Trinitrotoluene, proton transfer

This reaction between s-trinitrotoluene and ethoxide ion, elegantly studied by Caldin and Long (47), is a reversible one to form a purple solution. By spectroscopic techniques it is possible to measure the forward rate, the reverse rate and the equilibrium constant. Although the form of the kinetic equations does not permit a differentiatimi between a proton transfer process and an addition process, the authors favor the former explanation, attributing the purple color to formation of the anion XII, and citing as evidence the deuterium exchange experiment and the fact that trinitrotoluene, in the presence of ethoxide ion or pyridine, acts as a nucleophile toward benzaldehyde. The purple solution is decolorized by a series of weak acids at rates which are related to the dissociation constants of the acids and measurable at temperatures from — 80° to -f-20°. 

At higher initial concentrations of s-trinitrotoluene and ethoxide ion it is possible to detect a reaction which is approximately 50 times faster than the proton transfer reaction, with which it probably occurs concurrently (48). The product of this reaction, almost certainly a charge-transfer complex (49), is brown in color. 

The PMP acetal is quite susceptible to acid-catalyzed cleavage. In the following case a normally readily cleaved cyclopentylidene group could not be cleaved in preference to the PMP acetal. In a very creative move the authors prepared a charge transfer complex with the extremely electron-dehcient trinitrotoluene and the electron-rich PMP groups to suppress protonation... 

Mass spectrometry has been used to identily complexes formed in the gas phase by reaction of oxyanions and carbanions with 1,3,5-triazine. Spectroscopic studies and DFT calculations on the species formed by reaction of ethylamine with 2,4-dinitrotoluene in DMSO indicate that transfer of a methyl proton rather than nucleophilic attack is the major interaction. Two methods for the detection of 2,4,6-trinitrotoluene (TNT) using fluorescence techniques rely on the formation of its complexes with amines. Also cobalt-doped zinc sulfide quantum dots have been used to interact with complexes formed from TNT and L-cysteine allowing detection of the nitro compound. A complex between 2,4-dinitroanisole and L-cysteine methyl ester has been identified by surface-enhanced Raman spectroscopy. ... 

These studies by Caldin et al. thus explain, in a completely satisfactory manner, Foster s originally puzzling observation that the interactions of alkoxides with s-trinitrotoluene, s-trinitroanisole, and s-trinitrobenzene result in products having strikingly different visible absorption spectra. In the first case, XII is formed by abstraction of a proton from the methyl group in the second, the process is one of addition to give III in the third, a charge-transfer complex is formed. These are three distinctly different structural types, and it is to be expected that their visible spectra will differ. 

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