Thermochemical study of the monobromonitrobenzene isomers

J. Chem. Thermodynamics 42 (2010) 169–176

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Thermochemical study of the monobromonitrobenzene isomers Manuel A.V. Ribeiro da Silva *, Ana I.M.C. Lobo Ferreira, Ana Filipa L.O.M. Santos, Inês M. Rocha Centro de Investigação em Química, Department of Chemistry, Faculty of Science, University of Porto, Rua do Campo Alegre, 687, P-4169-007 Porto, Portugal

a r t i c l e

i n f o

Article history: Received 4 May 2009 Received in revised form 27 May 2009 Accepted 31 May 2009 Available online 12 June 2009 Keywords: Thermochemistry Energy of combustion Enthalpy of sublimation Enthalpy of formation Rotating bomb combustion calorimetry Knudsen effusion technique Vapour pressure Entropy of sublimation Gibbs energy of sublimation Cox scheme Bromonitrobenzene isomers

a b s t r a c t The standard (p° = 0.1 MPa) molar enthalpies of formation, of the 2-, 3-, and 4-monobromonitrobenzene isomers, in the crystalline phase, at T = 298.15 K, were derived from the standard massic energies of combustion, in oxygen, at T = 298.15 K, measured by rotating bomb combustion calorimetry. From the temperature dependence of the vapour pressures of these compounds, measured by the Knudsen effusion technique, their standard molar enthalpies of sublimation, at T = 298.15 K, were derived using the Clausius-Clapeyron equation. Dc U m ðcrÞ=ðkJ  mol 2-Bromonitrobenzene 3-Bromonitrobenzene 4-Bromonitrobenzene

1

2939.5 ± 1.0 2919.7 ± 1.1 2913.1 ± 0.9

Þ

Df Hm ðcrÞ=ðkJ  mol1 Þ

Dgcr Hm =ðkJ  mol1 Þ

26.9 ± 1.3 7.1 ± 1.3 0.5 ± 1.2

85.2 ± 0.3 86.8 ± 0.5 86.6 ± 0.6

The combination of the values of the standard molar enthalpies of formation in the crystalline phase, and of the standard molar enthalpies of sublimation, yielded the standard molar enthalpies of formation in the gaseous phase. The results were interpreted in terms of enthalpic increments, and compared with estimated values applying the empirical scheme developed by Cox. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction Organic bromine compounds represent a very important group of organic halogen compounds. The Tyrian, also known as Royal Purple, is a purple-red dye of which the main chemical is 6,60 -dibromoindigo extracted from Mediterranean Sea mollusks. Although these kinds of compounds can be produced naturally by marine and terrestrial plants, fungi, bacteria and marine animals, having a special interest to society as potential therapeutic drugs, they can also be prepared by synthesis. The bromine compounds participate in Suzuki [1] and Heck [2] reactions for aryl bromides synthesis, the Suzuki–Miyaura coupling reaction [3], playing also an important role as intermediates in the production of agrochemicals and pharmaceuticals. Some researchers have shown that the bromonitrobenzene isomers can stimulate the microbial dechlorination of polychlorinated biphenyl compounds, taking part of the natural restoration [4]. For all these reasons, the understanding of the reactivity and of the energetics of these compounds is of major relevance. The present work is part of a broad research project on the energetic effect

* Corresponding author. Tel.: +351 22 0402 521; fax: +351 22 0402 522. E-mail address: [email protected] (M.A.V. Ribeiro da Silva). 0021-9614/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jct.2009.06.008

of the substitution of bromine atoms in the aromatic ring of some benzene and heterocyclic derivatives like bromine substituted benzoic acids [5,6], anilines [7], pyridines [8] and indolines [9]. In this paper, we report the experimental thermochemical study of 2-, 3- and 4-bromonitrobenzenes. The standard (p° = 0.1 MPa) molar enthalpies of formation, of these compounds, in the crystalline phase, at T = 298.15 K, were derived from rotating-bomb combustion calorimetry experiments, whereas the vapour pressures measured as a function of temperature by the Knudsen effusion technique, yielded the standard molar enthalpies of sublimation, at T = 298.15 K, by application of the Clausius–Clapeyron equation. Hence, the standard molar enthalpies of formation, in the gaseous phase, at T = 298.15 K, were derived and compared with the estimated values using not only the enthalpic increments method but also applying the empirical scheme developed by Cox [10].

2. Experimental 2.1. Compounds and purity control The 2-bromonitrobenzene [CAS 577-19-5] and 3-bromonitrobenzene [CAS 585-79-5] were obtained commercially from Merck

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Chemicals, with an assessed minimum purity of 0.994 and 0.995 (mass fraction), respectively. Both compounds were purified firstly by sublimation at 0.1 Pa background pressure at T = (310 and 320) K, respectively, followed by recrystallization from petroleum ether (313 to 333) K, finishing with sublimation using the same conditions. The 4-bromonitrobenzene [CAS 586-78-7], with the assessed minimum purity of 0.994 (mass fraction), was supplied by Aldrich Chemical Co. and used for calorimetric measurements and Knudsen effusion technique without further purification. The final purity of the three isomers was checked by gas chromatography, using an Agilent 4890 apparatus equipped with an HP-5 column, cross-linked, 5% diphenyl and 95% dimethylpolysiloxane (15 m  0.530 mm i.d.  1.5 lm film thickness), and using nitrogen as carrier gas. The temperature of the injector was set at 423 K and the oven temperature was programmed with an initial period at T = 323 K, during 1 min, followed by a ramp at 10 K  min1, and finishing with a period of 5 min at T = 423 K. No impurities greater than 103 in mass fraction could be detected in the samples of the 2- and 3-bromonitrobenzene isomers used for the rotating bomb combustion calorimetry and Knudsen effusion technique. With the 4-bromonitrobenzene isomer no impurities were detected by GC. The specific densities used to calculate the true mass from the apparent mass in air, taken from the Merck catalogue [11] for 3bromonitrobenzene and from Alfa Aesar catalogue [12] for 4-bromonitrobenzene, were 1.70 g  cm3 and 1.95 g  cm3, respectively. The specific density of 2-bromonitrobenzene was determined as 1.85 g  cm3, estimated from the mass and the dimensions of the pellet of the crystalline compound, using a digital palmer (323511 N Mitutoyo). The relative atomic masses used in the calculation of all molar quantities were those recommended by the IUPAC Commission in 2005 [13], yielding 202.0055 g  mol1, for the molar mass of the bromonitrobenzene isomers. The benzoic acid NIST Standard Reference Material, sample 39j [14] was used without any further purification to calibrate the rotating bomb calorimeter. 2.2. Combustion calorimetry measurements The standard massic energies of combustion of the bromonitrobenzene isomers were measured with an isoperibol rotating bomb calorimeter that was formerly used at the National Physical Laboratory, Teddington, UK. The apparatus and operating technique have been previously described in the literature [15–18]. Since for the combustion of bromine containing compounds it is recommend the use of tantalum-lined bomb [19], the combustion experiments were performed with a twin-valve bomb lined with tantalum, having an internal volume of 0.329 dm3. After being filled with oxygen, the bomb was placed in the inverted position inside of the calorimeter. A Mettler PM11-N balance, with a sensitivity of ±101 g, was used to weigh the amount of distilled water added to the calorimeter from a weighted acrylic vessel, and for each experiment a correction for the deviation from 3969.2 g of mass of water added was made. Calorimetric temperatures were measured with an uncertainty within bounds of ±104 K every 10 s, using a quartz thermometer (Hewlett Packard HP 2804A) interfaced to a microcomputer programmed to compute the adiabatic temperature change, by means of a version of the LABTERMO program [20]. In the fore, main and after periods, the number of temperature readings was 125, 100, and 125, respectively. The calorimetric system was calibrated with benzoic acid (NBS Standard Reference Material 39j), having a standard massic energy of combustion, under bomb conditions, of (26434 ± 3) Jg1 [14]. According to the procedure proposed by Coops et al. [21], the cal-

ibration experiments were carry out in oxygen, at the pressure of 3.04 MPa, with 1.00 cm3 of deionised water added to the bomb. For each combustion experiment, the ignition temperature was chosen so that the final temperature would be close to T = 298.15 K. The ignition energy was measured by the change of the potential difference on the discharge of a capacitor (1281 lF) across a platinum wire (/ = 0.05 mm, Goodfellow, mass fraction 0.9999). The obtained value for the energy equivalent of the calorimeter was found to be e(calor) = (20369.0 ± 2.3) JK1, as the mean of 10 calibrations experiments, where the uncertainty quoted is the standard deviation of the mean. For bomb combustion calorimetry of organic bromine compounds, 4-bromobenzoic acid is recommended as test substance [22]. Hence, the accuracy of both the experimental procedure and of the calorimeter performance was checked in our laboratory, by other researchers measuring the energy of combustion of 4bromobenzoic acid in pellet form, enclosed in polyester bags made from Melinex (0.025 mm thickness) using the technique described by Skinner and Snelson [23], at pressure of 3.04 MPa and in the presence of 20.00 cm3 of aqueous solution of As2O3 (0.09 mol  dm3). The standard massic energy of combustion obtained for the reaction with HBr600H2O(l) as the single bromineDcu° = containing product in the final state was (15260.9 ± 2.2) Jg1 [24], which is in excellent agreement with the recommended value [22], Dcu° = (15261.0 ± 4.2) Jg1. The three bromonitrobenzene isomers were burned in pellet form, under a pressure of 3.04 MPa, in the presence of 30.00 cm3 of an aqueous solution of As2O3  (0.09 mol  dm3), in order to reduce all the free bromine produced in the combustion to hydrobromic acid. The extent of oxidation of As2O3 (aq) was determined by titration with a standardized iodine solution. Following the procedure described by Hu et al. [25], the calculation of the energetic term of the oxidation of As2O3 to As2O5, in aqueous solution, includes the enthalpy of oxidation of As2O3 by Br2 [26] and also the thermal effects of mixing of As2O5 with strong acids as HBr [27]. Some previous research [28] found no evidence for the oxidation of the aqueous solution of As2O3 (aq) within about 5 h, at room temperature in the presence of oxygen, at the pressure of 3.04 MPa. For the cotton thread fuse, with the empirical formula CH1.686O0.843, the massic energy of combustion is assigned to Dcu° = 16240 Jg1 [21], a value which was confirmed in our laboratory. For each combustion experiment of the three bromonitrobenzene isomers, the rotation of the bomb was started when the temperature had risen to 63% of its final value and then continued throughout the rest of the experiment. This procedure was described by Good et al. [29], which have shown that the frictional work due to the rotation of the bomb is automatically accounted in the temperature corrections for the work of the water stirring and for the heat exchanged with the surrounding isothermal jacket. The rotating mechanism allows the simultaneous axial and endover-end rotation of the bomb, causing the aqueous solution of As2O3 placed in the bomb to wash all internal surfaces of the bomb, yielding a homogeneous final solution. The isothermal jacket consists of a thermostatic bath containing a cavity of exactly the same shape as the calorimeter has, but 1 cm larger in overall dimensions, enclosed in a hollow lid. The jacket and the lid are filled with water maintained at T = (299.050 ± 0.001) K, using a temperature controller (Tronac PTC 41), so that the calorimeter was completely surrounded by a constant temperature. The nitric acid formed from the combustion of the bromonitrobenzene samples and from the traces of atmospheric N2 remaining inside the bomb was analyzed by the Devarda’s alloy method [30] and an energetic contribution based on 59.7 kJmol1 [31] for the standard molar energy of formation was made. The amount of

M.A.V. Ribeiro da Silva et al. / J. Chem. Thermodynamics 42 (2010) 169–176

H2PtBr4 formed was determined from the mass loss of the platinum crucible and its supporting ring, which correlates to an energy cor1 rection based on Df Hm ðH2 PtBr4 ; aqÞ ¼ ð368:2  0:1ÞkJ mol [31]. At the end of some experiments a small residue of carbon was found due to an insufficient pressing of the compound when the pellet was made. The correction for carbon soot formation was based on the standard massic energy of combustion of carbon, Dcu° = 33 kJg1 [21]. If the carbon soot formed stays only on the walls of the platinum crucible, and not in the combustion solution or on the walls of the bomb, an energy correction for the carbon soot was done by weighing the crucible before and after calcinations. Otherwise, the experiments were rejected. For each compound, the pressure coefficient of massic energy, (ou/op)T, at T = 298.15 K was assumed to be –0.2 Jg1  MPa1 [32], a typical value for most organic compounds. The standard state corrections, DUR, and the heat capacities of the bomb contents, ei and ef, were calculated by the procedure given by Bjellerup [33], using the solubility constants and energies of solution of CO2 and O2, as given by Hu et al. [25]. All the necessary weighings for the combustion experiments were made on a Mettler Toledo AE 245 balance, with a sensitivity of ±105 g, and corrections from the apparent mass to true mass were introduced.

171

The Clausius–Clapeyron equation ln(p/Pa) = a  b  (T/K)1, where a is a constant and b ¼ Dgcr Hm ðhTiÞ=R, was used to derive the standard molar enthalpies of sublimation at the mean temperature of the experimental temperature range, Dgcr Hm ðhTiÞ. Sublimation enthalpies at T = 298.15 K were derived using equation (3).

Dgcr Hm ð298:15 KÞ ¼ Dgcr Hm ðhTiÞ þ Dgcr C p;m ð298:15 K  hTiÞ:

ð3Þ

The heat capacity difference, Dgcr C p;m , for the bromonitrobenzenes isomers was calculated from the heat of capacity of both phases, where Dgcr C p;m ¼ C p;m ðgÞ  C p;m ðcrÞ. The standard molar entropies of sublimation were calculated by equation (4), where p° = 105 Pa, and the standard molar Gibbs energies of sublimation were calculated through equation (5), where all thermodynamic parameters are referred to the temperature of 298.15 K

Dgcr Sm ðT ¼ 298:15 KÞ ¼ Dgcr Sm fhTi; pðhTiÞg þ Dgcr C p;m lnð298:15 K=hTiÞ  R lnfp =pðhTiÞg;

ð4Þ

Dgcr Gm ¼ Dgcr Hm  298:15Dgcr Sm :

ð5Þ

3. Results 2.3. Vapour pressures measurements 3.1. Experimental enthalpies of formation The vapour pressures of the crystals of all the compounds studied were measured, at several temperatures, by mass-loss Knudsen effusion technique. The apparatus used enables the simultaneous operation of three different effusion orifices, making it possible to check the absence of any dependence of the measured pressures from the area of the effusion orifices. A detailed description of the apparatus, operating technique and results obtained for two test substances (benzoic acid and ferrocene) have already been reported [34]. Each cell was filled with a certain amount of pulverized sample (between 200 mg and 300 mg), which was compressed to form a smooth surface. Before and after each Knudsen effusion experiment, the cells were weighed on a Mettler AE163 analytical balance, with a sensitivity of ±105 g. The three effusion cells are placed on cylindrical holes in the aluminium blocks of the respective cold-fingers. It is assumed that the effusion cells are in internal equilibrium with a thermostatically controlled bath that contains a mixture of water and an anti-freezing, which allows the measuring of the vapour pressures at low temperatures. The measurements were performed over a temperature range of 20 K chosen to correspond to measured vapour pressures within the range (0.1 to 1.0) Pa. The experimental parameters of a Knudsen effusion experiment are the mass loss, Dm, of the crystalline sample, the temperature at which the sublimation occurs, T, and the time of the experiment, t. The vapour pressure is calculated according to the following equation:

p ¼ ðDm=Ao wo tÞð2pRT=MÞ1=2 ;

ð1Þ 1

1

where R is the gas constant (R = 8.314472 JK  mol ), M is the molar mass of the effusing vapour, Ao is the area of the effusion orifice and wo is the respective Clausing factor which is determined using Eq. (2): 1

wo ¼ fl þ ð3l=8rÞg :

ð2Þ

The diameters and Clausing factors of each effusion orifice, made from a platinum foil of 0.0125 mm thickness, are given as supplementary information (table S1).

Listed in table 1 are the detailed results for one typical combustion experiment for each isomer, where the symbols are as previously defined [25,32,35]. The internal energy for the isothermal bomb process, DU(IBP), was calculated according to equation (6):

DUðIBPÞ ¼ eðcalorÞcorr:  DT ad þ ðT i  298:15 KÞ  ei þ ð298:15 K  T i  DT ad Þ  ef þ DUðignÞ; where DTad is the calorimeter temperature change corrected for the heat exchange, the work of stirring and the frictional work of the bomb rotation and e(calor)corr = e(calor) + cp(H2O, l)  Dm(H2O, l). Detailed results for each combustion experiment performed for each studied isomer of monobromonitrobenzene are given in Supplementary information (tables S2, S3 and S4). The results of all combustion experiments of each compound, together with the mean value, hDcu°i, and its standard deviation of the mean are presented in table 2. These results are referred to the combustion reaction of the monobromonitrobenzene isomers yielding HBr600H2O(l) as the only bromine containing product in the final state, as described by equation (7)

C6 H4 NO2 BrðcrÞ þ 5:75O2 ðgÞ þ 598:5H2 Oð1Þ ! 6CO2 ðgÞ þ 0:5N2 ðgÞ þ HBr  600H2 OðlÞ:

ð7Þ

Shown in table 3 are the derived standard molar energies and enthalpies of combustion and the standard molar enthalpies of formation for the compounds in the crystalline phase, at T = 298.15 K. The values of Df Hm ðcrÞ were derived from Dc Hm ðcrÞ using the values of the standard molar enthalpies of formation of CO2(g), H2O(l) and HBr600H2O(l), at T = 298.15 K, which are –(393.51 ± 0.13) kJmol1 [36], –(285.830 ± 0.040) kJmol1 [36] and (120.924 ± 0.005) kJmol1 [31,36], respectively. 3.2. Vapour pressure measurements The standard molar enthalpies of sublimation at the mean temperature, h Ti of the studied compounds obtained by mass-loss Knudsen effusion technique were derived from the integrated form of the ClausiusClapeyron equation (8):

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TABLE 1 Typical combustion results at T = 298.15 K (p° = 0.1 MPa), for the monobromonitrobenzenes. Experiment

2-Bromonitrobenzene

3-Bromonitrobenzene

4-Bromonitrobenzene

m(cpd)/g m0 (fuse)/g Ti/K Tf/K DTad/K ei/(JK1) ef/(JK1) e(calor)corr./(JK1) Dm(H2O)/g DU(IBP)a/J DU(fuse)/J DU(HNO3)/J DU(As2O3)/J DU(H2PtBr4)/J DU(ign)/J DU(carb)/J DUR/J Dcu°/(Jg1)

1.33289 0.00268 297.1584 298.1786 0.97935 117.14 115.17 20359.8 2.2 20053.01 43.52 41.67 517.02 0.08 1.11 0.66 47.80 14557.53

1.20633 0.00189 297.2589 298.1853 0.88118 117.04 115.06 20348.9 4.8 18033.08 30.69 37.37 479.27 0.04 1.12 0.00 43.37 14459.01

1.15367 0.00243 297.2720 298.1457 0.83982 117.00 115.08 20369.4 0.1 17203.85 39.46 37.25 448.05 0.08 1.11 0.00 41.61 14421.28

m(cpd) and m0 (fuse) are the mass of the compound burnt and the mass of fuse (cotton) used, respectively, in each experiment; Ti is the initial temperature rise; Tf is the final temperature rise; DTad is the corrected temperature rise; ei is the energy equivalent of the contents in the initial state; ef is the energy equivalent of the contents in the final state; e(calor)corr. is the corrected energy equivalent of the calorimeter for the amount of water used; Dm(H2O) is the deviation of mass of water added to the calorimeter from 3969.2 g; DU(IBP) is the energy change for the isothermal combustion reaction under actual bomb conditions; DU(fuse) is the energy of combustion of the fuse (cotton); DU(HNO3) is the energy correction for the nitric acid formation; DU(As2O3) is the energy correction for the oxidation of the aqueous solution of As2O3; DU(H2PtBr4) is the energy correction for the formation of the platinum complex; DU(ign) is the electrical energy for ignition; DU(carb) is the correction energy for carbon soot formation; DUR is the standard state correction; Dc u is the standard massic energy of combustion. a DU(IBP) includes DU ign).

d½lnðp=p Þ=dð1=TÞ ¼ Dgcr Hm ðhTiÞ=R:

ð8Þ

The plots of ln p against 1/T for the global results obtained for the monobromonitrobenzene isomers are presented in figure 1. Presented in table 4 for each compound and for each effusion orifice used, are the results of the vapour pressure obtained from each effusion experiment during the effusion period, t, at the temperature T, together with the residuals of the values calculated through equation (9), where a is a constant and b ¼ Dgcr Hm ðhTiÞ=R.

lnðp=PaÞ ¼ a  b  ðT=KÞ1 :

ð9Þ

Table 5 contains, for each effusion orifice used and for the global treatment, the equilibrium pressure at the mean temperature, p(hTi) and the entropies of sublimation at equilibrium conditions,

TABLE 2 Individual values of the standard (p° = 0.1 MPa) massic energies of combustion,
c u , of the bromonitrobenzenes at T = 298.15 K. 2-Bromonitrobenzene 14557.53 14547.89 14553.56 14553.77 14550.15 14547.06

14551.7 ± 1.6 a

3-Bromonitrobenzene Dcu°/(Jg1) 14459.01 14457.12 14452.25 14446.01 14449.20 14452.38 14459.70 hDc u i=ðJ  g1 Þa 14453.7 ± 1.9

4-Bromonitrobenzene 14421.28 14425.96 14417.92 14419.52 14420.84 14418.87

14420.7 ± 1.2

Mean value and standard deviation of the mean.

Dgcr Sm ðhTi; pðhTiÞÞ ¼ Dgcr Hom ðhTiÞ=hTi. For each compound, the calculated enthalpies of sublimation obtained from each individual orifice are in agreement within the experimental error. Listed in table 6 are the (p, T) values calculated from the (p, T) equations for the crystalline compounds, within the experimental range of pressure used: (0.1 to 1.0) Pa. The heat capacity of the studied isomers, for the crystalline phase, was estimated by equation (10) using the Domalsky and Hearing method [37], which is an extension of the second-order group-additivity method firstly developed by Benson and co-workers [38].

C p;m ðBr—C6 H4 —NO2 Þcr ¼ 4  ½CB  ðCB Þ2 ðHÞcr þ ½CB  ðCB Þ2 ðBrÞcr þ ½CB  ðC B Þ2 ðNO2 Þcr ;

ð10Þ

where [CB–(CB)2(H)]cr = 20.13 JK1  mol1 and [CB–(CB)2(NO2)]cr = 50.96 JK1  mol1. The [CB–(CB)2(Br)]cr parameter was derived from the C p;m of bromobenzoic acid, as shown in equation (11)

C op;m ðBr—C6 H4 —COOHÞcr ¼ 4  ½CB  ðCB Þ2 ðHÞcr þ ½CB  ðCB Þ2 ðBrÞcr þ ½CB  ðCB Þ2 ðCOÞcr þ ½CO—ðCB Þ2 ðOÞcr þ ½O—ðHÞðCOÞcr ; ð11Þ where [CB–(CB)2(CO)]cr = 42.89 JK1  mol1; [CO–(O)(CB)]cr = 43.75 JK1  mol1; [O–(H)(CO)]cr = 44.60 JK1  mol1. The C p;m value of the 2-bromobenzoic acid, in the crystalline phase, 153.8 JK1  mol1 [5] was used. The heat capacity of the 3- and 4-bromobenzoic acids does not differ significantly between themselves. So, using the value of the 2-bromobenzoic acid, the estimated value of the [CB–(CB)2(Br)]cr was 27.82 JK1  mol1.

TABLE 3 Derived standard (p° = 0.1 MPa) molar values of the monobromonitrobenzene isomers, in the condensed phase, at T = 298.15 K. Compound


c U m ðcrÞ/(kJmol1)


c Hm ðcrÞ/(kJmol1)


f Hm ðcrÞ/(kJmol1)

2-Bromonitrobenzene 3-Bromonitrobenzene 4-Bromonitrobenzene

2939.5 ± 1.0 2919.7 ± 1.1 2913.1 ± 0.9

2937.6 ± 1.0 2917.8 ± 1.1 2911.2 ± 0.9

26.9 ± 1.3 7.1 ± 1.3 0.5 ± 1.2

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173

Due to the absence of the parameter [CB–(CB)2(NO2)]g in the method described for Domalsky [37], the heat capacity in the gaseous phase of the isomers was determined computationally by other researchers of the Group, as being 138.03 JK1  mol1. Hence, the heat capacity difference, Dgcr C p;m used for the calculation of the sublimation enthalpies and entropies of the studied isomers was –(21 ± 11) JK1  mol1. This value was confirmed using equation (12) derived by Chickos et al. [40], which gives a value of –(24 ± 33) JK1  mol1.

Dgcr C p;m ¼ f0:75  0:15C p;m ðcrÞg

FIGURE 1. Plots of ln(p/Pa) against (T/K)1 for 2-, 3- and 4-bromonitrobenzenes: e – small orifice; h – medium orifice; 4 – large orifice.

TABLE 4 Knudsen effusion results for the 2-, 3- and 4-Bromonitrobenzenes. T/K

t/s

102 Dln(p/Pa)

p/Pa Small

Medium

Large

Small

Medium

Large

0.1 0.0 1.2 1.5 2.0 0.6 1.1 0.1 0.9 3.0 0.7

1.5 1.0 1.8 0.5 2.4 0.2 0.5 1.9 1.5 0.2 1.1

0.9 1.2 1.9 0.3 1.0 0.8 1.9 2.2 0.6 1.7 1.8

275.15 277.15 279.16 281.17 283.17 285.17 287.21 289.17 291.14 293.14 295.16

23419 22258 21280 18961 12407 16912 14420 13320 11379 10104 12012

0.095 0.124 0.164 0.208 0.279 0.351 0.451 0.581 0.733 0.913 1.204

2-Bromonitrobenzene 0.094 0.094 0.123 0.123 0.165 0.165 0.210 0.212 0.280 0.277 0.353 0.350 0.459 0.448 0.592 0.594 0.729 0.735 0.939 0.925 1.210 1.218

279.57 281.21 283.14 285.16 287.19 289.39 291.16 293.15 295.17

10773 20819 20050 18632 17329 16524 16210 11967 11852

0.143 0.184 0.238 0.301 0.398 0.507 0.649 0.817 1.067

3-Bromonitrobenzene 0.142 0.141 0.183 0.181 0.237 0.236 0.299 0.294 0.385 0.381 0.506 0.496 0.650 0.635 0.815 0.790 1.054 1.033

1.1 2.4 2.8 0.0 2.1 1.5 1.2 1.0 2.2

1.8 1.7 2.2 0.4 1.3 1.6 1.4 0.4 0.9

2.1 2.1 0.5 2.0 2.4 2.1 3.7 0.9 0.1

289.15 291.12 293.14 295.15 297.14 299.15 301.13 303.14 305.14 307.14 309.15

23425 21711 17403 17116 15537 15906 14447 12634 12263 12104 13296

0.103 0.122 0.170 0.209 0.255 0.332 0.427 0.526 0.644 0.800 1.050

4-Bromonitrobenzene 0.101 0.099 0.119 0.120 0.165 0.161 0.209 0.202 0.255 0.248 0.337 0.333 0.410 0.409 0.537 0.520 0.653 0.624 0.809 0.788 1.036 1.010

4.4 3.0 5.2 1.9 2.0 1.1 3.4 1.2 1.1 1.6 3.5

2.4 5.7 2.2 1.6 1.8 2.7 0.9 3.2 0.2 0.5 2.1

0.8 4.9 0.2 1.7 4.7 1.3 1.1 0.0 4.3 3.2 0.4

However, this parameter can also be found by other way: the difference between the heat capacity of the bromobenzoic acid and benzoic acid gives [CB–(CB)2(Br)]cr  [CB–(CB)2(H)]cr. Using the C p;m values of the bromobenzoic acid [5] and benzoic acid [39] the [CB–(CB)2(Br)]cr found was 27.13 JK1  mol1. The value used in this work for [CB–(CB)2(Br)]cr was a average of these two estimated parameter: 27.48 JK1  mol1. Once found every parameter, the heat capacity of the bromonitrobenzene isomers, in crystalline phase, was found to be 158.96 JK1  mol1.

ð12Þ

Since both calculated values of Dgcr C p;m are equal, within of the uncertainty associated, the value used for the calculation of the enthalpies, entropies and Gibbs energies, at T = 298.15 K was – (21 ± 11) JK1  mol1. Table 7 presents the values of the standard molar enthalpies, entropies and Gibbs energies of sublimation, together with the values of the vapour pressures, at T = 298.15 K for the studied compounds. The standard molar enthalpies of formation, in the gaseous phase, at T = 298.15 K, derived from the experimental values of the standard molar enthalpies of formation, in the crystalline phase and from the values of the standard molar enthalpies of sublimation are summarized in table 8. 3.3. Enthalpies of formation estimated with the Cox scheme The experimental values of the standard molar enthalpies of formation in the gaseous phase for the three compounds studied can be compared with the values obtained by the empirical method suggest by Cox [10], based on the transferability of group enthalpic contributions in benzene derivatives. Cox assumed that each group, when substituted into a benzene ring, produces a characteristic increment in Df Hm ðgÞ and suggested a correction of +4 kJmol1 for each pair of substituents in ortho-position of the aromatic ring. To obtain the estimated values, Df Hm ðgÞ of benzene (82.6 ± 0.7) kJmol1 [41], bromobenzene (105.4 ± 4.1) kJmol1 [41] and nitrobenzene (67.5 ± 0.5) kJmol1 [41] were used in the scheme presented in figure 2. The estimated values of Df Hm ðgÞ derived from the application of the Cox scheme for the 2-, 3- and 4-bromonitrobenzenes are also listed in table 9, where D is the difference between the experimental and estimated values. 4. Discussion The standard molar enthalpies of formation, in the gaseous phase, of the monobromonitrobenzene isomers, presented in table 8, show that the 2-bromonitrobenzene is the more enthalpically unstable isomer when compared with the other two isomers. As was verified for 2-chloronitrobenzene [42], the steric effect caused by the proximity between the bromine atom and the nitro group in 2-bromonitrobenzene has a great effect in terms of enthalpic destabilisation. The nitro group is a strong electron withdrawing group by both inductive and resonance effects. These effects contribute for reducing the electronic density of the aromatic ring and, consequently, reducing the reactivity of the compound, giving it more stability. When the nitro group is coplanar with the aromatic p ring, it reaches the maximum capacity of electron withdrawing, as shown by the structures 3–5 of figure 3. So, the presence of steric repulsions between the bromine atom and the closest oxygen of the nitro group forces the rotation of the nitro group, favouring the non-planar arrangement, decreasing the double bond character of the CAN bond and, consequently, slightly

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TABLE 5 Experimental results for monobromonitrobenzene isomers, where a and b are from Clausius–Clapeyron, ln(p/Pa) = a  b/(T/K)1, and b ¼ Dgcr Hm ðhTiÞ=R; R = 8.314472 JK1  mol1. Orifices

a

b

Small Medium Large Global results

34.83 ± 0.18 35.15 ± 0.19 35.63 ± 0.21 35.01 ± 0.11

10229 ± 53 10318 ± 55 10298 ± 61 10282 ± 32

pðhTiÞ/Pa

hTi/K

Dgcr Hm ðhTiÞ/(kJmol1)

Dgcr Sm fhTi; pðhTiÞg/(kJK1mol1)

85.0 ± 0.4 85.8 ± 0.5 85.6 ± 0.5 85.5 ± 0.3

299.8 ± 1.1

87.3 ± 0.7 87.1 ± 0.7 86.7 ± 0.8 87.0 ± 0.5

302.7 ± 1.7

86.1 ± 1.0 87.2 ± 1.0 86.4 ± 0.8 86.6 ± 0.6

289.5 ± 2.0

2-Bromonitrobenzene

285.16

0.351

3-Bromonitrobenzene Small Medium Large Global results

35.61 ± 0.31 35.54 ± 0.31 35.36 ± 0.34 35.50 ± 0.20

10494 ± 89 10477 ± 88 10429 ± 98 10467 ± 56

287.37

0.397

4-Bromonitrobenzene Small Medium Large Global results

33.53 ± 0.41 33.94 ± 0.39 33.63 ± 0.34 33.70 ± 0.23

10359 ± 124 10486 ± 116 10397 ± 101 10414 ± 69

299.15

0.329

TABLE 6 Calculated (p, T) values from the vapour pressure equations for the compounds studied. p /Pa

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

288.0 289.2 302.8

289.5 290.6 304.4

290.7 291.9 305.8

291.8 293.0 307.0

292.8 293.9 308.1

293.7 294.8 309.0

T/K 2-Bromonitrobenzene 3-Bromonitrobenzene 4-Bromonitrobenzene

275.6 276.9 289.3

280.8 282.0 294.9

283.9 285.2 298.4

286.2 287.4 300.8

TABLE 7 Values of the standard (p° = 0.1 MPa) molar enthalpies, Dgcr Hm , entropies, Dgcr Sm , Gibbs energies, Dgcr Gm , of sublimation and vapour pressures, at T = 298.15 K, for the compounds studied. Compound

Dgcr Hm /(kJmol1)

Dgcr Sm /(kJK1mol1)

Dgcr Gm /(kJmol1)

pð298:15 KÞ/Pa

2-Bromonitrobenzene 3-Bromonitrobenzene 4-Bromonitrobenzene

85.2 ± 0.3 86.8 ± 0.5 86.6 ± 0.6

194.3 ± 1.1 198.5 ± 1.7 184.6 ± 2.0

27.3 ± 0.4 27.6 ± 0.7 31.6 ± 0.8

1.72 1.46 0.29

TABLE 8 Standard (p° = 0.1 MPa) molar enthalpies of formation, in both crystalline and gaseous phases, and standard molar enthalpies of sublimation, at T = 298.15 K, for the bromonitrobenzene isomers. Compound

Df Hm ðcrÞ/(kJmol1)

Dgcr Hm /(kJmol1)

Df Hm ðgÞ/(kJmol1)

2-Bromonitrobenzene 3-Bromonitrobenzene 4-Bromonitrobenzene

26.9 ± 1.3 7.1 ± 1.3 0.5 ± 1.2

85.2 ± 0.3 86.8 ± 0.5 86.6 ± 0.6

112.1 ± 1.3 93.9 ± 1.4 87.1 ± 1.3

blocking the p delocalization with the aromatic ring. So, the overlap between the pp orbitals of the ring and the nitrogen shows a strong dependence on the twist angle of the nitro group. From literature [43], it is possible to observe an effective twist angle of the nitro group not only for 2-bromonitrobenzene, but for all isomers, and a correlation, with equation 1

Df Hm ðgÞ=kJ  mol

¼ 1:0051uðC—NÞ= þ 68:619;

ð13Þ

can be found between the standard molar enthalpy of formation, in the gaseous phase, and this angle, as shown in figure 4. It is possible Br NO 2

_

+

= NO 2

to observe that the twist of the nitro group has a higher destabilizing effect in the equivalent of structures, 3–5 of figure 3, for the para- and ortho- bromonitrobenzene isomers, when compared with the meta-isomer, since the bromine atom is also electron withdrawing by inductive effect.

Br

FIGURE 2. Empirical scheme for the estimation of Df Hm (g) for the studied compounds.

TABLE 9 Experimental and estimated (Cox scheme) values of gas-phase enthalpies of formation of the three monobromonitrobenzene isomers. Compound

2-Bromonitrobenzene 3-Bromonitrobenzene 4-Bromonitrobenzene a

Da/(kJmol1)

Df Hm ðgÞ=ðkJ  mol1 Þ Experimental

Cox

112.1 ± 1.3 93.9 ± 1.4 87.1 ± 1.3

94.3 ± 4.2 90.3 ± 4.2 90.3 ± 4.2

Difference between the experimental and the estimated values.

17.8 ± 4.4 3.6 ± 4.4  3.2 ± 4.4

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M.A.V. Ribeiro da Silva et al. / J. Chem. Thermodynamics 42 (2010) 169–176 O–

O N+

–

O

1

O

–

O

O

–

O

O

–

O

O

N+

N+

N+

N+

2

3

4

5

FIGURE 3. Resonance structures of nitrobenzene.

higher than the value for 2-bromonitrobenzene. Allen et al. [47] studied the halogen (X)  O(nitro) interactions by the combination of crystallographic data and ab initio molecular orbital study and the essential conclusions were that the CX  O angle prefers to be linear, the angle between the X–C plane and the CNO2 plane would be less than 45°. So, the rotations of the nitro group for out-plane in 2-bromonitrobenzene difficults the interaction between bromine atom and nitro group, thus justifying the observed difference of the enthalpies of sublimation with 3- and 4-isomers. The experimental Dgcr Sm for the three isomers is described in expression (14)

Dgcr Sm ð3-bromonitrobenzeneÞ > Dgcr Sm ð2-bromonitrobenzeneÞ > Dgcr Sm ð4-bromonitrobenzeneÞ: ð14Þ Dgcr Sm

FIGURE 4. Relation between the Df Hm ðgÞ and the effective twist angle of the nitro group {u(C–N)} in 2-, 3- and 4-bromonitrobenzenes.; s – 4-bromonitrobenzene; h – 3-bromonitrobenzene; 4 – 2-bromonitrobenzene.

For the ortho isomer, the torsion angle was found to be 43°, which is very similar with the dihedral angle of the 2-chloronitrobenzene [44]. The effective torsion angle of the nitro group, on both 3- and 4- bromonitrobenzenes, is perhaps due to the steric effect of the bromine atom on the adjacent hydrogen atoms, these ones causing this torsion, but this hypothetical explanation can not yet be firmly confirmed. With the exception of the 2-bromonitrobenzene, the CAN bond lengths, presented in table 10, for the other isomers, became shortened with the entering of the bromine atom when compared with the CAN bond of nitrobenzene (CAN = 0.1468 nm [45]). The fact that the bromine atom is an electron donor for the resonance effect is responsible for the increasing of the negative charge in the carbon to which the nitro group is bound. So, the nitro group acts as an electron withdrawing substituent and polarizes the C–N(O)2, thus shorting the respective bond. The bond between the bromine atom and the ring of the studied compounds does not differ significantly from the same bond in bromobenzene (BrAC = 0.1899 nm [46]. As expected, the CAC(NO2)AC angles in the three isomers are slightly increased than the CAC(H)AC angles in the benzene ring, but similar to the nitrobenzene ð]C—CðNO2 Þ—C ¼ 123:5 Þ. The enthalpies of sublimation of both 3- and 4-bromonitrobenzenes are similar within the experimental error and slightly

The observed relation of for halogen substituted nitrobenzenes may be a consequence of the relation between their entropies in crystalline phase. The value of entropy of sublimation of the 4-bromonitrobenzene suggests a higher degree of orientational disorder in the crystalline phase than the other isomers, making the entropy difference between the crystal and the gaseous phase smaller. The steric interaction in ortho-isomers affected the atoms movement in crystal package. Since the non-plane structure does not allow a very efficient packing, the molecules will have a relative degree of orientational disorder. This explains the lower value of the entropy of sublimation of 2-bromonitrobenzene compared to the 3-bromonitrobenzene. As a consequence of the enthalpic and entropic effects, the volatility of the 4-bromonitrobenzene is lower than 2- and 3-isomers. As it can be seen in table 9, the values of Df Hm ðgÞ estimated by the Cox scheme [10] for 3- and 4-bromonitrobenzenes are in very good agreement with the experimental values. The differences, D, between the experimental and estimated values are –(3.2 ± 4.4) and (3.6 ± 4.4) kJmol1, for the meta and para isomers, which is within the usually accepted limit of 10 kJmol1 for agreement between experimental and estimated values by the Cox scheme. However, the Cox scheme fails completely in the estimation of Df Hm ðgÞ for 2-bromonitrobenzene, with D = (17.8 ± 4.4) kJmol1. Although the Cox scheme suggests an enthalpic correction of +4 kJmol1 due to the steric destabilization of the ortho-pair of substituents in benzene ring, it seems to be necessary to include a higher value of enthalpic correction of destabilization. As Ribeiro da Silva et al. suggested for 2-chloronitrobenzene [42], following the same behaviour observed in 3-chloro-4-nitroaniline [48], for 2-bromonitrobenzene it is necessary to consider a larger correction term than 4 kJmol1 suggested by Cox, but a correction of more 22 kJmol1 for benzene derivates with either a chlorine or a bromine atom in ortho position relative to the nitro group. Acknowledgments Thanks are due to Fundação para a Ciência e Tecnologia (FCT), Lisbon, Portugal and to FEDER for financial support to Centro de Investigação em Química, University of Porto. A.I.M.C.L.F. and A.F.L.O.M.S. thank FCT and the European Social Fund (ESF) under the Community Support Framework (CSF) for

TABLE 10 Geometrical Parameters of 2-bromonitrobenzene, 3-bromonitrobenzene and 4-bromonitrobenzene from ED. Compound

C–N(O)2/nm

C–Br/nm

]C—CðNÞ—C=

]C—CðBrÞ—C=

u=

2-Bromonitrobenzene 3-Bromonitrobenzene 4-Bromonitrobenzene

0.1494 0.1448 0.1454

0.1894 0.1865 0.1896

120.4 124.4 121.6

119.6 121.4 122.6

43.3 25.0 18.5

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JCT 09-144