General Acid Catalysis of the Fading of Photoisomerized 2,4-Dinitrotoluene

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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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