Chemical equilibrium study in the reacting system of (1-alkoxyethyl)benzene synthesis from isoalkanols and styrene

Article abstract of DOI:10.1039/b200312k

The chemical equilibrium of the reactive systems isoalkanol + styrene ? (1-alkoxyethyl)benzene (alkyl is isopropyl, sec-butyl, and cyclohexyl) was studied in the liquid phase in the temperature range 343 to 433 K using a cation exchanger as heterogeneous

Full text of DOI:10.1039/b200312k

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Chemical equilibrium study in the reacting system of
(1-alkoxyethyl)benzene synthesis from isoalkanols and styrene†
Sergey P. Verevkin* and Andreas Heintz
2
Department of Physical Chemistry, University of Rostock, Hermannstr. 14, 18051 Rostock,
Germany. E-mail: sergey.verevkin@chemie.uni-rostock.de; Fax: ϩ0049-381-498-6502
Received (in Cambridge, UK) 8th January 2002, Accepted 11th February 2002
First published as an Advance Article on the web 6th March 2002
The chemical equilibrium of the reactive systems isoalkanol ϩ styrene
(1-alkoxyethyl)benzene (alkyl is isopropyl,
sec-butyl, and cyclohexyl) was studied in the liquid phase in the temperature range 343 to 433 K using a cation
exchanger as heterogeneous catalyst. Enthalpies of reactions, ∆rHmo, of (1-alkoxyethyl)benzene synthesis in the liquid
phase were obtained from the temperature dependence of equilibrium constants measured in reactive mixtures with
an excess of alkanol and showed a good agreement with reaction enthalpies derived from values of the enthalpy of
formation ∆fHmo(l) of the participants in the reaction for the synthesis of (1-isopropoxyethyl)benzene, measured by
combustion calorimetry. The standard molar enthalpies of vaporization of (1-alkoxyethyl)benzenes were obtained
from the temperature dependence of the vapor pressure measured by the transpiration method. Resulting
values of ∆fHmo(g) were used to prove the consistency of the experimental data and to derive strain enthalpies
of (1-alkoxyethyl)benzenes. The strain effects were discussed in terms of deviations of ∆fHmo(g) from the group
additivity rules.
it is obvious that the slope of a plot R(ln K) vs. 1/T gives the
1
Introduction
standard enthalpy of reaction ∆rHmo if the temperature depend-
ence of the equilibrium constant K is known. From the
enthalpy of reaction, ∆rHmo the enthalpy of formation of one
particular species can be determined if those for the other
species are known. This procedure, involving study of the
changes of equilibrium constants with temperature, is referred
to as the second law method. Studies of chemical equilibria are
essentially less pretentious in comparison to direct calorimetric
methods, because mixtures of reactants are usually studied by
determining the equilibrium ratios Kx defined as:
Enthalpies of chemical reactions can be determined experi-
mentally by means of calorimetric measurements (in accord-
ance with the first law of thermodynamics) and from results
of studies of chemical equilibrium constants and their temper-
ature dependence (in accordance with the second law of
thermodynamics).
The first law method (combustion enthalpy measurement):
combustion calorimetry is the conventional method for measur-
ing ∆fHmo of stable species in the condensed state (solid or
liquid). Despite the fact that the accuracy of measurements of
the energy released in a combustion calorimeter is close to
0.01%, the reliability of the derived enthalpy of combustion
is limited by the purity of the sample used. Thus, a lot of
reactive, unstable compounds, or even isomers are excluded
from thermochemical investigation using this method due to
difficulties in their separation, purification or attestation for
purity.
(4)
where xi is the mole fraction of the component i. Kx defined by
eqn. (4) is related to K in eqn. (2) by
The second law method (equilibrium constant measure-
ments): for a chemical reaction in the liquid phase, e.g.,
(5)
A ϩ B
C
(1)
where γi is the activity coefficient of component i. γi depends on
the composition of the mixture. As a consequence, the meas-
urement of Kx is not sufficient for determining K, the know-
ledge of γi of each component at given composition is also
required. In some cases, however, the ratio of γi-values in eqn.
(5) is very close to unity, e.g. isomerization equilibria of alkyl-
benzenes.1 In this case equilibrium ratios Kx calculated from
mole fractions of reaction participants is nearly equal to the
true thermodynamic constant K. Hence, reaction enthalpy,
∆rHmo, can be calculated from the temperature dependence of
the ratio Kx.
the equilibrium constant, K, is defined as the ratio of the
activities ai of the products and the reagents under equilibrium
conditions:
(2)
From van’t Hoff’s relation
d(ln K)/dT = ∆rHmo/RT2
(3)
The quantitative analysis of the experimental results
becomes drastically more complex if one of components of a
reacting liquid mixture is associated by strong intermolecular
forces such as hydrogen bonding. An example is the reacting
mixture of methyl tert-butyl ether (MTBE) synthesis from
† Electronic supplementary information (ESI) available: experimentally
determined compositions of equilibrium mixtures in the reactions of
alkanols with styrene; results of combustion experiments. See http://
www.rsc.org/suppdata/p2/b2/b200312k/
728
J. Chem. Soc., Perkin Trans. 2, 2002, 728–733
This journal is © The Royal Society of Chemistry 2002
DOI: 10.1039/b200312k

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methanol and isobutene.2 In this case the equilibrium ratio Kx
[eqn. (4)] depends on the composition of the equilibrium
mixture indicating a particularly real (non-ideal) mixture.
Taking into account the large scale production of MTBE and
similar compounds used as a gasoline additives, elaboration of
a general procedure of the thermodynamic interpretation
of equilibrium results for associated reacting systems is of
increasing importance. In our previous works we developed a
2
Experimental
2.1 Materials
Styrene, isopropyl alcohol (iPrOH), sec-butyl alcohol (sec-
BuOH), and cyclohexanol (c-HexOH) (water content less than
0.01%) were purchased from Merck. GC analyses gave a purity
>99.9% in agreement with specifications. (1-Alkoxyethyl)-
benzenes were synthesized via alkylation of an appropriate
alkanol with styrene (see Scheme 1) in the presence of a
catalytic amount of cation exchange resin in the Hϩ form
(Amberlyst 15, Aldrich) at 343 K. Prior to the experiments the
cation exchange resin Amberlyst 15 in Hϩ form was dried for
eight hours at 383 K in a vacuum oven at reduced pressure.
Pure samples of (1-alkoxyethyl)benzenes were obtained by
repeated distillations at reduced pressure under N2, after being
dried with molecular sieves (0.4 nm). No impurities could be
detected in the samples by GC.
procedure to determine
K
and ∆rHmo for etherefication
reactions3–5 from experimental data of Kx without measuring
activity coefficients γi of the reactants. A detailed study of the
system α-methylstyrene ϩ methanol
methyl cumyl ether‡
including phase analysis of the liquid as well as vapor phase has
revealed that Kx is essentially identical to K in mixture composi-
tions containing an excess of methanol.3 Other systems behave
similarly leading to the conclusion that the study of ether syn-
thesis reaction equilibria at compositions with a considerable
excess of the alcoholic component allows determination of K
without additional measurements of single γi-values. The same
procedure is applied in this work to further etherification
reactions.
Besides widely used methyl tert-butyl ether and tert-amyl§
methyl ether, (1-alkoxyethyl)benzenes are synthesized in the
liquid phase over acid-functionalized ion-exchanged resin cat-
alysts from alkanols and styrene (see Scheme 1). Styrene and
2.2 Chemical equilibrium study
Glass vials with screw caps were filled two-thirds full with the
initial liquid mixture of alkanol and styrene. Cation-exchange
resin Amberlyst 15 (Aldrich) in the Hϩ form was added as a
solid catalyst. The quantity of catalyst was approximately 10%
from the weight of the mixture. The vial was thermostatted at
temperature T i 0.1 K and periodically shaken. After definite
time intervals the vial was cooled rapidly in ice and opened. A
sample for GC analysis was taken from the liquid phase using
a syringe. The vial was then kept at the same temperature.
Samples were withdrawn until no further change of the com-
position was observed, indicating that the chemical equilibrium
was established.
Scheme 1 Reaction of the (1-alkoxyethyl)benzene synthesis (R =
methyl, ethyl, isopropyl, propyl, butyl, sec-butyl and cyclohexyl).
The composition of the reaction mixtures was analysed using
a Hewlett Packard gas chromatograph 5890 Series II equipped
with a flame ionisation detector and Hewlett Packard 3390A
integrator. The carrier gas (nitrogen) flow was 12.1 cm3 sϪ1. A
capillary column HP-5 (stationary phase crosslinked 5% PH
ME silicone) was used with a column length of 30 m, an inside
diameter of 0.32 mm, and a film thickness of 0.25 µm. The
standard temperature program of the GC was T = 353 K for
180 s followed by a heating rate of 0.167 K sϪ1 to T = 523 K.
Response factors of all reagents were determined using cal-
ibration mixtures of the corresponding components prepared
gravimetrically.
alkanols are large scale products of the chemical industry and
their chemical reactions provide yields of ethers of over 50 mole
percent. Therefore (1-alkoxyethyl)benzenes are promising
substances for the aforementioned purpose. We have studied
systematically the chemical equilibrium in the reacting systems
of (1-alkoxyethyl)benzene synthesis such as isopropyl- (iPrEB),
sec-butyl- (sBuEB), and cyclohexyl- (c-HexEB) derivatives (see
Scheme 1). Values of the reaction enthalpies ∆rHmo were derived
from the temperature dependence of equilibrium ratios Kx
measured in the reactive mixtures with an excess of alkanol. In
order to test the validity of the procedure used, the results
obtained have been compared with those calculated from
difference of the enthalpies of formation of the reaction par-
ticipants. For this purpose, standard molar enthalpy of form-
ation in the liquid phase ∆fHmo(l) of (1-isopropoxyethyl)benzene
was additionally obtained from the calorimetrically measured
enthalpy of combustion. Further support of the reliability of
the procedures applied in this work is expected from the
quantitative analysis of standard molar enthalpies of form-
ation in the gaseous phase ∆fHmo(g). Values of ∆fHmo(g) of
(1-alkoxyethyl)benzenes were obtained from measured values
of ∆fHmo(l) and their enthalpies of vaporization measured by a
transpiration method.6 Thus, the systematic investigation of the
(1-alkoxyethyl)benzene synthesis reactions would be of value
for two reasons. First, if good agreement between the standard
reaction enthalpies ∆rHmo(l), obtained from the temperature
dependence of the equilibrium constants and those obtained
from combustion calorimetry, is found, it would confirm a
satisfactory reliability and consistency of our procedure. Sec-
ond, experimental values of ∆fHmo(g) would provide useful
information on the strain effects and the relation between
structure and properties of (1-alkoxyethyl)benzenes. The results
of the study can also contribute to an improvement of the
Benson7 group-contribution methodology.
2.3 Combustion calorimetry
An isoperibolic calorimeter3 equipped with a static bomb and
an isothermal water jacket was used for measuring the energy
of combustion of (1-isopropoxyethyl)benzene. The temper-
ature of the water jacket was maintained to within 0.0015 K
using a high precision mercury contact thermometer. To
exclude traces of water in the liquid samples used for the com-
bustion experiments, the purified samples were dried over
molecular sieves and distilled once more prior to combustion.
Each sample was sealed in a container to avoid oxidation and
contamination with moisture. In the present study, we used
commercially available polythene ampoules (Fa. NeoLab,
Heidelberg) of 1 cm3 volume as the sample container for the
liquids. The initial temperature of the combustion experiments
was 298.15 K for each experiment. The energy equivalent of the
calorimeter εcalor was determined with a standard reference
sample of benzoic acid (sample SRM 39i, N.I.S.T.). From seven
experiments, εcalor was determined to be 15296.0 2.3 J KϪ1
.
For converting the energy of the actual combustion process to
that of the isothermal process, and reducing to standard states,
the conventional procedure8 was applied. The sample mass of
(1-isopropoxyethyl)benzene was reduced to vacuum, taking
into consideration the density value ρ(293) = 0.915 g cmϪ3 which
was determined in a calibrated 10 cm3 pycnometer. The energy
‡ The IUPAC name for cumyl is α,α-dimethylbenzyl.
§ The IUPAC name for tert-amyl is tert-pentyl.
J. Chem. Soc., Perkin Trans. 2, 2002, 728–733
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Table 1 Thermochemical data for (1-alkoxyethyl)benzenes at T = 298.15 K (po = 0.1 MPa)
∆fHmo(l)/kJ molϪ1 ∆glHmo/kJ molϪ1 ∆fHmo(g)b/kJ molϪ1 ∆fHmo(g)(calc.)c/kJ molϪ1 HS d/kJ molϪ1
(1-Isopropoxyethyl)benzene
Ϫ233.3 1.0
Ϫ259.8 2.9a
Ϫ258.1 2.9a
Ϫ263.8 2.2a
55.41 0.27
58.70 0.46
59.13 0.48
69.75 0.45
Ϫ177.9 1.0
Ϫ201.1 2.9
Ϫ199.0 2.9
Ϫ194.1 2.3
Ϫ184.5
Ϫ206.0
Ϫ206.0
Ϫ207.8
6.6
4.9
7.0
S*,R*-(1-sec-Butoxyethyl)benzene
S*,S*-(1-sec-Butoxyethyl)benzene
(1-Cyclohexyloxyethyl)benzene
13.7
a Calculated from the results of the chemical equilibrium study (see text). b ∆fHmo(g) = ∆fHmo(l) ϩ ∆glHmo. c Calculated as the sum of strain-free
increments (see text). d Strain enthalpy of ethylbenzene derivatives HS = ∆fHmo(g)(exp.) Ϫ ∆fHmo(g)(calc.).
of combustion of the cotton thread ∆cuo(CH1.774O0.887) =
Ϫ(16945.2 4.2) J gϪ1 and polythene ∆cuo(CH1.930) = Ϫ46361.0
3.1 J gϪ1 were measured in our laboratory previously.
product diisopropyl ether (reaction 1.3). The dehydration of
isopropyl alcohol is irreversible, but the rate of the reversible
reaction 1.1 is rapid enough to reach chemical equilibrium of
this reaction even along with the slow decomposition of iso-
propyl alcohol. In contrast to reaction 1.1, reaction 1.3 is very
slow and the equilibrium of this reaction was generally not
reached during the time necessary for investigation of the goal
reaction 1.1.
2.4 Transpiration method
The molar enthalpies of vaporization of the (1-alkoxyethyl)-
benzenes were determined using the method of transpiration in
a saturated N2-stream and applying the Clausius–Clapeyron
equation to vapor pressures measured as function of the tem-
perature. The method has been described before6 and has given
results in excellent agreement with other established techniques
for determining vapor pressures of pure substances in the range
of 0.005 to ca. 10000 Pa and enthalpies of vaporization, ∆glHmo,
from the temperature dependence of the vapor pressure. Values
of ∆glHmo at T = 298.15 K are listed in Table 1.
3.1.2 The (1-isobutoxyethyl)benzene synthesis reaction.
Reactions observed in the system of (1-isobutoxyethyl)benzene
synthesis are presented in Scheme 3. The rate of the goal chem-
3
Results
3.1 Equilibrium constants and reaction enthalpies
The chemical equilibrium of the reactive systems isoalkanol ϩ
styrene
(1-alkoxyethyl)benzene was studied in the liquid
phase in the temperature range 343 to 433 K. In spite of the
fact that equilibrium studies have been performed at elevated
temperatures, any corrections of the reaction enthalpies are
negligible,3–5 taking into account the individual error bars of
about 1.5 to 3 kJ molϪ1 typical of the equilibrium measure-
ments. In further calculations it was assumed that the enthalpy
of reaction hardly changes on passing from the average
temperature of the experimental range to T = 298.15 K.
3.1.1 The (1-isopropoxyethyl)benzene synthesis reaction.
The linear alkanols such as ethanol, propanol, and butanol
react rapidly with the styrene without side reactions.5 In con-
trast, reaction of styrene with the branched alkanols is accom-
panied by a few side reactions. Reactions presented in Scheme 2
Scheme 3 Scheme of (1-sec-butoxyethyl)benzene synthesis.
ical reaction 2.1 of styrene with the sec-butyl alcohol is again
substantially less than with the n-butanol. The goal reac-
tion (2.1), produces two isomers S*,R*-(1-sec-butoxyethyl)-
benzene and S*,S*-(1-sec-butoxyethyl)benzene, which are able
to undergo the mutual isomerisation (reaction 2.4) under the
given reaction conditions. In presence of the acidic catalyst,
dehydration of the sec-butyl alcohol (2.2) produces the isomeric
butenes (C4H8). These products can react with the sec-butyl
alcohol leading to di-sec-butyl ether (reaction 2.3). The
dehydration of sec-butyl alcohol is irreversible, but the rate of
the reversible reaction 2.1 is rapid, and equilibrium of this reac-
tion could be reached in spite of the slow decomposition of sec-
butyl alcohol. In contrast, equilibration of the reaction 2.3 is
very slow and the equilibrium of this reaction was generally not
reached during the time necessary for investigation of the goal
to reaction 2.1.
Scheme 2 Scheme of (1-isopropoxyethyl)benzene synthesis.
have been observed in the system of (1-isopropoxyethyl)-
benzene synthesis. The rate of the goal chemical reaction of
styrene with the branched alkanol is substantially less then the
rate with the linear ones. Together with the goal reaction (1.1),
in the presence of the acidic catalyst, dehydration of isopropyl
alcohol occurs (1.2). The product of this reaction is propene
(C3H6). It reacts with the isopropyl alcohol and gives the
3.1.3 The (1-cyclohexyloxyethyl)benzene synthesis reaction.
Reactions observed in the system of (1-cyclohexyloxyethyl)-
benzene synthesis are presented in Scheme 4. The goal reaction
3.1 is also accompanied by a slow dehydration of cyclohexanol.
However, possibly due to high strain, the product of the reac-
tion of cyclohexanol with cyclohexene, dicyclohexyl ether, was
not found in the reaction products.
730
J. Chem. Soc., Perkin Trans. 2, 2002, 728–733

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benzene synthesis obtained previously are also presented in
Fig. 2. Experimental values of Kx have been approximated as a
function of temperature by the linear equation lg Kx = a ϩ
b(1000T /K)Ϫ1 using the method of least squares. The slopes
of these lines when multiplied by the gas constant give the
standard enthalpy of reaction ∆rHmo = Ϫb × 2.303R of the
(1-alkoxyethyl)benzene synthesis reaction in the liquid state in
kJ molϪ1 and the intercept gives the standard entropy of reac-
tion ∆rSmo = a × 2.303 R in J KϪ1 molϪ1. Numerical results are
presented in Table 2.
3.2 Energy of combustion and enthalpy of formation
The results for a typical combustion experiment of (1-isoprop-
oxyethyl)benzene are given in a table deposited as electronic
supplementary information. Standard molar enthalpies of
combustion ∆cHmo(l) and of formation ∆fHmo(l) (see Table 1)
were calculated according to the established procedure.8 The
assigned standard deviation of the mean value of ∆fHmo(l)
includes the uncertainties from calibration, from the combus-
tion energies of the auxiliary materials, and the uncertainties of
the enthalpies of formation of the reaction products H2O and
CO2.
Scheme 4 Scheme of (1-cyclohexyloxyethyl)benzene synthesis.
Experimentally determined compositions of equilibrium
mixtures (including mole fractions of by-products) and Kx-
values in the liquid phase, obtained from the results of the
chemical equilibrium study of the (1-alkoxyethyl)benzene
synthesis using eqn. (4) are listed in tables deposited as elec-
tronic supplementary information and are presented graphic-
ally in Fig. 1. Inspection of these tables and Fig. 1 shows that
4
Discussion
4.1 Comparison of the reaction enthalpies obtained from
equilibrium studies and from combustion experiments
Standard enthalpies of the synthesis reactions of (1-alkoxy-
ethyl)benzenes from isoalkanols and styrene have been
obtained in the liquid phase. The validity of these 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 ∆fHmo(l) of (1-isopropoxyethyl)bezene at 298.15 K
was measured by means of combustion calorimetry in this work
(Table 1). Further experimental data necessary for comparison
are available in the literature: for styrene9 ∆fHmo(l) = (103.4
0.9) kJ molϪ1, for isopropyl alcohol10 ∆fHmo(l) Ϫ(317.0 0.3) kJ
molϪ1, and for sec-butyl alcohol10 ∆fHmo(l) = Ϫ(342.7 0.6) kJ
molϪ1. Data for cyclohexanol ∆fHmo(l) = Ϫ(348.8 0.2) kJ molϪ1
were calculated from its gaseous enthalpy of formation11
Fig. 1 Equilibrium ratios Kx for (1-alkoxyethyl)benzene synthesis as a
function of alkanol mole fraction x in the equilibrium mixture at T =
353.1 K.
∆fHmo(g) = Ϫ(286.8
0.2) kJ molϪ1 and enthalpy of vapor-
Kx-values are almost independent of the mole fraction of alk-
anol if xAlkOH > 0.3. It can be also assumed that the presence of
the side reaction products has no significant influence on the on
the equilibration of the goal reactions. For all three systems
studied, a plot of lg Kx as a function of 1000/T with Kx-values
obtained from the glass vials technique at values of xAlkOH ≥ 0.3
give straight lines which are almost parallel to each other (see
Fig. 2). For comparison other reactions of (1-alkoxyethyl)-
ization12 ∆glHmo = 62.01 kJ mol.Ϫ1 These data were used to
o
calculate independently ∆ H
of the (1-isopropoxyethyl)-
m (first law)
f
benzene (iPrEB) synthesis reaction in the liquid phase:
∆rHmo(l)(first law) = ∆fHmo(l)(iPrEB) Ϫ ∆fHmo(l)(iPrOH) Ϫ
∆fHmo(l)(Styrene) = Ϫ(20.0 3.0) kJ molϪ1 (6)
The comparison with experimental values obtained from the
chemical equilibrium study is given in Table 2. The calculated
values of ∆rHmo(l)(first law) for the reaction of (1-isopropoxy-
ethyl)benzene is in agreement with those of ∆rHmo(l)(second law)
within the limits of the experimental uncertainties estimated for
both methods.
The thermodynamic consistency observed allows calculation
of the enthalpy of formation of other (1-alkoxyethyl)benzenes
(see Table 1) from the known enthalpies of formation of
alkanols, styrene and standard enthalpies of reaction obtained
from the temperature dependence of Kx (see Table 2).
Because of the very slow rate, the chemical equilibration of
the side reactions of formation of diisopropyl ether (1.3) in
Scheme 2 and di-sec-butyl ether (2.3) in Scheme 3 was reached
only at certain compositions of the reaction mixture. Equi-
librium ratios Kx of these reactions were derived at each tem-
perature of investigation, but the results for of these ratios are
less reliable in comparison with those of the goal reactions 1.1
Fig. 2 Equilibrium ratios Kx for (1-alkoxyethyl)benzene synthesis at a
mole fraction xalkanol > 0.3 as a function of temperature.
J. Chem. Soc., Perkin Trans. 2, 2002, 728–733
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Table 2 Thermodynamic functions ∆rHmo and ∆ Smo of the (1-alkoxyethyl)benzene synthesis reactions in the liquid phase at T = 298.15 K obtained
from lg Kx = a ϩ b(1000T /K)Ϫ1 and data of ∆ Hmorobtained from enthalpies of formation of the reaction participants
r
o
o
Alkyl
∆ H
a/kJ molϪ1
∆ H
b/kJ molϪ1
a
b
〈T 〉c/K
∆rSmo a/J KϪ1 molϪ1
m (second law)
m (first law)
r
r
Iso-Pr
sec-Bu
Cyclo-Hex
Ϫ20.9 0.5
Ϫ20.5 2.7
Ϫ18.4 2.0
Ϫ20.0 3.0
Ϫ2.57 0.06
Ϫ2.69 0.36
Ϫ2.03 0.27
1.09 0.02
1.07 0.14
0.97 0.10
383.1
393.1
383.1
Ϫ49.2 1.2
Ϫ51.5 7.0
Ϫ38.9 5.2
a Derived from the temperature dependence of Kx. b Calculated from the enthalpies of formation of the reaction participants. c The average
temperature of the equilibrium study.
(Scheme 2) and 2.1 (Scheme 3). Experimental values of Kx
were approximated as a function of temperature by the linear
equation lg Kx = Ϫ(4.34 0.36) ϩ (1.95 0.14)(1000T /K)Ϫ1 for
reaction 1.3 (Scheme 2) and lg Kx = Ϫ(4.21 0.34) ϩ (1.45
0.13)(1000T /K)Ϫ1 for reaction 2.3 (Scheme 3) and presented in
Fig. 3. The validity of these results obtained from the chemical
equilibrium study can be verified by comparison with the values
resulting values of ∆fHmo(g) of (1-alkoxyethyl)benzenes calcu-
lated as sum of ∆fHmo(l) and ∆1gHmo are shown in Table 1.
We define the strain enthalpy HS of a molecule as the differ-
ence between the experimental standard enthalpy of formation
∆fHmo(g) and the calculated sum of the strain-free Benson type
increments7 for this molecule. The strain-free increments for the
calculation of enthalpies of formation of alkanes,14 alkylbenz-
enes15 and ethers16,17 are already well established. By using
these group-additivity parameters and the values of ∆fHmo(g) of
ethylbenzene derivatives (Table 1) and their values of strain
enthalpies HS = [∆fHmo(g) Ϫ Σincrements] have been estimated
(Table 1).
(1-Isopropoxyethyl)benzene and (1-isobutoxyethyl)benzene
studied here are very similarly strained by about 5 kJ molϪ1
(Table 1). This fact is a further indication that the data for
∆fHmo(g) of (1-alkoxyethyl)benzenes, obtained by combining the
different experimental techniques (calorimetry, transpiration,
equilibrium study) generally consistently support confidence
in the experimental procedures used. What causes strain in
these molecules? Elucidation of the nature of strain in (1-
alkoxyethyl)benzene is aided by comparison with the strain of
similarly shaped isopropylbenzene:13 ∆fHmo(g) = Ϫ(3.9 1.1) kJ
molϪ1 and strain enthalpy HS = 5.0 kJ molϪ1. 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 the analogy with the
strain of isopropylbenzene, that the observed amount of
destabilization in alkoxyethylbenzene 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
the correction for HS = 5.0 kJ molϪ1 like in isopropylbenzene)
for the group-contribution correlation for ∆fHmo(g) of the simi-
larly shaped (1-alkoxyethyl)benzenes. It is obvious, that the
somewhat larger strain of the (1-cyclohexyloxyethyl)benzene
(HS = 13.7 kJ molϪ1, see Table 1) is caused by an additional
contribution from the strain of the cyclohexyl substituent
attached to the oxygen atom. For comparison, the strain of the
Fig. 3 Equilibrium ratios Kx for diisopropyl ether and di-sec-butyl
ether synthesis reactions as a function of temperature.
of the reaction enthalpies calculated from the formation
enthalpies of the reaction participants. For this purpose the
standard molar enthalpies of formation ∆fHmo(l) of appropriate
olefins and ethers were taken from the literature.13 The calcu-
lated values of ∆rHmo(l) = Ϫ38.9 kJ molϪ1 for the reaction 1.3
(Scheme 3) and ∆rHmo(l) = Ϫ28.4 kJ molϪ1 for reaction 2.3
(Scheme 3) are in very close agreement being within the limits
of experimental uncertainty with those of ∆rHmo(l) = Ϫ(37.3
2.6) kJ molϪ1 and ∆rHmo(l) = Ϫ(27.8 2.5) kJ molϪ1 derived
from the chemical equilibrium study.
Experimental values of Kx of the isomerisation reaction 2.4
of two isomers S*,R*-(1-sec-butoxyethyl)benzene and S*,S*-
(1-sec-butoxyethyl)benzene were approximated as a function of
temperature by the linear equation lg Kx = Ϫ(0.20 0.032) ϩ
(0.088
0.013)(1000T /K)Ϫ1. The enthalpy of this reaction
∆rHmo(l) = Ϫ(1.68 0.24) kJ molϪ1 was derived from the chem-
ical equilibrium study and used to obtain enthalpies of form-
ation ∆fHmo(l) of both isomers with help of the reaction 2.1 and
enthalpies of formation of its participants (see Table 1).
cyclohexanol itself is13 HS = 8.8 kJ molϪ1
.
References
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4.2 Strain enthalpies HS of (1-alkoxyethyl)benzenes
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 were derived from their gaseous standard molar
enthalpies of formation ∆fHmo(g) at 298.15 K. Indeed, the iso-
propoxy, sec-butoxy, and cyclohexyloxyethylbenzenes listed in
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into ∆fHmo(g) from the CH2-group should remain constant. In
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