Synthesis, characterization, and evaluation of 10-undecenoic acid-based epithio derivatives as multifunctional additives

Article abstract of DOI:10.1021/jf5033558

Novel epithio compounds from alkyl epoxy undecanoates (n-alkyl, C₁, C₄, and C₆; isoalkyl, C₃, C₄, and C₈) were synthesized using an ammonium thiocyanate in ionic liquid 1-methylimidazolium tetrafluoroborate/H₂O (2:1) solvent system in 85-90% yields by gas chromatographic (GC) analysis. The synthesized products were characterized by ¹H and ¹³C nuclear magnetic resonance spectroscopy, Fourier transform infrared spectroscopy (FTIR), gas chromatography, and GC mass spectral (GC-MS) analyses and evaluated for their antioxidant, extreme pressure (EP), and antiwear (AW) properties in three different base oils, namely, epoxy jatropha fatty acid n-butyl esters (EJB), di-2-ethylhexyl sebacate (DOS), and mineral oil (S-105). Among the synthesized products, n-butyl epithio undecanoate exhibited superior antioxidant property (229.2 °C) compared to butylated hydroxytoluene (BHT, 193.8 °C) in base oil DOS and comparable performance in EJB and S-105 base oils. All of the epithio derivatives exhibited significantly enhanced weld point for the base oils EJB and DOS at 2 wt % level and displayed moderate enhancement in S-105 base oil. Methyl epithio undecanoate at 0.6% concentration exhibited considerable improvement in the wear scar of DOS base oil. The synthesized epithio derivatives have potential as multifunctional additives in lubricant formulations.

Full text of DOI:10.1021/jf5033558

Article
pubs.acs.org/JAFC
Synthesis, Characterization, and Evaluation of 10-Undecenoic
Acid-Based Epithio Derivatives as Multifunctional Additives
Gorla Geethanjali, Korlipara V. Padmaja, Arukali Sammaiah, and Rachapudi B. N. Prasad*
Centre for Lipid Research, CSIRIndian Institute of Chemical Technology, Hyderabad 500 007, Andhra Pradesh, India
ABSTRACT: Novel epithio compounds from alkyl epoxy undecanoates (n-alkyl, C , C , and C ; isoalkyl, C , C , and C ) were
1
4
6
3
4
8
synthesized using an ammonium thiocyanate in ionic liquid 1-methylimidazolium tetrafluoroborate/H O (2:1) solvent system in
2
1
13
8
5−90% yields by gas chromatographic (GC) analysis. The synthesized products were characterized by H and C nuclear
magnetic resonance spectroscopy, Fourier transform infrared spectroscopy (FTIR), gas chromatography, and GC mass spectral
GC-MS) analyses and evaluated for their antioxidant, extreme pressure (EP), and antiwear (AW) properties in three different
(
base oils, namely, epoxy jatropha fatty acid n-butyl esters (EJB), di-2-ethylhexyl sebacate (DOS), and mineral oil (S-105). Among
the synthesized products, n-butyl epithio undecanoate exhibited superior antioxidant property (229.2 °C) compared to butylated
hydroxytoluene (BHT, 193.8 °C) in base oil DOS and comparable performance in EJB and S-105 base oils. All of the epithio
derivatives exhibited significantly enhanced weld point for the base oils EJB and DOS at 2 wt % level and displayed moderate
enhancement in S-105 base oil. Methyl epithio undecanoate at 0.6% concentration exhibited considerable improvement in the
wear scar of DOS base oil. The synthesized epithio derivatives have potential as multifunctional additives in lubricant
formulations.
KEYWORDS: 10-undecenoic acid, epithio derivatives, ionic liquid, extreme pressure additive, antiwear, antioxidant, biolubricant,
synthetic ester, mineral oil
INTRODUCTION
heterocyclic fatty acids containing an S atom were used as
intermediates leading to the preparation of thionitrile deriva-
tives. Kuranova et al. have prepared trisubstituted five-
■
The increasing use of non-petroleum lubricant base stocks will
require new environmentally friendly and multifunctional
lubricant additives. Currently, the majority of lubricant additives
are petroleum based, providing an opportunity for a biobased
lubricant additive that is eco-friendly, cost-effective, and of supe-
rior quality with compatibility for biolubricants. Using renew-
able raw material oils and fats of vegetable and animal origin
offers new synthesis for the chemical industry. The structural
modification of long-chain fatty acid derivatives such as addition
reactions to double bonds or epoxy groups gives compounds
containing either sulfur, phosphorus, or halogen, or a combina-
tion of these elements. These compounds may exhibit good
antiwear (AW) or extreme pressure (EP) lubricating character-
istics and, in some cases, function in both capacities. Organic
sulfur compounds have been widely used in lubricating compo-
sitions for their EP as well as AW properties. Sulfur compounds
17
membered sulfur heterocycles by reacting methyl 9,10:12,13-
18
diepithiostearate with HBr or Cl CCOOH. Preparation of a
3
number of epithio C fatty esters from methyl oleate, linoleate,
18
16
1
ricinoleate, and derivatives of ricinoleate is reported. Epoxy
fatty esters on treatment with dimethylthioformamide and
trifluoroacetic acid in dichloromethane were converted to their
2
1
corresponding 2,3-epithio fatty esters. 2,2 -Epoxy fatty esters
1
were transformed to 2,2 -epithio fatty esters by similar reaction
1
13
and were characterized by H and C NMR spectroscopy.
Methodologies reported for the synthesis of episulfides involve
the use of catalysts and reagents, namely, dichlorodisulfides,
iodothiocyanate, sulfuryl lithium, dimethylthioformamide, thio-
urea, potassium thiocyanate, and ammonium thiocyanate, using
3
−7
8
19
ruthenium catalyst and other methods that are either corrosive
can also be used as antioxidants, and some of them are con-
sidered natural antioxidants in lubricating oils.
or expensive. Furthermore, the reported methods suffer from
disadvantages such as moderate yields, elevated temperatures,
and requirement of stoichiometic amounts of catalyst. Further-
more, in view of developing new ecofriendly biolubricant multi-
functional additives, we report the synthesis and characterization
of novel epithio derivatives of 10-undecenoic acid, a commer-
cially available derivative of castor oil, followed by their evalu-
ation for their antioxidant, antiwear, and extreme pressure pro-
perties in three types of base fluids.
9
−11
Many
attempts were made by various workers on sulfurization of
vegetable oils to find replacement for sulfurized sperm whale oil,
a biobased antiwear and EP additive. These attempts were not
successful as the sulfurization of vegetable oils resulted in high
viscous resinous products that displayed a tendency to corrode
copper, excessive foaming, thickening during gear box tests, and
exhibit low thermal stability. These products were mostly insolu-
ble in base oils and required the addition of nonwax fatty acid
1
2,13
methyl esters to improve their solubility.
Epithio compounds are very effective lubricant additives, in
Received: July 14, 2014
particular EP/AW and antioxidant additives, and are superior to
Revised: October 31, 2014
Accepted: November 4, 2014
14
other sulfur products. The synthesis of C fatty acids contain-
1
8
1
5,16
ing a thiophene nucleus has been reported.
Three-membered
©
XXXX American Chemical Society
A
dx.doi.org/10.1021/jf5033558 | J. Agric. Food Chem. XXXX, XXX, XXX−XXX

Journal of Agricultural and Food Chemistry
Article
(
2 g, 1 wt % of KFA), and xylene, 100 mL, were taken in a four-necked
MATERIALS AND METHODS
■
round-bottom flask and stirred at 130−140 °C for a period of 8 h. The
reaction was monitored by TLC. The crude product was distilled at
1
The distilled product was extracted with hexane and washed with
water until it was acid free, passed through Na SO , concentrated, and
dried under vacuum. The product was further purified using basic
alumina column chromatography by elution with hexane to obtain
di-2-ethylhexyl sebacate with an acid value of <0.1. The yield of the
product was found to be 95%. The structure of the product was
Materials. 10-Undecenoic acid was obtained from Jayant Oil Mills
Mumbai, India). Methanol, propan-2-ol, butan-1-ol, 2-methylpropan-
-ol, hexan-1-ol, 2-ethylhexan-1-ol, hexane, and ethyl acetate were pro-
cured from Qualigens Fine Chemicals (Mumbai, India). Dichloromethane,
m-chloroperbenzoic acid (m-CPBA), aniline, butylated hydroxytoluene
BHT), paraffinoil, ammonium thiocyanate, 1-methylimidazole, and HBf4
were purchased from Sigma-Aldrich (St. Louis, MO, USA). Hydrogen
peroxide (30% aqueous solution), xylene, and p-toluenesulfonic acid
(
80 °C temperature and 5 mmHg to remove excess alcohol and xylene.
1
2
4
(
(
p-TSA) were purchased from M/s sd fine-chem Ltd., Mumbai, India.
1
characterized by FT-IR, EI-MS, and H NMR spectral studies.
Formic acid solution (85%) and sulfuric acid (AR grade) were purchased
from M/s Rankhem, New Delhi, India. Sebacic acid was procured from
Sisco Research Laboratories (Mumbai, India). Jatropha oil was obtained
from a local company. LZ 1359 and tricresyl phosphate (TCP) was used as
control in antiwear and extreme pressure analyses, respectively. All of the
chemicals were of analytical reagent grade and used as received.
Synthesis of Epoxy Jatropha Fatty Acid n-Butyl Esters (EJB). n-
21
Butyl esters of jatropha fatty acids (125 g, 0.385 mol), formic acid
(
26.6 g, 0.577 mol), and sulfuric acid (1.7 mL, 2 wt % of HCOOH and
hydrogen peroxide) were taken in a three-necked round-bottom flask
equipped with an overhead stirrer. A 30% aqueous hydrogen peroxide
solution (117.9 mL, 1.155 mol) was added slowly to the contents under
strong mechanical stirring at 15−20 °C over a duration of 1 h. After
addition, the temperature was raised to 55−60 °C for 5 h. The product
was extracted with ethyl acetate and washed with water until it was acid
free, then passed through anhydrous sodium sulfate, and dried under
reduced pressure to yield the product with oxirane oxygen content of
Methods. The analytical determinations of the sebacic acid and
jatropha oil derivatives were accomplished according to the official
methods of the American Oil Chemists’ Society (AOCS) as follows: acid
value (AOCS Cd 63), iodine value (IV, AOCS Da 15-48), and oxirane
value (OO, AOCS Cd 9-57). The base oil lubricant properties such as
density, viscosity, and viscosity index were evaluated per methods
4
.4%. The yield of the epoxy jatropha fatty acid n-butyl esters was found
20
1
13
described previously. H NMR and C NMR spectra were recorded
on Bruker AR X 400 spectrometer (400 MHz) and a Varian spectro-
meter at 75 MHz, respectively, and TMS was used as an internal
standard. IR spectra were recorded in chloroform on a PerkinElmer
Fourier transform (model Spectrum BX; Shelton, CT, USA) FT-IR
spectrometer neat as thin film. The spectrum was recorded in
absorbance mode.
to be 95%. The structure of the product was characterized by FT-IR and
H NMR spectral studies.
1
1
H NMR (400 MHz, CDCl ) δ 0.85−0.95 (t, −CH CH ), 1.2−1.4
3
2
3
(
m, −CH CH ), 1.4−1.5 (m, CH CHOCH−), 1.6−1.7 (m,
2
3
2
CH CH CO−), 2.8−3.1 (m, −CHOCH−), 2.3 (t, −CH CO),
2
2
2
−1
4
(
.05 (t, −CH OCO); IR (neat, cm ) 2927 (CH), 1740
2
CO), 826 (epoxy ring), 1172 (CC (O)O).
A gas chromatograph Agilent 6890N series was employed to analyze
the fatty acid composition using an HP-1 column (30 m × 0.25 mm ×
Preparation of Ionic Liquid [HMIM]BF . 1-Methylimidazole
4
(
6 g, 0.0738 mol) was taken into a three-necked round-bottom flask.
0.5 μm). The flame ionization detector and injector were at 300 °C. The
The temperature of the medium was sustained at 0 °C, and 48%
tetrafluoroboric acid (9.54 mL, 0.0738 mol) was added slowly to it for
oven temperature was programmed at 150 °C for 2 min and then
increased to 300 °C at 10 °C/min. The carrier gas used was nitrogen at a
flow rate of 1.5 mL/min. The GC-MS analysis was carried out using an
Agilent 6890 gas chromatograph connected to an Agilent 5973 mass
spectrophotometer at 70 eV (m/z 50−600; source at 230 °C and
quadruple at 150 °C) in the EI mode using an HP-1 MS capillary column
4
0 min, and then the reaction was further continued at room tempera-
ture for 2 h. After completion of the reaction, the contents were kept
under high vacuum to remove moisture at 80−90 °C. The product was
subjected to ethyl acetate washings to remove excess imidazole followed
by drying under reduced pressure to obtain a light yellow viscous liquid.
Synthesis of Alkyl 10-Undecenoates (2a-f). 10-Undecenoic acid
was esterified with different alcohols, namely, methanol, propan-2-ol,
butan-1-ol, 2-methylpropan-1-ol, hexan-1-ol, and 2-ethylhexan-1-ol, in
the presence of p-TSA (1 wt % of the acid) for a duration of 4−5 h to
yield alkyl 10-undecenoates 2a−f). The synthetic procedure is reported
(
1
30 m × 0.25 mm × 0.5 μm). The oven temperature was programmed at
50 °C for 2 min, 10 °C/min to 300 °C, and 20 min at 300 °C. The
carrier gas used was helium at a flow rate of 1.0 mL/min.
The antioxidant activity of the synthesized epithio derivatives in three
base oils was evaluated using a differential scanning calorimeter (DSC Q
2000, TA Instruments) under oxygen atmosphere. The oxidative onset
22
elsewhere.
temperature (OOT) was measured for base oils with and without added
additives in dynamic mode. The temperature programming was set at
Synthesis of Alkyl 10-Epoxyundecanoates (3a−f). Alkyl
0-undecenoates were epoxidized using m-chloroperbenzoic acid to
1
2
5−300 °C, ramped at 10 °C/min, and purified oxygen was passed
yield alkyl epoxy undecanoates. The synthetic procedure is reported
through the sample enclosure at 50 mL/min. The oxidative onset
temperature was determined from the start of the exothermic reaction
peak in the DSC thermogram.
22
elsewhere.
Typical Procedure for the Preparation of Butyl 10-
Epithioundecanoate (4a−f). n-Butyl 10-epoxyundecanoate (2 g,
The load-carrying capacity and antiwear properties of synthesized
additives were investigated using a Stanhope SETA four-ball tester. All
of the wear tests were conducted at a rotating speed of 1200 rpm under a
load of 40 kg, with a duration of 60 min. An optical microscope was used
to determine the wear scar diameters (WSD) of the three lower balls
with an accuracy of ±0.01 mm, and these diameters were averaged and
cited as the WSD reported in this paper. The extreme pressure tests were
run at 1475 rpm, ambient temperature at variable load, and test duration
for 1 min or until the balls welded, whichever occurred first. The extreme
pressure properties of three base oils with and without added additive at
different concentrations were evaluated as weld point loads and are
reproducible to within one loading increment (±10 kg for 10 kg loading
increments).
0
.0078 mol) and ammonium thiocyanate (0.594 g, 0.0078 mol) were
taken into a round-bottom flask. Ionic liquid [HMIM]BF (4 g) and
4
water (2 g) in a 2:1 ratio were added to the contents, and the resultant
reaction mixture was stirred at 60 °C for a period of 23 h. The progress of
the reaction was monitored by TLC using hexane/ethyl acetate (90:10
v/v). After completion of the reaction, the crude product was extracted
using ether, and the ethereal solution was washed with water and
dried over sodium sulfate. The combined ether extracts were concen-
trated using a rotary evaporator. The structure of the product was
established by GC-MS and GC (90%). The crude product was purified
using silica gel column chromatography using hexane and ethyl acetate
(
80/20) to afford n-butyl 10-epithioundecanoate (1.44 g, 68% of
1
isolated yield). The product was characterized by H NMR, IR, GC, and
GC-MS spectral data.
The surface analysis of wear scar developed on the steel balls during
the antiwear analysis was studied using a Hitachi S-3000N scanning
electron microscope (SEM) with energy/wavelength dispersion, at a
voltage of 20 kV.
1
Methyl 10-Epithioundecanoate (ESUME, 4a): 1.29 g, 60%; H
NMR (400 MHz, CDCl ) δ (TMS) 1.2−1.40 (m, 12 H, −CH ), 1.4−
3
2
1
.65 (m, 2H, CH2CH2CO), 1.75−1.85 (m, 2H, H CSCH
2
Synthesis of Di-2-ethylhexyl Sebacate. Sebacic acid (200 g,
.99 mol), 2-ethylhexanol (465 mL, 2.97 mol), p-toluenesulfonic acid
0
CH ), 2.15−2.18 (d, 2H, H CSCH), 2.25−2.35 (t, 2H, −COCH ),
2
2
2
B
dx.doi.org/10.1021/jf5033558 | J. Agric. Food Chem. XXXX, XXX, XXX−XXX

Journal of Agricultural and Food Chemistry
Article
RESULTS AND DISCUSSION
2
.48−2.52 (d, H CSCH), 3.6−3.65 (t, 2H −OCH ); 13C NMR (75
■
2
3
MHz, CDCl ) δ 24.5−30.0 (−CH −), 25.8 (−CH CHSCH ),
3
2
2
2
In the present study epithio undecanoates were synthesized from
epoxy undecanoates in a green reaction medium, ionic liquid.
The use of ionic liquid for the synthesis of epithio compounds
helps in carrying out the reaction under mild and neutral con-
ditions to afford epithio undecanoates in high yields. The perfor-
mance of the epithio derivative as multifunctional additives was
evaluated in three base oils: epoxy jatropha n-butyl esters (EJB),
di-2-ethylhexyl sebacate (DOS), and paraffin (S-105) base oil.
The epoxy undecanoates were used as starting material to
synthesize epithio undecanoates using ammonium thiocyanate
in stoichiometric ratio (Scheme 1). All of the reactions were
2
9.5 (−CH CHSCH ), 34.0 (−CH CO), 51.0 (CH 
2
2
2
3
OCO), 36.2 (−CH CHSCH ), 174.5 (−COO−); IR
2
2
−1
(
neat, cm ) 2929 (−CH stretching), 1736 (−CO), 612 (−C
S− stretching), 1042 (−CS− asymmetric stretching), 1172 (C
+
C(O)O); EIMS m/z 230 [M] , 215, 173, 199.
2-Propyl 10-Epithioundecanoate (ESIPUES, 4b): 1.29 g; 60%;
1
H NMR (400 MHz, CDCl ) δ (TMS) 1.20−1.36 (q, 6H,
3
−
CH(CH ) ), 1.42−1.66 (m, 14 H, −CH ), 1.75−1.85 (m, 2H,
3
2
2
H CSCHCH ), 2.15−2.18 (d, 2H, H CSCH), 2.2−2.3 (t, 2H,
2
2
2
−
COCH ), 2.48−2.52 (d, H CSCH), 3.14−3.18 (d, 2H, OCH );
2
2
2
1
3
C NMR (75 MHz, CDCl ) δ 14 (CH CH −), 21.7 ((CH ) 
3 3 2 3 2
CH−), 22.6−32.2 (−CH −), 34.3 (−CH CO), 36.5 (−CH 
2
2
2
Scheme 1. Synthesis of Epithio Undecanoates
CHSCH ), 30.5 (−CH CHSCH ), 174.5 (−COO−); IR
2
2
2
−1
(
neat, cm ) 2927 (−CH stretching), 1736 (−CO), 612 (−C
S− stretching), 1042 (−CS− asymmetric stretching), 1174 (C
+
C(O)O); EIMS m/z 281 [M + Na] , 259, 199, 181.
2
-Methyl-1-propyl 10-epithioundecanoate (ESC-4UES, 4c):
1
1
−
.20 g; 57%; H NMR (400 MHz, CDCl ) δ (TMS) 0.86−0.95 (t, 3H,
3
CH ), 1.18−1.65 (m, 18 H, −CH ), 2.2−2.3 (t, 2H, −COCH ),
3
2
2
1
.72−1.8 (m, 2H, H CSCHCH ), 2.12−2.17 (d, 2H, H CSCH),
2
2
2
2
.28−2.32 (t, 2H, −COCH ), 2.42−2.48 (d, U CSCH), 3.85−3.95
2
2
1
3
(
t, 2H −OCH (CH ) ); C NMR (75 MHz, CDCl ) 14 (CH 
2
2
2
3
3
CH−), 19 ((CH ) CH−), 21.7 ((CH ) CH−), 22.6−32.2
3 2 3 2
(
3
−CH −), 34.5 (−CH CO), 36.5 (−CH CHSCH ),
2
2
2
2
0.5 (−CH CHSCH ), 70.3 (−CHOCO), 173.5
2
2
−1
(
−COO−); IR (neat, cm ) 2929 (−CH stretching), 1736 (−CO),
6
19 (−CS−), 1042 (−CS− asymmetric stretching), 1163 (CC(
completed within 20−24 h, affording high yields of alkyl
O)O); EIMS m/z 295 [M + Na], 216, 199.
1
0-epithioundecanoates as revealed by GC analysis (85−96%).
Butyl 10-Epithioundecanoate (ESnBU-UES, 4d): 1.44 g; 68%;
The products were purified using silica gel column chromatog-
raphy using hexane and ethyl acetate (80:20) to afford pure alkyl
1
H NMR (400 MHz, CDCl ) δ (TMS) 0.9−1.0 (d, 3H, −CH ), 1.2−
3
3
1
.40 (m, 14 H, −CH ), 1.4−1.45 (m, 2H, CH2CH CO), 1.75−
2
2
1
0-epithioundecanoates with an isolated yield of 60−68%. The
FT-IR spectra of the epoxy undecanoate derivatives are char-
acterized by a stretching vibration frequency at 835 cm , which
is attributable to the epoxy stretching vibration frequency and
1
.85 (m, 2H, H CSCHCH ), 2.1−2.15 (d, 2H, H CSCH), 2.2−2.3
2
2
2
−
1
(
t, 2H, −COCH ), 2.45−2.5 (d, H CSCH), 4.0−4.1 (t, 2H, −O
2 2 2 3 3 2
2
2
13
CH (CH ) ); C NMR (75 MHz, CDCl ) 13.5 (CH CH −),
−
1
1
9.0 (CH CH −), 25.0−30.5 (−CH −), 34.2 (−CH CO), 36.5
asymmetric ring deformation at 916 cm . In contrast, the IR
spectral feature observed in epithio derivatives is the disap-
pearance of these peaks, indicating complete conversion of epoxy
to epithio group in the synthesized products. Two new peaks
3
2
2
2
(
(
−CH CHSCH ), 31.0 (−CH CHSCH ), 26.0
2
2
2
2
−CH CHSCH ), 64.1 (−CHOCO), 173.8 (−COO−);
2
2
−1
IR (neat, cm ) 2929 (−CH stretching), 1736 (−CO), 619
(
2
−CS−), 1163 (−CC(O)O); EIMS m/z 295 [M + Na],
−1
appeared at 619 and 1011 cm , which are attributable to the
16, 199.
23−25
symmetric and asymmetric epithio ring deformation,
ing the successful conversion of epoxide to epithio group. The
FT-IR spectra also exhibited the characteristic absorptions of
prov-
Hexyl 10-Epithioundecanoate (ESnHE-UES, 4e): 1.37 g; 66.5%;
1
H NMR (400 MHz, CDCl ) δ (TMS) 0.95−1.0 (d, 3H, −CH ), 1.3−
3
3
1
.50 (m, 18 H, −CH ), 1.45−1.5 (m, 2H, CH2CH CO), 1.8−
2
2
−
1
aliphatic carbons around 2987 and 2865 cm and an ester group
1
.85 (m, 2H, H CSCHCH ), 2.1−2.15 (d, 2H, H CSCH), 2.25−2.3
−1
2
2
2
(
−COO−) around 1735 cm .
1
(
t, 2H, −COCH ), 2.45−2.5 (d, H CSCH), 4.05−4.1(t, 2H, −O
In H NMR spectra of alkyl 10-epithioundecanoates, signals at
2
2
13
CH (CH ) ); C NMR (75 MHz, CDCl ) δ 14.1 (CH CH −),
2
2
2
3
3
2
2.2 and 2.5 ppm corresponding to methylene and methine
protons, respectively, confirmed the epithio ring introduction
into the molecule, respectively. In epithio undecanoate deriva-
tives the disappearance of peaks at 2.8−2.9 ppm corresponding
to −CH− protons of the epoxy carbon confirmed the epithio
ring formation. Both the infrared spectra and NMR spectra of
these epithio derivatives indicated the absence of an epoxy group
in the epithio derivatives. The GC-MS and GC chromatograms
also indicated the absence of an epoxy group, indicating complete
conversion into an epithio group. Signals in C NMR at 31.0 and
6.0 ppm confirm the presence of an epithio ring in the synthe-
sized products. The adjacent methylene group of the epithio
system was shifted to 36.0 ppm due to a β effect from the sulfur
atom, also confirming the product formation.
2
2.5 (CH CH −), 28.5−29.4 (−CH −), 25.5−26 (−CH CH
3
2
2
2
SCH ), 31.5 (−CH CHSCH ), 34−34.2 (−CH CO),
2 2 2 2
3
6.0 (−CH CHSCH ), 64.5 (−CHOCO), 174
2
2
−1
(
−COO−); IR (neat, cm ) 2929 (−CH stretching), 1738 (−C
O), 615 (−CS−), 1163 (−CC(O)O); EIMS m/z 323
+
[
M + Na] , 199.
2
-Ethylhexyl 10-epithioundecanoate (ES2ET-HE-UES, 4f): 1.3 g;
1
6
−
4%; H NMR (400 MHz, CDCl ) δ (TMS) 0.8−0.95 (t, 6H,
3
(CH ) ), 1.20−1.50 (m, 21 H, −CH ), 1.75−1.85 (m, 2H,
3
2
2
1
3
H CSCHCH ), 2.12−2.17 (d, 2H, H CSCH), 2.25−2.35 (t, 2H,
2
2
2
2
−
COCH ), 2.48−2.52 (d, H CSCH), 3.95−4.0 (t, 2H −O
2
2
13
CH (CH ) ); C NMR (75 MHz, CDCl ) δ 10.8 (CH CH −),
2 2 2 3 3 2
1
4 (CH CH −), 22.1−32.4 (−CH −), 30.5 (−CH CHS
3
2
2
2
CH ), 36.5 (−CH CHSCH ), 38.5 (−CH CO), 66.5
2 2 2 2
−1
Performance Evaluation of the Epithio Derivatives as
Antioxidants. The antioxidant activity of the synthesized
epithio derivatives and commercial antioxidant BHT were
(
−CHOCO), 174 (−COO−); IR (neat, cm ) 2929 (−CH
stretching), 1732 (−CO), 619 (−CS−), 1042 (−CS− asymmetric
+
stretching), 1171 (−CC(O)O); EIMS m/z 351 [M + Na] , 199.
C
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Journal of Agricultural and Food Chemistry
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evaluated using three base oils, EJB, DOS, and S-105. The
oxidative onset temperature (OOT) of the pure base oils was
found to be 164.4, 180.2, and 193.6 °C, respectively. The fatty
acid composition of synthesized biolubricant base oil EJB
revealed that almost all of the unsaturation converted to epoxy
group as follows: C18:1 epoxy, 44.1%; C18:2 mono epoxy, 3%;
and C18:2 di epoxy, 26%. The basic lubricant properties such as
viscosity, viscosity index, and density exhibited by the three base
oils are shown in Table 1. The additive percentage in base oils
Table 2. Antioxidant Property of the Synthesized Additives
(2 wt %) in Three Lubricant Base Stocks
oxidative onset temperature (OOT) (°C)
a
sample name
EJB
DOS
S-105
no additive
ESUME
164.4
177.6
162.7
156.1
180.6
169.7
180.3
180.2
195.1
206.2
200.1
229.2
199.5
193.8
193.6
216.5
214.4
215.9
242.0
199.2
243.3
ESIPAUES
ESC-4UES
ESnBU-UES
ES-2ET-HE-UES
BHT
Table 1. Physicochemical Properties of Base Oils
property
EJB
DOS
S-105
a
ESUME, methyl 10-epithoundecanoate; ESIPAUES, 1-(2-propyl) 10-
density 15 °C (g/cm3)
0.915
11.15
3.09
0.920
12.38
3.26
0.835
10.28
3.36
epithoundecanoate; ESC-4UES, 2-methyl-1-propyl 10-epithoundeca-
noate; ESnBU-UES, 1-butyl 10-epithoundecanoate; ES-2ET-HE-UES,
2-ethylhexyl 10-epithoundecanoate; BHT, butylated hydroxytoluene.
viscosity
40 °C, cSt
00 °C, cSt
1
viscosity index
126
126
117
additives in DOS base oil. In the case of S-105 base oil, as the
chain length increased from C to C , an increase in activity, and
was optimized using base oil (DOS) spiked with additive butyl
0-epithioundecanoate (4d) ranging from 0.5 to 4% and their
1
4
1
on further increase in chain length (C ), a decrease in activity
8
OOTs are 181.8, 188.1, 229.2, 230.5, and 232.5 °C, respectively.
The maximum antioxidant activity was observed at 2 wt % con-
centration, and even though the additive dosage was increased
to 3 and 4%, a negligible increase in antioxidant activity was
observed. Hence, the antioxidant efficacies of synthesized epithio
derivatives and commercial antioxidant, BHT, were evaluated
at 2 wt % concentration in the base oils. All of the epithio
undecanoates exhibited superior performance compared to BHT
in DOS base oil, whereas 4d exhibited comparable performance
with BHT in EJB and S-105 base oils. The nature of the ester
alkyl group of the epithio undecanoate derivatives displayed
different antioxidant activities in the three base oils tested.
Products prepared from short-chain alcohols (methyl and
n-butyl) exhibited better performance, and as the chain length
increased (2-ethylhexyl), the antioxidant activity decreased.
There is a clear effect of the length of the alkyl chain of the ester
moiety of epithio derivatives on antioxidant potency, resulting in
products (4a and 4d) with greater antioxidant ability to stabilize
base oil than their corresponding epithio derivative, 4e. Similar
results have been published earlier for ether derivatives of
were observed. Butyl 10-epithioundecanoate exhibited excellent
antioxidant activity among all of the synthesized additives in the
three base oils EJB, DOS, and S-105 with OOTs of 180.6, 229.2,
and 242 °C, respectively, followed by 4a. A typical dynamic DSC
curve is given in Figure 1, showing the dynamic curve of a base oil
without any additive and of a base oil with 4d at 2 wt %
concentration in EJB and S-105 base oil.
Performance Evaluation of the Synthesized Epithio
Derivatives as Extreme Pressure Additives. The weld point
(WP) in extreme pressure property can be defined as the load at
which welding of the balls due to the friction between the
stationary three balls and the rotating top ball is high enough to
weld. The WP of a test sample can be evaluated within ±10 kg by
increasing the smaller load close to the weld point. Higher WP
indicates the better EP properties of the test sample. Compounds
containing certain elements such as sulfur, phosphorus, and halo-
gens are known to be capable of participating in tribochemical
reactions with the friction surfaces and generating in situ tribo-
32
films that reduce friction under extreme pressure conditions.
Therefore, the synthesized alkyl epithio undecanoates were
evaluated for their EP activity using three base oils, EJB, DOS,
and S-105, and the results were compared with those of
commercial EP additive, TCP. In the absence of EP additives the
three base oils displayed weld points of 160, 120, and 120 kg,
respectively. The base oil doped with the additives was stirred
under magnetic stirring for 30 min at 40 °C to attain the
homogeneity of the sample. Butyl epithio undecanoate and base
oil DOS were used for optimization of additive concentration.
The improvement in weld point of the base oil was analyzed at
different concentrations ranging from 0.5 to 4%, and their weld
points were 130, 140, 170, 160, and 150 kg, respectively. From
the results 2 wt % was the optimum concentration at which the
maximum weld point was observed. Even though the concen-
tration was increased further to 3 and 4%, a decrease in weld
point was observed. The blending of EP additives with the base
oils resulted in increased weld point of three base oils as shown in
Table 3. As the chain length of the ester moiety of the episulfide
undecanoate increased from C to C , the weld point of the base
21,25
hydroxytyrosol and amino alcohols,
and in which the
shorter alkyl side chains improved the scavenger activity and
the reducing ability of hydroxytyrosol, and the presence of longer
26
linear chains (C −C ) decreased such antioxidant effects.
7
9
A cutoff effect in the antioxidant efficacy in relation to the alkyl
chain length was also reported by Sorensen et al. on alkyl
ferulates. The most efficient alkyl ferulates were methyl and butyl
ferulate, whereas octyl ferulate was pro-oxidative, and the pro-
oxidative effect increased further with an increment in the alkyl
chain length from C to C . Further elongation of the alkyl chain
8
12
length to C16 and C20 resulted in weak pro-oxidative effects to
27
weak antioxidative effects. This might be also due to readily
available sites for oxidation, making them more sensitive to
28−31
cleavage than shorter ester chain lengths.
The three epithio
derivatives prepared with iso alkyl epoxy undecanoates exhibited
less antioxidant activity compared to straight-chain epithio
derivatives. This could be related to the steric effect of side chains
present in the ester chain of isoalkyl epithio derivatives.
1
4
Epithio derivatives with methyl and butyl ester chain lengths
exhibited comparable antioxidant activities in base oil EJB. The
same trend was observed in DOS and S-105 base oils, and the
results are summarized in Table 2. The optimum chain length
oil EJB increased from 160 to 210 kg. Surprisingly, methyl and
butyl ester chains of episulfide undecanoate exhibited similar
weld points of 210 kg. In the case of the 2-ethyhexyl ester of
episulfide undecanoate the decrease in weld load of 180 kg was
observed. The same trend was observed in base oils DOS and
C exhibited superior OOT of 229 °C among the synthesized
4
D
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Journal of Agricultural and Food Chemistry
Article
Figure 1. Nonisothermal DSC curves of base oil with and without antioxidant.
S-105. This shows a decrease in load-carrying capacity with
increasing chain length of the alcohol moiety of the fatty acid
ester of epithio derivatives. These results demonstrate the effects
of molecular structure on lubricity property. This is in good
agreement with the general observation that shorter chain
lengths reduce molecular interactions and decrease the tem-
derivatives with these two oils than with mineral oil. Similar
enhancements of load-carrying capacity in biobased base stocks
relative to mineral oil have been reported. Results indicate that
the prepared compounds exhibited improved EP activity, and
they can be effectively used as EP additives for biobased lubricant
basestocks.
34
33
perature stability of a protective lubricant film. n-Butyl 10-
epithioundecanoate exhibited comparable performance with
commercial additive TCP in both base oils EJB and DOS and
inferior performance in S-105. The synthesized additives
exhibited better compatibility with biolubricant base oil (EJB)
and synthetic ester (DOS) compared to mineral base oil (S-105),
which indicates a better synergistic effect of the epithio
Performance Evaluation of Epithio Derivatives as
Antiwear Additives. The investigation of the antiwear ability
of methyl and butyl epithio undecanoates was done by varying
their concentration in base oil DOS. Figure 2 shows the variation
of the WSD of the lower steel balls with different additive
concentrations. It is observed from the figure that the addition
of additives in base stock, even at small concentrations, can
E
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Journal of Agricultural and Food Chemistry
Article
Table 3. Effect of 2 wt % Concentration of the Extreme
Pressure Additive on Three Base Oils
concentration. The slight increase in the WSD with increasing
additive concentration has also been reported in the litera-
3
5,36
ture.
This might be ascribed to corrosion of metal caused by
weld load (kg)
the sulfur element present in the additives.
sample name
EJB
DOS
S-105
Worn Surface Analysis. Figure 3 shows the SEM images of
the worn surface of steel balls lubricated with base oil DOS alone
and with 4a added oil at 0.6% concentration. The wear scar
obtained from the test lubricated with base oil alone exhibited
more scratches and deeper furrows compared with the scar
produced with base oil with 0.6 wt % epithio derivative (4a). It
can also be observed that the worn surface of steel balls lubricated
by base oil with 4a is shallower and smoother than that lubricated
by base oil alone. This result further verifies that 4a possesses
good AW ability.
no additive
ESUME
160
210
180
180
210
170
180
210
120
170
140
140
170
130
140
160
120
160
130
130
160
120
130
200
ESIPAUES
ESC-4UES
ESnBU-UES
ESnHE-UES
ES-2ET-HE-UES
TCP
In conclusion, we have synthesized a series of novel epithio
derivatives from 10-undecenoic acid. The addition of epithio
derivatives as multifunctional additives to the base oils resulted in
the enhancement of antioxidant, extreme pressure, and antiwear
properties. Among the reported products, butyl 10-epithounde-
canoate exhibited superior antioxidant and extreme pressure
properties.
AUTHOR INFORMATION
■
Corresponding Author
*
(R.B.N.P.) Phone: +91 040 27193370. Fax: +9140 27193370.
E-mail: rbnprasad@iict.res.in.
Notes
The authors declare no competing financial interest.
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Figure 2. Wear scar diameter of the synthesized antiwear additive at
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