We report a physical organic chemistry experiment for second-year college students. Students perform nucleophilic aromatic substitution (NAS) reactions on 5,10,15,20-tetrakis(2,3,4,5,6-pentafluorophenyl)porphyrin (TPPF<sub>20</sub>) using three different nucleophiles. Substitution occurs preferentially at the 4-position (para) because it is thermodynamically favored, and the 2- and 6-position (ortho) positions are kinetically disfavored because of steric interactions with the porphyrin ring. The activation energy depends heavily on the nucleophile. An open source software (ImageJ from NIH) can be used to quantify relative intensities of spots on a TLC plate obtained from different times and varying temperatures. This data can be used to generate Arrhenius plots allowing students to determine relative activation energies for three different primary nucleophiles: butanethiol, butylamine, and butanol. The experiment was developed by five undergraduates and evaluated by 40 organic chemistry II students and eight students in a physical chemistry laboratory. Students gained a deeper understanding of the relationships between the NAS mechanism, Arrhenius plots, and activation energy.

Experimental determination of activation energy of nucleophilic aromatic substitution on porphyrins

255th National Meeting (2018)

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