericyclic reaction (see below) may occur or not under a given condition. It was exactly this indispensable development that earned Dr. Roald Hoffmann his Nobel Prize in 1981 [3].
Dr. Roald Hoffmann: how to predict a chemical reaction?
A pericyclic reaction is an organic reaction that proceeds via a single concerted transition state, the geometry of which allows for the continuous overlap of pi and sigma bonds [2]. While a slightly complex topic to understand at the basic level, a pericyclic reaction can be explained efficiently using a breakdown of its main features. “Concerted” implies that it is a chemical reaction in which all bond breaking and bond making occurs in a single step. The “transition state” of the chemical reaction is the state corresponding to its highest level of potential energy in a reaction-energy diagram (also known as reaction coordinate (Figure 1)). Pi and sigma bonds are two types of covalent bonds (bonds formed by sharing of electrons instead of transfer) that differ in terms of bond strength.
Figure 1: Example of a reaction coordinate
The x-axis shows the progress of the reaction while the y-axis represents energy level. As can be interpreted from the graph, the energy of the reactants varies as the reaction progresses, reaching its peak at a level called the ‘activation energy’. It is only once this energy level is reached, that the conversion of reactants to products may begin. The final energy of the products is less than that of the initial reactants, indicating an exothermic (or heat-releasing) reaction.
Woodward and Hoffmann were the first to define a set of rules to determine the feasibility of reactions stereochemistry of products formed in pericyclic reactions. This discovery required an interdisciplinary approach and wide application of physical chemistry, essentially making Hoffmann responsible for why modern chemists today need to have working knowledge of quantum chemistry!
Dr. Hoffmann Explains: What Keeps Creativity Alive?
Dr. Hoffmann’s contributions to chemistry are well known. Less well known, however, is that over a career that spans nearly fifty years, Hoffmann has thought and written extensively about a wide variety of other topics, such as chemistry’s relationship to philosophy, literature, and the arts, that he has also articulated in the form of a book. In addition, he has also dabbled in the field of poetry, with many coveted collections.
He remains deeply fascinated by chemical reasoning, the role of symbolism and writing in science, and the relationship between art and craft and science. In fact, he believes that the way one writes is a subtle yet powerful indication of their character. Through his poems, he finds appreciation for the power of concise statements, something he already finds of abundant importance given his scientific background.
In Dr. Hoffmann’s view, creativity is kept alive by an unrelenting desire to expand one’s perceptions and branch out into other disciplines that not only give personal satisfaction but also help reinforce what one already knows. While many scientists may give higher importance to formulas and equations over language, Dr. Hoffmann believes that it is meant to serve and express simple things in wonderful ways, even if those things are energy and force.
Learn More
If you’d like to hear more about the journey toward a Nobel Prize, visit us on Spotify to listen to our ChemTalk podcast with Nobel Laureate Dr. Roald Hoffmann to learn more about how his love for chemistry began, his thoughts on nanotechnology, and his unlikely friendship with Oliver Sacks.
Find the ChemTalk podcast here.
Works Cited
[1] Press release. NobelPrize.org. Nobel Prize Outreach AB 2023. Tue. 31 Jan 2023. <https://www.nobelprize.org/prizes/chemistry/1981/press-release/>
[2] Explified. 6 October 2020. “Woodward Hoffmann Rules Explained.” YouTube.
<https://www.youtube.com/watch?v=MZAQcRLuDvw>
[3] Hoffmann, Roald. Personal Interview. Conducted by Scott Gietler. 2 June 2021.