Calorimetric study of bromoacetophenone isomers

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Highlights

  • A calorimetric study of bromoacetophenone isomers was performed.

  • Enthalpies of formation were derived by rotating-bomb combustion calorimetry.

  • Enthalpies of phase transition were determined by Calvet microcalorimetry.

  • Cox scheme was applied for the estimation of ΔfHmo (g) for the compounds.

  • The values of ΔfHmo (g) were compared with literature values for similar compounds.

Abstract

The standard (po = 0.1 MPa) molar enthalpies of formation of 2-, 2′-, 3′- and 4′-bromoacetophenones were derived from the standard molar energies of combustion in oxygen, to yield CO2 (g) and HBr·600H2O (l) at T = 298.15 K, measured by rotating bomb combustion calorimetry. The standard molar enthalpies associated with phase transitions of the isomers studied at T = 298.15 K, were obtained by high temperature Calvet microcalorimetry. The standard (po = 0.1 MPa) molar enthalpies of formation of all the bromoacetophenone isomers in the gaseous phase at T = 298.15 K were derived from the experimental results. The gas-phase enthalpies of formation were also estimated by the empirical scheme developed by Cox and the values obtained were compared with the experimental ones. The results are interpreted in terms of the energetic increments for the introduction of the substituents in the benzene ring.

Introduction

Brominated organic compounds are important technical products used as gasoline additives [1], flame-retardants [2] and fumigants, [3], playing also a significant role as intermediates in the pharmaceutical industry. They are characterized by their long life, chemical stability and have become a serious environmental issue. Because they are difficult to decompose completely by incineration, giving rise to dioxins many researchers have devoted attention to conducting photocatalytic reactions for decomposition of halogenated compounds [4], [5], [6]. Among these compounds, α-haloacetophenone derivatives have received considerable attention mainly in pharmaceutical industries, due to their ability to act as a novel class of potent, photo reversible, and membrane permeable inhibitors of protein tyrosine phosphatases (PTPs) [7], [8]. PTPs inhibitors may be promising drugs to treat some diseases such diabetes, obesity and cancer [9].

The establishment of relationships between the energetic and structural properties of compounds is relevant in explaining their behavior in terms of reactivity. Our research group, is involved in a systematic investigation of the energetic effects of the introduction of bromine atoms on the aromatic ring of some benzene and heterocyclic derivatives such as bromonitrobenzenes [10], [11], bromine substituted benzoic acids [12], [13], bromoanilines [14], bromoanisoles [15], bromophenols [16], [17], bromobenzonitriles [18], bromopyridines [19], bromoquinolines [20], bromonaphthalenes [21], bromo-8-hydroxyquinolines [22], bromouracil [23] and bromoindolines [24]. The aim of these studies is to provide an understanding of the effect of different substituents on the structure and energetic properties of brominated compounds, with the ultimate goal to develop predictive schemes, based on the transferability of group enthalpic contributions. In this context, should be possible to estimate thermochemical parameters for other compounds whose experimental study is not possible due to the difficulties in their synthesis and/or in obtaining them pure enough to be studied. Obviously, the availability of reliable data for the thermodynamic properties of the compounds on which group contributions can be based is essential.

This work reports the standard molar enthalpies of formation of the bromoacetophenone isomers, in the gaseous phase, at T = 298.15 K derived from measurements of the standard molar enthalpies of combustion in oxygen by rotating-bomb calorimetry and from the standard molar enthalpies associated with the phase transitions as measured by Calvet microcalorimetry.

The derived values of the standard molar enthalpies of formation of the compounds in the gaseous state are analysed and compared in terms of enthalpic increments due to the introduction of a bromine atom group into several benzene derivatives.

Section snippets

Compounds and purity control

The compounds were commercially obtained from Aldrich Chemical Co., with the following assessed mass fractions purities: 2-bromoacetophenone [CAS 70-11-1] 0.98; 2′-bromoacetophenone [CAS 2142-69-0] 0.99; 3′-bromoacetophenone [CAS 2142-63-4] 0.99 and 4′-bromoacetophenone [CAS 99-90-1] 0.98. The liquids 2′- and 3′-bromoacetophenones were purified by repeated distillation under reduced pressure and the solids 2- and 4′-bromoacetophenones were purified by successive sublimations under reduced

Results

Table 2 reports the results for one typical combustion experiment of each compound studied, where Δm(H2O) is the deviation from the reference mass of water added to the calorimeter; the reference mass of water the mass assigned to εcal; the remaining terms are as previously defined [32], [41], [45]. Detailed values of each combustion experiment performed for each studied compound are given in tables S1 to S4 of the Supplementary Information.

The values of the internal energy associated with the

Discussion

The values of ΔfHmo (g) obtained experimentally for 2′-, 3′- and 4′-bromoacetophenones are compared with the ones estimated using the Cox scheme [54] which assumes that the enthalpic increment for the introduction of a bromine atom in the different positions of acetophenone will be the same as in benzene with a correction term of 4 kJ · mol−1 that was applied for the ortho-pair of substituents. The standard molar gas-phase enthalpies of formation given in the literature for benzene, ΔfHmo (g) = (82.6

Acknowledgments

Thanks are due to Fundação para a Ciência e Tecnologia (FCT), Lisbon, Portugal and to Fundo Europeu de Desenvolvimento Regional (FEDER) - Portugal for financial support given to Centro de Investigação em Química da Universidade do Porto, (PEst-C/QUI/UI0081/2013), and to Programa Ciência 2008.

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