Thermochemical study of the ortho interactions in alkyl substituted anilines

J. Chem. Thermodynamics 2000, 32, 247–259 doi: 10.1006/jcht.1999.0587 Available online at http://www.idealibrary.com on

Thermochemical study of the ortho interactions in alkyl substituted anilines Sergey P. Verevkina Department of Physical Chemistry, University of Rostock, Hermannstr. 14, D-18051 Rostock, F.R.G. o (l or cr) at the The standard ( p o = 0.1 MPa) molar enthalpies of formation 1f Hm temperature T = 298.15 K were determined by means of combustion calorimetry for 2-ethyl-aniline, 2-iso-propyl-aniline, 2-tert-butyl-aniline, 2,6-di-methyl-aniline, 2,6-diethyl-aniline, 2,6-di-iso-propyl-aniline, and 2,4,6-tri-tert-butyl-aniline. The standard molar enthalpies of vaporization (or sublimation) of these compounds were obtained from the temperature dependence of the vapor pressure measured in a flow system. The resulting o (g) were obtained at the temperature T = 298.15 K and used to derive values of 1f Hm strain enthalpies of alkyl-anilines. The intra-molecular interactions of the substituents were o (g) from group additivity rules. These values discussed in terms of deviations of 1f Hm provide a further improvement on the group-contribution methodology for estimation of c 2000 Academic Press the thermodynamic properties of organic compounds.

KEYWORDS: enthalpy of combustion; enthalpy of sublimation; enthalpy of vaporization; enthalpy of formation; alkyl-anilines

1. Introduction Investigation of the energetic effects of interactions of substituents on the benzene nucleus has long been a popular endeavour for many of research groups.(1) Nevertheless, a quantitative description of the interaction between amino group and alkyl substituent is not possible, primarily because of the lack of thermochemical information. Only a few species of this type have been characterized thermochemically.(2–4) This paper extends our previous studies(5–9) on the systematic investigation of a series of substituted benzenes. The aim of this work was an experimental study of the thermochemical properties of alkylanilines at T = 298.15 K (figure 1) with a view to answering the following questions. What is the interaction energy between the alkyl and the amino-group in the ortho position of alkyl-anilines? What is the “buttress” interaction energy among the alkyls and the amino-group in their adjacent alkyl-NH2 -alkyl position on the benzene ring? Is there any dependence on the type of alkyl substituent (alkyl: ethyl-, iso-propyl-, and tert-butyl)? a (E-mail: [email protected]).

0021–9614/00/020247 + 13 $35.00/0

c 2000 Academic Press

248

S. P. Verevkin

NH2

NH2

NH2

A

B

C

NH2

NH2

NH2

NH2

D

E

F

G

FIGURE 1. Structures of anilines: A, 2-ethyl-aniline; B, 2-iso-propyl-aniline; C, 2-tert-butyl-aniline; D, 2,6-di-methyl-aniline; E, 2,6-di-ethyl-aniline; F, 2,6-di-iso-propyl-aniline; and G, 2,4,6-tri-tertbutyl-aniline.

To answer these questions, and in view of the paucity of thermochemical quantities for alkyl-anilines, we have obtained 1f Hmo (g) at the temperature T = 298.15 K for seven compounds (figure 1) by combining the results of combustion calorimetry, transpiration method, and d.s.c. Strain enthalpies 1Hm (strain) of alkyl-anilines were derived from their standard molar enthalpies of formation in the gaseous phase. The energies of the intramolecular interactions of substituents on the benzene nucleus in terms of departure of enthalpies of formation from the group additivity rules were discussed.

2. Experimental All compounds were purchased from Aldrich or Acros Ltd. G.l.c. analyses of the purchased samples gave an average mass fraction of 0.99. The liquid compounds were fractionally distilled under flowing dry N2 , after being dried with molecular sieves (0.4 nm). The determination of purity was carried out by g.l.c. No impurities (≤ 1 · 10−4 mass fraction) could be detected in combustion samples A to G purified by repeated distillation using a spinning-band column at reduced pressure. The solid compound was purified by repeated crystallization from petroleum ether or ethanol and then finally sublimed under reduced pressure to remove any traces of solvent. The degree of purity was determined by g.l.c. For all the solid compounds, mass fractions of 0.9998 were additionally established by d.s.c. measurements of the melting process.(10) No impurities (greater than mass fraction 1 · 10−4 ) could be detected in samples of A to G by g.l.c. To exclude traces of water from the liquid samples for combustion experiments, the purified samples were dried over molecular sieves and distilled once more prior to combustion. The samples were stored in a refrigerator, either at reduced pressure or under a nitrogen atmosphere. We used the following equipment: a g.l.c. (Carlo Erba Fraktometer Vega Series GC 6000) and Hewlett Packard Integrator 3390A (N2 -flow of 0.333 cm3 · s−1 , SE-30 capillary column of length 25 m). The standard temperature programme of the g.l.c. was T = 313 K

Thermochemistry of alkyl-anilines

249

for 60 s, followed by a heating rate of 0.167 K · s−1 to T = 523 K. Melting temperatures, enthalpies of fusion, and massic heat capacities were determined with a Perkin-Elmer DSC-2C. An isoperibol rotating-bomb macrocalorimeter described elsewhere(11, 12) was used for the measurement of energies of combustion. The energy equivalent of the calorimeter εcalor was determined with a standard reference sample of benzoic acid (sample SRM 39i, N.I.S.T.). On the basis of seven to ten experiments, values of εcalor were measured to be (25126.9 ± 1.9) J · K−1 for A, E, F, and G, and (25112.9 ± 1.9) J · K−1 for B, C, and D. Change in the energy equivalent between the various combustion series was due to repair to the bomb to cure a leak. Purified samples were colourless, and the absence of water was shown by Karl Fischer titration. From a practical point of view, careful encapsulation is important in combustion calorimetry of volatile and hygroscopic liquids such as amines. In the present study, we used commercially available polyethene bulbs (NeoLab, Heidelberg) of 1 cm3 volume as the sample container for liquids in order to reduce the capillary effect and make the encapsulation easier. Liquid specimens were transferred to polyethene bulbs (under an inert atmosphere in a glove box) with a syringe. The narrow neck of the container was compressed with special tweezers and was sealed outside the glove-box by heating with hot air. Then, the loaded container was placed in the bomb and burned in oxygen at a pressure of 3.04 MPa. The solid samples were compressed (under an inert atmosphere in a glove-box) into pellets of mass ≈ 400–500 mg, and were burned in oxygen. Completeness of the combustion of the crystalline sample was ensured by the addition of about 50 mg of oil to the pellet. The detailed procedure has been described previously.(11, 12) The combustion products were examined for carbon monoxide (Dr¨ager tube) and unburned carbon, but neither were detected. Corrections for nitric acid formation were based on the titration with 0.1 mol · dm−3 NaOH (aq). The atomic weights used were those recommended by the IUPAC Commission.(13) The densities of the commercially available liquid compounds were taken from the Aldrich catalogue. The density of the solid substance was determined by submerging a pellet of the substance in water in a calibrated 10 cm3 pycnometer. The enthalpy of fusion and the massic heat capacities were measured with a d.s.c. The temperature scale of the d.s.c. was calibrated by measuring the melting temperatures of the recommended high-purity standards: benzoic acid, tin, and indium.(10) The power scale was calibrated using sapphire as a standard material. A summary of the auxiliary quantities used for the combustion experiments and the necessary information for reducing apparent mass to mass, converting the energy of the actual bomb process to that of an isothermal one, and reducing to standard states(14) is given in table 1. The enthalpies of vaporization and the enthalpy of sublimation of alkyl-anilines were determined by the method of transpiration in a saturated N2 -stream(15, 16) and by using the Clausius–Clapeyron equation. About 0.5 g of a given sample was mixed with glass beads and placed in a thermostatted U-tube of 20 cm length and 0.5 cm diameter. A nitrogen stream was passed through the U-tube at constant temperature (±0.1 K), and the transported amount of material was collected in a cold trap. A nitrogen flow of (0.28 to 0.56) cm3 · s−1 was found to be optimal in achieving saturation of the carrier gas at

250

S. P. Verevkin

TABLE 1. Formula, density ρ(T = 293 K), massic heat capacity cp (T = 298.15 K), and expansion coefficients (δV /δT ) p of the materials used in the present study a ρ g · cm−3

cp

b

10−6 · (δV /δT ) p

J · K−1 g−1

dm3 · K−1

2-Ethyl-aniline

C8 H11 N

0.983

1.63

1.0

2-Iso-propyl-aniline

C9 H13 N

0.955

2.01

1.0

2-Tert-butyl-aniline

C10 H15 N

0.957

1.84

1.0

2,6-Dimethyl-aniline

C8 H11 N

0.906

1.97

1.0

2,6-Di-ethyl-aniline

C10 H15 N

0.984

2.05

1.0

2,6-Di-iso-propyl-aniline

C12 H19 N

0.940

1.63

1.0

2,4,6-Tri-tert-butyl-aniline

C18 H31 N

0.991c

2.13

0.1

Oild

CH1.940

0.880

0.84

1.0

Cottone

CH1.774 O0.887

1.50

1.67

0.1

Polyethene f

CH1.930

0.92

2.53

0.1

c

a Measured with a pycnometer. b From d.s.c. measurements. c Estimated. d From nine combustion experiments, 1c u o = −(46003.6 ± 4.0) J · g−1 , where 1c u o denotes the standard massic energy of combustion. e From ten combustion experiments, 1c u o = −(16945.2 ± 4.2) J · g−1 . f From eleven combustion experiments, 1c u o = −(46361.0 ± 3.1) J · g−1 .

each specified temperature. The amount of condensed substance was determined by g.l.c. analysis using an internal standard (n-C11 H24 or n-C13 H28 ). The vapor pressure p at each saturation temperature was calculated from the amount of product collected within a definite time period, to which was added the small value of the residual vapor pressure at the temperature of condensation. The latter was calculated from a linear correlation between ln p and T −1 obtained by iteration. To derive the standard molar enthalpy of vaporization or sublimation at the mean temperature hT i of the experimental temperature g g range, 1l Hmo (T ) or 1cr Hmo (T ), the integrated form of the linear Clausius–Clapeyron equation: ln( p/Pa) = a − b · (T /K)−1 ,

(1)

g 1cr Hmo (T )/R,

where b = was used. The observed enthalpies of vaporization or sublimation at the temperature T obtained by this procedure are listed in table 2. The experimental data were approximated by the linear equation ln p = f (T −1 ) (see table 2) using the method of least-squares. The errors in the thermodynamic functions were defined as deviations of experimental ln p from the linear correlation.

3. Results and discussion A summary of typical combustion experiments of alkyl-anilines is given in table 3. The individual values of the standard massic energies of combustion 1c u o , together with the mean and its standard deviation, are given in table 4. To derive 1f Hmo (l or cr) from 1c Hmo , molar enthalpies of formation of H2 O(l): −(285.830 ± 0.042) kJ · mol−1 and CO2 (g):

Thermochemistry of alkyl-anilines

251

TABLE 2. Results from measurements of the vapor pressure p by the transpiration method T /Ka

m/mgb

283.5 288.4 293.6 298.6 303.2

4.52 3.01 2.40 2.36 3.42

V (N2 )/dm3 c

p/Pad

T /Ka

m/mgb

V (N2 )/dm3

2-ethyl-aniline: ln( p/Pa) = 27.21 − 7255 · (T /K)−1 19.09 4.973 308.3 2.72 8.68 7.233 313.5 4.26 3.94 12.59 318.3 2.38 2.65 18.32 323.3 1.15 2.63 26.70

g

g

g

g

g

g

g

g

g

g

g

g

c

1.35 1.47 0.594 0.214

p/Pad 41.30 59.47 82.01 109.9

285.8 291.7 296.5 301.4 306.4

o (303.4 K) = (60.32 ± 0.86) kJ · mol−1 ; 1 H o (298.15 K) = (60.63 ± 0.86) kJ · mol−1 1l Hm l m 2-iso-propyl-aniline: ln( p/Pa) = 27.10 − 7378 · (T /K)−1 3.35 16.95 3.689 311.3 3.08 1.80 31.40 1.70 5.57 5.675 316.3 2.72 1.16 43.09 2.14 4.41 8.965 321.5 2.10 0.589 65.38 2.66 3.43 14.29 326.4 1.60 0.347 84.56 2.39 2.12 20.73

278.6 288.7 293.4 298.4

o (306.1 K) = (61.34 ± 0.89) kJ · mol−1 ; 1 H o (298.15 K) = (61.81 ± 0.89) kJ · mol−1 1l Hm l m 2-tert-butyl-aniline: ln( p/Pa) = 27.35 − 7542 · (T /K)−1 0.487 6.26 1.341 303.6 0.996 1.35 12.30 0.794 4.00 3.345 308.5 0.975 0.880 18.44 1.140 3.82 5.016 313.7 0.912 0.551 27.53 0.891 1.86 8.001 318.2 0.933 0.408 38.02

285.8 291.7 296.5 301.4 306.4

o (298.4 K) = (62.71 ± 0.36) kJ · mol−1 ; 1 H o (298.15 K) = (62.71 ± 0.36) kJ · mol−1 1l Hm l m 2,6-di-methyl-aniline: ln( p/Pa) = 26.83 − 7117 · (T /K)−1 5.29 16.46 6.730 311.3 4.40 1.75 51.56 2.99 5.41 11.47 316.3 4.14 1.13 75.31 3.55 4.28 17.11 321.5 2.99 0.572 107.2 4.04 3.33 24.98 326.4 2.56 0.337 155.3 3.67 2.06 36.55

283.5 288.5 293.5 294.7 298.4 303.4

o (306.1 K) = (59.17 ± 0.33) kJ · mol−1 ; 1 H o (298.15 K) = (59.64 ± 0.33) kJ · mol−1 1l Hm l m 2,6-di-ethyl-aniline: ln( p/Pa) = 27.83 − 7869 · (T /K)−1 1.27 20.74 1.033 308.4 1.45 2.23 10.84 1.81 18.07 1.679 313.4 1.54 1.72 14.90 0.962 5.78 2.783 318.3 2.02 1.55 21.67 0.842 4.42 3.183 323.2 2.22 1.14 32.37 1.10 4.23 4.345 328.2 1.54 0.555 46.14 1.21 2.98 6.783

283.5 293.5 298.4 303.3

o (305.8 K) = (65.45 ± 0.58) kJ · mol−1 ; 1 H o (298.15 K) = (65.89 ± 0.58) kJ · mol−1 1l Hm l m 2,6-di-iso-propyl-aniline: ln( p/Pa) = 28.28 − 8325 · (T /K)−1 0.465 19.90 0.3319 308.4 0.578 2.24 3.610 0.331 5.04 0.9224 313.3 0.831 2.16 5.380 0.451 4.46 1.419 318.4 0.457 0.766 8.350 0.531 3.26 2.281 323.2 0.590 0.666 12.39

333.3 338.3 343.6 348.5

o (303.3 K) = (69.22 ± 0.27) kJ · mol−1 ; 1 H o (298.15 K) = (69.53 ± 0.27) kJ · mol−1 1l Hm l m 2,4,6-tri-tert-butyl-aniline: ln( p/Pa) = 32.54 − 10748 · (T /K)−1 13.0 91.88 1.346 353.3 14.6 17.57 7.870 12.5 53.46 2.219 358.2 17.5 13.06 12.72 15.8 45.01 3.321 363.4 19.5 9.37 19.75 15.3 26.27 5.507 368.5 15.6 5.09 29.08 g

g

o (350.9 K) = (89.4 ± 1.1) kJ · mol−1 ; 1 H o (298.15 K) = (92.5 ± 1.1) kJ · mol−1 1cr Hm cr m a Temperature of saturation, N gas flow (0.26 to 0.52) cm3 · s−1 . b Mass of transferred sample condensed at T = 243 K. 2 c Volume of nitrogen used to transfer sample. d Vapor pressure at temperature T of experiment; corrections were made for

residual vapor pressure at T = 243 K.

252

S. P. Verevkin

−(393.51 ± 0.13) kJ · mol−1 were taken, as assigned by CODATA.(13) Table 5 lists the derived standard molar enthalpies of combustion, and the standard molar enthalpies of formation of the compounds in the condensed and gaseous states. The standard deviations assigned to the mean values of 1f Hmo (l or cr) include the uncertainties of the calibration, and those of the combustion energies of the auxiliary materials and the reaction products H2 O and CO2 . The experimental enthalpies of vaporization or sublimation at T = 298.15 K and the equations for the temperature dependence ln( p/Pa) = a − b · (T /K)−1 are recorded in table 2. Since the average temperatures of the measurements by the transpiration method deviate from T = 298.15 K, the observed values of the enthalpies of vaporization or sublimation of alkyl-anilines (see table 2) had to be adjusted to this reference temperature. The corrections were estimated with help of the “Sidgwick correction”:(17) g

g

{1l Hmo (hT i) − 1l Hmo (298.15 K)}/(kJ · mol−1 ) = −6 · 10−2 · {(hT i/K) − 298.15}. (2) With these corrections (the uncertainty of the correlation was not taken into account), and g g the measured values of 1l Hmo (T ) and 1cr Hmo (T ) from table 2, the standard enthalpies of vaporization or sublimation at T = 298.15 K were calculated (tables 2 and 5). Only a few para- and meta-alkyl substituted anilines have had their thermochemical properties reported in the literature. The experimental enthalpies of formation 1f Hmo (g) = 41.8 kJ · mol−1 of 4-methyl-aniline and 1f Hmo (g) = 61.1 kJ · mol−1 of 3-methyl-aniline were reported by Draeger.(32) There are hardly any reasons to justify the difference in the enthalpies of formation of 20 kJ · mol−1 between 4methyl-aniline and 3-methyl-aniline. Therefore, we have preferred the experimental standard molar enthalpies of formation of −30.1 kJ · mol−1 for 4-methyl-aniline(cr) and −5.4 kJ · mol−1 for 3-methyl-aniline(l), which were reported by Stull et al.(18) The g enthalpy of vaporization 1l Hmo = 57.4 kJ · mol−1 of 4-methyl-aniline was estimated by approximation(19) from the experimental ( p, T ) data.(20) Then, the enthalpy of g sublimation, 1cr Hmo = 74.7 kJ · mol−1 , was calculated by adding the enthalpy of fusion, l o 1cr Hm = 17.30 kJ · mol−1 , reported for this compound.(21) The resulting standard molar enthalpy of formation, 1f Hmo (g) = 43.6 kJ · mol−1 , of 4-methyl-aniline (table 6) was used for the further interpretation of interactions of substituents on the benzene nucleus. Since the boiling temperatures of 4-methyl-aniline (Tb = 473.7 K)(22) and 3-methyl-aniline (Tb = 476.5 K)(22) are very close, it was reasonable to assume an equality of their enthalpies of vaporization (this value is also in very close agreement with the value g 1l Hmo = 57.90 kJ · mol−1 and boiling temperature 473.4 K(22) for 2-methyl-aniline measured directly).(3) Hence, the standard molar enthalpy of formation of gaseous 3-methyl-aniline, 1f Hmo (g) = 52.0 kJ · mol−1 (table 6), may be estimated from those data. The use of the group-additivity method is straightforward and easy. It does not require computing resources as ab initio calculations do. Another advantage of using groupadditivity is the convenience of predicting thermodynamic properties for large molecules without loss of accuracy. The group-additivity method serves as a valuable tool for many scientists and engineers whose work involves thermodynamic characterization of elementary and overall reaction processes. The most successful empirical method for estimating

18.94 12666.12

−m 0 · 1c u 0 /Jd

−m 00 · 1c u 00 /Jd −38697.3

−39482.2

47.17

12597.24

16.16

19.59

−25.84

−43429.94

1.72941

0.271721e

0.000954

−40253.0

44.78

12131.98

16.13

18.83

−24.50

−43006.20

1.71254

0.261685e

0.000952

0.765595

aniline

aniline 0.779443

2-tert-butyl-

2-iso-propyl-

−38563.0

40.30

12949.02

17.65

15.48

−20.13

−35764.23

1.42416

0.279309e

0.001042

0.590214

aniline

2,6-dimethyl-

−40106.7

47.76

12510.45

16.75

19.52

−25.56

−44129.39

1.75626

0.269849e

0.000989

0.786876

2,6-di-ethyl-aniline

−41097.6

37.32

13588.77

17.30

16.99

−23.84

−41115.43

1.63724

0.293108e

0.001021

0.668590

aniline

2,6-di-iso-propyl-

−42863.4

23.29

3349.78

13.63

9.37

−13.74

−25795.79

1.02721

0.072815 f

0.000805

0.522870

butyl-aniline

2,4,6-tri-tert-

a For the definition of the symbols see reference 10, macrocalorimeter. T = 298.15 K; V (bomb) = 0.2664 dm3 ; p i (gas) = 3.04 MPa; m i (H O) = 0.78 g; h 2 i ) · (T i − 298.15 K) + 1U (ign) = 1.46 J; m(Pt) = 8.61 g. b Masses obtained from apparent masses. c 1Tc = T f − T i + 1Tcorr ; (εcont ) · (−1Tc ) = (εcont f (εcont ) · (298.15 K − T f + 1Tcorr ). d 1Ucorr , the correction to standard states, is the sum of items 81 to 85, 87 to 90, 93, and 94 in reference 10. e Combustion in polyethene. f Combustion with added oil.

1c u o (sub)/(J · g−1 )

57.91

22.64

1U (dec)/J

−26.93

1Ucorr /Jd

1.87555

1Tc /Kc

(εcont ) · (−1Tc )/J

0.273207e

m 00 (auxiliary)/gb

−47126.70

0.001118

m 0 (cotton)/gb

(εcalor ) · (−1Tc )/J

0.888604

m(substance)/gb

2-ethyl-aniline

TABLE 3. Results for typical combustion experiments at T = 298.15 K ( p o = 0.1 MPa)a

Thermochemistry of alkyl-anilines 253

39475.1 39482.2 39492.5 39496.9

39486.7 ± 4.9

38697.3 38711.2 38699.7 38689.5

38699.4 ± 4.5

40106.7 40114.5 40110.7 40113.2

40111.3 ± 1.7

h−1c u o i/(J · g−1 ) 38565.7 ± 1.5

2,6-di-ethyl-aniline

−1c u o /(J · g−1 ) 38565.2 38569.9 38564.6 38563.0

2,6-di-methyl-aniline

l l l l l l cr

State 4695.3 ± 1.4 5345.8 ± 1.7 6014.6 ± 1.6 4679.1 ± 1.0 5994.2 ± 1.3 7297.8 ± 1.8 11226.8 ± 3.4

o (l or cr) a −1c Hm kJ · mol−1

−24.8 ± 1.8 −53.6 ± 2.1 −64.2 ± 2.1 −41.0 ± 1.5 −84.7 ± 1.8 −139.7 ± 2.4 −286.8 ± 4.2

o (l or cr) 1f Hm kJ · mol−1

g

60.63 ± 0.86 61.81 ± 0.89 62.71 ± 0.36 59.64 ± 0.33 65.89 ± 0.58 69.53 ± 0.27 92.5 ± 1.1

kJ · mol−1

o or 1 H o −1l Hm cr m

g

41103.3 ± 2.9

41097.6 41099.3 41106.6 41109.6

b

2,6-di-iso-propylaniline

TABLE 5. Thermochemical results at T = 298.15 K ( p o = 0.1 MPa)

40248.1 ± 3.5

40237.7 40250.1 40251.6 40253.0

2-tert-butylaniline

35.8 ± 2.0 8.2 ± 2.3 −1.5 ± 2.1 18.6 ± 1.5 −18.8 ± 1.9 −70.2 ± 2.4 −194.3 ± 4.3

o (g) 1f Hm kJ · mol−1

42871.1 ± 5.4

42861.2 42863.4 42875.9 42884.0

2,4,6-tri-tert-butylaniline

a Calculated from the massic energies of combustion in table 4. b From the measurements of vapour pressure at different temperatures (table 2) using the Clausius– o = (19.38 ± 0.21) kJ · mol−1 (at T = 426.4 K). Clapeyron equation. c From d.s.c. measurements, 1lcr Hm

2-ethyl-aniline (A) 2-iso-propyl-aniline (B) 2-tert-butyl-aniline (C) 2,6-dimethyl-aniline (D) 2,6-di-ethyl-aniline (E) 2,6-di-iso-propyl-aniline (F) 2,4,6-tri-tert-butyl-aniline (G)c

2-iso-propylaniline

2-ethyl-aniline

TABLE 4. Values of individual massic energies of combustion 1c u o at T = 298.15 K ( p o = 0.1 MPa); h1c u o i denotes the mean value

254 S. P. Verevkin

−194.3 ± 4.3

2,4,6-tri-tert-butyl-aniline

22.8

−265.6

−79.4

−20.1

1Hm (strain)

71.3

9.2

1.3

−4.6

1.1

0.4

28.8

−2.6

4.2

2.2

−1.8

kJ · mol−1

(alkyl-aniline)b

1m (strain)

31.7

9.0

0.0

0.0

0.0

0.0

10.9

0.0

5.0

0.0

0.0

kJ · mol−1

(alkyl-benzene)c

39.6 ± 4.3

0.2 ± 2.4

1.3 ± 1.9

−4.6 ± 1.5

1.1 ± 0.9

0.4 ± 1.1

17.9 ± 2.1

−2.6 ± 0.5

−0.8 ± 2.3

2.2 ± 2.0

−1.8 ± 0.5

kJ · mol−1

1{1Hm (strain)}d

3.8 ± 4.3

1.8 ± 2.4

3.1 ± 1.9

−1.0 ± 1.5

kJ · mol−1

1Hm (buttress)e

a Calculated as the sum of strain-free increments (see text). b Strain enthalpy of alkyl-anilines 1H (strain) = 1 H o (g)(exp) − 1 H o (g)(calc). c Strain enthalpy m f m f m of alkyl-benzene 1Hm (strain), taken from earlier work (see text). d The sum of resulting interactions of alkyl substituents and the NH2 group in alkyl-anilines: 1{1Hm (strain)} = 1Hm (strain)(alkyl-aniline) − 1Hm (strain)(alkyl-benzene) (see text). The uncertainties of the interactions are suggested to be only equal o (g) of the alkyl-anilines. e The buttress effect from interactions of alkyl substituents and the NH group in alkyl-anilines: to those of the experimental 1f Hm 2 P 1Hm (buttress) = 1{1Hm (strain)} − 0i (see text).

−70.2 ± 2.4

2,6-di-iso-propyl-aniline

23.9 ± 0.9(30)

2,5-di-methyl-aniline 18.6 ± 1.5

22.8

23.2 ± 1.1(30)

2,4-di-methyl-aniline

−18.8 ± 1.9

22.8

−1.5 ± 2.1

2-tert-butyl-aniline

2,6-di-ethyl-aniline

187.0 −30.3

184.4 ± 0.5(2)

2-phenyl-aniline

2,6-di-methyl-aniline

4.0

8.2 ± 2.3

2-iso-propyl-aniline

33.6

35.8 ± 2.0

2-ethyl-aniline

55.0

53.2 ± 0.5(3)

kJ · mol−1

kJ · mol−1

2-methyl-aniline

o (g)/(calc)a 1f Hm

o (g)/(exp) 1f Hm

TABLE 6. Non-nearest-neighbor interactions 1{Hm (strain)} and buttress effects 1Hm (buttress) of alkyl groups with the amino group at T = 298.15 K

Thermochemistry of alkyl-anilines 255

256

S. P. Verevkin o (g) at TABLE 7. Strain free increments 0i for the calculation of 1f Hm T = 298.15 K of hydrocarbons and amines

Hydrocarbons(24)

Aliphatic amines(27)

Aromatic amines(27)

0i /(kJ · mol−1 ) CH3 [C]

−42.05

CH3 [N]

−42.05

CH2 [2C]

−21.46

CH2 [N, C]

−26.9

N[CB , 2H]

CH[3C]

−9.04

CH[N, 2C]

−20.0

N[CB , C, H]

C[4C]

−1.26

C[N, 3C]

−16.1

N[CB , 2C]

CB H[2CB ]a

13.72

N[C, 2H]

19.4

CB [C, 2CB ]a

23.51

N[2C, H]

64.1

CB [CB ]b

22.40

N[3C]

16.6 65.4 115.2

CB [N]

2.1

103.2

aC represents the aromatic C atoms.(26) B o (biphenyl, g) = (184.41 ± 0.54) kJ · mol−1 .(29) 1f Hm

b Calculated

from

1f Hmo (g) of gaseous species is based on the additivity of group properties. The concept of strain(23) and strain energy provides a basis that helps to correlate the structures, stabilities, and reactivities of molecules.(23) Strain enthalpy reflects a non-additive component of the enthalpy of a molecule, and appears to be unique for each molecule. We define the strain of a molecule as the difference between the experimental standard molar enthalpy of formation 1f Hmo (g) and the calculated sum of strain-free increments(24) of the Benson type(25) for the molecule. The system of strain-free increments is based on the standard enthalpies of formation 1f Hmo (g) of simple homologous (“strainless”) molecules. Strain-free group additivity increments for hydrocarbons,(24) arenes,(26) and amines(27) are well defined. Their advantage with respect to the classic Benson increments(25) is the possibility of determining strain enthalpies. All the increments necessary in this work are listed in table 7. By using these group-additivity parameters 0i and the values of 1f Hmo (g) of alkyl-anilines (table 5) derived this research, the values of the strain enP in thalpies 1Hm (strain) = 1f Hmo (g) − 0i of the alkylsubstituted anilines have been estimated. The resulting non-nearest-neighbour interactions of alkyl substituents with the NH2 -group in the gaseous state are listed in table 6. Elucidation of the nature of the strain in alkyl-anilines is aided by comparison of the strain with similarly shaped alkylbenzenes. The standard molar enthalpies of formation 1f Hmo (g) and strain enthalpies 1Hm (strain) of iso-propylbenzene (4.0 kJ · mol−1 and 9.0 kJ · mol−1 ); tert-butylbenzene (−24.42 kJ · mol−1 and 10.9 kJ · mol−1 ); 1,3-di-isopropylbenzene (−75.4 kJ · mol−1 and 9.0 kJ · mol−1 ); and 1,3,5,-tri-tert-butylbenzene (−238.8 kJ · mol−1 and 31.7 kJ · mol−1 ) were determined recently.(6, 28) These alkylbenzenes provide a relevant structural model of strain in the mono-, di-, and tri-alkyl substituted anilines studied. Their strain enthalpies describe the intrinsic strain of the alkylbenzenes due to steric repulsions of alkyl groups and the benzene ring attached to the tertiary or quaternary carbon atom. Comparison with the strain of alkylbenzenes allowed the derivation of the effects of non-nearest-neighbour interactions of alkyl

Thermochemistry of alkyl-anilines

257

substituents on the benzene ring with the NH2 -group directly. We calculated the differences 1{1Hm (strain)} between individual strains for each alkyl-aniline and the strain of the appropriate alkyl substituted benzenes or toluenes(1) (table 6). These values were interpreted as the sum of excess interactions of alkyl substituents on the benzene ring with the NH2 -group. The ortho-interactions of the methyl, ethyl, iso-propyl, and phenyl groups with the amino-group were negligible within the boundaries of the experimental uncertainties. Only the ortho-interactions of the tert-butyl substituent with the amino group led to a meaningful destabilization of mono tert-butyl substituted aniline (table 6) due to the steric repulsion of the NH2 -group with the bulky alkyl substituent. The correction terms 0 ortho (tert-alkyl - amino) = 17.9 kJ · mol−1 was suggested for the application of the group-contribution correlation for 1f Hmo (g) of the ortho-tert-alkyl substituted anilines. In our previous work(7) it was established that the steric repulsion between the OH-group and the tert-alkyl substituent is generally caused by interactions with the nearest-neighbour methyl-groups, whilst the longer carbon chain of the substituents is sterically devoid of interactions. Thus, the magnitude of the ortho-effect is suggested to be independent of the size (chain-length) of the tert-alkyl substituent. Therefore, the values of the ortho effects derived here could be propagated on the longer substituents as studied here. Unfortunately, the correct interpretation of the values for the pairwise interactions of the para- or meta-alkyl substituent with the NH2 -group is thwarted by the lack of reliable thermochemical information. Available results for the alkyl substituted aminobenzenes are summarized in table 8. They indicate very weak stabilization, though it is generally indistinguishable within the boundaries of the experimental uncertainties (about 4 kJ · mol−1 to 7 kJ · mol−1 ). Similar trends were already observed for the alkyl substituted phenols(5, 7) and nitrobenzenes.(9) The meaningful stabilization (−10 kJ · mol−1 ) of 4-methyl-aniline seems to be doubtful, taking into consideration that no additional stabilization was detected in 2,4-di-methyl-aniline (table 6) in comparison with the 2-methyl-aniline. Also, no additional impact of the methyl substituent in the meta position of 2,5-di-methyl-aniline (table 6) is evident, again comparing with the 2-methyl-aniline. Thus, no additional correction terms 0 para (alkyl - amino) or 0 meta (alkyl - amino) were suggested for the application of the group-contribution correlation for 1f Hmo (g) of the para- and meta-alkyl substituted anilines. The buttress effect(8) is the excess steric interactions of substituents in the case where there are three or more adjacent groups on three adjacent carbon atoms of the aromatic compound. The introduction of buttress corrections provides a substantial improvement of the predictive accuracy of the group-additivity methods.(31) Meaningful buttress interactions of tert-butyl substituents with the OH-group have been detected in phenols.(8) We calculated the differences 1{1Hm (strain)} between the individual strains for each alkyl-aniline and the strain of the appropriate alkyl substituted benzenes (table 6, column 7). The values of 1{1Hm (strain)} were interpreted as the sum of the excess interactions of the alkyl substituents on the benzene ring with the NH2 -group. In order to obtain the buttress effects for the studied compounds, the values of the ortho-, meta-, and para-interactions of substituents have to be removed from the resulting sum 1{1Hm (strain)}. The remainder, 1Hm (buttress), may then be interpreted as the buttress

258

S. P. Verevkin

TABLE 8. Non-nearest-neighbor interactions 1{1Hm (strain)} of alkyl groups with the amino group in para and meta positions on the benzene ring at T = 298.15 K o (g)/(exp) 1f Hm kJ · mol−1

o (g)/(calc) a 1f Hm kJ · mol−1

1Hm (strain) b kJ · mol−1

44.6d

55.0

−10.4

−10.4

68.9 ± 7.4(4)

69.5

−0.6

−0.6 ± 7.4

52.0d

55.0

−3.0

−3.0

N,N-dimethyl-3-toluidine

72.6 ± 7.3(4)

69.5

3.1

N-ethyl-3-toluidine

30.5 ± 3.8(4)

34.9

−4.4

4-methyl-aniline N,N-dimethyl-4-toluidine 3-methyl-aniline

1{1Hm (strain)} c kJ · mol−1

3.1 ± 7.3 −4.4 ± 3.8

a Calculated as the sum of strain-free increments (see text). b Strain enthalpy of alkyl-aminobenzenes 1H (strain) = m o (g)(exp) − 1 H o (g)(calc). c The sum of resulting interactions of alkyl substituents with the NH group in alkyl1f Hm f m 2 aminobenzenes: 1{1Hm (strain)} = 1Hm (strain)(alkyl-aminobenzenes) − 1Hm (strain)(alkyl-benzene) (see text). The o (g) of the alkyl-anilines. d Values uncertainties of the interactions are assumed to be equal to those of the experimental 1f Hm were assessed in this work (see text)

effect of the excess interactions of substituents in the sequence alkyl-NH2 -alkyl on the benzene nucleus. No significant interactions (buttress effect) were detected for all the sequences of substituents alkyl-NH2 -alkyl, studied in this work, within the boundaries of the experimental uncertainties. Thus, no additional correction terms are suggested for the application of the group-contribution correlation for 1f Hmo (g) of the 2,6-alkyl-substituted anilines. The investigation of the compounds A to G (figure 1) covered a broad range of structures of alkyl-anilines. The experimental standard molar enthalpies of formation fill a gap in our knowledge of the thermochemical data for anilines. The derived group-contribution values allow the prediction of the thermochemical properties of new compounds with similar structures. REFERENCES 1. Pedley, J. P.; Naylor, R. D.; Kirby, S. P. Thermochemical Data of Organic Compounds: 2nd edition. Chapman and Hall: London. 1986. 2. Steele, W. V.; Chirico, R. D.; Nguyen, A.; Hossenlopp, I. A.; Smith, N. K. J. Chem. Thermodynamics 1991, 23, 101–134. 3. Steele, W. V.; Chirico, R. D.; Nguyen, A.; Knipmeyer, S. E. J. Chem. Thermodynamics 1994, 26, 515–544. 4. Ribeiro da Silva, M. A. V.; Ribeiro da Silva, M. D. M. C.; Monteiro, M. F. B. M.; Gomes, M. L. A. C. N.; Chickos, J. S.; Smith, A. P.; Liebman, J. F. Struct. Chem. 1996, 7, 367–373. 5. Verevkin, S. P. Ber. Bunsenges. Phys. Chem. 1998, 102, 1467–1474. 6. Verevkin, S. P. J. Chem. Thermodynamics 1998, 30, 1029–1040. 7. Verevkin, S. P. J. Chem. Thermodynamics 1999, 31, 559–585. 8. Verevkin, S. P. J. Chem. Thermodynamics 1999, 31, 1397–1416. 9. Verevkin, S. P. J. Chem. Thermodynamics 1999, accepted. 10. Hemminger, W. F.; Cammenga, H. K. Methoden der Thermischen Analyse. Springer: Berlin. 1989, 269. 11. Beckhaus, H.-D.; Kratt, G.; Lay, K.; Geiselmann, J.; R¨uchardt, C.; Kitschke, B.; Lindner, H. J. Chem. Ber. 1980, 113, 3441–3455.

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12. Verevkin, S. P.; Beckhaus, H.-D.; R¨uchardt, C. Thermochim. Acta 1990, 197, 27–39. 13. CODATA Key Values for Thermodynamics. Cox, J. D.; Wagman, D. D.; Medvedev, V. A.: editors. Hemisphere: New York. 1989; Atomic weights of the elements. Pure and Appl. Chem. 1994, 66, 2423–2444. 14. Hubbard, W. N.; Scott, D. W.; Waddington, G. Experimental Thermochemistry. Rossini, F. D.: editor. Interscience: New York. 1956, 75–127. 15. Cox, J. D.; Pilcher, G. Thermochemistry of Organic and Organometallic Compounds. Academic Press: London. 1970. 16. Chickos, J. S.; Hesse, D. G.; Hosseini, S.; Liebman, J. F.; Mendenhall, G. D.; Verevkin, S. P.; Rakus, K.; Beckhaus, H.-D.; R¨uchardt, C. J. Chem. Thermodynamics 1995, 27, 693–705. 17. Chickos, J. S.; Hesse, D. G.; Panshin, S. Y.; Rogers, D. W.; Saunders, M.; Uffer, P. M.; Liebman, J. F. J. Org. Chem. 1992, 57, 1897–1902. 18. Stull, D. R.; Westrum, E. F., Jr.; Sinke, G. C. The Chemical Thermodynamics of Organic Compounds. Wiley: New York. 1969. 19. Reid, R. C.; Prausnitz, J. M.; Sherwood, T. K. The Properties of Gases and Liquids: 3rd edition. McGraw-Hill: New York. 1977, 249. 20. Stull, D. R. Ind. Eng. Chem. 1947, 39, 517–540. 21. Meva’a, L. M.; Lichanot, A. Thermochim. Acta 1990, 158, 335–345. 22. CRC Handbook of Data on Organic Compounds: 2nd edition. Weast, R. C.; Grasselli, J. G.: editors. CRC Press: Boca Raton, FL. 1989. 23. Liebman, J. F.; Greenberg, A. Strained Organic Molecules. Organic Chemistry Series 38. Academic: New York. 1978. 24. Schleyer, P. v. R.; Williams, J. E.; Blanchard, K. B. J. Am. Chem. Soc. 1970, 92, 2377–2386. 25. Benson, S. W. Thermochemical Kinetics. Wiley: New York. 1976. 26. Beckhaus, H.-D. Chem. Ber. 1983, 116, 86–96. 27. Verevkin, S. P.; Beckhaus, H.-D.; Sch¨ule, U.; R¨uchardt, C. Struct. Chem. 1998, 9, 1–7. 28. Verevkin, S. P. Thermochim. Acta 1998, 316, 131–136. 29. Chirico, R. D.; Knipmeyer, S. E.; Nguyen, A.; Steele, W. V. J. Chem. Thermodynamics 1989, 21, 1307–1331. 30. Verevkin, S. P. J. Chem. Thermodynamics 1997, 29, 891–899. 31. Wu, Y. G.; Patel, S. N.; Bozzelli, J. W. Thermochim. Acta 1993, 222, 153–185. 32. Draeger, J. A. J. Chem. Thermodynamics 1984, 16, 1067–1073. (Received 3 June 1999 and accepted for publication 29 August 1999)

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