Louis Plack Hammett

Louis Plack Hammett (February 21, 1894 – February 19, 1980) was an American physical chemist renowned for his contributions to physical organic chemistry, particularly the development of the Hammett equation and the Hammett acidity function. His work laid foundational principles for quantifying the effects of substituents on reaction rates and equilibria, influencing both academic research and industrial chemistry.

Early life and education

• Born in Mount Vernon, Iowa, United States.
• Earned a Bachelor of Science in chemistry from the University of Illinois at Urbana‑Champaign in 1915.
• Completed a Ph.D. in physical chemistry at the same institution in 1919 under the supervision of Arthur A. Noyes.

Academic and professional career

• Remained at the University of Illinois as a faculty member, eventually attaining the position of professor of chemistry.
• Served as department head of chemistry from 1947 to 1959.
• Conducted research primarily in the fields of reaction kinetics, thermochemistry, and the quantitative description of substituent effects.

Major contributions

1. Hammett equation (1937)

   • Introduced a linear free‑energy relationship correlating reaction rates and equilibrium constants with substituent constant (σ) values, expressed as:
     $$ \log\frac{k}{k_{0}} = \rho\sigma $$
   • Provided a systematic method for evaluating electronic effects of substituents on aromatic compounds, becoming a central tool in physical organic chemistry.

2. Hammett acidity function (H₀)

   • Developed an acidity scale applicable to extremely strong acid media where the conventional pH scale is inadequate.
   • Defined by the equation:
     $$ H_{0} = pK_{\mathrm{BH^{+}}} + \log\frac{[B]}{[BH^{+}]} $$
   • The function is widely used in superacid research and industrial processes involving highly acidic environments.

3. Thermodynamic data compilation

   • Produced extensive tables of thermodynamic properties (enthalpies, entropies, free energies) for a broad range of organic compounds, facilitating predictive calculations in chemistry.

Awards and honors

• Elected member of the National Academy of Sciences (1945).
• Fellow of the American Academy of Arts and Sciences.
• Recipient of the Priestley Medal, the highest honor of the American Chemical Society (1978).
• Awarded the National Medal of Science for his contributions to chemical science (1963).

Personal life

• Married with two children; maintained a lifelong association with the University of Illinois community.
• Retired from active research in the early 1970s but continued to advise graduate students and collaborate on scholarly publications.

Death

• Died on February 19, 1980, in Champaign, Illinois, United States.

Legacy

• The Hammett equation remains a staple in undergraduate and graduate curricula, and its concepts are integral to modern computational chemistry and drug design.
• The Hammett acidity function continues to be employed in the study of superacid chemistry and catalysis.
• Several chemical societies and academic institutions have established lectureships and awards in his name to honor contributions to physical organic chemistry.