George S. Hammond

George Simms Hammond (May 22, 1921 – October 5, 2005) was an American scientist and theoretical chemist who developed "Hammond's postulate", and fathered organic photochemistry, His research garnered him the Norris Award in 1968, the Priestley Medal in 1976, the National Medal of Science in 1994, and the Othmer Gold Medal in 2003. He served as the executive chairman of the Allied Chemical Corporation from 1979 to 1989.

He was a chemist at the California Institute of Technology, and subsequently headed both the Departments of Chemistry and Chemical Engineering at the university. He conducted research at the University of Oxford and University of Basel as a Guggenheim Fellow and National Science Foundation Fellow, respectively. He served as the foreign secretary of the National Academy of Sciences from 1974 to 1978.

A native of Maine, he was born and raised in Auburn; he attended nearby Bates College in Lewiston, Maine, where he graduated magna cum laude with a B.S. in chemistry in 1943. He completed his doctorate at Harvard University in 1947, under the mentorship of Paul Doughty Bartlett, and a postdoctorate at University of California, Los Angeles with Saul Winstein in 1948.

Early life and education George Simms Hammond was born on May 22, 1921, in Auburn, Maine. As a result, he was homeschooled until the sixth grade. Afterwards, he was educated at various Auburn public schools before graduating in 1938. After graduating he took a gap year to continue operating his dairy farm. After his educational hiatus he applied to and was accepted into Bates College, in Lewiston, Maine. He graduated with a Bachelors of Science in chemistry magna cum laude and Phi Beta Kappa in January 1943.

Life outside of academia Aside from the academic world, during all these years, George Hammond, "made many public speeches on controversial themes, both political (e.g., the invasion of Cambodia, delivered in 1971 at a public rally on Caltech's Olive Walk) and scientific (e.g., the future of chemistry)" The way this excerpt was written says many things about George Hammond, starting with his passionate character. Hammond fought for everything he believed in. He cared about his nation and he was also a little reckless about the consequences he could suffer for defying the government. Also, in the excerpt, a sarcastic side of Hammond can be perceived, a man of strong character with the ability to recognize when he is wrong.

Later pursuits He was appointed as the Foreign Secretary of the National Academy of Sciences in 1974 and served for one term retiring in 1978. He also gave notable speeches on political issues such as the invasion of Cambodia,

Therefore, the geometric structure of a state can be predicted by comparing its energy to the species neighboring it along the reaction coordinate. For example, in an exothermic reaction the transition state is closer in energy to the reactants than to the products. Therefore, the transition state will be more geometrically similar to the reactants than to the products. In contrast, however, in an endothermic reaction the transition state is closer in energy to the products than to the reactants. So, according to Hammond's postulate the structure of the transition state would resemble the products more than the reactants. This type of comparison is especially useful because most transition states cannot be characterized experimentally.

Hammond's postulate also helps to explain and rationalize the Bell–Evans–Polanyi principle. Namely, this principle describes the experimental observation that the rate of a reaction, and therefore its activation energy, is affected by the enthalpy change of that reaction. Hammond's postulate explains this observation by describing how varying the enthalpy of a reaction would also change the structure of the transition state. In turn, this change in geometric structure would alter the energy of the transition state, and therefore the activation energy and reaction rate as well.

The postulate has also been used to predict the shape of reaction coordinate diagrams. For example, electrophilic aromatic substitutions involves a distinct intermediate and two less well defined states. By measuring the effects of aromatic substituents and applying Hammond's postulate it was concluded that the rate-determining step involves formation of a transition state that should resemble the intermediate complex.

During the 1940s and 1950s, chemists had trouble explaining why even slight changes in the reactants caused significant differences in the rate and product distributions of a reaction. In 1955 George S. Hammond, a young professor at Iowa State University, postulated that transition-state theory could be used to qualitatively explain the observed structure-reactivity relationships. However, Hammond's version has received more attention since its qualitative nature was easier to understand and employ than Leffler's complex mathematical equations. Hammond's postulate is sometimes called the Hammond-Leffler postulate to give credit to both scientists.

Interpreting the postulate Effectively, the postulate states that the structure of a transition state resembles that of the species nearest to it in free energy. This can be explained with reference to potential energy diagrams:

In case (a), which is an exothermic reaction, the energy of the transition state is closer in energy to that of the reactant than that of the intermediate or the product. Therefore, from the postulate, the structure of the transition state also more closely resembles that of the reactant. In case (b), the energy of the transition state is close to neither the reactant nor the product, making none of them a good structural model for the transition state. Further information would be needed in order to predict the structure or characteristics of the transition state. Case (c) depicts the potential diagram for an endothermic reaction, in which, according to the postulate, the transition state should more closely resemble that of the intermediate or the product.

Another significance of Hammond's postulate is that it permits us to discuss the structure of the transition state in terms of the reactants, intermediates, or products. In the case where the transition state closely resembles the reactants, the transition state is called “early” while a “late” transition state is the one that closely resembles the intermediate or the product.

An example of the “early” transition state is chlorination. Chlorination favors the products because it is an exothermic reaction, which means that the products are lower in energy than the reactants. When looking at the adjacent diagram (representation of an "early" transition state), one must focus on the transition state, which is not able to be observed during an experiment. To understand what is meant by an “early” transition state, the Hammond postulate represents a curve that shows the kinetics of this reaction. Since the reactants are higher in energy, the transition state appears to be right after the reaction starts.

An example of the “late” transition state is bromination. Bromination favors the reactants because it is an endothermic reaction, which means that the reactants are lower in energy than the products. Since the transition state is hard to observe, the postulate of bromination helps to picture the “late” transition state (see the representation of the "late" transition state). Since the products are higher in energy, the transition state appears to be right before the reaction is complete.

One other useful interpretation of the postulate often found in textbooks of organic chemistry is the following:

Assume that the transition states for reactions involving unstable intermediates can be closely approximated by the intermediates themselves.

This interpretation ignores extremely exothermic and endothermic reactions which are relatively unusual and relates the transition state to the intermediates which are usually the most unstable.

Structure of transition states

SN1 reactions

Hammond's postulate can be used to examine the structure of the transition states of a SN1 reaction. In particular, the dissociation of the leaving group is the first transition state in a SN1 reaction. The stabilities of the carbocations formed by this dissociation are known to follow the trend tertiary > secondary > primary > methyl.

Therefore, since the tertiary carbocation is relatively stable and therefore close in energy to the R-X reactant, then the tertiary transition state will have a structure that is fairly similar to the R-X reactant. In terms of the graph of reaction coordinate versus energy, this is shown by the fact that the tertiary transition state is further to the left than the other transition states. In contrast, the energy of a methyl carbocation is very high, and therefore the structure of the transition state is more similar to the intermediate carbocation than to the R-X reactant. Accordingly, the methyl transition state is very far to the right.

SN2 reactions Substitution, nucleophilic bimolecular reactions are concerted reactions where both the nucleophile and substrate are involved in the rate limiting step. Since this reaction is concerted, the reaction occurs in one step, where the bonds are broken, while new bonds are formed. Therefore, to interpret this reaction, it is important to look at the transition state, which resembles the concerted rate limiting step. In the "Depiction of SN2 Reaction" figure, the nucleophile forms a new bond to the carbon, while the halide (L) bond is broken.

E1 reactions

An E1 reaction consists of a unimolecular elimination, where the rate determining step of the mechanism depends on the removal of a single molecular species. This is a two-step mechanism. The more stable the carbocation intermediate is, the faster the reaction will proceed, favoring the products. Stabilization of the carbocation intermediate lowers the activation energy. The reactivity order is (CH3)3C- > (CH3)2CH- > CH3CH2- > CH3-.

Furthermore, studies describe a typical kinetic resolution process that starts out with two enantiomers that are energetically equivalent and, in the end, forms two energy-inequivalent intermediates, referred to as diastereomers. According to Hammond's postulate, the more stable diastereomer is formed faster.

E2 reactions Elimination, bimolecular reactions are one step, concerted reaction where both base and substrate participate in the rate limiting step. In an E2 mechanism, a base takes a proton near the leaving group, forcing the electrons down to make a double bond, and forcing off the leaving group-all in one concerted step. The rate law depends on the first order concentration of two reactants, making it a 2nd order (bimolecular) elimination reaction. Factors that affect the rate determining step are stereochemistry, leaving groups, and base strength.

A theory, for an E2 reaction, by Joseph Bunnett suggests the lowest pass through the energy barrier between reactants and products is gained by an adjustment between the degrees of Cβ-H and Cα-X rupture at the transition state. The adjustment involves much breaking of the bond more easily broken, and a small amount of breaking of the bond which requires more energy. This conclusion by Bunnett is a contradiction from the Hammond postulate. The Hammond postulate is the opposite of what Bunnett theorized. In the transition state of a bond breaking step it involves little breaking when the bond is easily broken and much breaking when it is difficult to break. National Medal of Science (1994) Glenn T. Seaborg Medal (1994) Othmer Gold Medal (2003)