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Photochemical Reaction Mechanisms of 2‐Nitrobenzyl Compounds in Solution, I. 2‐Nitrotoluene: Thermodynamic and Kinetic Parameters of the aci ‐Nitro Tautomer
Author(s) -
Schwörer Markus,
Wirz Jakob
Publication year - 2001
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/1522-2675(20010613)84:6<1441::aid-hlca1441>3.0.co;2-w
Subject(s) - chemistry , tautomer , nitro , kinetic energy , computational chemistry , photochemistry , medicinal chemistry , organic chemistry , alkyl , physics , quantum mechanics
The largely reversible, light‐induced tautomerization of 2‐nitrotoluene ( 1 ) to the quinonoid aci ‐nitro tautomer aci ‐ 1 was studied by flash photolysis as a benchmark for comparison with the widely used nitrobenzyl phototriggers (`caged compounds'). The pH‐rate profile for the decay of aci ‐ 1 in aqueous solution exhibits downward curvature at pH 3 – 4, which is attributed to pre‐equilibrium ionization of the nitronic acid aci ‐ 1 to its anion 1 − (p K a =3.57). Two regions of upward curvature, at pH ca . 6 and <0 ( H 0 ≈−1), each indicate a change in the reaction mechanism. The elementary reactions that dominate between the curved regions are assigned on the basis of kinetic isotope effects and the observation of general acid catalysis: Hydronium ions regenerate 2‐nitrotoluene by C‐protonation of 1 − in the pH range of 0 – 6, and H 2 O is the proton source at pH>6. A hird, irreversible Nef ‐type isomerization of aci ‐ 1 prevails in highly acidic solutions (pH<0). The equilibrium constant for the thermal tautomerization of 1 to aci ‐ 1 is estimated as p K T =17.0±0.2 based on kinetic data.