Thermochemical properties of branched alkylsubstituted benzenes

Article abstract of DOI:10.1006/jcht.1998.0370

The standard (p○ = 0.1 MPa) molar enthalpies of formation Δ_fH○_m (1 or cr) at the temperature T = 298.15 K were measured using combustion calorimetry for tert-butylbenzene, 4-methyl-1-tert-butylbenzene, 3,5-dimethyl - tert-butylbenzene, 5-methyl-1,3-di-tert-butylbenzene, and 1,4-di-tert-butylbenzene. The standard molar enthalpies of vaporization Δ^g₁H○_m, or sublimation Δ^g_crH○_m, of these compounds, and also of 3-methyl-1-tert-butylbenzene, 1,3-di-tert-butylbenzene, and 1,3,5-tri-tert-butylbenzene were obtained from the temperature function of the vapour pressure measured in a flow system. Enthalpies of fusion Δ¹_crH○_m of solid compounds were measured by d.s.c. Strain enthalpies of branched alkylbenzenes were derived from the measured enthalpies of formation of the gaseous compounds. These experimental results, together with the data available from the literature, provided a quantitative understanding of the interrelations of structure and energetics of alkylbenzenes, and a further improvement on the group-contribution methodologyfor estimation of thermodynamic properties of organic compounds.

Full text of DOI:10.1006/jcht.1998.0370

J. Chem. Thermodynamics 1998, 30, 1029᎐1040
Article No. ct980370
Thermochemical properties of branched
alkylsubstituted benzenes
Sergey P. Verevkina
Institute of Physical Chemistry, Uni¨ersity of Rostock, Hermannstr. 14,
18051 Rostock, F.R.G.
T
Ž
.
Ž
.
The standard pЊ s 0.1 MPa molar enthalpies of formation ⌬f Hm l or cr at the temperature
T s 298.15 K were measured using combustion calorimetry for tert-butylbenzene, 4-methyl-
1-tert-butylbenzene, 3,5-dimethyl-1-tert-butylbenzene, 5-methyl-1,3-di-tert-butylbenzene, and
1,4-di-tert-butylbenzene. The standard molar enthalpies of vaporization ⌬gl HTm , or sublimation
T
g
⌬
cr Hm , of these compounds, and also of 3-methyl-1-tert-butylbenzene, 1,3-di-tert-
butylbenzene, and 1,3,5-tri-tert-butylbenzene were obtained from the temperature function of
the vapour pressure measured in a flow system. Enthalpies of fusion
T
l
⌬cr Hm of solid
compounds were measured by d.s.c. Strain enthalpies of branched alkylbenzenes were derived
from the measured enthalpies of formation of the gaseous compounds. These experimental
results, together with the data available from the literature, provided
a quantitative
understanding of the interrelations of structure and energetics of alkylbenzenes, and a
further improvement on the group-contribution methodology for estimation of thermodynamic
properties of organic compounds.
ᮊ 1998 Academic Press
KEYWORDS: enthalpy of combustion; enthalpy of sublimation; enthalpy of vaporization;
enthalpy of formation; enthalpy of fusion; alkylbenzenes
1. Introduction
Traditionally, relatively little characterization has been done on branched molecules
in each class of organic compounds. Recently, systematic investigations have been
done in the Thermochemical Laboratory of the University of Freiburg with
measurements on branched ethers,Ž1. esters,Ž2. nitriles,Ž3. ketones,Ž4. and hydro-
carbons.Ž5. This paper follows this course and is concerned with the investigation of
branched alkylbenzenes with tert-butyl substituents. There is a paucity of
thermochemical data for such compounds in the literature.Ž6. Substituted
branched alkylbenzenes are represented only by tert-butylbenzenesŽ7. and tri-
tertbutylbenzenes.Ž8. This set of thermochemical information is too limited to
provide a reliable concept for the prediction of enthalpies of formation of poly-
alkylsubstituted benzenes. We report here a systematic study of the thermochemical
Ž
.
properties of a series of tert-butylbenzenes figure 1 . The resulting standard molar
Ž .
enthalpies of formation in the gas phase ⌬f HTm g of branched alkylbenzenes were
aŽ
.
E-mail: sergey.verevkin@chemie.uni-rostock.de .
0021᎐9614r98r081029 q 12 $30.00r0
ᮊ 1998 Academic Press

1030
S. P. Verevkin
FIGURE 1. Structures of alkylbenzenes: 1a, tert-butylbenzene; 1b, 3-methyl-1-tert-butylbenzene; 1c,
4-methyl-1-tert-butylbenzene; 1d, 3,5-dimethyl-1-tert-butylbenzene; 2a, 1,3-di-tert-butylbenzene; 2b, 5-
methyl-1,3-di-tert-butylbenzene; 2c, 1,4-di-tert-butylbenzene; 2d, 1,3,5-tri-tert-butylbenzene.
obtained from calorimetrically measured enthalpies of combustion and enthalpies
of sublimation, or vaporization by a transpiration method. These experimental
results, together with the data available from the literature, provide a quantitative
understanding of the relation between structure and properties of alkylbenznes.
The results of the study also could be aimed at an improvement of the Benson
group-contribution methodology.
2. Experimental
Ž
.
Pure Aldrich, Acros, and Merck products figure 1 , each with a mass fraction
purity of about 0.99, were purified by repeated distillation under reduced pressure.
The solid samples were purified by recrystallization from ethanol and petroleum
ether and then sublimed at reduced pressure to remove any traces of solvent. Only
3-methyl-1-tert-butylbenzene was synthesized. This was done via isomerization of
4-methyl-1-tert-butylbenzene with a catalytic amount of AlCl3 at room temperature.
The degree of purity was determined by g.l.c. For all solid compounds, mass
fraction purities of 0.9999 were additionally established by d.s.c. measurements of
y4
the melting process.Ž9. No impurities greater than mass fraction 1 10
could be
.
Ž
.
Ž
.
detected in tert-butylbenzenes figure 1 by either method. The following equipment
Ž
.
was used: g.l.c. Carlo Erba Fraktometer Vega Series GC 6000 , Hewlett Packard
Integrator 3390A, N2-flow of 0.333 cm sy1, SE-30 capillary column of length 25 m.
.
3
The standard temperature programme of the g.c. was T s 313 K for 5 min followed

Thermochemistry of substituted benzenes
1031
Ž
.
TABLE 1. Results from typical combustion experiments at T s 298.15 K pЊ s 0.1 MPa , where 1a s
tert-butylbenzene, 1c s 4-methyl-1-tert-butylbenzene, 1d s 3,5-dimethyl-1-tert-butylbenzene, 2b s 5-
methyl-1,3-di-tert-butylbenzene, and 2c s 1,4-di-tert-butylbenzene a
1a
1c
1d
2b
2c
b
Ž
.
m substance rg
0.471137
0.001074
0.034480e
0.85202
0.593995
0.001107
0.271660
1.53859
0.378116
0.000867
0.268297
1.15803
0.371088
0.000845
0.333575
0.000909
b
Ž
.
mЈ cotton rg
b
Ž
.
mЉ polyethene rg
⌬TcrKc
0.65653
y16487.16
0.58914
y14785.51
.
Ž
Ž
. Ž
.
.
␧
y⌬Tc rJ y21357.40
y⌬Tc rJ
y38638.05
y29081.12
calor
.
. Ž
␧
y12.00
9.04
18.20
y20.91
16.01
18.75
y15.59
10.77
14.68
y8.33
5.90
14.31
y7.75
5.28
15.40
cont.
⌬ucorr.rJd
d
.
ymЈ ⌬c uЈrJ
d
.
ymЉ ⌬c uЉrJ
788.06
.
y43623.6
12594.41
y43819.1
12438.50
y43984.6
y1
.
Ž
. Ž
⌬c uЊ sub r J g
y44393.3
y44281.3
a For the definition of the symbols see reference 11: Th s 298.15 K; Vbomb s 0.2664 dm ; p gas s
3
iŽ
.
iŽ
.
Ž .
3.04 MPa; m H 2O s 0.78 g; ⌬Uign s 1.46 J; m Pt s 8.61 g.
b Masses obtained from apparent masses.
c ⌬Tc s T f y T i q ⌬Tcorr.
;
␧
calor s 25066.7 " 1.8 J K
for substance 1ay; 1
␧
calor s 25112.6 "
y1
.
Ž
.
Ž
y1
.
Ž
.
.
.
Ž
1.9 J K
for substances 1c, 1d, and 2b; and
␧
s 25096.64 " 0.54 J K
for substance 2c.
.
Ž
.
Ž
.
Ž
.
Ž calor
.
.
i
i
f
f
.
␧
y⌬Tc s ␧
и T y 298.15 K q ␧
и 298.15 K y T q ⌬Tcorr
cont
cont
cont
d ⌬ucorr , the correction to standard states, is the sum of items 81 to 85, 87 to 90, 93, and 94 in
reference 11.
e Combustion in Mylar.
y1
.
by a heating rate of 0.167 K s to T s 523 K. Melting temperatures and enthalpies
of fusion were determined with a Perkin-Elmer DSC-2C.
For meausurements of the energies of combustion of tert-butylbenzenes, an
isoperibolic macrocalorimeter with a static bomb and a stirred water-bath was
used. The substances were placed in polyethene capsules or pressed into pellets of
mass f 400 mg and were burned in oxygen at p s 3.04 MPa pressure. To ensure
equilibrium in the final state, 0.78 g of water was added to the bomb. The detailed
procedure has been described previously.Ž2,10. The combustion products were
Ž
.
examined for carbon monoxide Draeger tube and unburnt carbon, but neither
was detected. The energy of ignition was determined to be 1.46 J. The energy
Ž
.
equivalent of the calorimeter ␧
see table 1 was determined with a standard
caloŽr
.
reference sample of benzoic acid sample SRM 39i, N.I.S.T. . For the reduction of
the data to standard conditions, conventional proceduresŽ11. were used. The atomic
weights used were those recommended by the IUPAC Commission.Ž12. The densities
of the commercially available liquid compounds were taken from the Aldrich
catalogue. The densities of solid substances were determined by submerging tablets
of the substances in water in a calibrated 10 cm3 pycnometer. The massic heat
capacities were measured with the d.s.c. The summary of auxiliary quantities for
the combustion experiments and information necessary for reducing apparent mass
to mass is given in table 2.
The enthalpies of vaporization of tert-butylbenzene, 3-methyl-1-tert-butyl-
benzene, 4-methyl-1-tert-butylbenzene, 3,5-dimethyl-1-tert-butylbenzene, and 1,3-

1032
S. P. Verevkin
T
l
Ž
.
TABLE 2. Temperature of fusion Tfus, standard molar enthalpy of fusion ⌬cr Hm , density ␳ T s 293 K ,
Ž
.
Ž
.
massic heat capacity cp T s 298.15 K , and expansion coefficients ␦ ¨r␦T of the materials used in
p
the present study
a
c
a
T
b
a
1
10y6 Ѩ ¨r␦T
.
Tfus
⌬
cr Hm
␳
cp
y1 y1
Ž
.
p
y1
y3
y1
kJ mol
dm3
K
.
.
.
.
.
K
g cm
J K
g
Tert-butylbenzene
4-Methyl-1-tert-
butylbenzene
3,5-Dimethyl-1-tert-
butylbenzene
0.867
1.73
1.32
1.48
1.0
1.0
1.0
0.853
0.867
5-Methyl-1,3-di-tert-
butylbenzene
307.55
350.45
343.15
18.43
21.76
11.92
0.922
0.943
1.28
1.52
0.1
0.1
1,4-Di-tert-butylbenzene
1,3,5-Tri-tert-butylbenzene
Cottond
1.500
0.920
1.38
1.67
2.53
1.32
0.1
0.1
0.1
Polyethene e
Mylar f
a
From d.s.c. measurements; the usual error bars of ⌬cr HTm measurements are "0.21 kJ mol
.
l
y1
.
b Measured with a pycnometer. c Estimated. From 10 combustion experiments, ⌬c uЊ CH 1.774O0.887
s
d
Ž
.
y16945.2 " 4.2 J g
,
where ⌬c uЊ denotes standard massic energy of combustion. eFrom 11
y1
.
.
Ž
f
combustion experiments, ⌬c uЊ CH 1.930 s y46361.0 " 3.1 J gy1. From seven combusion experiments,
.
.
Ž
.
Ž
⌬c uЊ C10 H 8O4 s y22838.8 " 4.8 J gy1. Calculation of mass of the dry mylar film using the
.
.
Ž
.
Ž
dependence of mass on humidity: mdry s mmoist 1 y 4.64 10y5 rH , where rH denotes relative humidity.
.
Ž
.
di-tert-butylbenzene, and enthalpies of sublimation of 5-methyl-1,3-di-tert-
butylbenzene, 1,4-di-tert-butylbenzene, and 1,3,5-tri-tert-butylbenzene were
determined with the method of transference in a saturated N2 streamŽ5,10,13. using
the Clausius᎐Clapeyron equation. About 0.5 g of the sample was mixed with glass
beads and placed in a thermostatted U-tube of length 20 cm and diameter 0.5 cm.
Ž
.
At constant temperature "0.1 K , a nitrogen stream was passed through the
U-tube and the transported amount of material was collected in a cold trap. The
3
y1
.
Ž
.
flow region 0.28 to 0.56 cm s of the nitrogen stream was optimised in order to
reach the equilibrium of saturation of the transporting gas at each temperature of
the investigation. The amount of condensed substance was determined by g.c.
Ž
.
analysis using an internal standard hydrocarbons, C11H 24 or C13H 28 . The vapour
pressure p at each saturation temperature was calculated from the amount of
product collected within a definite time period with the help of the ideal gas
equation p V s n R T:
.
. .
.
.
.
2
p s m R Tr V N M ,
1
Ž .
Ä
4
Ž
.
y1
y1
.
.
mol ; m is the mass of transported compound; V N2
Ž
.
where R s 8.31451 J K
is the volume of transporting gas; M is the molar mass of the compound; and T is
the saturation temperature. The measured value of the vapour pressure p at each
Ž
temperature was independent of the gas flow in the broad range from 0.1 to
0.8 cm sy1, which proved that the transporting gas was saturated with the
3
.
.
compound. The vapour pressure p was corrected for the residual vapour pressure

Thermochemistry of substituted benzenes
1033
TABLE 3. Results from measurements of the vapour pressure p by the transpiration method
3 c
3 c
TrK a
mrmgb
V N2 rdm
prPad
tert-butylbenzene 1a
TrK a
mrmgb
V N2 rdm
prPad
Ž
.
Ž
.
e
Ž
.
278.1
281.7
283.0
286.9
288.1
291.7
6.12
2.92
5.86
3.73
6.70
3.95
1.739
0.644
1.146
0.555
0.881
0.414
69.70
88.51
99.12
129.0
145.1
181.2
293.1
296.7
298.1
303.1
306.7
308.2
8.91
3.96
11.8
8.11
7.25
9.15
0.844
0.303
0.782
0.404
0.280
0.328
199.6
246.2
283.4
375.1
482.9
519.7
g
g
⌬l HTm 293.1 K s 47.80 " 0.36 kJ moly1; ⌬l Hm 298.15 K s 47.50 " 0.36 kJ mol
T Ž
.
Ž
.
y1
.
.
Ž
.
Ž
.
e
Ž
.
1-tert-butyl-3-methylbenzene 1b
278.7
283.5
288.6
293.4
0.893
0.958
1.13
1.784
1.313
1.044
0.640
8.954
12.78
18.72
27.61
298.3
303.3
308.4
313.5
1.10
1.08
1.08
0.845
0.471
0.337
0.236
0.141
39.72
54.40
77.43
100.8
1.03
g
g
⌬l HTm 296.1 K s 51.40 " 0.60 kJ moly1; ⌬l Hm 298.15 K s 51.27 " 0.60 kJ mol
T Ž
.
Ž
.
y1
.
.
Ž
.
Ž
.
e
Ž
.
1-tert-butyl-4-methylbenzene 1b
278.4
283.4
288.3
293.4
1.97
2.48
2.95
2.83
1.613
1.334
1.084
0.712
21.20
31.82
46.23
67.20
298.3
303.1
308.2
313.2
2.87
2.24
2.18
1.90
0.497
0.279
0.186
0.125
97.25
135.0
196.8
254.4
g
g
⌬l HTm 295.8 K s 52.31 " 0.46 kJ moly1; ⌬l Hm 298.15 K s 52.17 " 0.46 kJ mol
T Ž
.
Ž
.
y1
.
.
Ž
.
Ž
.
e
Ž
.
1-tert-butyl-3,5-dimethylbenzene 1d
283.6
288.3
293.4
298.1
2.50
3.46
4.27
3.95
3.530
3.200
2.610
1.670
11.20
16.86
25.35
36.49
303.3
308.4
313.2
318.2
4.01
4.65
4.59
4.63
1.120
0.877
0.653
0.469
54.99
81.25
107.7
151.0
g
g
⌬l HTm 300.9 K s 56.47 " 0.60 kJ moly1; ⌬l Hm 298.15 K s 56.63 " 0.60 kJ mol
T Ž
.
Ž
.
y1
.
.
Ž
.
Ž
.
e
Ž
.
1,3-di-tert-butylbenzene 2a
288.3
293.1
297.9
303.2
308.1
3.58
4.67
5.84
7.29
6.500
5.440
4.620
3.890
4.060
7.243
11.24
16.53
24.47
37.49
313.4
318.0
322.9
328.0
333.1
11.1
12.0
14.6
11.1
11.3
2.740
2.090
1.760
1.020
0.750
52.91
74.50
107.9
142.2
197.0
11.7
g
g
T Ž
⌬l HTm 310.7 K s 58.85 " 0.48 kJ moly1; ⌬l Hm 298.15 K s 59.60 " 0.48 kJ mol
.
Ž
.
y1
.
.
Ž
.
Ž
.
at the condensation temperature. The latter was calculated from a linear correlation
between ln p and Ty1 obtained by iteration. The molar enthalpies of sublimation
were calculated from the slope of the linear Clausius᎐Clapeyron correlation
g
T
T Ž .
g
y1
Ž
.
⌬cr Hm s yR dln prdT . The observed enthalpies of vaporization ⌬l Hm T , or
T Ž .
g
sublimation ⌬cr Hm T , at the temperature T obtained by this procedure are listed
in table 3.
For the purpose of comparing enthalpies of formation of alkylbenzenes measured
by combustion calorimetry with data obtained by another method, results from our
previous equilibrium studyŽ14,15. have been included in this paper. The enthalpies
Ž
.
of reactions figure 2 of isomerization and transalkylation of liquid tert-

1034
S. P. Verevkin
TABLE 3ᎏcontinued
3 c
3 c
TrK a
mrmgb
V N2 rdm
prPad
TrK a
mrmgb
V N2 rdm
prPad
Ž
.
Ž
.
e
Ž
.
1,3-di-tert-butyl-5-methylbenzene 2b
274.7
278.8
284.0
289.1
2.23
1.96
2.81
3.64
87.82
44.44
33.52
23.83
0.3119
0.5395
1.020
294.3
297.5
301.1
3.16
2.65
2.18
10.79
6.37
3.60
3.556
5.046
7.335
1.858
T Ž
.
Ž
.
Ž
.
Ž
.
cr Hm 287.9 K s 82.42 " 0.47 kJ moly1; ⌬cr HTm 298.15 K s 81.80 " 0.47 kJ mol
g
g
y1
.
.
⌬
e
Ž
.
1,3-di-tert-butyl-5-methylbenzene 2b
308.9
313.9
318.7
323.6
1.95
2.43
2.85
2.44
1.540
1.310
1.050
0.680
15.39
22.55
32.89
43.58
328.4
333.4
338.3
2.23
2.08
1.57
0.430
0.280
0.150
62.79
90.03
126.7
g
g
⌬l HTm 323.6 K s 61.75 " 0.90 kJ moly1; ⌬l Hm 298.15 K s 63.27 " 0.90 kJ mol
T Ž
.
Ž
.
y1
.
.
Ž
.
Ž
.
e
Ž
.
1,4-di-tert-butylbenzene 2c
288.2
293.2
298.0
303.3
308.2
1.13
1.89
2.32
2.31
2.84
20.85
18.60
13.58
7.52
0.7055
1.324
2.229
3.994
6.706
313.3
318.2
323.1
328.2
333.2
3.53
5.18
5.64
4.80
4.60
4.14
3.78
2.44
1.36
0.800
11.09
17.82
30.09
45.93
74.89
5.52
T Ž
.
Ž
.
Ž
.
Ž
.
cr Hm 310.7 K s 82.08 " 0.36 kJ moly1; ⌬cr HTm 298.15 K s 82.83 " 0.36 kJ mol
g
g
y1
.
.
⌬
e
Ž
.
1,3,5-tri-tert-butylbenzene 2d
297.7
303.3
308.5
313.4
318.3
2.72
4.04
5.14
6.05
9.57
27.20
22.11
16.12
11.71
11.47
1.009
1.841
3.210
5.207
8.402
323.4
328.4
333.4
338.4
341.3
8.33
11.4
8.85
7.11
6.08
6.150
5.450
2.720
1.460
0.970
13.64
21.07
32.76
49.02
63.12
T Ž
.
Ž
.
Ž
.
Ž
.
cr Hm 319.5 K s 79.93 " 0.26 kJ moly1; ⌬cr HTm 298.15 K s 81.21 " 0.26 kJ mol
g
g
y1
.
.
⌬
a Temperature of saturation, N2 gas flow 0.26 to 0.52 cm
condensed at T s 243 K.
s
.
b Mass of transferred sample
3
y1
.
Ž
.
c
d
Volume of nitrogen used to transfer sample.
Vapour pressure at
temperature T, calculated from m and the residual vapour pressure at T s 243 K. e For explanation
of number in parenthesis, see figure 1.
butylbenzenes have been determined in our previous work.Ž14. In order to provide
clarity, the methods used here are briefly discussed in this paper. The study was
performed in the liquid phase, in glass bulbs, which were at least two-thirds filled
with the initial mixture of benzene and tert-butylbenzenes. The catalyst used was
Ä
Ž
.4
AlBr3 mole fraction 0.01 to 0.03 . The bulbs were thermostatted at Ti " 0.2 K. At
the end of each experiment, the glass bulbs were opened in ice water, the organic
products were separated, and the compounds dissolved in water and extracted with
ether; the organic products were combined with the ether extracts and analysed by
g.c. It was established experimentally that the treatment of the samples did not
Ž
shift the equilibrium. The study was performed in the temperature range 272 to
.
373 K. At each experimental temperature the equilibrium concentrations of
benzene, 1,3- and 1,4-di-tert-butylbenzene, and 1,3,5-tri-tert-butylbenzene varied by

Thermochemistry of substituted benzenes
1035
FIGURE 2. Reactions investigated by chemical equilibrium method.
Ž
.
Ž
.
a factor of 4 to 7 . The concentration equilibrium constants of reactions 1 to 3
Ä
Ž
.
4
were independent within 3 to 7 per cent of the composition of the equilibrium
mixtures, and can thus be assumed to be equal to the thermodynamic constants.
The experimental data were approximated with the linear equation lg KN s a q
b Ty1, using the method of least squares. The reaction of the positional
.
isomerization of 1-tert-butyl-4-methylbenzene to 1-tert-butyl-3-methylbenzene
Ž15.
Ž
.
figure 2 has been investigated
in the same manner.
3. Results and discussion
The results for a typical combustion experiment on each compound are given in
table 1. The individual values of the standard massic energy of combustion ⌬c uЊ,
together with the mean and its standard deviation, are given in table 4. To derive
⌬f HTm from ⌬c HTm , the following molar enthalpies of formation were

1036
S. P. Verevkin
Ž
.
TABLE 4. Values of standard massic energies of combustion ⌬c uЊ at T s 298.15 K pЊ s 0.1 MPa .
²
:
⌬c uЊ denotes mean value. 1a s tert-butylbenzene, 1c s 4-methyl-1-tert-butylbenzene, 1d s 3,5-
dimethyl-1-tert-butylbenzene, 2b s 5-methyl-1,3-di-tert-butylbenzene, and
2c s 1,4-di-tert-butylbenzene
1a
1c
1d
2b
2c
y1
.
Ž
.
.
y⌬c uЊr J g
43623.6
43616.9
43640.8
43635.8
43615.3
43631.6
43627.3
"4.2
43799.7
43810.3
43819.1
43825.0
43821.7
43984.6
43975.5
43987.8
43974.2
44393.3
44395.0
44386.8
44398.7
44388.2
44281.3
44283.9
44279.8
44273.9
44274.8
43977.8y1
.
²
: Ž
y⌬c uЊ r J g
43815.2
"4.6
43980.0
44392.4
"2.2
44278.7
"0.9
"2.7
y1
used:Ž12. for H 2O l : y 285.83 " 0.04 kJ mol , and for CO2 g : y 393.51 "
.
Ž .
Ž
.
Ž .
Ž
0.13 kJ moly1. Table 5 lists the derived standard molar enthalpies of combustion
and standard molar enthalpies of formation of the compounds in the condensed
and gaseous states. The assigned standard deviations of the mean include the
uncertainties from calibration and combustion energies of the auxiliary materials.
The only previous determination of the standard molar enthalpy of formation
.
.
⌬f HTm
l
of tert-butylbenzene was made by Prosen et al.Ž7. using combustion
Ž .
y1
.
Ž
.
calorimetry. Their value of y 70.7 " 1.2 kJ mol is in agreement with our value
y1
.
Ž
.
of y 71.92 " 0.71 kJ mol within the bounds of the uncertainties.
The experimental enthalpies of vaporization ⌬gl HTm , or sublimation ⌬gcr HTm , at
T s 298.15 K are recorded in table 3. Because of the deviations from T s 298.15 K
of the average temperatures of measurements by the transpiration method, the
observed values of the enthalpies of vaporization, or sublimation of alkylbenzenes
Ž
.
see table 3 had to be corrected to this reference temperature. The corrections
were estimated with help of the ‘‘Sidgwick correction’’:
g
l
⌬g HT
y⌬ HT 298.15 K r kJ mol
sy6 10
Ä
4
,
y1
y2
.
.
.
² :
T
² :
T rK y298.15
Ä
4
Ž
m Ž
.
m Ž
.
.
Ž
.
l
2
Ž .
following the recommendation of Chickos et al.Ž16. With these corrections and the
g
Ž .
T
T Ž .
and ⌬cr Hm T from table 3, the standard molar
measured values of ⌬l HTm
g
Ž
enthalpies of vaporization or sublimation at T s 298.15 K were calculated tables 3
.
and 5 .
Determinations of the enthalpies of vaporization and of sublimation of
alkylbenzenes have been made by a number of groups in recent years. The values
are very consistent with this research as can be seen by reference to table 6. The
T Ž .
for 1,4,-di-tert-butylbenzeneŽ17. and 1,3,5-tri-tert-
g
observed values of ⌬cr Hm
T
butylbenzeneŽ18. were extrapolated to T s 298.15 K in the same manner as our
results, using the ‘‘Sidgwick correction’’. The enthalpy of sublimation at T s
298.15 K of 1,4,-di-tert-butylbenzene, derived from ebulliometry and d.s.c.
measurements recently,Ž19. is in a good agreement with this work within the
boundaries of the experimental uncertainties.

Thermochemistry of substituted benzenes
1037

1038
S. P. Verevkin
TABLE 6. Comparison at T s 298.15 K with earlier values of enthalpy of vaporization or sublimation
⌬gl HTm
⌬cr Hm
T
g
Reference
y1
y1
.
.
kJ mol
kJ mol
Tert-butylbenzene
48.1 " 0.2
47.7 " 0.2
47.5 " 0.4
47.5 " 0.4
63.0 " 0.6
7
23
24
This work
17
1,-4-Di-tert-butylbenzene
1,3,5-Tri-tert-butylbenzene
83.2a
84.5 " 0.8
82.8 " 0.4
79.5 " 0.4b
81.2 " 0.3
19
This work
18
This work
a
y1
.
.
Ž²
T
Ž²
T
:
:
.
The observed value
The observed value
s 305 K 82.8 kJ moly1 was extrapolated to T s 298.15 K.
b
.
s 294 K 79.7 kJ mol
was extrapolated to T s 298.15 K.
The following resultsŽ14. were obtained from the study of the equilibria of
reactions 1 to 3 see figure 2 : lg KN s 0.3268 y 77.3 TrK y1, ⌬r Hm s
.
Ž
. Ž
.
Ž
.
y1
y1
y1
.
.
.
Ž
.
Ž
.
Ž .
for reaction 1 ;
1.48 " 0.33 kJ mol , and ⌬r Sm s 6.26 " 0.73 J K
mol
y1
lg KN s 0.5436 q 157.5 TrK , ⌬r Hm s y 3.02 " 0.72 kJ moly1, and ⌬r Smys1
.
.
Ž
.
Ž
.
y1
y1
.
.
.
Ž
mol
Ž
.
Ž .
.
10.41 " 1.66 J K
mol for reaction 2 ; and lg KN s 0.5752 y 52.3 TrK
,
y1
y1
⌬r Hm s 1.00 " 1.08 kJ moly1, and ⌬r Sm s 11.01 " 3.62 J K
for
.
.
.
.
Ž
.
Ž
Ž .
reaction 3 .
Ž
.
The values of the enthalpies ⌬r Hm and entropies ⌬r Sm of reactions 1 to 3
were derived for the average temperature 313 K. In further calculations it was
assumed that the enthalpies of the reaction hardly change on passing from the
average temperature of the experimental range to T s 298.15 K.
Ž .
Reaction 4 was investigated in the liquid phase and in the temperature range
Ž15.
Ž
.
293 to 383 K. The following experimental results were derived
for the average
.
temperature 338 K by this study: lg KN s 0.3291 y 1.04 TrK y1, ⌬r Hm s
.
Ž
y1
y1
0.02 " 0.13 kJ mol , and ⌬r Sm s 6.30 " 0.40 J K
moly1. The errors in the
.
.
.
.
Ž
.
Ž
thermodynamic functions are given by the uncertainty for the meaningful level
0.05. They were derived in accordance with the requirements of the IUPAC
Commission and include the errors in the equilibrium constants KN as well as
y1
.
Ž
.
deviations of KN from linear dependence lg KN s a q b TrK
.
Ž .
Using the experimental enthalpy of reaction 1 , and the enthalpy of formation
of liquid 1,4-di-tert-butylbenzene, ⌬f HTm l s ⌬f Hm cr q ⌬cr Hm s y190.23
T
l
Ž .
T Ž .
y1
.
Ž
.
kJ mol
from tables 2 and 5 , the enthalpy of formation in the liquid
phase of 1,3-di-tert-butylbenzene was calculated to be ⌬f HTm l s y 188.75 "
Ž .
Ž
0.57 kJ moly1. The valuable check of the experimental data for mutual consistency
.
.
allows the calculation of ⌬f HTm l of 1,3-di-tert-butylbenzene from the experimental
Ž .
Ž .
T Ž .
enthalpy of reaction 2 and the enthalpies of formation of benzene ⌬f Hm l s
Ž6.
y1
.
Ž
.
Ž
.
49.0 " 0.6 kJ mol
and tert-butylbenzene see table 5 . Calculated in this way,
the value ⌬f HTm l of 1,3-di-tert-butylbenzene y 189.8 " 1.2 kJ mol is in a very
y1
.
Ž .
Ž
.
Ž .
good agreement with the calculation from data for reaction 1 .
Ž .
Using the experimental enthalpy of reaction 3 and the enthalpies of formation
Ž
.
of tert-butylbenzene, and 1,3- and 1,4-di-tert-butylbenzene see table 5 , the

Thermochemistry of substituted benzenes
1039
enthalpy of formation ⌬f HTm
l
Ž .
Ž
of 1,3,5-tri-tert-butylbenzene y 308.1 "
y1
.
.
1.4 kJ mol
was calculated. By taking into account the enthalpy of fusion
T
l
y1
.
Ž
.
.
⌬cr Hm s 11.92 kJ mol
of 1,3,5-tri-tert-butylbenzene table 2 , our value for
⌬f HTm cr agrees satisfactorily with the value of y 321.9 " 3.8 kJ mol measured
y1
Ž .
Ž
.
by combustion calorimetry.Ž8. The experimental enthalpy of reaction 4 allowed
Ž .
the calculation of ⌬f HTm
of 3-methyl-1-tert-butylbenzene, y 109.74 "
Ž .
l of
Ž .
l
Ž
0.87 kJ moly1, from the known table 5 enthalpy of formation ⌬f HTm
.
.
Ž
.
4-methyl-1-tert-butylbenzene. A summary of the standard enthalpies of formation
derived in this investigation is given in table 5.
We define the strain of a molecule as the difference between the experimental
standard enthalpy of formation ⌬f HTm g and the calculated sum of the strain-free
Ž .
increments of the Benson typeŽ20. for this molecule. The system of strain-free
increments is based on the standard enthalpies of formation ⌬f HTm g of simple
Ž .
Ž
.
homologous ‘‘strainless’’ molecules. Strain-free group additivity increments for
hydrocarbonsŽ21. and arenesŽ22. are well defined. Their advantage with respect to
the classic Benson incrementsŽ20. is the possibility to determine strain enthalpies.
All the increments necessary in this work are as follows:Ž21,22. CH 3 C s
w x
y1
y1
y1
.
.
.
w
x
w
x
y42.05 kJ mol
;
CH 2 2C s y21.46 kJ mol
;
CH 3C s y9.04 kJ mol
;
y1
y1
.
.
w
x
w
x
w x
C 4C s y1.26 kJ mol
;
C B H 2C B s 13.72 kJ mol
; C B C, 2C B s 23.51
y1
.
Ž
.
kJ mol
where CB represents the aromatic C atoms . Using these group-additivity
parameters and the values of ⌬f HTm g of alkylbenzenes table 5 derived in this
Ž .
Ž
.
Ž
.
Ä
T Ž .
4
research, the values of strain enthalpies ⌬ Hm strain s ⌬f Hm g y ⌺ increments
of alkylsubstituted benzenes have been estimated. These resulting interactions are
listed in table 5. All studied tert-butylbenzenes are strained. The value of
y1
.
Ž
.
⌬ Hm strain s 10.88 kJ mol for the tert-butylbenzene has been included in each
molecule studied. This value describes the intrinsic strain of the tert-butylbenzenes
due to steric repulsions of methyl groups and the benzene ring attached to the
quaternary carbon atom. The comparison with this strain allowed the derivation of
the effects of non-nearest-neighbour interactions of alkyl substituents on the
benzene ring directly. We calculated the differences between individual strains for
Ž
.
each compound and the strain of tert-butylbenzene table 5 . These values were
interpreted as the sum of excess interactions of alkylsubstituents on the benzene
ring. The quantities of the pairwise interactions of methyl and tert-butyl substituents
y1
y1
.
.
Ž
.
indicated
a
very weak stabilization about 0.9 kJ mol
to 1.8 kJ mol
,
.
Ž
independent of the type of substitution meta- or para- position of alkyl substituents
of the benzene ring. The interactions of the two tert-butyl groups led to a weak
y1
.
Ž
.
destabilization 2.0 kJ mol
both in the meta- and para- substitution. The
y1
y1
.
.
Ž
.
was detected by
resulting weak stabilization about 0.5 kJ mol to 0.9 kJ mol
the location of the three alkyl substituents on the benzene ring in the 1,3,5-positions.
It is significant that all the effects of interactions for meta- and para- alkyl
Ž
.
substituted benzenes alkyl: methyl and tert-butyl detected here are not
y1
.
significantly larger than the individual error bars of about "1.0 kJ mol of the
thermochemical measurements, which were performed in this work. The same
trend was observed for the methyl-, ethyl-, and propyl-substituted benzenes.Ž25.
Because of this, one can be assured that no new group-additivity parameters or
correction terms are necessary for the application of the group-contribution

1040
S. P. Verevkin
correlation for ⌬f HTm g of the meta-, para-, and 1,3,5-alkylsubstituted as alkyl:
Ž .
Ž
.
methyl, ethyl, propyl, and tert-butyl benzenes.
The author expresses his gratitude to Professor Dr C. Ruchardt and Dr H.-D.
¨
Beckhaus for supporting the thermochemical measurements and for the exceedingly
creative atmosphere in their laboratory. The author is grateful to the staff of the
Institute of Organic Chemistry at Freiburg University, where the experimental part
of this work was done. Thanks are due to the Deutsche Forschungsgemeinschaft
and Alexander von Humboldt-Stiftung for their financial support.
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(
)
Recei¨ed 15 January 1998; accepted 16 March 1998
O-713