CC BY
52
YflK 544.3+536.7
BecTHHK Cn6ry. Cep. 4. 2013. Bun. 1
V. N. Emel'yanenko, S. P. Verevkin
THERMODYNAMIC STUDY OF PURE METHYLBENZALDEHYDES AND THEIR MIXTURES WITH IONIC LIQUIDS*
1. Introduction. Ionic liquids (ILs), organic salts that are liquid under ambient conditions, are being heralded as possible replacement solvents for volatile organics primarily because of the lack of any appreciable vapor pressures, which in turn may greatly reduce emission and worker exposure hazards. Numerous types of reactions have been carried out using ILs as solvents with similar or enhanced reaction rates, extents of reaction, and selectivities compared to those of their volatile organic solvent counterparts [1]. For ILs to be used effectively as solvents, it is essential to know how they interact with different solutes. These interactions should be intensive enough in order to keep solubility, but they should be also weak enough to alleviate a recovery of organic products from ionic liquids. A quantitative measure of such interactions is given by the partial pressures of the solute in the mixture or by activity coefficient yj, which describes the degree of nonideality for species i in a mixture [2]. The activity coefficients at infinite dilution, y°°, is especially important because describes the extreme case in which only solute-solvent interactions contribute to nonideality [3]. Separation processes for removing dilute impurities, as encountered in many environmental applications, require knowledge of for design purpose.
This work continues our study of thermodynamic properties of mixtures of solutes in ILs [4, 5]. In order to understand basics of organic products recovery from reaction mixtures containing ILs, we studied a phase behavior in the model systems containing ortho-, meta, and para-methylbenzaldehyde in the ionic liquids [BMIM][NTf2] and [OMIM][BF4]. Methylbenzaldehydes are important intermediates in oxidation reactions of ortho-xylene [6], para-xylene [7] and are important for manufacture of phthalic anhydride [8] and tereph-thalic acid [7]. For improvement a selectivity of an overall reactions and separation of the products, knowledge of the thermodynamic properties of methylbenzaldehydes is important. Additionally, thermochemical properties of the pure methylbenzaldehydes have been studied by using combustion calorimetry and transpiration method and structure-property relations in substituted benzenes have been discussed.
2. Experimental.
2.1 Materials. The liquid samples of methylbenzaldehydes (purchased from Merck) having a mass-fraction purity of about 0.99 were purified by repeated distillation in vacuum. Examination of the samples using GC showed no discernible amounts of impurities. The products were analyzed with a Hewlett Packard gas chromatograph 5890 Series II equipped with a flame ionization detector. No impurities (greater than mass fraction 0.0005) could be detected in the samples of methylbenzaldehydes used for the investigation.
The ionic liquids 1-methyl-3-butyl-imidazolium bis (trifluoromethyl-sulfonyl) imide [BMIM][NTf2] and 1-methyl-3-octyl-imidazolium tetrafluoroborate [OMIM][BF4] (1), were of the commercial origin (Iolitec). Before using, the samples of ionic liquids were subjected
V. N. Emel'yanenko — Department of Physical Chemistry, University of Rostock, Rostock, Germany.
Sergey P. Verevkin — PhD, docent, Faculty of Interdisciplinary Research, Department "Life, Light and Matter", University of Rostock; e-mail: [email protected]
* This work has been supported by the German Science Foundation (DFG) in frame of the priority program SPP 1191 "Ionic Liquids".
© V. N. Emel'yanenko, S. P. Verevkin, 2013
to vacuum evaporation at 333 K over 24 h to remove possible traces of solvents and moisture. Chromosorb W/AW-DMCS 100/120 mesh was used as solid support for the ionic liquids in the GC-column. Before using, chromosorb was subjected to vacuum treatment with heating in order to remove traces of adsorbed moisture.
N N M^^ ^Bu
(CFSOAN
©
Me"
"Oct
BF
©
(1)
2.2 Experimental procedures. The methods applied were as follows: gas-chromato-graphic method [4] for determination of activity coefficients at infinite dilution, transpiration method for study VLE of binary mixtures containing ILs [5], transpiration method for vapor pressures measurements of pure methylbenzaldehydes [9, 10] and combustion calorimetry for determination of enthalpies of formation of pure compounds [11]. We also used quantum-chemical calculations with G3MP2 [12] and G4 [13] methods from the Gaussian 09 package [14].
3. Results and discussion.
3.1 Activity coefficients at infinite dilution. The values of of ortho-, meta, and para-methylbenzaldehyde in the ionic liquids [BMIM][NTf2] and [OMIM][BF4] were determined by gas chromatography (GC) at 385 K using the ionic liquids as stationary phase.
According to Cruickshank et. al. [15] the following equation for the data treatment has been used:
ln y:
i,3
, (n zRT\
111 teJ
B11 - V
i „0
RT
Po +
2 ¿>12 - Vf
RT
■ Jpo,
(2)
where y°°3 is the activity coefficient of component i at infinite dilution in the stationary phase (index 3), is the vapor pressure of the pure liquid solute; n3 is the number of moles of the stationary phase component on the column and VN is the standardized retention volume obtained by the GC retention time measurements. The second and third term in Eq. (2) are correction terms which arise from the non-ideality of mobile gaseous phase. En is the second virial coefficient of the solute; B12 the mixed virial coefficient of the solute with the carrier gas nitrogen; V® is the liquid molar volume of pure solute and V™ is the partial molar volume of solute in the ionic liquid at infinite dilution. The factor J appearing in Eq. (2) corrects for the influence of the pressure drop along the column. Results are presented in Table 1.
0
Table 1
Activity coefficients at infinite dilution (y|° ) of methylbenzaldehydes at 385 K from GC-measurements
Ionic liquid 2-methylbenz aldehyde 3-methylbenz aldehyde 4-methylbenzaldehyde
[BMIM][NTf2] 0.931 0.875 0.891
[OMIM][BF4] 1.30 1.21 1.25
It is apparently, that values of of ortho-, meta, and para-isomers are very close on both ILs studied, following, there are no specific interactions between methylbenzaldehydes and IL due to position of methyl substituent in the benzene ring. Thus, a selective separation of ortho-, meta-, and para-methylbenzaldehyde from any reacting mixture containing these
isomers is hardly possible. However, the values of of ortho-, meta-, and para-isomers in [BMIM][NTf2] and [OMIM][BF4] are around unity. This behavior indicates that these solutes interact with ionic liquids very moderate, following a recovery of methylbenzaldehydes from their mixtures with ILs will be not difficult.
3.2 Vapor—liquid equilibrium measurements of the binary mixtures (Solute +IL). Activity coefficients at infinite dilution are good indicator for solute-solvent interactions, but for more profound information can be obtained only from measurements of vapor pressures covering the whole range of concentration of solute + ionic liquid mixtures. We have been performed two series of experiments where first ortho- and than para-methyl-benzaldehyde where mixed with ionic liquid and partial pressures of solutes were studed at different temperatures (see Tables 2, 3). Partial pressure of the ionic liquid in the experimental conditions was not detectable. We checked every system under study for repeatability of the measurements and it was governed within 1-3 % by accuracy of the GC analysis [5]. The VLE measurements help to evaluate the affinity between a solute and a solvent. Such knowledge is important to assess a degree of recovery of organic products from ionic liquids. Fig. 1-4 show the vapor pressures alteration of systems methylbenzaldehydes + IL. The weak ion-dipole interaction between the ionic liquid and methylbenzaldehyde evokes a weak positive deviation from Raoult's law. The anion in [OMIM][BF4] shows a stronger interaction with both methylbenzaldehydes then anion in [BMIM][NTf2]. This means that the separation of methylbenzaldehydes from [BMIM][NTf2] should be easier, then from [OMIM][BF4]. Comparison of the results at two temperature levels — 298 K and 313 K reveals that positive deviation from Raoult's law increase with the increasing temperature for both ILs, however the degree of interaction of ortho-methylbenzaldehyde and para-methylbenzaldehyde is very close, thus both ILs are not suitable for separation of these isomers.
Table 2
VLE in the system o-methylbenzaldehyde + [BMIM][NTf2] and p-methylbenzaldehyde + [BMIM][NTf2]
T, K o-methylbenz aldehyde p-methylbenzaldehyde
Xl -^exp 5 P& -Pexp/-Fb Xl -^exp 5 P& -Pexp/fb
298.2 0.1567 9.40 0.15 0.1651 7.00 0.15
0.1591 8.98 0.15 0.1667 7.11 0.15
0.2220 13.16 0.22 0.3018 12.71 0.26
0.2257 13.39 0.22 0.3036 12.87 0.27
0.4787 28.00 0.46 0.4448 19.68 0.41
0.4812 28.82 0.47 0.5883 27.25 0.57
0.6385 38.01 0.62 0.5907 27.58 0.57
0.7648 46.51 0.76 0.7474 34.95 0.73
0.7668 46.38 0.76 0.9216 44.99 0.93
0.9050 58.93 0.97 0.9442 45.86 0.95
0.9501 46.12 0.96
0.9840 47.81 0.99
303.2 0.4737 39.73 0.46 0.1563 10.39 0.15
0.4758 38.59 0.45 0.1598 10.65 0.16
0.1500 14.47 0.17 0.2964 19.95 0.29
0.1530 13.61 0.16 0.2995 19.47 0.29
0.2171 19.14 0.22 0.4440 30.13 0.44
0.2200 19.31 0.22 0.5829 39.25 0.58
0.6359 54.57 0.64 0.5859 39.90 0.59
T, K o-methylbenz aldehyde p-methylbenz aldehyde
Xl -^exp 5 P& -Pexp/fb Xl -^exp 5 P& -Pexp/fb
303.2 0.7606 69.68 0.81 0.7431 52.80 0.78
0.9042 84.27 0.98 0.7447 52.89 0.78
0.9046 84.90 0.99 0.9201 64.25 0.95
0.9448 65.04 0.96
0.9498 65.68 0.97
308.2 0.1464 19.70 0.17 0.1498 13.74 0.14
0.2121 25.77 0.22 0.1531 14.74 0.16
0.2147 26.90 0.23 0.2860 27.89 0.29
0.4690 57.26 0.48 0.2895 29.90 0.32
0.4713 59.39 0.50 0.4396 42.29 0.45
0.6329 74.95 0.63 0.5765 59.67 0.63
0.7559 100.38 0.84 0.5799 57.28 0.60
0.7584 100.89 0.85 0.7414 75.71 0.80
0.9034 118.77 1.00 0.8804 86.95 0.92
0.9038 118.48 0.99 0.9191 87.07 0.92
0.9196 87.85 0.93
0.9452 91.29 0.96
313.2 0.1393 26.02 0.16 0.1432 20.75 0.16
0.1421 25.72 0.16 0.1470 20.69 0.16
0.2059 37.52 0.23 0.2782 40.03 0.31
0.2095 36.85 0.22 0.2827 39.54 0.30
0.4633 82.91 0.50 0.4371 64.00 0.49
0.4667 84.04 0.51 0.5728 84.61 0.65
0.6292 108.72 0.66 0.7360 106.99 0.82
0.7534 138.09 0.84 0.7380 109.77 0.84
0.9024 163.38 0.99 0.5686 75.68 0.58
0.9030 166.73 1.01 0.8769 119.93 0.92
0.9179 123.55 0.94
0.9186 124.33 0.95
318.2 0.1329 38.85 0.17 0.1352 29.68 0.17
0.1365 37.89 0.17 0.1395 30.36 0.17
0.1972 51.37 0.23 0.2688 56.11 0.32
0.2022 50.51 0.23 0.2738 56.14 0.32
0.4552 120.81 0.54 0.4342 84.24 0.47
0.4599 116.05 0.52 0.5607 114.80 0.65
0.6243 143.23 0.64 0.5647 119.67 0.67
0.7406 207.59 0.93 0.7322 153.33 0.86
0.7459 200.80 0.90 0.7343 149.49 0.84
0.9020 232.85 1.04 0.8776 170.98 0.96
0.8792 169.02 0.95
0.9140 171.72 0.97
0.9173 174.86 0.99
323.2 0.1245 52.74 0.18 0.1270 41.13 0.17
0.1292 53.23 0.18 0.1312 40.55 0.17
0.1852 72.89 0.24 0.2585 80.81 0.33
0.1921 69.18 0.23 0.2637 80.31 0.33
0.4433 168.88 0.56 0.4309 115.28 0.47
0.4503 167.02 0.56 0.5520 154.13 0.63
0.7267 291.90 0.97 0.5568 155.55 0.64
T, к o-methylbenz aldehyde p-methylbenz aldehyde
Xl -^exp ? P& -Pexp/fb Xl -^exp ? P& -Pexp/fb
0.7349 287.71 0.96 0.7272 204.84 0.84
0.9013 316.60 1.05 0.7300 211.43 0.87
323.2 0.8786 234.69 0.96
0.9150 233.93 0.96
0.9159 232.16 0.95
Table 3
VLE in the system o-methylbenzaldehyde + [OMIM][BF4] and p-methylbenzaldehyde + [OMIM][BF4]
T, К o-methybenz aldehyde p-methylbenzaldehyde
Xl -^exp ? P& -Pexp/fb Xl -^exp ? P& -Pexp/fb
298.2 0.2166 13.73 0.22 0.2020 9.79 0.20
0.3322 23.51 0.39 0.3553 18.55 0.39
0.4746 34.78 0.57 0.4695 26.05 0.54
0.5389 39.15 0.64 0.5660 31.58 0.66
0.8207 55.49 0.91 0.7315 40.23 0.84
0.9308 61.88 1.01 0.8467 43.77 0.91
303.2 0.2135 20.66 0.24 0.1867 13.76 0.20
0.4717 48.96 0.57 0.2003 14.89 0.22
0.5368 57.33 0.67 0.3339 28.89 0.43
0.6832 69.92 0.81 0.3531 29.05 0.43
0.8200 80.85 0.94 0.4677 37.01 0.55
0.9306 87.04 1.01 0.5644 46.04 0.68
0.6920 56.40 0.83
0.7301 57.36 0.84
0.8461 66.15 0.97
308.2 0.2095 31.45 0.26 0.1985 22.21 0.23
0.4684 71.75 0.60 0.3503 41.39 0.44
0.5345 87.87 0.74 0.4644 54.87 0.58
0.6807 109.95 0.92 0.5614 66.61 0.70
0.8192 117.04 0.98 0.6901 77.04 0.81
0.7287 85.15 0.90
0.8454 95.30 1.00
313.2 0.2064 44.68 0.27 0.1964 31.14 0.24
0.4640 102.00 0.62 0.3472 61.09 0.47
0.5310 123.25 0.75 0.4606 78.80 0.60
0.6782 152.94 0.93 0.5582 99.55 0.76
0.8181 163.20 0.99 0.6882 117.20 0.89
0.7268 123.06 0.94
0.8445 130.58 1.00
318.2 0.2029 63.18 0.28 0.1937 44.38 0.25
0.4587 138.99 0.62 0.3435 85.79 0.48
0.5262 163.95 0.74 0.4563 108.08 0.60
0.6747 213.17 0.96 0.5541 139.83 0.78
0.8356 223.36 1.00 0.6856 159.98 0.89
0.7244 171.21 0.95
0.8434 187.58 1.04
323.2 0.1985 85.24 0.28 0.1906 63.29 0.26
T, к o-methylbenzaldehyde p-methylbenz aldehyde
«i -fexp ? P& -Pexp/fb «i -fexp ? P& -Pexp/fb
0.4522 194.33 0.65 0.3393 116.49 0.48
0.5197 232.19 0.77 0.4515 147.29 0.60
323.2 0.6704 298.93 0.99 0.5488 194.94 0.80
0.6823 224.49 0.92
0.7216 238.02 0.98
0.4 0.6 Mole fraction
Fig. 1. The comparison of vapor pressures of o-methylbenzaldehyde and p-methylbenzalde-hyde in the mixture with [BMIM][NTf2 ] as function of mole fraction xi (methylbenzaldehyde) at 298.2 K:
1 — o-methylbenzaldehyde; 2 — p-methylben-zaldehyde; solid line — ideal solution; P — vapor pressure of solution; Po — vapor pressure of solute
0.4 0.6 Mole fraction
Fig. 2. The comparison of vapor pressures of o-methyl-benzaldehyde and p-methylbenzaldehyde in the mixture with [BMIM][NTf2 ] as function of mole fraction x1 (methylbenzaldehyde) at 313.2 K:
1 — o-methylbenzaldehyde; 2 — p-methylbenzalde-hyde; solid line — ideal solution; P — vapor pressure of solution; Po — vapor pressure of solute
3.3. Vapor pressures and enthalpies of vaporization of pure methylbenzaldehydes. Values of saturated vapor pressures p0 of the pure liquid solutes are required for calculation of and VLE. Vapor pressures of individual pure ortho-, meta, and para-methylbenzaldehyde have been measured by the transpiration method. Our experimental vapor pressures of methylbenzaldehydes have been measured in the temperature range about 30 K and possibly close to ambient temperatures. The following equation:
7 ГТ1
Д1пр,о = а+- + Д?Ср1п- (3)
T To
was fitted to the experimental p, T data using a and b as adjustable parameters. T0 appearing in Eq. (3) is an arbitrarily chosen reference temperature (which has been chosen to be
Fig. 3. The comparison of vapor pressures of o-methylben-zaldehyde and p-methylbenzaldehyde in the mixture with [OMIM][BF4] as function of mole fraction xi (methylbenzaldehyde) at 298.2 K:
1 — o-methylbenzaldehyde; 2 — p-methylbenzaldehyde; solid line — ideal solution; P — vapor pressure of solution; Po — vapor pressure of solute
0.2 0.4 0.6 0.8 Mole fraction
Fig. 4. The comparison of vapor pressures of o-methylbenzaldehyde and p-methylbenzalde-hyde in the mixture with [OMIM][BF4] as function of mole fraction xi (methylbenzaldehyde) at 313.2 K:
1 — o-methylbenzaldehyde; 2 — p-methylben-zaldehyde; solid line — ideal solution; P — vapor pressure of solution; Po — vapor pressure of solute
0.4 0.6 Mole fraction
298.15 K). Consequently, from Eq. (3) the expression for the vaporization enthalpy at temperature T is derived:
A? Hm(T ) = -b + AfCpT. (4)
Values of A? Cp have been derived using the isobaric molar heat capacities C? of methylben-zaldehydes calculated according to a procedure developed by Chickos and Acree [16]. The data approximations with Eq. (3) are given below:
2-methylbenzaldehyde; AgHm(298.15 K) = 52.51 ± 0.24 kJ/mol
, , 275.09 71860.63 64.9 / T, K In(p, Pa) = —----- — In
R R ■ (T, K) R V298-15
3-methylbenzaldehyde; A?Fm((298.15 K) = 53.67 ± 0.25 kJ/mol
, , ^ N 278-05 73020-24 64-9 / T, K In(p, Pa) = —----- — In
R R ■ (T, K) R \298-15
4-methylbenzaldehyde; A?Hm((298.15 K) = 53.53 ± 0.43 kJ/mol
, , ^ N 276-51 72882-63 64-9 f T, K In(p, Pa) = —----- — In
R R ■ (T, K) R \298.15
Enthalpies of vaporization of methylbenzaldehydes have been determined for the first time. Value of A®ffm(298.15 K) of orifeo-methylbenzaldehyde is slightly lower then those for meta- and para-methylbenzaldehyde due to steric interaction of substituents in the benzene ring disturbing organizing of the structure of the liquid phase. Values of Af Hm(298.15 K)
for meta- and para-methylbenzaldehyde are indistinguishable in consonance with those for another pairs of meta - and para-substituted benzenes [17].
3.4 Enthalpies of formation of 2- and 4-methylbenzaldehydes. Results of combustion experiment with 2- and 4-methylbenzaldehyde are summarized in Tables 4-6. Values of the molar enthalpies of formation, Af H^ (l) of compounds under study have been obtained from the enthalpic balance for reactions
C8 H8O + 9.5O = 8CO2 +4H2O. (5)
Table 4
Results for combustion experiments on 2-methylbenzaldehyde T = 298.15 K (p° = 0.1 MPa)*
m(substance), g** 0.205805 0.263267 0.250304 0.228353 0.250895
to'(cotton), g** 0.001253 0.0012 0.001367 0.001343 0.000974
to'(polythen), g" 0.370947 0.398213 0.382616 0.389026 0.351328
Tc, K*** 1.63797 1.85702 1.77751 1.74652 1.68288
£calor( Tc) 1 J -24383.8 -27644.8 -26461.1 -25999.7 -25052.3
£cont ( Tc) 1 J -26.07 -30.37 -29.02 -28.15 -26.98
A [/(dec) HNO3, J 45.39 50.17 42.41 46.59 64.5
Af/corr, J 8.6 10.24 9.79 9.37 9.69
—m'Acu', J**** 21.23 20.33 23.16 22.76 16.5
—m'Acu', J**** 17196.1 18460.08 17737.04 18034.19 16286.62
Acu° (substance), Jg_1 -34686.2 -34696 -34668.6 -34661 -34683.7
* For the definition of the symbols see reference [18]; Th = 298.15 K; V(bomb) = 0.32 dm3; pl(gas) = 3.04 MPa; mi(H2O) = 1.00 g.
** Masses obtained from apparent masses.
*** ATC = Tf - Ti + ATcon-; econt • (-Tc) = 4ont№ - 298.15 K) + efont(298.15 K - Tf + ATcorr). **** AUcorr, the correction to standard states, is the sum of items 81 to 85, 87 to 90, 93, and 94 in reference [19]; Ac«°(polyethene) = -(46357.3±3.6) Jg"1; e = 14886.5±0.9 JK"1; water content 20'000 ppm.
Table 5
Results for combustion Experiments on 4-Methylbenzaldehyde T = 298.15 K (p° = 0.1 MPa)
TO(substance), g* 0.28384 0.2371 0.259741 0.252171 0.261568
to'(cotton), g* 0.001203 0.001061 0.001262 0.001494 0.001479
to' (polythen), g* 0.399039 0.39896 0.375157 0.365701 0.343918
Tc, K" 1.90607 1.79588 1.77459 1.72859 1.68282
^calor ( Tc) ? J -28374.8 -26734.5 -26417.7 -25732.6 -25051.3
£cont ( Tc), J -31.21 -29.23 -29.02 -27.89 -27.21
A [/(dec) HNO3, J 51.96 48.98 47.78 46.29 45.99
A[/corr, J 10.64 9.72 9.82 9.47 9.31
—m'Acu', J*** 20.38 17.98 21.38 25.32 25.06
—m'Acu', J*** 18498.37 18494.71 17391.27 16952.91 15943.11
Acu° (substance), Jg_1 -34613.5 -34552.4 -34559.2 -34605.6 -34618.2
* Masses obtained from apparent masses.
** ATc = Tf - Ti + ATcorr; econt • (-Tc) = elcont(Ti - 298.15 K) + efont(298.15 K - Tf + ATcorr). *** AUcorr, the correction to standard states, is the sum of items 81 to 85, 87 to 90, 93, and 94 in reference [19]; Ac«°(polyethene) = -(46357.3±3.6) Jg"1; e = 14886.5±0.9 JK"1; water content 2'300 ppm.
Results for combustion experiments on 4-methylbenzaldehyde T = 298.15 K (p° = 0.1 MPa)
2-methylbenzaldehyde -384.256282 -384.247170 -384.665554 -384.656587
3-methylbenz aldehyde -384.259376 -384.249966 -384.668623 -384.659397
4-methylbenzaldehyde -384.259671 -384.250313 -384.669198 -384.659950
benzene -231.829758 -231.824309 -232.093987 -232.088586
benz aldehyde -345.019787 -345.012355 -345.384990 -345.377674
acetophenone -384.672801 -384.663987
toluene -271.068985 -271.061629 -271.377414 -271.370151
methane -40.422100 -40.418284 -40.465302 -40.461486
ethane -79.651199 -79.646714 -79.738106 -79.733658
methanol -115.552221 -115.547932 -115.651767 -115.647493
according to the Hess's law using the molar enthalpies of formation of H2O (l) and CO2 (g) as assigned by CODATA [18]. The total uncertainty was calculated according to the guidelines presented by Olofs-son [19]. The uncertainty assigned to Af Hi is twice the overall standard deviation and include the uncertainties from calibration,
Table 7
Experimental thermochemical results at T = 298.15 K (p° = 0.1 MPa), kJ/mol
Compound AfH°m (1)* A9fHrn** AfH°m (g)*"
2-methylbenzaldehyde -121.0 ±2.1 52.5 ± 0.2 -68.5 ±2.1
3-methylbenz aldehyde - 53.7 ±0.3 -
4-methylbenzaldehyde -131.8 ±3.7 53.5 ± 0.4 -78.3 ±3.8
* From combustion experiments.
** From vapor pressure measurements by transpiration. *** Experimental values derived as the sum AfHm (g) Af Hm (l)+Ag Hm.
from the combustion energies of the auxiliary materials, and the uncertainties of the enthalpies of formation of the reaction products H2O and CO2. Enthalpies of formation of 2- and 4-methylbenzaldehyde (see Table 7) were measured for the first time.
3.5 Calculation of the gaseous enthalpies of formation of 2- and 4-methylben-zaldehydes. Enthalpies of formation of 2- and 4-methylbenzaldehyde and their values of vaporization enthalpies can now be used for further calculation of the standard enthalpies of formation, Af H^ (g) at 298.15 K (Table 7). The resulting values for methylbenzaldehydes are given in Table 7 ready for comparison with the theoretical values from quantum-chemical calculations.
3.6 Quantum chemical calculations for methylbenzaldehydes. First principle calculation of the enthalpy of formation of methylbenzaldehydes have not been yet reported in the literature. Recent development of the quantum chemistry methods have been designed for prediction enthalpies of formation AfH0 with "chemical accuracy" which is usually defined as within ±4 kJ/mol of the experimental value [20]. From practical point of view the best first-principles method should be accurate but not very time consuming. It has turned out that two composite methods G3MP2 and G4 can meet this expectation for organic molecules with about 15 heavy atoms. The computational cost of these methods follow the order: G3MP2 < G4. For example, for 2-methylbenzaldehyde the CPU consuming times were 23 min (G3MP2), 9 h 47 min (G4) by using one Intel(R) Xeon(R) CPU X5670 2.93 GHz processor. Our results are presented in the Table 8. In standard Gaussian theories, theoretical enthalpies of formation are calculated through atomization, bond separation or isodesmic reactions [20]. In this work we calculated the enthalpies of formation of
methylbenzaldehydes with help of atomization (AT) procedure:
C8H8 O = 8C + 8H + O,
bond separation reaction:
C8H8O + 17CH4 = CH3OH + 12C2H6,
(7)
and isodesmic reaction:
\/H C 1 CH3 3
+ il
V
CH3 +
(8)
Table 8
Results of calculation of gaseous A/Hi0n of methylbenzaldehydes, kJ/mol
Compound AfHl (g) exp AfH°n (g) calc
G3MP2 G4
AT Eq. (7) Eq. (8) AT Eq. (7) Eq. (8)
2-methylbenz aldehyde -68.5 ±2.1 -69.2 -72.3 -62.2 -65.1 -74.8 -61.8
3-methylbenz aldehyde - -76.6 -79.6 -69.6 -72.4 -82.2 -69.2
4-methylbenz aldehyde -78.3 ±3.8 -77.5 -80.5 -70.5 -73.9 -83.6 -70.7
acetophenone -86.7 ±1.5 [22] -89.3 - - -84.5 - -
benzaldehyde -36.7 ±2.8 [22] -43.1 - - -38.6 - -
Each type of reaction has advantages and disadvantages. For example atomization and bond separation reactions are based on the elements or molecules with well established thermochemical data. Isodesmic reactions rely upon the similarity of bonding environments in the reactants and products that leads to cancellation of systematic errors in the ab initio calculations [21]. As a rule, carefully selected isodesmic reactions provide the the least deviations of theoretical Af H0 values. From our experiences, however, the quantum chemistry is not a desired remedy and each particular first-principles method requires careful validation with the reliable experimental data.
Using enthalpies of reactions (7) and (8), calculated by G3MP2 and G4 and together with enthalpies of formation, Af H0 (g), for methane, ethane, methanol, benzene, toluene, and benzaldehyde recommended by Pedley et al. [22] enthalpies of formation of all three isomeric methylbenzaldehydes have been calculated (see Table 8). To our surprise results of quantum chemical calculations collected in Table 8 contradict our expectation. While enthalpies of formation of methylbenzaldehydes calculated with help of AT and the bond separation reaction (7), for both G3MP2 and G4 method, are in good agreement with the experimental values derived in this work, enthalpies of formation of methylbenzaldehydes estimated with help of the isodesmic reaction (8) systematically deviate from experimental results by about 7 kJ/mol. There is no apparent reason for such deviation and as a matter of fact the isodesmic reaction (8) is clerarly the best possible isodesmic reaction for the substituted benzenes studed in this work. Such unexpected ambiguity of Af H0 values
obtained from isodesmic reactions in contrast to the less favorable atomization and bond separation reactions chelenges in the future for further investigation of this artifact with another disubstituted benezene derivatives. However, within the current study in order to be sure that the atomization is the best way to get theoretical enthalpies of formation, we have successfully checked both the G3MP2 and the G4 method with recommended by Pedley et al. [22] experimental values for benzaldehyde and acetophenone (see Table 8). Thus, in this work enthalpies of formation derived with help of the atomization procedure for both G3MP2 and G4 method are considered to be more reliable for analysis of substituent effects in methylbenzaldehydes.
3.7. Analysis of substituent effects in for methylbenzaldehydes. The methyl and carbonyl groups are generally referred to as electron donating substituents in aromatic system. The CH3 interacts with nearby systems via hyperconjugation, while the CO shares its lone pair electrons with electrons in the ring. The counterplay of the electronic delocalization is expected to be of a moderate energetic in the case of the meta-and para- derivatives. Intensive sterical interactions of the methyl and CO due to close proximity of substituents in the ortfeo-position on the benzene ring is also not expected because of their relatively small size. There are several possibilities to derive substituents effect on the benzene ring [23]. However, the isodesmic reaction (7) is especially convenient for benzene derivatives, because enthalpy of the distribution reaction like (7), expresses energetic of the mutual interaction of substituents on the benzene ring directly. For example, the enthalpy of reaction (7) applied for 2-methylbenzaldehyde allows to derive the strain of this molecule in comparison to benzene, toluene, and benzaldehyde, or in other words, this amount of interaction is caused by steric and electronic interactions of methyl and the carbonyl substituents. We calculated substituent effects in methylbenzaldehydes with the help of reaction (7) using experimental enthalpies of formation, Af H0 (g), of benzene, toluene, and benzaldehyde [22] and Af H0 (g) of methylbenzenes calculated using the atomization procedure. These calculations (see Table 9) have revealed that 2-methylbenzaldehyde is slightly destabilized by about 4 kJ/mol according to G4 calculations, which are also in agreement with the experiment (last column in Table 9).
Table 9
Results of calculation of the standard enthalpies of reactions for reactions involving methylbenzaldehydes in the gaseous phase at 298.15 K, kJ/mol
In contrast, a weak stabilization of about 5-9 kJ/mol is calculated in 4-methylbenzaldehyde. 3-methyl-benzaldehyde is observed to be somewhat less stabilized at the level of 3-7 kJ/mol. These effects are not very large, but they are a important for understanding general energetic interplay of sub-stituents on the benzene ring.
Conclusion. A comprehensive study of thermodynamic properties of pure methylbenzaldehydes and their mixtures was performed in this work. Experimental results were used for validation of quantum chemical calculations and the atomization procedure was recommended for G3MP2 and G4 for calculation of gas enthalpies of formation.
Compound G4 (AT) G3MP2 (AT) exp.
2-methylbenzaldehyde 3.8 -0.3 0.4 ±3.7
3-methylbenz aldehyde -3.5 -7.7 -
4-methylbenz aldehyde -5.0 -8.6 -9.4 ±4.9
Authors are grateful to Prof. Morachevsky and Prof. Smirnova for their textbooks, which helped to open the door to the world of thermodynamics.
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Статья поступила в редакцию 10 сентября 2012 г.
