HAMMETT, LOUIS PLACK

(b. Wilmington, Delaware, 7 April 1894; d. Medford, New Jersey, 23 February 1987),

chemistry.

Hammett is considered one of the founding fathers of physical organic chemistry in the United States. He is best known for his work with strong acids that led to the Hammett acidity function, his work with linear free energy relationships that led to the Hammett equation, and his 1940 book, Physical Organic Chemistry, which gave a name to the subdiscipline and provided a research agenda for workers in the field for the next twenty years.

Early Life and Education . Although he was born in Wilmington, Delaware, Hammett—the oldest of three children of Philip and Marie (Plack) Hammett—was raised in Portland, Maine. His father, a Harvard graduate with two additional years at the Massachusetts Institute of Technology (MIT) studying mechanical engineering, was an official of the Maine Central Railroad. Hammett’s home was filled with books, and he became an avid reader. His father and an uncle who was an architect introduced him to the use of tools early in life. This early introduction to instruments may account for his willingness to use new technology throughout his research career.

Hammett attended Harvard and was strongly influenced by the organic chemistry lectures of Elmer Peter Kohler. Kohler, in Hammett’s words, “inspired his students to be very much interested in problems of structure and reactivity, whatever one could tell about mechanisms without studying reaction rates” (Gortler, 1978, p. 6). This inspiration, along with the belief that reaction rates could provide significant information about the detailed pathways of chemical reactions, was to remain with Hammett throughout his career. James B. Conant, who became one of the country’s most eminent organic chemists and later served as president of Harvard, was Hammett’s lab instructor in organic chemistry at Harvard. Hammett would have occasion to interact with Conant several times during his career.

After graduation in 1916, Hammett, who had won a Sheldon Traveling Fellowship, spent a year in Zürich at the Eidgenössische Technische Hochschule working with Hermann Staudinger. On his return to the United States, he took a position at the National Bureau of Standards laboratories doing war-related research on cellulose derivatives that were used to coat airplanes. The supervisor for the chemical group was Hal Beans, a physical chemist and a professor at Columbia University, who became a good friend and a mentor.

Shortly after the war, Hammett married Janet Thorpe Marriner of Portland, Maine; they eventually had two children, Philip and Jane. He went to work for the private laboratory of E. C. Worden and S. Isermann in Millburn, New Jersey, where he continued to work on organic materials, primarily dyestuffs and pharmaceuticals.

In 1919 Hammett visited Columbia, was offered a teaching position, and then did his doctoral thesis work (on the hydrogen electrode) with Hal Beans. He credited Beans with teaching him the art of research. After obtaining his PhD in 1923, Hammett was offered an assistant professorship at Columbia. Most of his teaching in the early years was in qualitative analysis; he published a textbook, Solutions of Electrolytes: With Particular Application to Qualitative Analysis, in 1929. A second edition of this text appeared in 1936.

The Acidity Function . In 1928 Hammett published an article titled “The Theory of Acidity” in which he advanced several new concepts, one of which concerned the leveling effect of water on acidity. His work in qualitative analysis, his newly developed ideas on acidity, and his reading of the work of J. N. Brønsted and A. Hantzsch led to his first major research contribution, the concept of superacids and the acidity function. This work, a search for a quantitative measure of the acidity of strong acid solutions, began with Alden Deyrup and Nicholas Dietz, his first graduate students. Hammett and Deyrup worked in concentrated sulfuric acid, measuring the ability of the acid to protonate a series of weak neutral bases. The reaction is shown below, where B is a weak base and BH⁺ is the protonated form of the base.

Hammett and Deyrup were constrained in their choice of weak bases by the limitations imposed by the technology of the time. They had to choose compounds (in this case, substituted anilines) that were sufficiently colored in either the protonated or unprotonated state, in order to determine concentrations colorimetrically. After putting a weak base in an acid medium, they would determine the ratio of the protonated and unprotonated species. They would then do the same experiment using more concentrated acid, continuing this process with the same base until the ratio of BH⁺/B became too large to measure accurately. Then they would switch to a weaker base and continue the process. It soon became clear that an equation could be written that described the ability of the strong acid to protonate weak bases:

(pK _HB⁺ is the measured ionization constant of BH⁺ in water).

This relationship eventually became known as the Hammett acidity function. Surprisingly, the H_o scale shows that 100 percent sulfuric acid is ten billion times more acidic than 10 percent acid. The rates of many acid-catalyzed reactions demonstrate a linear relationship with H_o, expressed by the equation H_o + log k = constant (where k is the rate constant for a given reaction). Hammett initially thought that his acidity function was unique, but subsequent work by many chemists, using assorted strong acids and different base systems, has established over four hundred different acidity functions.

Structure-Reactivity Relationships . In 1933 Hammett and his student Helmuth Phluger published a paper describing the reaction of trimethylamine and a series of methyl esters.

They found a linear relationship between the logarithms of the specific rates of the reactions and the logarithms of the ionization constants (equilibrium constants) of the carboxylic acids

whose methyl esters were being studied, that is, plotting log k (the logarithms of the rate constants) versus log K(the logarithms of the ionization constants) gave a straight line. J. N. Brønsted and K. Pedersen had shown a similar relationship between the rates of some acid-catalyzed reactions and the acidity constants of the catalyzing acids in 1924.

By 1935 Hammett, as well as G. N. Burkhardt in England, had discovered a large number of similar linear free energy relationships in the reactions of substituted benzene derivatives. In a Chemical Reviews article that year, Hammett illustrated these relationships with a number of graphs of log k versus log K for a variety of reactions, most of which involve the side-chain reactions of meta- and para- substituted benzene derivatives.

In a 1937 article Hammett generalized these structure-reactivity relationships with the equation log k/k_o = σρ, or log K/K_o= σρ, where k and k_o are the rates of the substituted and unsubstituted benzene derivative, respectively, K and K_o are equilibrium constants, σ is a substituent constant that measures the effect of replacing H by a given substituent in the meta- or para- position, and ρ, the reaction constant, is the slope of the line characteristic of the reaction being studied. A reaction that is very sensitive to substituent changes will have a much larger absolute ρ value than a reaction that is only mildly affected by substituent changes.

By defining ρ = 1 for the ionization of benzoic acids in water at 25°C, and setting σ = 0 for X = H, Hammett was able to tabulate fifteen σ values for various substituents (see Figure 1). Using these values and a series of reactions that obey the Hammett equation, he was able to determine additional σ values. In the 1937 paper, Hammett listed twenty-nine σ values and thirty-eight reactions whose rates or equilibria correlated well with the equation.

After the 1937 paper, Hammett did no more work on linear free energy relationships, although there were chapters on the subject in his 1940 and 1970 books. However, work in this area continued in many laboratories throughout the world for several decades and prompted an enormous amount of discussion and research regarding the causes of substituent effects on rate changes.

Hammett continued to study the rates of a variety of organic reactions, mainly nucleophilic substitution reactions, through the late 1930s and the 1940s. He was, of course, looking at organic problems through the eyes of a physical chemist and, in so doing, he was establishing a methodology for others to follow.

The Book: Physical Organic Chemistry . In 1940 Hammett published Physical Organic Chemistry, a text for an advanced course in chemistry. In its preface, Hammett says that the name he has chosen “implies the investigation of the phenomena of organic chemistry by quantitative and mathematical methods,” essentially the methods that Hammett himself had used in the previous twelve years. The book was subtitled Reaction Rates, Equilibria, and Mechanisms, reflecting Hammett’s primary interests.

In the preface to the book, Hammett wrote of an earlier era, “For a time, it was almost a point of honor with both physical and organic chemists to profess ignorance of the other’s field.” In the 1966 article “Physical Organic Chemistry in Retrospect,” he wrote, “To many physical chemists in the 1920s and early 1930s, the organic chemist was a grubby artisan engaged in an unsystematic search for new compounds, a search which was strongly influenced by the profit motive” (p. 464) Hammett, through his research and his writing, was bridging the gap between these two subdivisions of chemistry.

Hammett was not the only one applying the “methods” of physical chemistry to organic problems. There had been a growing community of such researchers from the late 1920s through the 1930s in England and the United States. Many were organic chemists who were, in a sense, rebelling from classical organic chemistry that had been almost exclusively involved with the preparation of new compounds and the determination of structure. Many were students who had been trained by the first wave that included Christopher Ingold, James Conant, Morris Kharasch, Howard Lucas, William Young, and Hammett. These—mostly young—chemists were seeking to understand the reactions of organic chemistry. Their work had not yet been defined as a subdiscipline, and Hammett’s book, if nothing else, provided them with a name, a corporate identity, “physical organic chemists.”

Figure 1

In the book, Hammett described and defined the classic work in physical organic chemistry. The book also set a research agenda for decades. Hammett was not only describing the important problems, he was saying, here is how to look at them and here are some of the difficulties. The book was still being cited in research papers forty years after it was first published.

The War Years . In 1941 Hammett wanted to contribute to the war effort. The United States had not yet entered the war, but research groups were being established, and he asked to join a group being organized by James Conant for work on military explosives. The National Defense Research Committee sent Hammett to England during the summer of 1941 to find out about the British wartime research efforts. When he returned he became the associate research director of the Explosives Research Laboratory in Bruceton, Pennsylvania. When George Kistiakowsky, the laboratory director, moved to Los Alamos, New Mexico, where he was involved with the development of the atomic bomb, Hammett took over as the director of the Bruceton laboratory, which made several important contributions to the war effort.

Postwar Career . After World War II, Hammett returned to Columbia and restarted his research program, investigating, in particular, the hydration of olefins. He also did some work on the use of ion exchange resins in acid catalysis and, finally, he worked with several collaborators on the use of the stirred flow reactor in the study of fast reactions. This use of a new technique was an appropriate career-ending exercise for a chemist who had pioneered in the use of various spectrophotometric techniques.

In 1951 Hammett became chairman of the Chemistry Department at Columbia, a position he held for six years. During his tenure as department chair he hired Cheves Walling, Gilbert Stork, and Ronald Breslow, making the department one of the major organic chemistry centers in the country. Hammett was also active in the American Chemical Society, served a number of years on the board of directors, and was chairman of the board in 1961. He retired from Columbia in 1961 but continued to do some consulting and writing, and lectured at a number of universities. He published the second edition of Physical Organic Chemistry in 1970.

Hammett was the recipient of the William H. Nichols Medal in 1957, the James Flack Norris Award for Outstanding Achievement in the Teaching of Chemistry in 1960, the James Flack Norris Award in Physical Organic Chemistry in 1966, the Priestley Medal and the Willard Gibbs Medal in 1961, the G. N. Lewis Medal in 1967, and the Chandler Medal and the National Medal of Science in 1968. He was also a member of the National Academy of Sciences and an Honorary Fellow of the Royal Society of Chemistry.

BIBLIOGRAPHY

A complete list of Hammett’s publications, along with autobiographical notes, may be obtained from the American Institute of Physics, Center for History of Physics, College Park, Maryland.

WORKS BY HAMMETT

“The Theory of Acidity.” Journal of the American Chemical Society 50 (1928): 2666–2673.

With Alden J. Deyrup. “A Series of Simple Basic Indicators. I. The Acidity Functions of Mixtures of Sulfuric and Perchloric Acids with Water.” Journal of the American Chemical Society 54 (1932): 2721–2739. The parent paper on the acidity function.

With Helmuth Phluger. “The Rate of Addition of Methyl Esters to Trimethylamine.” Journal of the American Chemical Society 55 (1933): 4079–4089. Hammett’s first paper on structure-reactivity relationships.

“Some Relations between Reaction Rates and Equilibrium Constants.” Chemical Reviews 17 (1935): 125–136.

“The Effect of Structure upon the Reactions of Organic Compounds. Benzene Derivatives.” Journal of the American Chemical Society 59 (1937): 96–103. The foundation paper for the Hammett equation.

“Physical Organic Chemistry in Retrospect.” Journal of Chemical Education 43 (1966): 464–469. An autobiographical address on the occasion of his receipt of the James Flack Norris Award in Physical Organic Chemistry.

Physical Organic Chemistry: Reaction Rates, Equilibria, and Mechanisms. New York: McGraw-Hill, 1940; 2nd ed., 1970.

OTHER SOURCES

Gortler, Leon. Oral Histories with Louis Hammett, 1 May 1978 and 22 May 1978. American Institute of Physics, Center for History of Physics, College Park, MD.

Shorter, John. “Hammett Memorial Lecture.” In Progress in Physical Organic Chemistry, vol. 17, edited by Robert W. Taft. New York: John Wiley and Sons, 1990.

——. “The Centenary of the Birth of Louis Hammett. ” Pure and Applied Chemistry 67 (1995): 835–840.

——. “The Prehistory of the Hammett Equation.” Chemické Listy 94 (2000): 210–214.

Westheimer, Frank H. “Louis Plack Hammett.” Biographical Memoirs of the National Academy of Sciences 72 (1997): 137–149.

Leon B. Gortler