Synthesis, antioxidant properties, and reaction kinetics of aliphatic diamine bridged hindered phenols

Article abstract of DOI:10.1134/S1070363216120422

A series of aliphatic diamine bridged hindered phenols was synthesized. Their antioxidant activity was evaluated for assessing the role of bridging groups in trapping 1,1-diphenyl-2-picrylhydrazyl radical (DPPH?) and in 2,2'-azodi(isobutyronitrile) (AIBN) induced oxidation of styrene. The study of reaction kinetics of scavenging of the peroxyl radicals demonstrated that the scavenging ability of the DPPH free radical decreased when length of the bridging groups increased. However, the ability to protect styrene from AIBN-induced oxidation increased with increased length of the bridging groups.

Full text of DOI:10.1134/S1070363216120422

ISSN 1070-3632, Russian Journal of General Chemistry, 2016, Vol. 86, No. 12, pp. 2797–2803. © Pleiades Publishing, Ltd., 2016.
Synthesis, Antioxidant Properties, and Reaction Kinetics
of Aliphatic Diamine Bridged Hindered Phenols1
C. Q. Li*, S. Y. Guo, J. Wang, and W. G. Shi
Provincial Key Laboratory of Oil and Gas Chemical Technology, College of Chemistry and Chemical Engineering,
Northeast Petroleum University, University str. 99, Daqing, Heilongjiang, 163318 China
*e-mail: dqpilicuiqin@126.com
Received June 4, 2016
Abstract—A series of aliphatic diamine bridged hindered phenols was synthesized. Their antioxidant activity
was evaluated for assessing the role of bridging groups in trapping 1,1-diphenyl-2-picrylhydrazyl radical
(DPPH•) and in 2,2'-azodi(isobutyronitrile) (AIBN) induced oxidation of styrene. The study of reaction kinetics
of scavenging of the peroxyl radicals demonstrated that the scavenging ability of the DPPH free radical
decreased when length of the bridging groups increased. However, the ability to protect styrene from AIBN-
induced oxidation increased with increased length of the bridging groups.
Keywords: hindered phenol, bridged group, antioxidant ability, DPPH, kinetic behavior
DOI: 10.1134/S1070363216120422
Free radicals, produced by oxidation, act as the
main factor of performance loss during service life of
many organic synthetic materials [1, 2]. Antioxidants
are widely used to stabilize organic compounds as they
can provide hydrogen atoms or electrons to terminate
free radicals [3, 4]. Activity of phenolic antioxidants
depends on the number of phenolic hydroxyl groups,
position and kind of substituents [5, 6], and generally
increases with the increase in number of phenolic
hydroxyl groups [7]. Kajiyama and co-workers [8]
studied the effect of para-substituents on the activity
of phenolic antioxidants and the accumulated data
demonstrated that those localize and stabilize the
phenoxy radical electron in p-position increasing the
activity of phenols. Phenolic antioxidants with tert-
butyl and/or methyl substituents on both o-positions
exhibited high antioxidant activity [9]. Electron
donating substituents increased the antioxidant activity
of phenolic antioxidants [10], while electron-with-
drawing substituents diminished it. However, effect of
length of para-substituents on antioxidant activity has
not been reported so far. The ideal antioxidant was
derived from Irganox 1098, produced by Giba–Geigy
(Switzerland). Antioxidant 1098 contains 1,6-hexane-
diamine as the bridging group.
diamine, 1,4-butanediamine, 1,6-hexanediamine and
1,8-diaminooctane as the bridging groups. Their struc-
tures were characterized by FT-IR, H NMR and LC
Mass spectra. The effect of length of the bridging
groups on the activity of antioxidants was investigated
using the DPPH assay [11] and online oxygen con-
sumption determination [12]. Kinetics of scavenging
of peroxyl radicals (ROO•) produced in the oxidation
of styrene was evaluated by calculating the inhibition
rate (kinh) and the number of trapping ROO• (n).
1
RESULTS AND DISCUSSION
Synthesis and characterization of aliphatic
diamine bridged hindered phenols. Four kinds of
aliphatic diamine bridged hindered phenols were
synthesized using ehtylenediamine, 1,4-butanediamine,
1,6-hexanediamine, and 1,8-diamineoctane as the
bridging groups (Scheme 1). Those phenols were
based on the Irganox 1098. 3-(3,5-Di-tert-butyl-4-
hydroxyphenyl)propionate (3,5-propionate) was
substituted by more active 3,5-propionyl chloride.
Concentration of 3,5-propionyl chloride was much
higher than that of aliphatic diamine. Unreacted 3,5-
propionyl chloride was removed by washing the solid
product with toluene. Hindered phenols were
Based on that material and our recent study, four
kinds of hindered phenols were synthesized with ethylene-
1
characterized by IR, H NMR and EI-MS spectra. The
spectral characteristics of other hindered phenols were
similar.
1 The text was submitted by the authors in English.
2797

2798
LI et al.
Scheme 1. Synthesis of aliphatic diamine bridged hindered phenols.
NH2(CH2CH2)nNH2 +
HO
CH2CH2COCl
HO
CH2CH2CONH(CH2CH2)nHNOCH2CH2
OH
n = 1–4.
Scavenging the DPPH free radical ability of the
aliphatic diamine bridged hindered phenols. With
an increase in concentration of hindered phenols,
inhibition increased rapidly to a certain level. At the
concentration 4×10–5 mol/L the maximum rate of
inhibition was lower than 100% due to reversible
nature of the process [13]. Inhibition decreased with an
increase in the length of the bridging groups (Fig. 1).
decreased. The total scavenging ability of ethylene-
diamine bridged hindered phenol was five times higher
than that of 1,8-diaminooctane analogue due to not
only donating of H atoms but also the molecular size
of the antioxidant. The higher the ability of donating of
H atoms the higher was the scavenging ability. The
larger molecule size the smaller was the reaction
probability between the antioxidant and the DPPH free
radical. Total scavenging ability of the hindered
phenols decreased with an increase in length of the
bridging group.
Scavenging the ROO• ability of the aliphatic
diamine bridged hindered phenols. Initiation of AIBN-
induced oxidation of styrene led to accumulation of the
corresponding radical which combined with oxygen to
give the peroxyl radical. The latter abstracted H atom
from another styrene molecule. The process of radical
Inhibition was affected not only by concentration of
the phenol hydrogen but also by scavenging time for
the hindered phenols [14] (Fig. 2).
The EC50, TC50 and AE values of the hindered
phenols were estimated on the basis of Figs. 1 and 2
(Table 1).
With the increase in length of the bridging group,
EC50 and TEC50 values also increased, while AE values
Concentration of phenol hydrogen
group (×10–5 mol/L)
t, min
Fig. 2. Effect of time on scavenging ability at concentration
4×10–5 mol/L and 30°C: (1) C2-phenol, (2) C4-phenol,
(3) C6-phenol, and (4) C8-phenol.
Fig. 1. Effect of aliphatic diamine bridged hindered phenols
on scavenging ability: (1) C2-phenol, (2) C4-phenol,
(3) C6-phenol, and (4) C8-phenol.
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SYNTHESIS, ANTIOXIDANT PROPERTIES, AND REACTION KINETICS
2799
Table 1. Scavenging parameters of aliphatic diamine bridged
hindered phenols on the DPPH radical
propagation caused oxidation of styrene and peroxide
formation. The hindered phenols inhibited oxidation in
the course of styrene polymerization. With the increase
of molar concentration of phenols the amount of
oxygen consumption decreased (Fig. 3).
Antioxidant
C2-Phenol
C4-Phenol
C6-Phenol
C8-Phenol
EC50, mmol/L
0.0076
TC50, min AE×10–3
21.43
28.04
45.14
70.74
6.14
4.05
1.79
1.01
0.0088
Hindered phenols demonstrated the same trend in
oxygen consumption in the course of styrene
polymerization. Probably the monomer free radical
initiated by AIBN combined with oxygen supporting
its consumption [15]. Upon addition of the hindered
phenol antioxidants, the phenolic hydroxyl group
terminated the free radical and decreased the oxygen
consumption rate. With the increase of concentration
of hindered phenol the effective phenol hydroxyl
number and the ability to provide H atoms increased.
Therefore, oxygen consumption decreased indicating
that phenols could inhibit styrene chain propagation.
0.0124
0.0140
at concentration of 17.55 µM, while that of the 1,8-
diaminooctane bridged hindered phenol was 3453 s
indicating the higher inhibiting ability of the latter. The
effect of inhibiting ROO• oxidation was opposite to
that of scavenging the DPPH radical. Possibly styrene
generated not only ROO•, but also long chain radicals
during the polymerization process and probability of
hindered phenols contact with the longer chain alkyl
radical increased.
Inhibition period of antioxidants (tinh) in the
initiation of AIBN-induced oxidation of styrene could
be measured by the cross-point from the tangent lines
for inhibition and oxidation periods (Figs. 3a and 4).
The tinh values became higher when concentration and
bridging group length increased. Inhibition period of
ethylenediamine bridged hindered phenol was 2869 s
Reaction kinetics of aliphatic diamine bridged
hindered phenols. Reaction kinetics of inhibition of
aliphatic diamine bridged phenols (AH) was studied.
Inhibition rate (Rinh) of aliphatic diamine bridged
phenols for the chain propagation reaction could be
expressed by Eq. (1), where kinh, kp, and [LH] represent
(a)
(b)
1
2
1
2
3
3
t, s
t, s
(c)
(d)
1
1
2
2
3
3
t, s
t, s
Fig. 3. Oxygen consumption curves [(1) 1.75, (2) 8.75, (3) 17.55 μM] of the initiation of AIBN-induced oxidation of styrene. (a) C2-
phenol, (b) C4-phenol, (c) C6-phenol, and (d) C8-phenol.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 86 No. 12 2016

2800
LI et al.
Upon depletion of all antioxidants at the point B,
oxygen consumption rate increased rapidly. Based on
the steady-state kinetic treatment, the rate of oxygen
consumption in the period of propagation, Rp, could be
expressed by Eq. (4), where kp/(2kt)0.5 referred to
oxidizability of the substrate, representing the suscep-
tibility of styrene to undergo peroxidation. The kinetic
chain length, kcl, that defined the cycle number of
chain propagation in the periods of inhibition (kclinh)
and propagation (kclp) was deduced from Eqs. (5) and (6).
Rp = –d[O2]/dt = [(kp/(2kt)0.5]Ri0.5[LH],
(4)
(5)
(6)
C2-phenol C4-phenol C6-phenol C8-phenol
kclinh = Rinh/Ri,
kclp = Rp/Ri.
Fig. 4. Inhibition period (tinh) of aliphatic diamine bridged
hindered phenols.
For many phenol antioxidants scavenging of free
radicals was slow and the mechanism was very com-
plicated. Therefore, it was important to explore
scavenging of the peroxy free radical mechanisms by
antioxidants.
the susceptibility of the peroxy radical to capture the
hydroxyl atom from antioxidants, the rate constant of
chain propagation (238 M–1 s–1 at 50°C) and con-
centration of styrene [16, 17]. Inhibition period could
be deduced from the AB segment (Fig. 4). The
stoichiometric factor, n, refers to the number of peroxy
free radicals trapped by every antioxidant molecule
and can be expressed by Eq. (2) [18].
According to the accumulated data (Table 2)
phenols acted as antioxidants with clear inhibition
period during which the rate of chain propagation and
the kinetic chain length were significantly decreased.
Oxygen consumption rate (Rinh), kinetic chain length
(kclinh) and oxidizability [kp/(2kt)0.5/10–2] decreased
with increased antioxidant concentration due to the
fact that more hydrogen was provided by the phenolic
–d[O2]/dt = Rinh = kp[LH]/(tinhkinh),
(1)
(2)
(3)
n = (Ritinh)/[AH],
Ri = Rg = 2.64×10–6[AIBN].
Table 2. Scavenging parameters of aliphatic diamine bridged hindered phenols on the ROO radicala
Concentration,
kinh×105,
Antioxidants
C2-Phenol
Rp×10–8, M/s Rinh×10–8, M/s tinh, s kp/(2kt)0.5/10–2
kclp
kclinh
μM
M/s
6.95
10.03
8.69
6.24
8.38
6.98
5.85
7.66
6.30
5.63
6.63
5.61
1.75
8.75
24.8
16.7
15.6
24.5
17.6
17.3
23.5
18.4
17.5
24.1
16.1
14.9
16.7
8.1
1568
2241
2869
1811
2412
3179
1908
2499
3318
1993
2690
3453
0.128
0.086
0.081
0.127
0.091
0.089
0.122
0.095
0.091
0.125
0.083
0.077
3.91
2.63
2.47
3.86
2.77
2.72
3.71
2.90
2.76
3.84
2.53
2.35
2.63
1.27
1.15
2.53
1.41
1.29
2.57
1.49
1.37
2.55
1.61
1.48
17.55
1.75
7.3
C4-Phenol
C6-Phenol
C8-Phenol
16.1
9.0
8.75
17.55
1.75
8.2
16.3
9.5
8.75
17.55
1.75
8.7
16.2
10.2
9.4
8.75
17.55
a Ri = Rg = 2.64×10–6[AIBN] s–1 = 6.34×10–8 M/s at concentration of AIBN = 24 mM, [LH] = 0.765 mol/L.
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SYNTHESIS, ANTIOXIDANT PROPERTIES, AND REACTION KINETICS
2801
2ROO·
HO
O
CH2CH2CONH(CH2CH2)nHNOCCH2CH2
·
CH2CH2CONH(CH2CH2)nHNOCCH2CH2
H-delocalization
O
OH
O·
ROOH
ROOH
a. Donation of a second
hydrogen atom
·
·
CHCH2CONH(CH2CH2)nHNOCCH2CH
HO
OH
CHCH2CONH(CH2CH2)nHNOCCH2CH
O
2ROO·
c. Complexation
2ROO·
ROOH
CHCH2CONH(CH2CH2)nHNOCCH2CH
ROOH ROOH
·
·
HO
CHCH2CONH(CH2CH2)nHNOCCH2CH
OH
HO
OH
b. Dimerization
·
HO
HO
CHCH2CONH(CH2CH2)nHNOCCH2CH
OH
OH
HO
HO
CHCH2CONH(CH2CH2)nHNOCCH2CH
CHCH2CONH(CH2CH2)nHNOCCH2CH
OH
·
CHCH2CONH(CH2CH2)nHNOCCH2CH
·
O
O
CCH2CONH(CH2CH2)nHNOCCH2CH
OH
CCH2CONH(CH2CH2)nHNOCCH2C
CCH2CONH(CH2CH2)nHNOCCH2C
O
O
ROOH
·
CCH2CONH(CH2CH2)nHNOCCH2CH
OH
ROOH
Fig. 5. Proposed mechanism for aliphatic diamine bridged hindered phenols/DPPH· reaction
hydroxyl which could be trapped by the ROO• radical.
With the increase in concentration of hindered phenols
and the bridged group, the tinh value increased (Table 2).
On the basis of Ri data, the relationship between tinh
and concentration of hindered phenols could be
obtained as listed in Table 3. The n values of the
hindered phenols were similar due to their close
chemical structures. Generally, one hydroxyl group
could trap only one DPPH radical. However, the n
value did not correspond to the number of the OH
groups hydrogen atoms available for donation and was
higher than that of the OH groups of phenols. This was
in agreement with the results of presented by Kurechi
[19] who determined that compounds with the hyd-
roxyl group, sterically hindered by the tert-butyl group,
demonstrated high antioxidant ability.
There could be considered three possible
approaches to explaining the antioxidant ability of
aliphatic diamine hindered bridged phenols [20]. After
the first reaction, phenol radicals could form a kenolic
compound by donation of two hydrogen atoms or two
DPPH molecular radicals combined with two aryl
radicals as shown in steps a (n = 4) and b (n = 4) (Fig. 5).
The third approach referred to dimerization between
two phenoxyl radicals [21]. Upon dimerization it could
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LI et al.
Table 3. The equation of tinh [antioxidant] and n of an anti-
Aliphatic diamine bridged hindered phenols. mp
211.8–212.4 (C2-phenol), 205.1–205.6 (C4-phenol),
161.2–162.0 (C6-phenol), and 156.0–162.1°C (C8-phenol).
IR spectrum, ν, cm–1: 3500–3650, 1530, 1640, 1240,
oxidant in protecting styrene against AIBN-induced oxidationa
t
inh (s) = (n/Ri)
Antioxidant
n
[antioxidant (μM)] + constant
1
1390. H NMR spectrum of C2-phenol, δ, ppm: 5.08 s
C2-Phenol tinh = 81.8 [antioxidant] + 1460.4
(r2 = 0.9853)
5.18
(2H, Ar-OH), 1.39–1.42 m [36H, C(CH3)3], 6.97–7.05
m (4H, Ar-H), 3.36–3.41 m (4H, Ar-CH2), 2.83 s (4H,
Ar-C–CH2), 7.32 t (2H, CONH), 2.42–2.46 t (4H,
O=C–N–CH2–CH2–N–C=O). The spectral data for
other synthesized aliphatic diamine bridged hindered
phenols were similar. The chemical shifts of protons of
the bridging groups [O=C–N–C–(CH2)n–C–N–C=O]
were in the range of 5.67–5.72. ESI-MS (m/z): 581.4
[M + 1]+ (C2), 609.5 [M + 1]+ (C4), 637.5 [M + 1]+
(C6), and 665.5 [M + 1]+ (C8).
C4-Phenol tinh =86.6 [antioxidant] + 1657.5
5.49
5.67
5.83
(r2 = 0.9997)
C6-Phenol tinh =89.4 [antioxidant] + 1739.1
(r2 = 0.9985)
C8-Phenol tinh =92.1 [antioxidant] + 1850.2
(r2 = 0.9968)
a Ri = Rg = 6.34×10–8 M/s.
interact for the second time with DPPH radicals to
produce the bi-quinonoid structure, as presented in
steps c (n =5) and d (n = 6).
Measurement of the scavenging DPPH free
radical. Scavenging capacity of aliphatic diamine
bridged hindered phenols on DPPH free radical were
determined by the modified method reported by Brand–
Williams [23]. DPPH free radical was dissolved in
ethanol and the solution was stored in darkness.
Ethanol solution of aliphatic diamine bridged hindered
phenol was added to the DPPH radical solution and the
mixture was equilibrated for 5 min at 25°C. Ab-
sorbance at 517 nm was recorded at various time
intervals until the reaction reached equilibrium. The
scavenging ability of hindered phenols was expressed
in terms of % inhibition and antioxidant ability (AE).
Percent inhibition was calculated by Eq. (7).
EXPERIMENTAL
Materials. Analytical reagent grade ethylenediamine,
1,4-butanediamine, 1,6-hexanediamine, 1,8-diamino-
octane, and toluene were purchased from Tianjin
Kemiou Chemical regent development center (China).
Dibutyltin dilaurate was purchased from Changchun
Chemical (China). DPPH was purchased from Beijing
Jingkehongda Biotechnology Co., Ltd. (China).
Antioxidant 1098 was purchased from Ciba Specialty
Chemicals (Shanghai, China). 3-(3,5-di-tert-butyl-4-
hydroxyphenyl)propionyl chloride (3,5-propionyl
chloride) was synthesized in our laboratory [22].
A0 − At
× 100,
(7)
% Inhibition =
A0
Synthesis of the aliphatic diamine bridged
hindered phenols. Aliphatic diamine was added to
toluene and heated slowly to 25°C and stirred for
10 min under the atmosphere of N2. Dibutyltin
dilaurate (2% of aliphatic diamine) and 3,5-propionyl
chloride (the molar ratio of 3,5-propionyl chloride to
aliphatic diamine 4 : 1) were added to the above mix-
ture and heated slowly to 40°C. The reaction lasted for
15 h. The solvent and by-product methanol were
removed by rotary evaporation at 40°C. The residue
was stored at 25°C for 8 h and filtered off as white
solid, washed three times with toluene (100 mL) and
dried under vacuum for 24 h at 60°C. Four aliphatic
diamine bridged hindered phenols were synthesized
with ethylenediamine, 1,4-butanediamine, 1,6-hexane-
diamine, and 1,8-diamineoctane as bridging groups,
named C2-phenol, C4-phenol, C6-phenol (Irganox
1098), and C8-phenol, respectively.
where A0 and At were the absorbance at 517 nm of the
mixture solution at the beginning and at the certain
time.
The antioxidant ability was calculated by Eq. (8)
[24].
1
EC50TC50
,
AE =
(8)
where EC50 was the amount of hindered phenol needed
to decrease the initial DPPH radical concentration by
50%, and TC50 was the time needed to reach the steady
state at EC50 concentration.
Measurement of the ROO free radical scavenging
ability. Free-radical reactions in the absence and
presence of antioxidants were monitored by oxygen
uptake determination, and the free-radical resource was
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2803
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supplied by decomposition of 2,2'-azodi(isobutyro-
nitrile) (AIBN) at 50°C. Briefly, the rate of oxygen
uptake was measured under the oxygen atmosphere in
a closed stainless steel autoclave. AIBN and the syn-
thesized hindered phenol antioxidants were dissolved
directly in toluene. The process of AIBN-induced
peroxidation of styrene was monitored by an oxygen
consumption apparatus equipped with an oxygen
pressure gauge sensitive to oxygen pressure. Styrene in
the toluene solution and antioxidant solution were
poured into a stainless steel autoclave which had been
filled in with dry nitrogen three times and then purged
by oxygen. The mixture was stirred for 10 min at 50°C.
The AIBN solution was injected to the mixture with a
syringe in order to initiate the peroxidation of styrene.
Initial concentrations of AIBN and styrene were 24 mM
and 0.765 M, respectively. All experiment were repeated
not less than three times with the standard deviation
within 10% and the final data were calculated as
average of three independent measurements.
CONCLUSIONS
Four hindered phenol antioxidants with aliphatic
diamines as the bridging groups were synthesized by
acylamidation. Radical scavenging activity of anti-
oxidants was tested by stable DPPH• and inhibiting
AIBN induced oxidation of styrene. The experimental
data indicated that the hindered phenols had high
scavenging ability on the DPPH radical that increased
with increasing concentration of an antioxidant and
decreased with higher length of bridging groups at the
same concentration. The scavenging effect of hindered
phenols on the ROO• radical was opposite to that of
the DPPH radical.
ACKNOWLEDGMENTS
This work was supported by the financial founda-
tion of National Nature Science Foundation of China
(no. 51303020). We are grateful to State Key Lab of
Inorganic Synthesis and Preparative Chemistry, Jilin
University for the characterization work.
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RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 86 No. 12 2016