EI fragmentation of E,Z-3-iodo-2-alkenoic acids and methyl E,Z-3-iodo-2-alkenoates

Abstract

The electron ionization mass spectra of some E,Z-3-iodo-2-alkenoic acids and methyl E,Z-3-iodo-2-alkenoates were investigated. A semi-empirical method was used to examine the thermochemical and molecular properties of the species. Two fragmentation paths of the molecular ions are determined by the geometry of the carbon-carbon double bond: (i) heterolytic loss of I Á radical, and ( The 3-halo-2,3-unsaturated acids and esters are valuable intermediates in organic synthesis. They present biological properties such as the defoliating agent of cotton plants 1 and they also have given rise to concern due to impact on the environment.2 Knowledge of the mass spectrometric behaviour of these compounds is important especially in the case of geometric isomers. Mass fragmentation studies have been carried out only for C 3 and C 4 alkenoates substituted with chloride and/or bromine atom.2-4 The mass fragmentation of iodo-substituted alkyl and aromatic compounds have been reported. 5,6 The usefulness of mass spectral techniques in differentiating E-and Z-isomers across the carbon-carbon double bond has been reviewed. 7In a previous work, concerned with the synthesis and characterization of 3-halo-2,3-unsaturated acids, we reported that E,Z-3-iodo-2-butenoic acids have a significant difference in the abundances of their molecular ions upon electron ionization. 8 We examined a group of E,Z-3-iodo-2-alkenoic acids and methyl E,Z-3-iodo-2-alkenoates in order to investigate the generality of that geometric effect. The results of this investigation are the subject of this paper.The application of the semi-empirical PM3 method, 9 to calculate theoretical thermochemical and molecule orbital data, provides insight into reactions of the studied molecular ions. PM3 has been utilized in the study of carbocations, 10 and the mass spectra of carbonyl, 11 aromatic 12,13 and alkyltrimethylammonium 14 compounds, as well as in the determination of proton affinities. 15 EXPERIMENTALThe mass spectra were obtained on individual compounds by using gas chromatographic introduction into a quadrupole mass detector. The gas chromatograph was a HewlettPackard 5890 Series II (Hewlett Packard, Avondale PA, USA) equipped with a cross-linked methyl silicone capillary column (Ultra 1, 25 m  0.2 mm i.d. 0.11 mm thickness), and the mass detector was a Hewlett-Packard Model HP 5971A. The injector, transfer line and mass detector temperatures were 200, 280 and 176°C respectively. The electron ionization energy was 70 eV.The Z-3-iodo-2-alkenoic acids were prepared by heating for 12 hours 2,3-acetylenic acids with an excess of concentrated aqueous HI, in a sealed glass ampoule at 100°C, following the method reported by Noble.16 The E-isomers were obtained by heating for 14 hours the pure Z-isomer in a sealed glass ampoule at 130°C. The esters were prepared by esterification of the carboxylic acid according to the method of Marshall et al. 17 The purity and configuration of the compounds were verified as explained elsewhere.2 The following twelve compounds were p...