Conversion of benzoic acid into phenol under CI condition: investigation of the reactant species

BEGALA, MICHELA

2012-01-01

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Abstract

Convertion of benzoic acid into phenol under CI conditions: investigation of the reactant species Michela Begala Dipartimento di Scienza della Vita e dell’Ambiente, Macrosezione di Scienze del Farmaco, Università degli Studi di Cagliari, Via Ospedale 72, 09124 Cagliari, Italy e-mail: michelabegala@unica.it Benzoyl cations are common and normally abundant fragment ions in the mass spectra of acylbenzenes and derivatives. Eberlin and co-workers reported that the benzoyl cations can work as structurally diagnostic fragment ions (SDFI) [1]. In particulary, they used the gas phase reactivity of the benzoyl cation and their dissociation behaviour to differentiate positional isomers. Unfortunately, in the case of isomeric chlorobenzoic acids, the benzoyl cations under EI and EI-CID conditions fragment identically. In this work, it is reported the results obtained when benzoyl cations are generated under CI condition (CH3OH) in an ion trap. In fact, protonation of the carboxyl group generally leads, spontaneously or induced by collisions (CID), to the elimination of a water molecule, thus resulting in the formation of a benzoyl cation. Interestingly, the CI spectrum of the benzoic acid shows, together with the protonated molecular ion (m/z 123) and the expected benzoyl cation (m/z 105), a rather small amount of an ion at m/z 94, attribuited to phenol by CID experiments. Ranasinghe and Glish demostred that the reaction of phenylium cation with methanol in an ion trap mass spectrometer leads to the formation of protonated anisole and, to a larger extent, to phenol radical ion [2]. In agreement with the cited results, this work shows that the CID spectrum of the benzoyl cation rather than displaying the characteristic phenylium ion (m/z 77) shows the formation of aboundant phenol, thus suggesting that benzoic acid could be formally converted in to phenol via phenylium ion. Conversion of isomeric benzoic acids into their phenolic derivatives using methanol, followed by CID of the reaction products, could therefore function has a method to assign the exact configuration of the starting positional isomers based on structurally diagnostic phenolic ions (SDPI). To this aim, ortho, meta and para isomers of disubstituted acylbenzenes were converted into the correspondent disubstituted phenols. Subsequently, CI-CID experiments on the resultant phenolic products revealed if the positional information of the starting benzoyl cation is retained or if isomerization by H-ring walking occurs in some extent. [1] L.A.B. Moraes, A.A. Sabino, E.C. Meurer, M. Eberlin. Am. Soc. Mass Spectrom. 2005, 16 431-36 [2] Y.A. Ranasinghe, G.L. Glish. Am. Soc. Mass Spectrom. 1996, 7 473-81 Financial support from Ministero dell’Università, dell‘Istruzione e della Ricerca MIUR-PRIN 2008 (prot. F21J10000010001) is gratefully acknowledged.