Electron donating or accepting power of organic substituents is an important parameter affecting many properties of parent molecules, most notably their reactivity and pKa of ionizable groups. These substituent properties are usually described by Hammett sigma constants obtained by measuring ionization of substituted benzoic acids. Although values of these constants have been measured for the most common functional groups, data for many important substituents are not available. Some time ago we reported a method to calculate substituent descriptors compatible with Hammett sigma constants using quantum chemically derived parameters. The present publication revisits the older study by applying more sophisticated methodology and a larger training data set, as well as introduces a free web tool allowing to calculate substituent descriptors compatible with Hammett sigma constants available at https://bitly.com/getsigmas .

the equilibrium constants in this relationship come from thermodynamics but the reaction rate constants come from chemical kinetics , and neither discipline In organic chemistry, the Hammett equation describes a linear free-energy relationship relating reaction rates and equilibrium constants for many reactions involving benzoic acid derivatives with meta- and para-substituents to each other with just two parameters: a substituent constant and a reaction constant. This equation was developed and published by Louis Plack Hammett in 1937 as a follow-up In organic chemistry, the Hammett equation describes a linear free-energy relationship relating reaction rates and equilibrium constants for many reactions involving benzoic acid derivatives with meta- and para-substituents to each other with just two parameters: a substituent constant and a reaction constant. This equation was developed and published by Louis Plack Hammett in 1937 as a follow-up to qualitative observations in his 1935 publication. The basic idea is that for any two reactions with two aromatic reactants only differing in the type of substituent, the change in free energy of activation is proportional to the change in Gibbs free energy. It is important to understand, that the equilibrium constants in this relationship come from thermodynamics but the reaction rate constants come from chemical kinetics , and neither discipline predicts such relationship. Instead, it was introduced by Hammett empirically. Hammett equation belongs to a larger set of correlations between the rate of a chemical reaction and its driving force. The basic equation is: the hydrolysis of substituted cinnamic acid ester in ethanol/water (+1.267) the ionization of substituted phenols in water (+2.008) the acid catalyzed esterification of substituted benzoic esters in ethanol (−0.085) the… In organic chemistry, the Hammett equation describes a linear free-energy relationship relating reaction rates and equilibrium constants for many reactions involving benzoic acid derivatives with meta- and para-substituents to each other with just two parameters: a substituent constant and a reaction constant. This equation was developed and published by Louis Plack Hammett in 1937 as a follow-up to qualitative observations in his 1935 publication. The basic idea is that for any two reactions with two aromatic reactants only differing in the type of substituent, the change in free energy of activation is proportional to the change in Gibbs free energy. It is important to understand, that the equilibrium constants in this relationship come from thermodynamics but the reaction rate constants come from chemical kinetics , and neither discipline predicts such relationship. Instead, it was introduced by Hammett empirically. Hammett equation belongs to a larger set of correlations between the rate of a chemical reaction and its driving force. The basic equation is: the hydrolysis of substituted cinnamic acid ester in ethanol/water (+1.267) the ionization of substituted phenols in water (+2.008) the acid catalyzed esterification of substituted benzoic esters in ethanol (−0.085) the acid catalyzed bromination of substituted acetophenones (Ketone halogenation) in an acetic acid/water/hydrochloric acid (+0.417) the hydrolysis of substituted benzyl chlorides in acetone-water at 69.8 °C (−1.875). The reaction constant, or sensitivity constant, ρ, describes the susceptibility of the reaction to substituents, compared to the ionization of benzoic acid. It is equivalent to the slope of the Hammett plot. Information on the reaction and the associated mechanism can be obtained based on the value obtained for ρ. If the value of: ρ>1, the reaction is more sensitive to substituents than benzoic acid and negative charge is built during the reaction (or positive charge is Roberts and Moreland studied the reactivities of 4-substituted bicyclo[2.2.2]octane-1-carboxylic acids and esters. In such a molecule, transmission of electrical effects of substituents through the ring by resonance is not possible. Hence, this hints on the role of the π-electrons in the transmission of substituent effects through aromatic systems. Reactivity of 4-substituted bicyclo[2.2.2]octane-1-carboxylic acids and esters were measured in 3 different processes, each of which had been previously used with the benzoic acid derivatives. A plot of log(k) against log(KA) showed a linear relationship. Such linear relationships correspond to linear free energy relationships, which strongly imply that the effect of the substituents are exerted through changes of potential energy and that the steric and entropy terms remain almost constant through the series. The linear relationship fit well in the Hammett Equation. For the 4-substituted bicyclo[2.2.2.]octane-1-carboxylic acid derivatives, the substituent and reaction constants are designated σ’ and ρ’. where R {\displaystyle R} is the gas constant, T {\displaystyle T} is the absolute temperature, and k R {\displaystyle k_{R}} and k H {\displaystyle k_{H}} are the rate constants for the substituted and unsubstituted reactions, respectively. This expression relates the Hammett product σ ρ {\displaystyle \sigma \rho } to the scalar product between a substituent-dependent force and a reaction-dependent displacement. In this interpretation, the substituent constant σ {\displaystyle \sigma } can be interpreted as a relative effective force characteristic of the substituent, while the reaction constant ρ {\displaystyle \rho } can be interpreted as a relative effective reaction-path length. The approach was illustrated for the alkaline hydrolysis of substituted ethyl benzoates, for which substituent-induced forces projected along the path from the reactant to the transition state reproduced density functional theory changes in activation energy.

Louis Plack Hammett (April 7, 1894 – February 9, 1987) was an American physical chemist. He is known for the Hammett equation, which relates reaction rates to equilibrium constants for certain classes of organic reactions involving substituted aromatic compounds. He is also known for his research into superacids and his development of a scheme for comparing their acidities based on what is now kno Louis Plack Hammett (April 7, 1894 – February 9, 1987) was an American physical chemist. He is known for the Hammett equation, which relates reaction rates to equilibrium constants for certain classes of organic reactions involving substituted aromatic compounds. He is also known for his research into superacids and his development of a scheme for comparing their acidities based on what is now known as the Hammett acidity function. The Curtin–Hammett principle bears his name. The awards he obtained included the Priestley Medal in 1961, the Willard Gibbs Award in 1961, the National Medal of Science in 1967, and in 1975 the Barnard Medal for Meritorious Service to Science. Hammett grew up in Portland, Maine, and studied in Harvard and Switzerland. He earned his Ph.D. at Columbia University. He authored an influential textbook on physical organic chemistry, and is credited with coining the term. Louis Plack Hammett (April 7, 1894 – February 9, 1987) was an American physical chemist. He is known for the Hammett equation, which relates reaction rates to equilibrium constants for certain classes of organic reactions involving substituted aromatic compounds. He is also known for his research into superacids and his development of a scheme for comparing their acidities based on what is now known as the Hammett acidity function. The Curtin–Hammett principle bears his name. The awards he obtained included the Priestley Medal in 1961, the Willard Gibbs Award in 1961, the National Medal of Science in 1967, and in 1975 the Barnard Medal for Meritorious Service to Science. Hammett grew up in Portland, Maine, and studied in Harvard and Switzerland. He earned his Ph.D. at Columbia University. He authored an influential textbook on physical organic chemistry, and is credited with coining the term. == References == == Further reading == Hammond, George S. (1997) Physical organic chemistry after 50 years: It has changed, but is it still there? IUPAC V9. 69, No. 9, pp. 1919–1922. Westheimer, F. H. (1997) Biographical Memoirs V72, pp. 136–149. Archived PDF Version (Archived and PDF Links added 15 May 2024) Young, Robin V., Sessine, Suzanne (1999) World of Chemistry Thomson Gale.