Abstract
Short-lived colored species are formed when a water solution of 2,4-dinitrotoluene is exposed to ultraviolet light. The colored species displayed an acid—base equilibrium and the absorption spectra of the acid form and the base form were measured. The half-life time for the first-order fading reaction was varied over a 105-fold range (20 μsec to 1 sec) by changing the acidity and temperature. The fading reaction was found to be general acid-catalyzed over the pH region 2–13. The experiment supports a mechanism having the following essential features. The nitro form is photochemically isomerized to an aci-nitro form which is in equilibrium with the anion. The rate-determining step in the fading reaction is the neutralization of this anion. The rate constants for the neutralization by various acids have been measured at 30°C to be 1.8×10—2 (H2O), 4.8×103 (HAc), 2.4×104 (+NH3CH2COOH), 7.4×104 (H+) liter sec—1 mole—1.
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