Determining Hammett sigma and rho values: Improvements on a published student experiment

Abstract

In a recently published organic chemistry laboratory text, a series of experiments are described in which students are able to relate the structure of a compound to its reactivity (7). The experiments are 1. Determining the pTfa's of substituted benzoic acids. 2. Preparing the benzoic acid methyl esters, 3. Measuring the rates of base hydrolysis of the methyl esters.

Determination of Hammett Equation Rho Constant for the Hydrolysis of p-Nitrophenyl Benzoate Esters

“…The positive ρ values of 2.1-2.4 are consistent with the reported ρ value of 2.23-2.45 for the base hydrolysis of methyl benzoate (2,7,12,13). Similar k app values were obtained when the ester concentrations were in the range of 50 μM to 150 μM.…”

“…Many experiments that were developed to illustrate the structure-reactivity relationship focused on the linear Gibbs energy relationship of the Hammett equation. These experiments include the determination of acidity constants of organic acids (1,2), the determination of chemical shifts of aromatic compounds by nuclear magnetic resonance (NMR) spectroscopy (3)(4)(5)(6), and the determination of rate constants via titration (2,7). We developed a two-part laboratory that combines organic synthesis and kinetics studies to investigate the base hydrolysis rates of p-nitrophenyl benzoate esters.…”

Substituent effect on the radical scavenging activity of 6-chromanol derivatives

“…The reaction constants (r) were then obtained from the slope values. 35 For meta-substituted 6-chromanols, a linear relationship between log k and s was observed (r meta ¼ À3.71, r 2 ¼ 0.91). The ortho-monosubstituted 6-chromanols (r 5-ortho ¼ À0.86, r 2 ¼ 0.89; r 7-ortho ¼ À0.87, r 2 ¼ 0.89) and the 5,7-disubstituted 6-chromanols (r 5,7-diortho ¼ À0.47, r 2 ¼ 0.90) also exhibited linear relationships between their s and Gc scavenging activities.…”

A Robust, “One-Pot” Method for Acquiring Kinetic Data for Hammett Plots Used To Demonstrate Transmission of Substituent Effects in Reactions of Aromatic Ethyl Esters

“…Previous papers ,− in this Journal have used between four and 11 substituents (in addition to the unsubstituted parent compound) to demonstrate the Hammett relationship, with many using parallel collection of data by allocating students to work with a particular substituted compound, then pooling data from the whole class. ,, This approach is vulnerable to variations in the quality of data across the class due to differing skill levels of students, or variations in conditions, in particular temperature variations for kinetic experiments. These issues, in particular the consequences of temperature variation across multiple experiments, are eliminated when data for all substrates are recorded in a single reaction mixture (‘one pot’).…”