Odd-even effect in melting properties of 12 alkane-α,ω-diamides

Article abstract of DOI:10.1016/j.jct.2006.04.004

Fusion and solid-to-solid transitions of a homologous series of 12 linear alkane-α,ω-diamides H₂NCO-(CH₂)_(n-2)-CONH₂, where n = (2 to 12 and 14), were investigated by differential scanning calorimetry (d.s.c.). The temperatures of fusion of even terms decreased from T_fus ≈ 572 K to about 460 K, whereas those of odd terms remained substantially constant at about 450 K. Solid-to-solid transitions were also detected for oxamide, malonamide, succinamide, adipamide, suberamide, and dodecanediamide. Regular odd-even alternation was displayed by the temperature, enthalpy, and entropy of fusion values, terms with even number of carbon atoms showing higher values than odd terms. This behaviour was attributed to different crystal packing allowing consonance between hydrogen bonding and dispersive interaction in even terms, which are characterised by multilayer structure, whereas in odd terms a strained three-dimensional network results in looser packing. Parallel alternation of densities in solid alkane-α,ω-diamides supports this interpretation. Comparison was made with literature values for temperatures, enthalpies, and entropies of fusion of isoelectronic linear alkanes, dicarboxylic acids, and alkyldiamines.

Full text of DOI:10.1016/j.jct.2006.04.004

J. Chem. Thermodynamics 38 (2006) 1546–1552
www.elsevier.com/locate/jct
Odd–even effect in melting properties of 12 alkane-a,x-diamides
a,
b
Elena Badea a,1, Giuseppe Della Gatta *, Domenico D’Angelo ,
c
d
´
Bruno Brunetti , Zuzana Recˇkova
a
Department of Chemistry IFM, University of Torino, Via P. Giuria 9, 10125 Turin, Italy
Department of General and Applied Organic Chemistry, University of Torino, Via P. Giuria 7, 10125 Turin, Italy
b
c
Department of Chemistry, University ‘‘La Sapienza’’, P.le A. Moro 5, 00185 Rome, Italy
Department of Physical Chemistry, Institute of Chemical Technology, Technicka´ 5, 16628 Prague 6, Czech Republic
d
Received 15 November 2005; received in revised form 17 April 2006; accepted 19 April 2006
Available online 5 May 2006
Abstract
Fusion and solid-to-solid transitions of a homologous series of 12 linear alkane-a,x-diamides H2NCO–(CH2)(nÀ2)–CONH2, where
n = (2 to 12 and 14), were investigated by differential scanning calorimetry (d.s.c.). The temperatures of fusion of even terms decreased
from Tfus ꢀ 572 K to about 460 K, whereas those of odd terms remained substantially constant at about 450 K. Solid-to-solid transitions
were also detected for oxamide, malonamide, succinamide, adipamide, suberamide, and dodecanediamide. Regular odd–even alternation
was displayed by the temperature, enthalpy, and entropy of fusion values, terms with even number of carbon atoms showing higher val-
ues than odd terms.
This behaviour was attributed to different crystal packing allowing consonance between hydrogen bonding and dispersive interaction
in even terms, which are characterised by multilayer structure, whereas in odd terms a strained three-dimensional network results in
looser packing. Parallel alternation of densities in solid alkane-a,x-diamides supports this interpretation. Comparison was made with
literature values for temperatures, enthalpies, and entropies of fusion of isoelectronic linear alkanes, dicarboxylic acids, and
alkyldiamines.
Ó 2006 Published by Elsevier Ltd.
Keywords: Alkane-a,x-diamides; Differential scanning calorimetry; Fusion; Odd–even effect; Packing pattern
1. Introduction
have studied solid-to-liquid and solid-to-solid phase transi-
tions of alkane-a,x-diamides (hereinafter named alkyldia-
mides), characterised by both hydrogen bonding and
dispersive forces that belong to the ubiquitous interactions
of biological structures. The thermodynamics of phase
transitions of alkyldiamides has only been investigated
for propane-1,3-diamide (malonamide) so far [2]. We there-
fore used d.s.c. to measure the temperatures and enthalpies
of fusion and solid-to-solid transitions of the alkyldiamides
from C2 to C14, except C13, as a part of our research pro-
gramme on the thermodynamic properties of homologous
series of model compounds [3–8].
Interactions between molecules have attracted increas-
ing attention since the emergence and rapid development
of supramolecular chemistry and molecular recognition.
They are ruled by intermolecular forces whose energetic
and geometric properties are less well understood than
those of chemical bonds between atoms in individual mol-
ecules. Melting temperature is the typical intensive thermo-
dynamic parameter related to the breaking of these forces
in solid-state molecular assemblies [1]. In this paper, we
The values of the thermodynamic parameters of fusion
displayed the odd–even effect already described for linear
alkanes and many terminally substituted linear alkyl deriv-
atives. In particular, we found an odd–even fusion
*
Corresponding author. Tel.: +39 011 670 7571; fax: +39 011 670 7855.
E-mail address: giuseppe.dellagatta@unito.it (G.D. Gatta).
On leave from the Faculty of Chemistry, University of Craiova, Calea
1
Bucureßsti 165, Craiova 1100, Romania.
0021-9614/$ - see front matter Ó 2006 Published by Elsevier Ltd.
doi:10.1016/j.jct.2006.04.004

E. Badea et al. / J. Chem. Thermodynamics 38 (2006) 1546–1552
1547
behaviour for homologous series of alkyl-a,x-diamines [8],
alkyl-a,x-dinitriles and alkyl-a,x-diols [9]. The odd–even
effect also concerns other solid-state properties, such as sol-
ubility, density, molar volume, and enthalpy of sublimation
[10–16]. Recent studies on crystal structures and their pack-
ing patterns provided an explanation for the melting tem-
perature alternation in some cases [17–20]. Solid linear
alkanes are solely held together by dispersive interactions
and their melting temperature alternation has been recently
explained in terms of a simple geometrical model [19]. In
the case of alkane-a,x-diols (n = 2 to 10) [20] and (n = 6
to 16) [9], and alkane-a,x-diamines (n = 3 to 12) [8], char-
acterised by both hydrogen bonds and dispersive forces,
the difference in crystal packing is the main reason invoked
for such alternation. For the alkane-a,x-dicarboxylic acids,
it was shown that crystal-packing requirements for odd
terms impose restrictions on molecular conformations
which tend to lower the temperatures of fusion by respect
to even terms [16].
was made in 3 cm3 of anhydrous DMSO solution in the
presence of an excess of H2O2 30% (ꢁ0.5 cm3) and
K2CO3 (ꢁ0.14 g) as catalyst in accordance with Katritzky
et al. [31]. This method, conceived for the hydrolysis of
mononitriles [32], was suitably modified by us to obtain ter-
minal alkyldiamides from the corresponding terminal
dinitriles. All alkyldiamides were purified by successive
crystallisations from appropriate ethanol, ethylacetate or
their mixed solutions and pure bidistilled water [33]. The
n.m.r. analysis was performed for all synthesised com-
pounds and their spectra were compared with those in
the literature. The final mass-fraction purity of all diamides
was always better than 0.99, as determined by the d.s.c.
peak-profile method [34].
High-purity indium and tin from NBS-ICTA, naphtha-
lene (CRM No. M16-03, mass fraction 0.9999), and ben-
zoic acid (CRM No. M16-06, mass fraction 0.9999) from
NPL were used as reference materials.
The enthalpies of solid-to-solid transition and fusion,
and the corresponding onset temperatures of alkyldiamides
were measured with a Setaram DSC 111 differential scan-
ning calorimeter at the heating rate 16.7 mK Æ sÀ1, over
the temperature range 300 K to 600 K. Further DSC runs
at higher scanning rates were performed to check revers-
ibility of solid-to-solid transitions and possible decomposi-
tion during fusion. The calorimetric output was both
graphically recorded by a 8210 Sefram recorder and digi-
tally acquired through a home-made software whose data
were processed with Microcal Origin 6.1 software. Enthal-
pies associated with solid-to-solid transitions and fusion
were obtained from d.s.c. peak areas measured with a Sal-
2. Experimental
The homologous series of 12 alkane-a,x-diamides
H2NOC–(CH2)(nÀ2)–CONH2 examined in this work are
listed in table 1 with their number of C atoms, molar
masses, commercial origins and fusion temperatures, crys-
tal structures, relevant spatial groups, and densities. Some
were commercial products from Fluka and Aldrich, but
most were prepared by hydrolysis of parent alkane-a,x-
dinitriles, except tridecanediamide whose parent dinitrile
is commercially unavailable. For 1 g of dinitrile, reaction
TABLE 1
List of the linear alkane-a,x-diamides with number of carbon atoms n, molar mass M, commercial origin and temperature of fusion Tfus, crystal structure,
spatial group, and density
Compound
na
Mb/(g Æ molÀ1
)
Originc
Tfus/K
Crystal structure
Spatial group
Density/(g Æ cmÀ3
)
d
Oxamide
2
3
4
5
6
7
8
9
10
11
12
14
88.066
102.092
116.119
130.146
144.173
158.200
172.226
186.253
200.280
214.307
228.334
256.387
Fluka
Fluka
Fluka
>573 (dec.)
441–445
>535 (dec.)
Triclinicd
P1ꢀ
1.667d
1.430e
1.442f
1.280g
1.360h
Malonamide
Succinamide
Glutaramide
Adipamide
Pimelamide
Suberamide
Azelamide
Monoclinice
Monoclinicf
Monoclinicg
Monoclinich
Monoclinici
Monoclinicj
Monoclinick
Monoclinicl
P21/ce
C2/cf
C2/cg
P21/ch
C2/ci
C2/cj
C2/ck
P21/cl
Synthesis
Aldrich
Synthesis
Synthesis
Synthesis
Synthesis
Synthesis
Synthesis
Synthesis
499–502
1.230j
1.200k
Sebacamide
Undecanediamide
Dodecanediamide
Tetradecanediamide
a
Number of carbon atoms in the molecule.
b
c
d
e
f
Based on the 1999 IUPAC table of standard atomic weights of the elements and its 2001 revision [21].
Syntheses made in our laboratory from parent terminal dinitriles.
Reference [22].
Reference [23].
Reference [24].
Reference [25].
Reference [26].
Reference [27].
Reference [28].
Reference [29].
Reference [30].
g
h
i
j
k
l

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E. Badea et al. / J. Chem. Thermodynamics 38 (2006) 1546–1552
moiraghi 237 high-precision manual planimeter, the uncer-
tainty being evaluated as less than 0.3% for areas larger
than 1 Æ 103 mm2. The enthalpy values obtained by the
treatment of digitally acquired data were in very good
agreement.
The actual temperature T of the samples as well as
enthalpy changes associated with the phase transition was
obtained by using the empirical equations supplied by the
manufacturer as described earlier [7,8].
Several melting runs with certified reference materials
(naphthalene, benzoic acid, indium, and tin) were per-
formed under the same conditions as the experimental
determinations, and showed agreement with recently rec-
ommended IUPAC values [35–37] within 0.08% for tem-
perature and 0.25% for enthalpy (table 2).
ent endothermic effects requiring different calorimetric sen-
sitivities. However, all values reported are averaged from
the results of at least four distinct first runs (on fresh sam-
ples). In addition, 2nd and sometimes 3rd runs were per-
formed to check both reversibility of solid-to-solid
transitions and possible decomposition during fusion, and
gave thus rise to a total number of 8–10 runs. Crimped alu-
minium crucibles were generally employed, but sealed
stainless steel crucibles were needed for oxamide and succi-
namide that decompose during melting. Sample mass was
(1 to 10) Æ 10À3 g as determined with a Mettler electrobal-
ance AE163 whose sensitivity is 1 Æ 10À5 g.
3. Results and discussions
All measurements were performed in static air condi-
tions. Eight to ten runs with fresh samples were made for
each compound to determine solid-to-solid transitions
and fusion thermodynamic quantities. Separate runs for
solid-to-solid transition and fusion were adopted due to
the substantial departure between corresponding onset
temperatures (more than 40 K) and the relevant very differ-
3.1. Temperature of fusion
The experimental temperatures, enthalpies, and the
derived entropies of fusion and solid-to-solid transitions
for the 12 alkyldiamides are presented in table 3, together
with the literature values for malonamide, the only alkyl-
diamide investigated so far [2].
TABLE 2
Comparison between temperatures of fusion Tfus and molar enthalpies of fusion DfusHm measured with our d.s.c. apparatus and literature values for the
four selected reference materials
Compound
Origin
Na
Tfus/K
D )
fusHm/(kJ Æ molÀ1
This work
Literature
This work
Literature
Naphthalene
Benzoic acid
Indium
NPL
NPL
NBS-ICTA
NBS-ICTA
4
4
4
4
353.11 0.09
395.23 0.12
429.40 0.08
504.89 0.02
353.39 0.03b
395.55 0.03c
429.748 0.00034d
505.06 0.01d
19.072 0.045
18.017 0.075
3.266 0.05
7.130 0.01
19.05 0.04b
18.06 0.05c
3.273 0.022d
7.148 0.022d
Tin
Uncertainties are expressed as twice the standard deviation of the mean.
a
Number of calorimetric runs.
Reference [36].
Reference [37].
Reference [35].
b
c
d
TABLE 3
Temperatures, molar enthalpies and entropies of solid-to-solid transition (Ttrs,DtrsHm,DtrsSm) and fusion (Tfus,DfusHm,DfusSm) of alkane-a,x-diamides
Compound
Oxamide
Na
Ttrs/K
D
trsHm/(kJ Æ molÀ1
)
D
trsSm/
Tfus/K
D
fusHm/
DfusSm/
(J Æ KÀ1 Æ molÀ1
)
(kJ Æ molÀ1
)
(J Æ KÀ1 Æ molÀ1
)
10
356.15 0.90
455.45 1.24
494.25 0.87
418.89 1.67
393.0b
1.88 0.13
3.56 0.31
62.40 3.32
1.89 0.01
1.90b
5.28 0.37
7.82 0.68
12.63 6.72
4.51 0.02
4.83b
ꢁ572 (dec.)
ꢁ50
Malonamide
8
444.19 0.34
443.0b
29.85 0.43
35.8b
67.19 0.97
80.81b
Succinamide
Glutaramide
Adipamide
Pimelamide
Suberamide
Azelamide
Sebacamide
Undecandiamide
Dodecandiamide
Tetradecandiamide
8
8
10
8
10
8
10
8
485.92 .077
6.08 0.86
12.51 1.77
ꢁ536 (dec.)
ꢁ46
453.95 0.07
499.09 0.07
446.82 0.02
493.23 0.38
450.41 0.22
484.31 0.17
451.20 0.04
466.07 0.56
469.26 0.56
38.36 0.04
52.72 0.90
44.56 0.21
58.37 1.60
55.02 0.71
68.75 0.76
64.38 1.48
73.73 1.29
77.45 1.83
84.50 0.09
105.63 1.80
99.73 0.47
118.34 3.24
122.16 1.58
141.93 1.57
142.69 3.28
158.20 2.77
165.05 3.90
458.39 1.27
431.28 0.36
1.59 0.34
5.45 0.35
3.46 0.74
12.64 0.81
10
10
422.79 0.77
5.09 0.28
12.03 0.66
Uncertainties are expressed as twice the standard deviation of the mean.
a
Total number of calorimetric runs.
Reference [2].
b

E. Badea et al. / J. Chem. Thermodynamics 38 (2006) 1546–1552
1549
In figure 1, the fusion temperatures of the 12 alkyldia-
mides are shown as a function of the number of carbon
atoms n in their alkyl chains. Comparison is made with
the (approximately) isoelectronic linear alkanes H3C–
(CH2)(n+2)–CH3 [16]. The Tfus values of the isoelectronic
dicarboxylic acids HOOC–(CH2)(nÀ2)–COOH [38] are
included as they show a remarkable similarity with the
behaviour of alkyldiamides.
The Tfus values of the alkyldiamides are higher than
those of the corresponding nearly isoelectronic linear alk-
anes due to the presence of H-bond interactions whereas
only dispersion forces are present in alkanes. The Tfus val-
ues of dicarboxylic acids are lower than those of alkyldia-
mides due to the difference in shared H-bonds: four for
dicarboxylic acids [17] against eight for alkyldiamides
[22–30].
For both alkyldiamides and dicarboxylic acids, temper-
atures of fusion of even terms are higher than those of odd
terms. This odd–even effect is more pronounced than for
linear alkanes. Moreover, a common tendency to level
out on increasing n was clearly observed. The Tfus values
of even terms decreased on increasing n, whereas odd terms
displayed only very slight increase or even rather constant
values. This gave rise to a Tfus trend in contrast with that of
alkanes [19], alkyldiamines [8,20], dithiols [39] and diols
[9,20], where Tfus values of both even and odd terms clearly
increase on increasing n. This trend correlates well with the
density values of alkyldiamides (table 1), namely even
terms possess slightly higher Tfus and densities than the cor-
responding odd terms and both properties show a clear
descending tendency on increasing n. Nevertheless, the
crystal structure of even and odd alkyldiamides is similar,
as shown by X-ray studies of terms with n = (2 to 10)
[22–30]. All alkyldiamides from C3 to C10, in fact, display
a common monoclinic structure, except oxamide (triclinic
structure) and randomly belong to two spatial groups,
P21/c and C2/c. There is no parallel alternation of spatial
group with fusion properties as we found for alkyldiamines
[8]. Difference in crystal structure cannot thus be invoked
as the reason for alternating Tfus in alkyldiamides. How-
ever, parallel alternation of densities and decreasing trend
of temperatures of fusion suggest that the odd–even effect
may be related to different packing patterns in odd and
even terms, if any. Refinement of the crystal structures of
the even terms, oxamide, succinamide, adipamide, subera-
mide, and sebacamide, showed that molecules are tied into
layers by a net of hydrogen bonds. There are two sets of
non-equivalent hydrogen bonds, whose length, however,
is almost the same. In oxamide [22] and succinamide [24],
the adjacent molecules form an eight-numbered ring
around a centre of symmetry, whereas in adipamide [26],
suberamide [28] and sebacamide [30], a chain structure is
formed. Moreover, adjacent rings or chains aggregate into
layers through both hydrogen bonds and dispersive inter-
actions. The spacing between molecular layers, connected
by dispersive forces only, slightly increases from oxamide
(0.3007 nm) to suberamide (0.37 nm), a distance typical
for van der Waals forces. All these structural features can
reasonably explain the higher values of Tfus for C2 and
C4 as well as their decreasing trend for the even
alkyldiamides.
The X-ray studies of the odd terms, namely malona-
mide, glutaramide, pimelamide and azelamide, revealed a
three-dimensional network characterised by a specific posi-
tion of the mean plane of methylene chain and those of
amide groups and hydrogen bonds linking parallel mole-
cules [23,25,27,29]. The molecules are no longer coplanar
as found for even terms and spacing between mean planes
is ruled by twisting amide groups. This network results in
looser packing attributable to the torsional conformations
of the molecules, which also hinder cooperation between
hydrogen bonding and dispersive interaction. Strained
molecular conformations, in fact, can be assumed to be
responsible for the lowering of both temperatures of fusion
and crystal densities of odd as opposed to even terms.
X-ray diffraction analyses of dicarboxylic acids showed
that hydrogen bonding and dispersive interaction operate
in consonance in even terms, and therefore culminate in
dense packing, but run into geometrical conflicts and lead
to looser packing in odd terms [17]. These structural fea-
tures also appear to be responsible for the rather constant
Tfus values within odd series of both alkyldiamides and
dicarboxylic acids.
Besides, it is worth noting that Tfus of even diamides can
be modelled quite well by a hyperbolic function depending
on the number of methylene groups in the molecule, as in
the case of other homologous series, such as hydrocarbons,
cycloalkanes, functionalised alkanes, and symmetrically
substituted derivatives [40]. As expected, both even and
odd terms appear to converge in the limit to the melting
temperature of polyethylene [8,40]. Moreover, the hyper-
bolic behaviour exhibited by the melting temperature in
most series seems to be characteristic of molecules that
pack similarly in the solid state [40].
FIGURE 1. Plot of temperatures of fusion Tfus as a function of the
number of carbon atoms n in the alkyl chain: j, alkane-a,x-diamides
H2NCO–(CH2)(nÀ2)–CONH2; d, isoelectronic alkane-a,x-dioic acids
HOOC–(CH2)(nÀ2)–COOH [37]; m, isoelectronic linear alkanes H3C–
(CH2)(n+2)–CH3 [15]. Void symbols: estimated values.

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E. Badea et al. / J. Chem. Thermodynamics 38 (2006) 1546–1552
3.2. Fusion enthalpy and entropy (molar and massic)
Molar enthalpies of fusion are higher for alkyldiamides
than for alkanes: in both series higher values are associated
with even terms, in agreement with observed Tfus values.
The intensity of the zig-zag effect within alkyldiamides
tends to decrease, whereas it increases within alkanes.
The average increase of molar enthalpies of alkyldiamides
is due to the progressively increased dimension of the mol-
ecules and corresponding increase of dispersive interaction
per mol. The almost constant difference between DfusHm
values of nearly isoelectronic alkyldiamides and alkanes
is in connection with the presence of a constant number
of H-bonds in alkyldiamides. The comparison of DfusHm
of isoelectronic alkyldiamines [8] (values not represented
in figure 2) showed their unexpected superposition to the
corresponding values of alkyldiamides, despite their four
H-bonds per molecule as opposed to eight. In this regard,
it must be stressed that comparison between net values of
enthalpies of fusion of different homologous series of alkyl
compounds cannot be fully connected to their crystal struc-
tures (and relevant energies) since fusion is a transition
between two condensed phases. The variety of the interac-
tions in the liquid phase prevents the assignation of a spe-
cific physical meaning to the DfusHm values, as can be done
for a solid–gas transition, if sublimation is congruent. Very
recent data, in fact, have shown lower DfusHm values for
dicarboxylic acids [38] compared with alkyldiamines and
alkyldiamides, i.e. practically superposed to those of alk-
anes. This apparently anomalous behaviour has been inter-
preted as resulting from the cyclisation of dicarboxylic
acids in the liquid phase and formation of intramolecular
hydrogen bonded species. The superposition of DfusHm of
the homologous series of alkyldiamides and alkyldiamines
can be thus interpreted to be due to the presence of some
residual solid-state interactions in liquid alkyldiamides.
The DfusSm values of the alkyldiamides (figure 3), also
lower than expected, are equally ascribable to the presence
of residual H-bonding structures in the liquid phase.
Our results can be usefully presented as plots of the mas-
sic enthalpies and entropies of fusion as a function of n
(figures 4 and 5) as for alkyldiamines in a previous paper
[8]. The odd–even effect is still present for massic enthalpy
values and its trend is similar to that of Tfus, which is an
intensive quantity, too. This behaviour can be ascribed to
the decrease of the specific relative contribution of the H-
bonds to crystal stability. By contrast, the slight increase
of the massic enthalpies of fusion of alkanes can be attrib-
uted to the long-range cumulative effect of the CH2 disper-
sive interactions. In alkyldiamides, any possible effect of
this kind is clearly overwhelmed by the presence of a strong
hydrogen bonded network. The massic entropy trend
(figure 5) is in agreement with the interpretation given for
the massic enthalpies. Moreover, the superposition of mas-
sic entropy values to those of alkanes indicates that the loss
of order accompanying fusion in alkyldiamides is lower
than for alkyldiamines [8]. This provides further evidence
of the presence of residual H-bonding structures in their
liquid phase.
In figures 2 and 3, the molar enthalpies DfusHm and
entropies DfusSm of fusion of alkyldiamides are reported
as a function of n and compared with those of the isoelec-
tronic linear alkanes [11,16]. The common odd–even effect
is an evident aspect of these plots as expected. Even so, the
trend of both DfusHm and DfusSm values showed an average
linear increase, in contrast with Tfus values (figure 1). The
enthalpies of fusion of oxamide and succinamide are
roughly evaluated because of extensive decomposition dur-
ing melting, but are in agreement with the zig-zag pattern
of the series. Adipamide and tetradecanamide showed
slight decomposition only at the end of melting and
computation of their DfusH was not affected. Suberamide,
sebacamide, and undecanamide were totally free from
decomposition.
FIGURE 2. Plot of molar enthalpies of fusion DfusHm as a function of the
number of carbon atoms n in the alkyl chain: j, alkane-a,x-diamides
H2NCO–(CH2)(nÀ2)–CONH2; m, isoelectronic linear alkanes H3C–
(CH2)(n+2)–CH3 [15]. Void symbols: estimated values.
FIGURE 3. Plot of molar entropies of fusion DfusSm as a function of the
number of carbon atoms n in the alkyl chain: j, alkane-a,x-diamides
H2NCO–(CH2)(nÀ2)–CONH2; m, isoelectronic linear alkanes H3C–
(CH2)(n+2)–CH3 [15]. Void symbols: estimated values.

E. Badea et al. / J. Chem. Thermodynamics 38 (2006) 1546–1552
1551
FIGURE 6. Plot of molar enthalpies of fusion DfusHm against molar
entropies of fusion DfusSm to illustrate the interplay. For the alkyldia-
mides: h, n is even; j, n is odd. For linear alkanes: n, n is even; m, n is
odd.
FIGURE 4. Plot of massic enthalpies of fusion Dfush as a function of the
number of carbon atoms n in the alkyl chain: j, alkane-a,x-diamides
H2NCO–(CH2)(nÀ2)–CONH2; m, isoelectronic linear alkanes H3C–
(CH2)(n+2)–CH3. Void symbols: estimated values.
boxylic acids [38], which were drawn by us and gave slopes
of about 401 K and 355 K, respectively. The departure of
the odd alkyldiamide term slope values from those of the
even terms could be referred to the presence of residual
associations in the liquid phase as was found for dicarbox-
ylic acids [38].
3.4. Solid-to-solid transitions
Solid-to-solid transitions were observed for almost all
even alkyldiamides C2, C4, C6, C8, and C12 plus C3 (table 3).
Oxamide showed three transitions in the solid phase, but
only one, that at the lowest temperature, was irreversible.
For malonamide, C3, the only alkyldiamide of this series
so far investigated, an irreversible transition at higher tem-
perature than that reported in the literature was found, but
the associated enthalpy value was identical [2]. One irre-
versible transition was found for adipamide and subera-
mide, and one reversible transition for succinamide and
FIGURE 5. Plot of massic entropies of fusion Dfuss as a function of the
number of carbon atoms n in the alkyl chain: j, alkane-a,x-diamides
H2NCO–(CH2)(nÀ2)–CONH2; m, isoelectronic linear alkanes H3C–
(CH2)(n+2)–CH3. Void symbols: estimated values.
3.3. Enthalpy–entropy of fusion interplay
The interplay plot DfusHm = f(DfusSm) gave straight lines
for both even and odd alkyldiamides (figure 6). In our ear-
lier work on alkyldiamines, the slope of the molar
enthalpy–entropy interplay was assumed to correspond to
the temperature of fusion (DfusHm/DfusSm = Tfus) of the
solid CH2 group [8]. For linear even and odd alkanes in
close ranges of n, Tfus[CH2] = 380 K was compared with
the temperature of fusion of polyethylene (415 K) consid-
ered as the limiting value of the interplay slope for linear
alkanes with the longest chains [41–44]. Our present mea-
surements for alkyldiamides indicate that slope for even
terms (400 K) is close to that of alkanes, whereas the slope
(450 K) of the odd terms is higher. A similar behaviour can
be observed for the interplay plots of even and odd dicar-
FIGURE 7. Plot of temperatures of fusion Tfus, j, and temperatures of
solid-to-solid transitions, Ttrs, d, of even alkane-a,x-diamides as a
function of the number of carbon atoms n in the alkyl chain.

1552
E. Badea et al. / J. Chem. Thermodynamics 38 (2006) 1546–1552
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dodecanediamide. All these transitions occur well before
the temperature of fusion (40 K to 60 K), except for malo-
namide. As a consequence, the fusion enthalpy value of
malonamide reported in figure 2 is the sum of enthalpies
of both solid-to-solid and solid-to-liquid phase transitions.
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This research was partially funded by the Ministero
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´
ana Recˇkova collaboration was rendered possible by a
3-month Socrates-Erasmus postgraduate mobility grant.
The authors gratefully acknowledge the comments and
suggestions on crystal structure of alkyldiamides from
Dr. Raffaella Puliti, CNR, Naples, Prof. Carlo Mattia,
University of Salerno, and Prof. Piero Ugliengo, University
of Turin. Authors are also grateful to Anna Boguszewska
from University of Lublin, Poland, for her participation
in chemical syntheses of alkyldiamides at the Department
of General and Applied Organic Chemistry, University of
Turin, within the scope of a 6-months Socrates-Erasmus
undergraduate mobility grant.
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JCT 05-287