America n Journal of Analy tic al Chemistry, 2011, 2, 324-331
doi:10.4236/ajac.2011.23040 Published Online July 2011 (http://www.scirp.org/journal/ajac)
Copyright © 2011 SciRes. AJAC
A Study of the Behavior of Alkyl Side Chains Phenols and
Arenes in Polar and Nonpolar GC Stationary Phases
Pavel Straka1*, Petr Bury an 2
1Institute of Rock Structure and Mechanics, Academy of Sciences of the Czech Republic, Prague, Czech Republic
2Institute of Chemical Technology, Prague, Czech Republic
E-mail: straka@irsm.cas.cz
Received July 17, 2010; revised January 10, 2010; accepted J anuar y 20, 2011
Abstract
Gas chromatographic measurements of the relative retention times of alkyl-substituted arenes and phenols on
capillary columns at temperatures of 125˚C - 140˚C have shown that logarithms of retention times increase
bi-linearly with the number of carbon atoms in the molecule. It was found that in a high density stationary
phase, the longer alkyl side chains of compounds in question are subject to cyclization as a result of the re-
sistance force of this phase affecting molecules during their thermal and diffusion motion. Consequently,
common conventional aromatic-aliphatic molecules become new molecules with quasi-alicyclic rings. In
comparison with the conventionally conceived molecules, the resulting aromatic-quasi-alicyclic molecules
are characterized by rather different, possibly even completely different non-covalent interactions between
the molecules, which then affect the retention characteristics. Moreover, cyclization facilitates the mixing of
n-alkyl arenes or n-alkyl phenols with high-molecular stationary phases, because the thermodynamic condi-
tion for mixing is better fulfilled.
Keywords: Alkyl Phenols, Alkyl Benzenes, Reten ti on Times, Mol ecu lar Mec hanics, Van der Waa ls Forces
1. Introduction
The following considerations presented were based on
gas c hromato gr aphic measurements on capillary columns
(50 m, an internal diameter of 0.25 mm) with stationary
phases Apiezon K (nonpolar) and tri (2,4-xylenyl) phos-
phate-phosphoric acid (95:5) (polar) [1-6]. The mea-
surements of the relative retention times (
, t rel
R
) of
n-alkyl phenols and n-alkyl benzenes C7-C12 on these
capillary columns with both polar and nonpolar station-
ary phases at temperatures of 125°C - 140°C showed that
the logarithms of these retention times increase
bi-linearly with the number of carbon atoms in a mole-
cule (
z
) (Figure 1) [1]. Two linear areas in the consecu-
tive intervals C7-C9 and C9-C12, with the slopes of each
line being different, were proven in the re lation
,
log
t rel
Raz b= +
(where
a
and
b
are constants), al-
though only one line had been anticipated to be found.
The complete dependence was thus of a bi-linear charac-
ter. This dependence was also found under the mentioned
conditions in the case of n-alkyl phenol methylethers [1].
In classic considerations on the dependence of reten-
tion t imes on the number o f carbo ns in differe nt ho- mo-
logous series of gas chromatographically separated sub-
stances, purely linear dependence without any diver-
gence is always considered, be it for isotherm or non-
isotherm separatio ns [2-6].
Howe ve r, the divergence detected in the dependence
,
log
t rel
Raz b= +
suggests that the behavior of mole-
cules o f the c ompound s in q uestio n duri ng sepa rations in
stationary phases is more complicated than has been as-
sumed by the classic considerations. The relation
,
log
t rel
Raz b= +
should be expected to be purely linear
as a r esult o f the gr owing number of ca rbon a toms i n the
molecule [2-6] and the corresponding increase of
, t rel
R
or more precisely
,
log
t rel
R
with boiling points (b
T
) of
the given homologs (
,
log
t relb
Ra bT= +
). The men-
tioned purely linear relation has frequently been applied
in the identification of unknown substances using data
acquired through gas chromatographic separations, but
the valid i ty of t his u s age is questionable.
In the study [1], the relative retention times were ex-
pressed as a logarithm of the ratio of the retention ti me of
the given n-alkyl phenol substituted in the ortho, meta
and para positions and pheno l, and in the case of n-alkyl
benzenes as a logarithm of the ratio of the retention time
P. STRAK A ET AL.
Copyright © 2011 SciRes. AJAC
325
of n-alkyl benzene and benzene. The mentioned bi-line-
arity in the increase of mentioned retention characteris-
tics was always observed at C9.
In a preliminary work [7], the following possibilities
were considered for explanation of this phenomenon.
Firstly, cyclization of alkyl side chains was taken into
account. In a high density stationary phase, the longer
alkyl side chains of n-alkyl phenols and n-alkyl benzenes
are subject to cyclization as a result of the resistance
force of this phase affecting the molecules of the com-
pounds during their thermal and diffusion motion. Con-
sequently, common conventional aromatic-aliphatic mo-
lecules become new molecules with quasi-alicyclic pa rts.
In comparison with the conventionally conceived mole-
cules, the resulting aromatic-quasi-alicyclic molecules
are likely to be characterized by rat her d i fferent, possibly
even completely different non-covalent interactions be-
tween the molecules, chiefly van der Waals interactions,
which then affect the retention characteristics.
Further, association of the molecules was considered.
The molecules of the compounds in question are of an
aromatic-aliphatic character. Such molecules tend, in a
dense environment (e.g. in organic gels), to form mo-
lecular aggregates, as was discovered during the research
Figure 1. The dependence of the relative retention time
(
) (lines 1 and 2) and bi-linear dependence of log
(lines 3 and 4) on the number of carbons in the molecule
(
z
) for the s tation ary phase of Apiez on K at a temperature
of 130˚C in the case of n-alkyl phenols. (Column oven tem-
perature 1 30˚C, liquid injector 300˚C, carrier gas: nitrogen,
FID.) Full points-ortho n-alkyl phenols; empty points-para
n-alkyl ph en ol s. S i mil ar de pe nd enc e s wer e f ound al so i n th e
case of n-alkyl benzenes and n-alkyl phenol methylethers.
Based on [ 1], adjus ted.
on the formation of coal structures [8]. A similar phe-
nomenon may occur in the environment of a dense sta-
tionary phase. The molecules in question may associate,
whic h woul d l ea d to a c hange in retention characteristics.
The association could also increase with the growing
length of the side alkyl chain and might strongly mani-
fest itself in the case of molecules containing more than
nine carbons. Therefore, a close association was investi-
gated both in the case of n-alkyl phenols and n-alkyl
benz enes for the system of two mole cule s . T hi s was done
by ca lcula tin g the ene rgie s o f the van der Waals interact-
tions (
vdWaals
E
) usin g the molecular mechanics modeling.
Two molecules were conformed until the state of the
energy minima was reached and the calculated energies
were compared with the energies of the van der Waals
interactions in two non-associated (isolated) molecules.
It was fo und that intensive nonbonding interactions occur
in the association of the two molecules, however, no de-
pendence of the
vdWaals
E
values on the length of the n-
alkyl chain was registered. Thus, th is way of influencing
of the rete ntion charac te ristics is u nlikely.
Finally, non-covalent interactions with a stationary
phase were taken into account. With the increasing num-
ber of carbons in the molecule of n-alk yl phenols and n-
alkyl benzenes, the intermolecular interactions between
the compounds in question and the stationary phase might
change non-linearly, but these interactions must manifest
themselves differently in polar and nonpolar phases.
Nevertheless, this was not observed. The character of the
dependences wa s the same, bi-linear, measured in the
case of alkyl benzenes for both the polar and nonpolar
phases. The same was found with alkyl phenols. So, this
Figure 2. Concepts of the cyclized forms in the case of ortho
n-propyl phenol (a) and n-propyl benzene (b). The cycliza-
tion is realized on the terminal methyl group (-CH3 cycli z a-
tion). Concepts (c) and (d) are depicted as the models for
energies calculations.
P. STRAK A ET AL.
Copyright © 2011 SciRes. AJAC
326
interpretation of the observed retention phenomena wa s
abandoned. Therefore, it wa s the cyclization alkyl chains
of the considered compounds that was preferred. The
preference for the cyclizatio n ari ses also from the fact that
alicyclic-aromatic ethers with five- and six-membered
alicyclic rings with oxygen and an interconnected aro-
matic ring (Figure 2), which are thermodynamically sta-
ble compounds, are formed.
The mentioned phenomena can be evaluated and com-
pared using method s of comput at ional che mistr y. In order
to assess cyclization, association and different interac-
tions, the energies of the co valent bond s and non-covalent
interactions, mainly van der Waals, can be computed. For
the purpose of this assessment, molecular conformations
which are realistic and energetically advantageous or pos-
sible and are in correlation with the ascertained chroma-
tographic data can also be estimated. On the basis of ex-
perience with the evaluation of aromatic, aromatic-ali-
phatic, aromatic-alicyclic and phenolic structures in terms
of energy using the methods of molecular mechanics [7-
9], two methods of molecular mechanics were selected
for the calculations (see below). Recently, the molecular
mechanics method was successfully used to study cycli-
zation reactions [10].
The aim of the presented work is to explain the bi- li-
nearity in the logarithmic dependence of the relative re-
tention time on the number of carbons in the molecule in
the case of n-alkyl phenols and n-alkyl benzenes by mo-
lecular mechanics modeling and to clarify the beha vior of
these compo und s in the environment o f a stationar y phase
of a ca pilla ry co lumn. T he potential energies of the bonds
and interactions have been considered for the cal- cula-
tions.
2. Calculations
In our considerations, the retention data obtained on a
nonpolar stationary phase of Apiezon K and a polar sta-
tionar y phase of trixylenylpho spha te-phosphoric acid (95:
5) at a temperature of 130˚C according to [1] were taken
into account .
For the calculations, the molecular mechanics methods
were selected [11-13] and two force fields, MM+ and
AMBER, were used. The reason is that these methods use
analytical and relatively simple potential energy functions
for describing the interactions between a set of atoms;
further, they are empirical and accurate and very suitable
for small organic molecules. What is important is that the
atom types, not atoms, are the fundamental basis for cal-
culat ing the interactions. In these methods, the interaction
potential describes both bonding and non-bonding inter-
actions. In the potential, the following energetic terms
were calculated:
- bond stretching (
bond
E
), which is associated
with the deformation of a bond from its standard equilib-
rium length;
- bond angle bendi ng (
angle
E
), which is associated
with the deformation of an angle from its normal value;
- stretch-bend (
stretch bend
E
)—the bond-stretching
and angle-bending cross term, which includes coupling
between the bond stretching and angle bending;
- dihedral s (
dihedral
E
)—the torsional energy, which
is associated with the tendency of dihedral angles to have
a cer tain n-fold symmetry and to have minimum ener gy;
- van der Waals (
vdWaals
E
), which describes the
repulsive forces keeping two non-bonded atoms apart at
close range and attrac tive forces drawing them toge ther at
long range;
- electrostatic (elst
E), which describes the non-
bonded electrostatic interactions, particularly dipole-di-
pole interactions.
These energetic terms were calculated both by the
MM+ and AMBER methods, with the exception of
stretch bend
E
, which was calculated only by means of the
MM+ method.
The mentioned potential energies of the covalent bonds
and non-covalent interactions were calculated for com-
mon (co nventional) n-alkyl phenols and n-alkyl benzenes
and also for the models of the cyclized forms of these
compounds. The concepts of the cyclized forms were
formulated on the basis of the study of the distribution of
electron densities (atomic charges in a molecule) in
common and cyclized molecules. As expected, in n-alkyl
phenol molecules, a high electron density was found on
the oxygen of the hydroxyl gro up and a very low elec tron
density on hydrogens of the terminal methyl group. The
closing of the alicyclic ring was thus easily implemented
by the hydrogen bridge as shown in Figure 2(a). The
concept of the cyclized form of alkyl benzenes was more
complicated. The electron density on the methyl carbon
was disc o vered to b e c o nsiderab l y higher tha n o n the ot h-
er c arbons and, e specially, o n hydrogen atoms of the me-
thyl group in an alkyl chain. The closing of the ali- cyclic
ring was thus implemented by an interaction of methyl
hydrogen with the ascribed charge
δ
+
and the benzene
ring with the ascribed charge
δ
(Figure 2(b)). The
basic concepts of cyclized forms and models of these
forms, for the sake of calculations demonstrated on n-
propyl phenol and n-propyl benzene, are shown in Fig-
ures 2(c) and 2(d), namely for the case of alicyclic ortho
n-propyl phenol and alicyclic n-propyl benzene. In the
latter case, n-propylbenzene is cyclized to the ortho posi-
tion with respect to the propyl. The cyclization here is
implemented on the terminal methyl group (-CH3 cycliza-
tion).
Besides the -CH3 case, cyclization was also considered
P. STRAK A ET AL.
Copyright © 2011 SciRes. AJAC
327
Table 1. Reduction of the maximum molecular size (
) during cyclization of the side alkyl chain. Comparison of
values for common and cyclized for ms of n-al kyl phe nol s.
Molecule
max
D
(nm)
Posit ion of subs tituent:
ortho
meta
para
Common:
Methyl phenol
0.56
0.59
0.67
Ethyl phenol
0.72
0.72
0.80
n-Propyl phenol
0.82
0.83
0.91
n-Butyl phenol
0.93
0.95
1.04
n-Pentyl phenol
1.06
1.08
1.15
n-Hexyl phenol
1.19
1.21
1.28
Cyclized on -CH3:
Methyl phenol
0.56
0.49
0.47
Ethyl phenol 0.65 0.55 0.48
Propyl phenol
0.72
0.61
0.57
Butyl phenol
0.72
0.66
0.54
Pentyl phenol
0.80
0.70
0.61
Hexyl phenol
0.78
0.71
0.73
and calcul ated for the -CH2- gro up neighboring CH3- (i.e.
in the
α
position with respect to CH3-, -CH2-
α
cycli-
zation) and for the -CH2- group in the
β
position with
respect to the CH3- group (-CH2-
β
cyclization). These
cyclizations were considered for molecules from methyl-
up to hexyl phenol and from methyl- up to hexyl benzene.
With n-alkyl phenol s and n-alkyl benzenes, these cycliza-
tions were considered and calculated for the ortho, meta
and para positions. The calculated energies of covalent
bonds and non-covalent interactions for common and
model cyclic forms were compared, but no significant
changes in energetic terms were found. Moreover, in the
case of the -CH2-
α
and -CH2-
β
cyclizations, the cal-
culated vdWaals
E values were similar to common n-alkyl
phenols and n-alkyl benzenes and no relevant depend-
ences on
z
were found.
The HyperChem program also makes it possible to
measure the distance and the maximum distance between
atoms of defined molecule (maximum size,
max
D
). This
possibility was utilized for the measurement and com-
parison of the maximum sizes of the common and cy-
clized molecules of the examined compounds. The
Atomic Charges part of the program was used for the
study of the above-mentioned distribution of the electron
dens itie s in t h e mole c u l es of the o bs erved compou nds.
3. Results and Discussion
The side-chain cyclization was based on the concept that
in a high-density stationary phase, the longer side alkyl
chains of n-alkyl phenols and n-alkyl benzenes are subject
to deformation as a result of the resistance of this phase
affecting the molecules of these compounds in motion.
Convent ional a romatic-al ipha tic mo lecules ar e thus tra ns-
formed into aromatic-quasi-alicyclic molecules. Cycliza-
tion is then accompanied by a decrease in the effective
size of molecules, which is significant for C9 and larger
molecules. Aromatic-quasi-alicyclic molecules of a s-
maller size are more easily mixed with the dense station-
ary pha se, and the formed system is, in compari son with a
system with common aromatic-aliphatic molecules, more
homogenous and thus thermodynamically more stable.
The reduction of the maximum size (
max
D
) of molecules
duri ng the cyclizatio n of the side chai n is shown in Table
1. The change in size of the molecules moving through a
chromato graphic column is the n accompanied by c hange s
Figure 3. The dependence of the van der Waals energies
(and conse quently forc es) in effect on the number of carbons
in the cyclized molecules of para n-alkyl phenols (the
AMBER method, -CH3 cyclization).
P. STRAK A ET AL.
Copyright © 2011 SciRes. AJAC
328
Table 2 . Pote ntial en ergi es of cov alent b ond s and non-co vale nt i nteracti ons i n meta n-alkyl phenols (common and c ycl ize d on
CH3-). Calc ulated by the MM+ program. For symbols see text.
n-alkyl phenols
bond
E
angle
E
stretch bend
E
(kJ/ mol)
dihedral
E
vdWaals
E
elst
E
max
D
(nm)
Phenol
4.57
9.43
0.44
23.32
12.11
0
0.56
Common:
Methyl phenol
Ethyl phenol
n-Propyl phenol
n-Butyl phenol
n-Pentyl phenol
n-Hexyl phenol
6.90
8.80
10.70
12.60
14.50
16.40
9.68
9.68
9.68
9.68
9.68
9.69
0.59
0.58
0.58
0.57
0.57
0.57
26.33
26.92
26.92
26.92
26.92
26.92
14.57
39.44
42.49
45.48
48.66
51.84
0.11
0.11
0.11
0.11
0.11
0.11
0.59
0.72
0.83
0.95
1.08
1.21
Cyclized on CH
3
:
Methyl phenol
Ethyl phenol
Propyl phenol
Butyl phenol
Pentyl phenol
Hexyl phenol
10.26
13.81
29.75
31.13
19.09
16.64
38.69
36.63
64.02
59.18
29.00
4.66
0.96
3.06
3.70
1.73
0.58
0.04
389.88
369.97
243.10
1.34
13.30
10.80
32.45
95.81
385.13
3173.3
2941.5
8006.8
0.00
0.49
0.16
0.05
0.04
0.05
0.49
0.55
0.61
0.66
0.69
0.71
Table 3. Potential e nergies o f covalent bonds and non-covalent interactions in n-alkyl ben zenes cy clized into t he position met a
with respect to the substituent (co m mon a nd cyclized on CH3-). Calculated by the MM+ pr o gra m. For symbol s see text.
n-alkyl benzenes
bond
E
angle
E
stretch bend
E
(kJ/ mol)
dihedral
E
vdWaals
E
elst
E
max
D
(nm)
Ben zene
4.57
9.43
0.44
23.32
12.11
0
-
Common:
Methyl benzene
Ethyl benzene
n-Propyl benzene
n-Butyl benzene
n-Pentyl ben zene
n-Hexyl benzene
7.06
8.96
10.86
12.76
14.66
16.56
0.25
0.25
0.25
0.25
0.25
0.25
0.14
0.13
0.13
0.13
0.13
0.13
26.33
26.92
26.92
26.92
26.92
26.92
14.85
39.73
42.78
45.77
48.96
52.14
0
0
0
0
0
0
0.59
0.72
0.82
0.95
1.06
1.19
Cyclized on CH
3
:
Met hyl benzene
Ethyl benzene
Propyl benzene
Butyl benzene
Pentyl benzene
Hexyl benzene
7.83
10.27
12.86
27.34
30.44
17.22
123.13
29.47
21.61
50.05
49.67
5.30
3.58
2.27
1.06
2.48
0.98
0.12
194.74
292.85
305.63
185.38
2.50
0.57
41.50
34.23
54.08
257.72
3102.9
-
0
0.56
0.24
0.16
0.13
0.13
0.47
0.51
0.57
0.62
0.66
0.69
in the intermolecular interactions and subsequently by a
change in the relative retention time.
The calculatio n results for the energies of the covalent
bonds and non-covalent interactions are demonstrated for
meta n-alkyl phenols (common and cyclized on CH3-)
and n-al kyl b enzene s both c ommo n and c yclized i nto the
meta position with respect to the substituent (again cy-
cliz atio n thr oug h CH3-), are summarized in Table s 2 and
3. From Tabl es 2 and 3 it is evident that cyclization doe s
not cause significant changes in covalent bonds, only
small changes in electrostatic interactions and some ex-
pected changes in the
angle
E
term was registred. How-
ever , subst ant ial c ha nges too k p lace in the va n de r W aals
interactions between the non-bonded atoms inside the
molecules. Typical dependence of the van der Waals
forces on the number of carbons (
z
), shown in the ex-
ample of para n-alkyl phenols, is pictured in Figure 3.
This is of the similar character as the detected depend-
ence of the logarithm of the relative retention time on
z
(Figure 1, lines 3 and 4). This finding is in agreement
with the fact that the same dependences were detected
both in the cases of the polar and nonpolar phases and
also in the cases of alkyl phenols and alkyl benzenes,
because the retention data and their changes are in the
given case related to intermolecular forces rather than to
the structure of the compounds being considered. The
similar results have been obtained in the case of ortho
phenols cyclized on the -CH3 group and also in the case
of the n-alkyl benzenes cyclized on the -CH3 group into
ortho and para positions with respect to the substituent.
Therefore, attention was focused on systematic calcula-
tions of the energies of the van der Waals interactions.
The numerical values of these energies showed the
strength of the van der Waals forces; the results are
summarized in Tables 4 and 5. From the data in these
tables, it i s clear that the bi-linearity in these interactions
occurs at
9z=
(to be more specific when the phenol or
benzene are substituted by n-propyl) during cyclization.
However, this result was ascertained only for n-alkyl
phenols and n-alkyl benzenes cyclized on the terminal
methyl group. In the case of the -CH2-
α
and -CH2-
β
P. STRAK A ET AL.
Copyright © 2011 SciRes. AJAC
329
Table 4. Energies of van der Waals interactions (
vdWaals
E
, kJ /mol) in the molecules of n-alkyl phenols and n-alkyl benzenes.
Calculated by the MM+ program. Positions ortho”, meta”, “para” at n-alkyl benzenes mean cyclization into the positions
with respe ct to the substituent.
n-alkyl phenols
ortho
meta
para
n-alkyl benzenes
ortho
meta
para
Common:
Methyl phenol
Ethyl phenol
n-Propyl phenol
n-Butyl phenol
n-Pentyl phenol
n-Hexyl phenol
17.12
41.25
44.24
47.23
50.40
53.58
14.57
39.44
42.49
45.48
48.66
51.84
14.60
39.47
42.51
45.51
48.69
51.87
Common:
Met hyl benzene
Ethyl benzene
n-Propyl benzene
n-Butyl benzene
n-Pentyl ben zene
n-Hexyl benzene
14.85
39.73
42.78
45.77
48.96
52.14
14.85
39.73
42.78
45.77
48.96
52.14
14.85
39.73
42.78
45.77
48.96
52.14
Cyclized on CH
3
:
Methyl phenol
Ethyl phenol
Propyl phenol
Butyl phenol
Pentyl phenol
Hexyl phenol
11.39
12.09
27.61
49.25
210.20
-
32.45
95.81
385.1
3173
2942
8007
44.96
109.4
411.3
1037
1132
-
Cyclized on CH
3
:
Met hyl benzene
Ethyl benzene
Propyl benzene
Butyl benzene
Pentyl benzene
Hexyl benzene
10.40
11.23
14.25
27.64
64.51
476.8
41.50
34.23
54.08
257.7
3103
-
23.99
46.87
67.42
339.3
832.2
905.6
Table 5. Energies of van der Waals interactions (
vdWaals
E
, kJ /mol) in the molecules of n-alkyl phenols and n-alkyl benzenes.
Calculat ed by the A MBER pr o gra m. Positio ns ortho”, meta”, para” at n-alkyl benzenes mean cyclization into the positions
with respect to the substituent.
n-alkyl phenols
ortho
meta
para
n-alk yl benzenes
„ortho“
„meta“
„para“
Common:
Methyl phenol
Ethyl phenol
n-Propyl phenol
n-Butyl phenol
n-Pentyl phenol
n-Hexyl phenol
17.54
36.17
36.54
37.13
37.90
38.71
13.53
35.04
35.48
36.07
36.86
37.67
13.58
35.09
35.53
36.13
36.91
37.72
Common:
Met hyl benzene
Ethyl benzene
n-Propyl benzene
n-Butyl benzene
n-Pentyl ben zene
n-Hexyl benzene
13.90
35.43
35.87
36.48
37.26
38.07
13.90
35.43
35.87
36.48
37.26
38.07
13.90
35.43
35.87
36.48
37.26
38.07
Cyclized on CH
3
:
Methyl phenol
Ethyl phenol
Propyl phenol
Butyl phenol
Pentyl phenol
Hexyl phenol
14.93
11.05
19.08
38.38
703.1
-
43.83
141.6
2976
4*106
4*106
4*10
10
65.28
175.1
6247
9*104
-
-
Cyclized on CH
3
:
Met hyl benzene
Ethyl benzene
Propyl benzene
Butyl benzene
Pentyl benzene
Hexyl benzene
12.92
14.35
11.52
16.90
61.29
-
55.41
40.43
61.62
1083
6*106
-
24.34
53.46
74.65
4183
35913
-
cyclizations, the detected
vdWaals
E
values were si milar to
common n-alkyl phenols and n-alkyl benzenes and no
relevant dependences on
z
were found, as stated
above.
An increase in the van der Waals interactions inside
the cyclized molecules (i.e. intramolecularly) must also
be reflected in an increase of the forces between mole-
cules (i.e. intermolecular ly). A more intense e ffect of the
attraction forces between molecules should then manifest
itself through a corresponding increase of the boiling
temperatures of the C9-compounds and be higher in both
types of molecules in question. The increase of the in-
termolecular forces caused by cyclization was confirmed
by a comparison of the boiling points of normal (–42.1˚C
- 125.5˚C) and alic yclic (33.5˚C - 150˚C) hydrocarbons
C3-C8. The results are shown in Table 6. The values of
the boiling p oints were taken fro m the co mpendium [14]
and verified against the compendium [15]. From the
values shown in Table 6, it is evident that the boiling
points of alicyclic hydrocarbons increase with the num-
ber of carbons
z
in the molecule similarly to van der
Waals forces.
If a change in the relative retention time as a result of
cyclization is caused by a change in the size of the mo-
lecule moving in a chromatographic column and a
chan ge i n bo th intr a mole c ula r a nd inte r mol ecula r van de r
Waals forces, then the related cyclization of the alkyl
chain could really be, in the case of n-alkyl phenols, ac-
companied by the formation of a hydrogen bridge be-
tween the oxygen in the phenolic group -OH and a hy-
drogen of the terminal group -CH3 of the alkyl; in other
words, in the case of n-alkyl phenols, the tendency to
form hydrogen bridges resulting in a heterocyclic ring
with six or more members being formed including also
one C-C bond of the aromatic ring of phenol as shown in
Figure 2 could actually be considered. Similarly, in the
case of alkyl benzenes, a tendency to integrate could
exist between the benzene ring (
δ
) and a hydrogen of
the terminal group -CH3 of the alkyl (
δ
+
), resulting in
the formation of a five-membered or more-membered
ring, i ncl ud i ng al so o ne C -C b o nd o f t he a r o ma tic r i ng of
benzene (Figure 2). F rom the thermodynamical aspect, it
P. STRAK A ET AL.
Copyright © 2011 SciRes. AJAC
330
Table 6. Boiling points (
b
T
) of normal and alicyclic hydro-
carbons C3–C8 and energies of intramolecular van der
Waals interactions.
Alkane
b
T
(˚C)
vdWaals
E
(MM+)
(kJ/mol)
vdWaals
E
(AMBER)
(kJ/mol)
Propane
42.1
8.47
1.84
n-Butane
0.5
11.87
2.80
n-Pentane
36.15
15.13
3.67
n-Hexane
68
18.36
4.51
n-Heptane
98.34
21.58
5.36
n-Octane
125.5
24.79
6.19
Cyclopropane
33.5
0.13
0.07
Cyclobutane
13
11.27
1.23
Cyclopentane
49.26
14.77
2.40
Cyclohexane
81
22.12
5.34
Cycloheptane
118.48
204.8
721.5
Cyclooctane 150 501.1 7840
is the affinity of these low-molecular compounds to the
stationary high-molecular phase that is important when
they are being mixed with this phase. The degree of af-
finity is the change in the Gibbs free energy of mixing
when the molecules of alkyl phenol or alkyl benzene are
blended with high-molecular chains or molecules of the
stationary phase (
mix
G
) at constant temperature and
pressure:
0
mix mixmix
GH TS∆=∆−∆ < (1)
where mix
H is the enthalpy of mixing, mix
S the
entro p y of mi xing a nd
T
the temperature of the separa-
tion ( K). T he more negati ve the mix
G is, the better the
mixing of the compounds will be (for the stationary
phase). Since the entropy of the system always increases
when the components are mixed, and the entropic term
of the Equation (1) is thus always negative
(mix
TS−∆ <
0), the mixing/solubility depends mainly on
the value of mix
H. The thermodynamic condition of
the solubility of a low-molecular element in the statio-
nary phase , or the mixing of the two subst ances is then:
mix mix
H TS∆ <∆
(2)
The blending will thus be the best in the case mix
H
0=
, when also the solubility of one component in the
other will be maximal as well. However, this is an ideal
case. The parameters of the solubility
δ
were intro-
duced for practical purposes, numerically characterizing
the solubility of low-molecular and high-molecular sub-
stances [16]. A lo w-molecular substance with a so lub ility
parameter identical to the solubility parameter of the
high-molecular substance will achieve maximal dissolu-
tion during the mixing, because
0
mix
H∆=
in this case.
Since cyclization increases
δ
in the case of hydrocarbons,
e.g. in the ca se o f hexan/cyclohexan fr om 14.9 (hexan) to
16. 8 (cyklo hexan) [ 17], the mixing is improved, because
this parameter for the high-molecular stationary phases
[18] can be approximately 16 - 18. Cyclization th us faci-
litates the mixing of al kyl phe nols or alkyl b enze nes wit h
high-molecular stationary phases, because the thermo-
dynamic condit ion for mixing is better fulfilled.
On the whole, the data obtained can both serve an
analytical methodology for the analysis of aromatics and
phenolics, which is still being discussed [19], and pro-
vide deeper insight into the problem of the van der Waals
forces/non-covalent interactions, which is also a topic of
interest [20] .
4. Conclusions
The side alkyl chains of n-alkyl phenol s beco me cyclized
in both the polar and nonpolar stationary phases of ca-
pillary columns, with a possible formation of hydrogen
bridges between oxygen of the phenolic OH group and
hydrogen of the methyl group of the side alkyl chain. In
the case of n-alkyl benzenes, cyclization is made possible
because of the interaction between benzene ring (
δ
)
and hydrogen of the terminal methyl group (
δ
+
) of the
alkyl chain. In the case of the formed aromatic-quasi-
alicyclic molecules, the effect of the van der Waals
forces thus increases not only intramolecularly but also
intermolecularly. In the case of n-alkyl phenols and n-
alkyl benzenes this effect is strong with the molecules
having the number of carbons higher than nine. This re-
sults in bi-linearity in the retention characteristics of
these compounds, observed in the dependence of the
logarithm of the relati ve retention time on the total num-
ber of carbons in the molecule.
5. Acknowledgements
This work was supported by the Grant Agency of the
Academy of Sciences of the Czech Republic in the fra me
of project No. IAA300460702 a nd the Institute Research
Plan, Identification Code AVOZ30460519.
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