J ournal of Chemical and Engineering Data, Vol. 46, No. 4, 2001 989
is the temperature averaged over the temperature range
zenes calculated as the sum of ∆fHo (l) and ∆gHo are
shown in Table 7.
We define the strain enthalpy HS of a molecule as the
m
l
m
covered by the experiments. With these corrections (the
uncertainty of the correlation was not taken into account)
and the measured values of ∆gl Hmo ( T ) and ( T ) from
Table 8, the standard enthalpies of vaporization at T )
298.15 K were calculated (Tables 7 and 8).
difference between the experimental standard enthalpy of
formation ∆fHo (g) and the calculated sum of the strain-
m
free Benson-type increments10 for this molecule. The strain-
free increments for the calculation of enthalpies of forma-
tion of alkanes,11 alkylbenzenes,12 and ethers13,14 are
already well established. By using these group additivity
4. Discu ssion
4.1. Com pa r ison of th e Rea ction En th a lpies Ob-
ta in ed fr om Equ ilibr iu m Stu d ies a n d fr om Com bu s-
tion Exper im en ts. We investigated the chemical equilib-
rium of reactions of the synthesis of (1-alkoxyethyl)benzenes
from n-alkanols and styrene in the temperature range (343
to 423) K and derived the standard enthalpies of these
reactions in the liquid phase. The validity of the results
obtained from the chemical equilibrium study can be
verified by comparison with the values of the reaction
enthalpies calculated from the formation enthalpies of the
reaction participants. For this purpose the standard molar
enthalpy of formation ∆fHom(l) of (1-methoxyethyl)- and (1-
butoxyethyl)benzenes at 298.15 K was measured by means
of combustion calorimetry in this work (Table 7). Further
experimental data necessary for comparison are available
parameters and the values of ∆fHo (g) of ethylbenzene
m
derivatives (Table 7), their values of strain enthalpies HS
) {∆fHo (g) - ∑increments} have been estimated (Table
m
7).
All (1-alkoxyethyl)benzenes studied here are very simi-
larly strained by about 5 kJ ‚mol-1 (Table 7). This fact is a
further indication that the data for ∆fHo (g) of (1-alkoxy-
m
ethyl)benzenes, obtained by combining the different ex-
perimental techniques (calorimetry, transpiration, and
equilibrium study), are generally consistent, supporting the
confidence in the experimental procedures used. What
reasons cause strain in these molecules? Elucidation of the
nature of the strain in (1-alkoxyethyl)benzenes is aided by
comparison with the strain of the similarly shaped isopro-
pylbenzene:15 ∆fHmo (g) ) -(3.9 ( 1.1) kJ ‚mol-1 and strain
enthalpy HS ) 5.0 kJ ‚mol-1. Isopropylbenzene is a relevant
structural pattern of strain in the (1-alkoxyethyl)benzenes
studied. Its strain enthalpy is a reflection of the intrinsic
strain of the molecule due to steric repulsions of methyl
groups and the benzene ring attached to the central
tertiary carbon atom. It is expected from analogy with the
strain of isopropylbenzene that the observed amount of
destabilization in (alkoxyethyl)benzene derivatives could
most likely be attributed to the steric repulsions of methyl
groups and the benzene ring attached to the central
tertiary carbon atom. Therefore, it can be concluded that
no additional group additivity parameters or correction
terms are necessary (besides those of the correction for HS
) 5.0 kJ ‚mol-1 like in isopropylbenzene) for the group
in the literature: for styrene7 ∆fHo (l) ) (103.4 ( 0.9)
m
kJ ‚mol-1, for methanol8,18 ∆fHo (l) -(239.5 ( 0.2) kJ ‚
m
mol-1, and for ethanol8,18 ∆fHo (l) ) -(277.0 ( 0.3) kJ ‚
m
mol-1. Data for propanol, ∆fHom(l) ) (-302.5 ( 0.2) kJ ‚
mol-1, and for butanol, ∆fHmo (l) ) (-327.0 ( 0.2) kJ ‚mol-1
,
were taken from Mosselman and Dekker.9,18 These data
were used to calculate independently ∆rHo (calorimetry) of the
m
(1-alkoxyethyl)benzene synthesis reaction in the liquid
phase [e.g., for (1-methoxyethyl)benzene]:
∆rHmo (l)(calorimetry) ) ∆fHom(l)(MeEB) - ∆fHmo (l)(MeOH)
-
∆fHom(l)(Styrene) ) -(25.4 ( 2.4) kJ ‚mol-1
The comparison with experimental values obtained from
the chemical equilibrium study is given in Table 5. The
contribution correlation for ∆fHo (g) of (1-alkoxyethyl)-
m
benzenes.
calculated values of ∆rHo (l)(calorimetry) for the reactions of
m
(methoxyethyl)- and (butoxyethyl)benzene are in very close
agreement, being within the limits of the experimental
Liter a tu r e Cited
uncertainties with those of ∆rHo (l)(equilibrium) derived from
(1) Heintz, A.; Verevkin, S. P. Simultaneous study of chemical and
vapor-liquid equilibria in the reacting system of the methyl
cumyl ether synthesis from methanol and R-methyl-styrene. Fluid
Phase Equilib. 2001, 179 (1-2), 85-100.
(2) Verevkin, S. P.; Heintz, A. Chemical equilibria study in the
reacting system of the alkyl cumyl ether synthesis from n-alkanols
and R-methyl-styrene. J . Chem. Eng. Data 2001, 46, 41-46.
(3) Hubbard, W. N.; Scott, D. W.; Waddington, G. In Experimental
Thermochemistry; Rossini, F. D., Ed.; Interscience: New York,
1956; pp 75-127.
(4) Atomic weights of the elements. Pure Appl. Chem. 1994, 66,
2423-2444.
m
the chemical equilibrium studies.
The thermodynamic consistency observed allows one to
calculate the enthalpy of formation of (1-ethoxyethyl)-
benzene, ∆fHom(l) ) -(196.2 ( 1.1) kJ ‚mol-1, and of (1-
propoxyethyl)benzene, ∆fHom(l) ) -(222.3 ( 2.7) kJ ‚mol-1
,
from the known enthalpies of formation of alkanols and
styrene and standard enthalpies of reaction obtained from
the temperature dependence of KX (see Table 7).
4.2. Str a in En th a lpies HS of (1-Alkoxyeth yl)ben -
zen es. An important test to establish the validity of the
experimental and calculation procedures presented in this
paper provides the comparison of strain enthalpies of (1-
alkoxyethyl)benzenes, which could be derived from their
(5) CODATA Key Values for Thermodynamics; Cox, J . D., Wagman,
D. D., Medvedev, V. A., Eds.; Hemisphere: New York, 1989.
(6) Chickos, J . S.; Hosseini, S.; Hesse, D. G.; Liebman, J . F. Heat
Capacity Corrections to a Standard State: A Comparison of New
and Some Literature Methods for Organic Liquids and Solids.
Struct. Chem. 1993, 4, 271-278.
gaseous standard molar enthalpies of formation ∆fHo (g)
(7) Prosen, E. J .; Rossini, F. D. Heats of formation and combustion
m
of 1,3-butadiene and styrene. J . Res. NBS 1945, 34, 59-63.
at 298.15 K (Table 7). Indeed, the (methoxyethyl)-, (ethoxy-
ethyl)-, (propoxyethyl)-, and (butoxyethyl)benzenes listed
in Table 7 present a typical example of the homologue
series. It is well established that for such series as alkanes
or alkanols9 the enthalpic contribution into ∆fHom(g) from
the CH2 group should remain constant. In other words, no
additional strain interactions in a molecule are expected
by passing from (methoxyethyl)- to (butoxyethyl)benzene.
(8) Chao, J .; Rossini, F. D. Heats of combustion, formation, and
isomerization of nineteen alkanols, J . Chem. Eng. Data 1965, 10,
374-379.
(9) Mosselman, C.; Dekker, H. Enthalpies of formation of n-alkan-
1-ols. J . Chem. Soc., Faraday Trans. 1 1975, 417-424.
(10) Benson, S. W. Thermochemical Kinetics; Wiley: New York, 1976;
p 274.
(11) Schleyer, P. v. R.; Williams, J . E.; Blanchard, K. B. The evaluation
of strain in hydrocarbons. The strain in adamantane and its
origin. J . Am. Chem. Soc. 1970, 92, 2377-2386.
The resulting values of ∆fHo (g) of (1-alkoxyethyl)ben-
m