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Synthesis and photolysis of aromatic nitrate esters Csizmadia, Imre Gyula

Abstract

Nitrate esters of aromatic alcohols were synthesized by esterification which involved competition between 0-nitration and aromatic C-nitration. TLC analysis gave a pattern of adsorption affinities for the nitroxy group and other substituents consistent with the molecular conformations. The NMR frequency of the α-protons showed a linear correlation with the accepted group electronegativities of the substituents in molecules with rigid carbon skeletons and gave a value of 4.18 kcal/mole for the nitroxy group. The symmetric and asymmetric IR stretching frequencies of nitroxy groups in dilute cyclohexane solution were shifted to higher values by steric interaction between contiguous groups when the C-ONO₂ bonds were constrained to coplanarity. The UV spectra showed benzenoid, π→π*, and n→π* bands and a solvent perturbation effect assigned to a solvent → solute charge-transfer interaction. The nitrate esters reacted with the solvent when irradiated in solution in the wavelength range of the n→π* excitation. Product analysis indicated that C-C bond cleavage occurred via intermediate alkoxyl radicals. Rate studies showed the following order of reactivity: benzyl nitrate < dl-hydrobenzoin dinitrate < meso-hydrobenzoin dinitrate, < trans-1, 2-acenaphthenediol nitrate < cis-1, 2-acenaphthenediol dinitrate. The rate measurements and ESR spectra gave evidence of intramolecular energy transfer from the naphthalene moiety to the nitroxy groups in the 1, 2-acenaphthenediol dinitrates assigned as a singlet→singlet transfer. Calculations from the apparent first-order rate constants and spectra showed that benzyl nitrate, and meso- and dl-hydrobenzoin dinitrates photolysed with a quantum yield of about 2 in benzene solution. A solvent effect caused k[subscript Et₂O] > k[subscript EtOH] >k[subscript PhH]. On the basis of product analysis, rate measurements, estimated quantum yields and ESR spectra a mechanism for the nitrate ester photolysis was proposed.

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