Negative chemical ionisation and collision induced fragmentations of deprotonated hydroperoxides

Abstract

The mass spectra and the collisional induced dissociations (CID) of deprotonated hydroperoxides, generated by negative chemical ionisation (NCI) with methane as ionising reagent, have been examined. The chemical ionisation mass spectra of hydroperoxides show that the base peak corresponds to the [(MÀH)ÀO]À ion for all the compounds studied with the exception of t-butyl hydroperoxide. The O 2 H À ion is present in the spectra of all of these compounds that do not possess a phenyl group. This study provides detailed information on the fragmentation behaviour of deprotonated hydroperoxides, [ Received 26 October 1998; Revised 14 November 1998; Accepted 15 November 1998 An important class of organic peroxides are the hydroperoxides (RÀOÀO-H). They have a well-earned reputation for instability both to shock and heat. Nevertheless, many are stable to both distillation and sublimation.1 The structures and amounts of hydroperoxides are usually determined by physical methods (gas chromatography, adsorption and liquid-liquid partition chromatography, paper and thin layer chromatography, and polarography), and also by chemical reduction methods and colorimetric and photometric methods.1,2 Although alkyl hydroperoxides have been studied by various mass spectrometric techniques, 3-7 there appear to be no reported studies on the gas-phase chemistry of negative ions of hydroperoxides produced by chemical ionisation (CI) (CH 4 ). For a comprehensive review of the fragmentation behaviour of deprotonated organic molecules, [MÀH] À , see Ref. 8. In this work, the fragmentation mechanism of deprotonated hydroperoxides, generated by NCI with methane as ionising reagent, is investigated. EXPERIMENTALCompounds 1 and 2 (Fig. 1) were prepared by catalytic oxidation of the corresponding alkanes in the presence of the Keggin-type heteropolytungstate (TBA) 4 PW 11 Fe(H 2 O)-O 39 Á2H 2 O and hydrogen peroxide (30 wt.% solution in water). After 9 hours of reaction, cyclohexane gave 57% of cyclohexyl hydroperoxide (1), 6% of cyclohexanol and 13% of cyclohexanone. 9 In the case of cyclooctane, we obtained 68% of cyclooctyl hydroperoxide (2) and 32% of cyclooctanone after 12 hours of reaction at 80°C in acetonitrile. 10The reactions were stopped with aqueous sodium hydrogen sulfite solution, extracted with dichloromethane, and the organic layer was dried with anhydrous sodium sulphate. Compounds 3, 4 and 5 (Fig. 1) were purchased from Aldrich and used without further purification.Negative CI mass spectra and tandem mass spectra were acquired with a VG AutoSpecQ (Micromass, UK). Methane was used as the CI reagent gas at a pressure of 5 Â 10 À4 mbar. The ion source temperature was maintained at 220°C. Compounds 1, 2, 3 and 5 were introduced into the mass spectrometer using a Konik gas chromatograph (HRGC-3000C). The main reason for this is that, since the compounds are extremely volatile, the S/N ratio of the massanalysed ion kinetic energy (MIKE) spectra, when using the direct insertion probe, was low. A 30 m DB-5 capillary column was used....

Sandwich-type tungstophosphates in the catalytic oxidation of cycloalkanes with hydrogen peroxide

“…All products were identified by GC-MS. Both hydroperoxides had been previously identified in our group by mass spectrometry [41].…”

Oxidation of cycloalkanes with hydrogen peroxide in the presence of Keggin-type polyoxotungstates

“…The cyclohexyl and the cyclooctyl hydroperoxides were identified by GC-MS with methane negative ion ionisation and tandem mass spectrometry [19]. Other possible reaction products detectable by GC were not observed.…”

A comparative study between Keggin-type tungstophosphates and tungstosilicates in the oxidation of cyclooctane with hydrogen peroxide

“…Aliquots were taken from reaction mixtures, at regular intervals, for peroxide determination. The cyclooctyl hydroperoxide was identified by two different methods: (a) reduction with triphenylphosphine to Scheme 1. give cyclooctanol [2,13] and (b) GC-MS, using previous identification of cyclooctyl hydroperoxide mass spectrum [5].…”

“…A possible way to improve selectivity can be through the selective decomposition of alkyl hydroperoxides formed during the oxidation reactions. One of the targets of our recent research is the finding of Keggin-type polyoxotungstates that can afford good selectivity towards formation of alkyl hydroperoxides in the catalytic oxidation of cycloalkanes [4][5][6][7]. Several catalysts have been described for the oxygenation of alkanes with tert-butyl hydroperoxide or hydrogen peroxide [1,2,8,9].…”

“…A brief reference to the formation of cyclooctyl hydroperoxide in the presence of [PW 11 Fe(H 2 O)O 39 ] 4− was made before [5]. Results obtained using an excess of hydrogen peroxide and using a H 2 O 2 /substrate molar ratio near the stoichiometric value are described and compared.…”