The standard molar enthalpies of formation of some alkyladamantanes

Article abstract of DOI:10.1006/jcht.2000.0691

The standard massic energies of combustion of three alkyl-derivative of adamantane were measured at T = 298.15 K by static-bomb combustion calorimetry. The standard molar enthal pi es of formation in the liquid and gaseous states were obtained from these data. The enthalpies of some reactions of isomerization were calculated from the equilibrium study and compared with the results of calorimetric measurements.

Full text of DOI:10.1006/jcht.2000.0691

J. Chem. Thermodynamics 2000, 32, 1311–1317
doi:10.1006/jcht.2000.0691
Available online at http://www.idealibrary.com on
The standard molar enthalpies of formation of
some alkyladamantanes
S. V. Melkhanova, S. M. Pimenova, V. P. Kolesov,a
Chemistry Department, Moscow State University, Moscow 119899, Russia
A. A. Pimerzin, and V. S. Sarkisova
Samara State Technical University, Galaktionovskaya 141, 443010, Samara,
Russia
The standard massic energies of combustion of three alkyl-derivative of adamantane were
measured at T = 298.15 K by static-bomb combustion calorimetry. The standard molar
enthalpies of formation in the liquid and gaseous states were obtained from these data. The
enthalpies of some reactions of isomerization were calculated from the equilibrium study
c
and compared with the results of calorimetric measurements. ꢀ 2000 Academic Press
KEYWORDS: thermochemistry; calorimetry; enthalpies of combustion; enthalpies of
formation; alkyladamantanes
1. Introduction
The thermochemistry of hydrocarbons with a cage structure is of special interest. Among
them, adamantane and its derivatives represent a very specific family of bridge-ring
diamond-like compounds. Their properties have been studied in numerous papers.(1) For
example, the standard enthalpy of formation of adamantane itself has been the object of
several independent investigations; this value, recommended by Pedley et al.,(2) is equal
to −(194.1 ± 2.2) kJ · mol−1 for the crystalline state and to −(134.1 ± 2.3) kJ · mol−1
for the gaseous state. But there are notable gaps in thermochemical data of adamantane
derivatives.
The enthalpies of combustion and formation of some methyladamantanes have been
measured by Steele and Watt(3) and by Clark et al.;(4) some discrepancies between their
results were discussed in reference 4. The enthalpies of isomerization of some methyl- and
ethyladamantanes have been derived from equilibrium studies.(5, 6)
This paper reports the standard energies of combustion and the standard
enthalpies of formation of 1,3-dimethyladamantane, 1,3,5-trimethyladamantane and
1-ethyladamantane at T = 298.15 K. These data are discussed together with the results of
previous works.
a
To whom correspondence should be addressed (E-mail: kolesov@capital.ru).
c
0021–9614/00/101311 + 07 $35.00/0
ꢀ 2000 Academic Press

1312
S. V. Melkhanova et al.
TABLE 1. Physico-chemical properties of the compounds at T = 298.15 K
b
T
M
ρ
T
fus
vap
a
20
d
Compound
n
D
Mass fraction purity
I
−1
−3
K
K
g · mol
g · cm
c
g.l.c.
calorim.
(1)
(11)
(12)
(1)
(1)
(13)d
1,3-DMA 164.29080 0.9016
1,3,5-TMA 178.31768 0.8834
1.4783
0.9995 0.9997
0.9993 0.9998
243
295
477
478
1121
1121
1135
1133
e
(14)
(11)
(14)
(15)
(3)
(1)
(13)d
1.4751
1.4753
(15) f
(11)
e
255.61
484.6
(16)
(15)
251
483.305
(1)
(1)
(1, 16)
(1)
(13)d
1-EA
164.29080 0.9350
1.4931
0.9995
215
492
1224
1225
e
a
1,3-DMA, 1,3-dimethyladamantane; 1,3,5-TMA, 1,3,5-trimethyladamantane; 1-EA, 1-
b
ethyladamantane. The molar masses for all compounds were calculated from the table of relative
atomic masses recommended by IUPAC.
(10) c
d
Found in this work. Kovats indices on silica fase SE-30.
e
f
Reduced to T = 378.2 K using temperature coefficient from reference 13. Found in this work at
g
T = 378.2 K. The triple point temperature.
2. Experimental
1,3-Dimethyladamantane and 1,3,5-trimethyladamantane were prepared by isomerization
on aluminium chloride of perhydroacenaphthene and perhydrofluorene, respectively, by
procedures published elsewhere.(1, 7, 8) 1-Ethyladamantane was synthesized from methyl-
1-adamantylketone.(1, 9) Samples of compounds were subjected to rigorous purification by
multiple vacuum rectification. Purity was checked by g.l.c. (all samples) and by the triple
point depression method (except for 1-ethyladamantane).
Physical properties of the compounds and the values of the parameters used for
converting the measured quantities to the isothermal bomb process, and to the standard
states are tabulated in table 1.
For 1,3,5-trimethyladamantane the triple point temperature Ttr = (255.61±0.05) K (see
table 1) was measured in a low-temperature adiabatic calorimeter,(15) whereas Steele and
Watt found for this substance Tfus = 295.0 K by d.s.c.(13) The contradiction between the
results of these measurements is difficult to explain. Steele and Watt stated their sample to
be solid; on the other hand, we had a liquid sample, as did other authors,,(1, 11, 16) who also
determined a structure of the compound. The identity of our samples was confirmed by the
agreement of physico-chemical constants and Kovats indices, measured in our laboratory,
with data taken from reference 13 (see table 1).
The energy of combustion was determined using a static bomb isoperibolic
macrocalorimeter. The temperature rise was measured with a copper resistance ther-
mometer and a bridge circuit.(17) The uncertainty of the temperature measurements
was about 5 · 10−5 K. The energy equivalent ε of the calorimeter was determined by
combustion of the thermochemical standard benzoic acid (the sample K-1, supplied by
the Russian Scientific Research Institute of Metrology) with a certified massic energy of

Enthalpies of formation of alkyladamantanes
1313
combustion of −(26434.0 ± 2.2) J · g−1 under the test conditions at T = 298.15 K. It
−1
was equal to (54266.2 ± 10.3) J ·  for experiments with 1,3-dimethyladamantane and
−1
(54329.4 ± 10.9) J ·  for all other experiments. The difference was due to a repair of
the calorimeter. The uncertainty of the mean ε value (as well as of 1cuo) was calculated
as ±t · s, where s is a standard deviation of the mean value, and t is a Student’s coefficient
at the 0.05 significance level and contain the uncertainty in the energy of combustion of
benzoic acid. The samples of compounds were sealed in weighted Terylene-film bags and
placed into a small platinum cup. The bomb, with 1 cm3 of added water, was charged with
purified oxygen to a pressure of 3.04 MPa. The initial temperature did not differ from
T = 298.15 K by more than 0.02 K. Provision for ignition was made using a platinum
wire heated by the discharge of a capacitor. After each run, the combustion products
were analysed for carbon dioxide by the Rossini method.(18) No soot was observed after
combustion experiments. Qualitative tests for CO with indicator tubes were negative
within the limits of their sensitivity {mole fraction x(CO) < 1 · 10−6}.
3. Results
Results of typical combustion experiments for each compound are summarized in table 2.
The standard massic energy of combustion of Terylene film, 1cuo = −(22927.9 ± 6.3)
J · g−1, and the resulting ratio of the mass of carbon dioxide to that of the film (2.2897 ±
0.0006), were measured previously.(19) The value (∂u/∂p)T = −0.167 J · g−1 · MPa−1
for all three substances was estimated in reference 3. The average recovery of CO2 was
as follows: 1.0000 ± 0.0003 for 1,3-dimethyladamantane, 1.0000 ± 0.0004 for 1,3,5-
trimethyladamantane and 1.0000 ± 0.0003 for 1-ethyladamantane.
The derived quantities are summarized in table 3. The overall random uncertainties
also included the uncertainty of calibration. Other contributions such as uncertainty
of combustion energy of Terylene film were found to be negligible. The standard
molar enthalpies of formation of CO2(g) and H2O(l), recommended by CODATA,(20)
−(393.51 ± 0.13) kJ · mol−1 and −(285.830 ± 0.040) kJ · mol−1, were employed in the
calculation of 1fHmo values.
4. Discussion
The enthalpies of combustion and formation of 1-ethyladamantane have been measured in
this work for the first time. The 1fHmo (g) values of 1,3-dimethyladamantane and of 1,3,5-
trimethyladamantane, obtained formerly by Steele and Watt,(3) are compared with our data
in table 4. The experimental data for some other methylated adamantane compounds are
also presented in table 4.
It can be seen from this table that the enthalpies of formation of 1-methyladamantane
and 1,3-dimethyladamantane are substantially higher than the enthalpies of formation of
their respective isomers, 2-methyladamantane and 2,2-dimethyladamantane. This implies
that the enthalpy increment for replacement of one tertiary hydrogen atom (a bridgehead
position) by a methyl group is greater than for the same replacement of one secondary
hydrogen atom. Clark et al.,(4) using their 1fHmo value for adamantane, calculated the

1314
S. V. Melkhanova et al.
TABLE 2. Summary of typical calorimetric experiments at T = 298.15 K
1,3-dimethyladamantane 1,3,5-trimethyladamantane 1-ethyladamantane
m(compound)/g
m(f)/g
0.241068
0.012988
0.203113
0.218745
0.014535
0.185478
0.226746
0.013634
0.191866
1R /
corr
0
ε · 1R /J
11022.4
10077.1
10424.2
corr
1U(f)/J
297.8
0.9
333.3
1.3
312.6
1.2
1U(HNO )/J
3
1U(std)/J
3.8
3.5
3.7
o
−1
)
−1 u (J · g
44468.4
44522.2
44572.8
c
m(CO , expt)/m(CO , theor)
0.9999
1.0000
1.0000
2
2
n
7
8
8
o
−1
)
h−1 u i(J · g
44465.6 ± 9.8
44521.5 ± 11.1
44568.9 ± 13.8
c
The symbols in this table have the following meanings: m(compound), mass of sample; m(f), mass of
0
Terylene film; 1R , increase in the thermometer resistance corrected for heat exchange; ε , the energy
corr
0
equivalent corrected for heat capacity of the bomb contents after combustion; ε · 1R , total energy
corr
change of the isothermal bomb process; 1U(f), correction for the combustion energy of Terylene film;
1U(HNO ), correction for the energy of formation of aqueous nitric acid from nitrogen, oxygen and water
3
o
−1
−3
(was based on 1 U = −59.8 kJ · mol for the concentration 0.1 mol · dm ); 1U(std), standard-state
r
m
(18)
o
correction, calculated using the approach proposed by Hubbard et al., ; 1 u , standard massic energy
c
of combustion of the compounds; m(CO , expt)/m(CO , theor), ratio of the mass of CO recovered after
2
2
2
the experiment to the mass of CO expected on the basis of the mass of the compound; n, number of
2
o
experiments; h1 u i, the average value of the standard massic energy of combustion.
c
TABLE 3. Derived thermodynamic quantities at T = 298.15 K
o
o
o
o
o
1
H
−1 U
−1 H
1 H (l)
1 H (g)
f
m
vap
c
c
f
m
m
m
m
Compound
−1
−1
−1
−1
−1
kJ · mol
kJ · mol
kJ · mol
kJ · mol
kJ · mol
−213.0 ± 2.3
−255.5 ± 2.7
−190.4 ± 3.0
(12)
1,3-dimethyladamantane 7305.3 ± 1.6 7317.7 ± 1.6 −262.7 ± 2.3 49.71 ± 0.20
(15)
1,3,5-trimethyladamantane 7939.0 ± 2.0 7952.6 ± 2.0 −307.2 ± 2.7 51.74 ± 0.20
a
1-ethyladamantane
7322.3 ± 2.3 7334.7 ± 2.3 −245.7 ± 2.8 55.3 ± 1.1
a
Determined in the Thermochemistry Laboratory by R. M. Varushchenko and L. L. Pashchenko.
bridgehead methyl group increments to be −36.86 kJ · mol−1 and −37.03 kJ · mol−1 for
1-methyladamantane and 1,3,5,7-tetramethyladamantane, respectively.
All experimental 1fHmo (g) values obtained in reference 4 are lower than those from
reference 3 (see table 4). Supposing that four bridgehead hydrogen atoms in adamantane
are equivalent, Clark et al.(4) claimed that there are inconsistences in the data of
Steele and Watt,(3) and predicted the 1fHmo (g) values of 1,3-dimethyladamantane and
1,3,5-trimethyladamantane on the basis of their average methyl group increment to be
−206.6 kJ · mol−1 and −243.5 kJ · mol−1, respectively (table 4).

Enthalpies of formation of alkyladamantanes
1315
TABLE 4. Comparison of the experimental gas-phase standard enthalpies of
formation of methyladamantanes
o
−1
Compound
−1 H /kJ · mol
f
m
(3)
(4)
Steele and Watt
Clark et al.
This work
a
1-methyladamantane
2-methyladamantane
174.9 ± 2.1
156.5 ± 2.1
169.9 ± 1.4
149.2 ± 2.6
(174.4)
b
(154.3)
2,2-dimethyladamantane
1,3-dimethyladamantane
1,3,5-trimethyladamantane
182.8 ± 2.8
218.8 ± 2.8
254.8 ± 4.2
b
(206.6)
213.0 ± 2.3
255.5 ± 2.7
b
(243.5)
a
1,3,5,7-tetramethyladamantane 295.0 ± 3.8
281.0 ± 2.1
(293.6)
a
o
(2)
Estimated using the 1 H values of adamantane and experimental data obtained in
f
m
b
this work. Estimated in reference 4 on the base of their experimental data.
The 1fHmo (g) value obtained in this work for 1,3,5-trimethyladamantane is close
to the data of Steele and Watt.(3) For 1,3-dimethyladamantane our result is between
the experimental data of Steele and Watt(3) and the data predicted in reference 4.
The reliability of the results obtained in this work is testified by the high purity of
the substances and complete combustion of the samples (table 2). Using these results
and the recommended value of the standard enthalpy of formation of adamantane,
1fHmo (g) = −(134.6 ± 2.3) kJ · mol−1 (2)
,
we calculated the methyl group increments
for replacement of one hydrogen atom in a bridgehead position: −39.2 kJ · mol−1
from 1fHmo (g) of 1,3-dimethyladamantane and −40.3 kJ · mol−1 from 1fHmo (g) of
1,3,5-trimethyladamantane; the average value is −39.75 kJ · mol−1. The 1fHmo (g) of
1-methyladamantane and 1,3,5,7-tetramethyladamantane, estimated on the basis of this
value, are equal to −174.4 kJ · mol−1 and −293.6 kJ · mol−1, respectively (see table 4).
Both these values are also close to the data of Steele and Watt.(3)
Using the above-mentioned value of 1fHmo (g) for 1-methyladamantane and the group
additivity values proposed by Cohen,(21) we estimated the enthalpy of formation of 1-
ethyladamantane to be −195.3 kJ · mol−1. This estimate does not differ substantially from
the experimental value, −(190.4 ± 3.0) kJ · mol−1 (see table 3).
It is interesting to compare the methyl group increments quoted above with the
analogous increments in bicyclo[2,2,1]heptane (norbornane), a bridge-ring compound,
the bridgehead positions of which resemble the corresponding positions in adamantane.
The 1fHmo (g) values of some methylated norbornane compounds have been determined
previously in our laboratory by Kozina et al.(22, 23) The experimental data for 1,4-
dimethylnorbornane were discussed in reference 4. The data for all methylated norbornane
compounds studied previously in the Thermochemistry Laboratory are listed in table 5.
It is obvious that the replacement of tertiary hydrogen by methyl group causes virtually
the same enthalpy increment in methylated adamantane compounds and norbornane
compounds (compounds 1 to 4). All enthalpy increments for the replacement of secondary
hydrogen (compounds 5 to 9) are smaller, and except for two 2-methylnorbornanes in

1316
S. V. Melkhanova et al.
TABLE 5. Comparison of the enthalpies of formation and the methyl
group increments in methylated adamantanes and norbornanes at T =
a
298.15 K
o
b
0
1 H
f
m
n
Compound
References
−1
−1
kJ · mol
kJ · mol
1
2
3
4
5
6
7
8
1-methylnorbornane
−92.8
−40.8
−38.3
−39.2
−40.3
−28.7
−29.1
−24.6
−21.9
−19.7
−24.1
23
1,4-dimethylnorbornane
1,3-dimethyladamantane
−128.6
23
−213.0 ± 2.3
this work
1,3,5-trimethyladamantane −255.5 ± 2.7
this work
exo-2-methylnorbornane
endo-2-methylnorbornane
7,7-dimethylnorbornane
2-methyladamantane
−80.7
−81.1
23
23
23
3
−101.2 ± 1.8
−156.5 ± 2.1
−154.3
4
9
2,2-dimethyladamantane
o
−182.8 ± 2.8
3
a
The 1 H value of norbornane, recommended in reference 24, (−52.0±2.3)
f
m
−1
kJ · mol , was used in the calculation of methyl group increments of methylated
b
norbornanes. Methyl group increments.
which the methyl group is attached to a cyclohexane ring fixed in a bath conformation, there
is no noticeable difference between them. The methyl group increments, estimated by the
group additivity method(21) for 1- and 2-methyl-substituted adamantane (or norbornane)
do not differ remarkably: they are equal to −32.2 kJ · mol−1 and to −30.9 kJ · mol−1
,
respectively. Hence, the enthalpy difference between these compounds (see table 5) cannot
be explained only by a difference in substitution pattern. Engler et al.(6) noticed, discussing
their equilibrium data, that the enthalpy difference between 1- and 2-methyladamantane
was caused mainly by axial methyl strain in 2-methyladamantane. The axial methyl strain,
analogous to that in axial methylcyclohexane, could also cause a decrease in methyl group
increments in other compounds (5 to 9) listed in table 5.
The enhanced enthalpy increase for a methyl substitution in a bridgehead position was
also established earlier from equilibration of some methyladamantanes.(5, 6) Two of the
authors have recently studied the isomerization of some alkyladamantanes and calculated
enthalpies and entropies of some isomerization reactions from experimental equilibrium
constants.(24, 25) These results also confirmed that the enthalpy of formation is more
negative for all alkyl-substituted adamantanes, containing alkyl in a bridgehead position.
The enthalpy and entropy of reaction:
1-ethyladamantane = 1,3-dimethyladamantane
(1)
first published in reference 24 were defined more accurately in an additional study of
equilibrium;(25) the enthalpy of reaction (1) at T = 388 K was found to be equal to
−(18.0 ± 1.1) kJ · mol−1 and −(23.7 ± 1.1) kJ · mol−1 for the liquid and the gaseous
states, respectively.

Enthalpies of formation of alkyladamantanes
1317
(2)
For the reaction of similar type:
1-methyl-3-ethyladamantane = 1,3,5-trimethyladamantane
the enthalpy for the liquid and the gaseous states at T = 396 K was equal to −(19.0±1.0)
kJ · mol−1 and −(24.2 ± 1.0) kJ · mol−1, respectively.(25) All these results are rather close
to the corresponding values, calculated for reaction (1) from the data listed in table 3
(−17.0 kJ · mol−1 and −22.6 kJ · mol−1, respectively at T = 298.15 K).
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