Synthesis and Characterization of Dialkyl Carbonates Prepared from Mid-, Long-Chain, and Guerbet Alcohols

Article abstract of DOI:10.1007/s11746-004-0897-4

A series of organic carbonates derived from mid-, long-chain, and Guerbet alcohols were synthesized in good yields (69-80% isolated yields after purification by distillation or recrystallization) through a carbonate interchange reaction by heating the linear or Guerbet alcohols with diethyl carbonate in the presence of catalyst, n-dibutyltin oxide. The carbonates were isolated by Kuegelrohr distillation or recrystallization from acetone and characterized using FTIR, GC, ¹¹H and ¹³C NMR spectroscopy, and GC-MS. El and positive Cl mass-spectral techniques were used to characterize the carbonates. Such materials are currently of commercial interest as lubricants and additives in various industrial applications.

Full text of DOI:10.1007/s11746-004-0897-4

Synthesis and Characterization of Dialkyl Carbonates
Prepared from Mid-, Long-Chain, and Guerbet Alcohols
James A. Kenar*, Gerhard Knothe, and Ashley L. Copes
Food and Industrial Oil Research, NCAUR, ARS, USDA, Peoria, Illinois 61604
ABSTRACT: A series of organic carbonates derived from mid-, the better-known methods of preparing dialkyl carbonates in-
long-chain, and Guerbet alcohols were synthesized in good
yields (69–80% isolated yields after purification by distillation or
recrystallization) through a carbonate interchange reaction by
heating the linear or Guerbet alcohols with diethyl carbonate in
the presence of catalyst, n-dibutyltin oxide. The carbonates were
isolated by Kügelrohr distillation or recrystallization from acetone
and characterized using FTIR, GC, 1H and 13C NMR spec-
troscopy, and GC–MS. EI and positive CI mass-spectral tech-
niques were used to characterize the carbonates. Such materials
are currently of commercial interest as lubricants and additives in
various industrial applications.
clude oxidative carbonylation of alcohols with carbon monox-
ide (13); reaction of alcohols with phosgene (14); reaction of
urea with alcohols in the presence of metal salt catalysts (15);
and reaction of alkyl halides with metal carbonate salts such as
K2CO3 or KHCO3 (16). More recently, carbonate interchange
reactions between small-chain dialkyl carbonates and alcohols
have received much attention (17). Dialkyl carbonates also
have been prepared enzymatically using lipases (18,19). Al-
though proprietary work has been carried out using various di-
alkyl carbonates, to our knowledge, a systematic analytical
characterization, especially of linear longer-chain and branched
dialkyl carbonates, is not available.
Paper no. J10742 in JAOCS 81, 285–291 (March 2004).
KEY WORDS: Carbonate interchange reaction, dialkyl carbon-
ates, fatty alcohols, Guerbet alcohols, Guerbet carbonates, long-
chain carbonates, mass spectrometry, nuclear magnetic reso-
nance.
In the present work, nine linear and branched dialkyl car-
bonates were prepared in good yields through a carbonate in-
terchange reaction between various mid-, long-chain, or Guer-
bet alcohols and diethyl carbonate using dibutyltin oxide as cat-
alyst. The carbonates were subsequently characterized by 1H
and 13C NMR, FTIR, as well as EI and positive CI GC–MS.
Dialkyl carbonates are generally safe noncorrosive molecules
employed in numerous commercial and synthetic applications
(1–3). For example, short-chain dialkyl carbonates such as di-
methyl and diethyl carbonate are currently being evaluated as
oxygen-containing additives to reduce exhaust emissions in
gasoline and diesel engines (4). Both diethyl and dimethyl car-
bonates are gaining popularity as nontoxic replacements for haz-
ardous chemical reagents such as phosgene and dimethyl sul-
fate (5). Recently, Rüsch gen. Klaas and Warwel (6) showed
that dimethyl and diethyl carbonates can be used as reactive sol-
vents to simultaneously extract and transesterify oilseed oils.
Longer-chain and branched (Guerbet) dialkyl carbonates
also may find application in lubricant (7,8), cosmetic (9,10),
plasticizer (11), and fuel (12) compositions. In the case of di-
alkyl carbonate-based lubricants, the carbonate group is postu-
lated to interact with metallic surfaces, and tribological testing
has shown them to have oiliness properties comparable to or
better than mineral oil and traditional synthetic esters with the
same viscosities, as well as better seal compatibility (3).
EXPERIMENTAL PROCEDURES
Materials. All chemicals were purchased from Aldrich Chemi-
cal Co. (Milwaukee, WI) and used without further purification
unless otherwise noted. Oleyl alcohol (85% technical grade,
Aldrich) was found to contain approximately 20% trans-iso-
mer as determined by integration of the cis- (5.35–5.39 ppm)
and trans- (5.31–5.35 ppm) olefinic hydrogen signals in the
NMR spectra. Methylene chloride was obtained from Fisher
Scientific Co. (Fairlawn, NJ). Isolauryl (2-butyloctanol, Jarcol
I-12) and isocetyl (2-hexyldecanol, Jarcol I-16) alcohols were
obtained from Jarchem Industries, Inc. (Newark, NJ).
Melting points. Melting points were determined on a Fisher–
Johns melting point apparatus and are uncorrected.
1
NMR. H and 13C NMR spectra were recorded using a
Bruker ARX 400 spectrometer (Billerica, MA) with a 5-mm
dual proton/carbon probe (400 MHz H/100.61 MHz 13C)
1
using CDCl3 as the solvent in all experiments.
The excellent properties and apparent commercial applica-
tions of dialkyl carbonates have led to a sizable body of patent
literature reporting dialkyl carbonate syntheses (1,2). Some of
FTIR. IR spectra were obtained using a PerkinElmer (Nor-
walk, CT) Spectrum RX FTIR system as either a film on NaCl
plates (liquids) or in a KBr matrix (solids).
GC. GC was performed with a Hewlett-Packard 5890 Series
II gas chromatograph (Palo Alto, CA), equipped with an FID and
an autosampler/injector. Analyses were conducted on an HP-
5MS capillary column, 30 m × 0.25 mm i.d. (Hewlett-Packard).
*To whom correspondence should be addressed at USDA, ARS, NCAUR,
1815 N. University St., Peoria, IL 61604.
E-mail: kenarja@ncaur.usda.gov
Copyright © 2004 by AOCS Press
285
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J.A. KENAR ET AL.
TABLE 1
Reaction Conditions, Yields, and b.p./m.p. of Long-Chain and Guerbet Carbonates 4a–ia
Temp.
(°C)
Time
(h)
Yieldb
(%)
b.p.
(°C/Torr)
m.p.
(°C)
4a, R = CH3(CH2)7–
4b, R = CH3(CH2)8–
4c, R = CH3(CH2)9–
115
125
24
19
79.8
77.5
100/0.370
125/0.480
—
—
120
120
130
20
28
25
78.8
77.8
77.3
142/0.140
—
46–47
56–57
4d, R = CH3(CH2)15
–
—
—
4e, R = CH3(CH2)17
–
4f, R = CH3(CH2)7CH=CH(CH2)8–
(cis/trans ca. 3.76:1)
4g, R = CH3(CH2)3CH(C2H5)CH2–
(2-ethylhexyl)
4h, R = CH3(CH2)5CH(C4H9)CH2–
(2-butyloctyl)
4i, R = CH3(CH2)7CH(C6H13)CH2–
(2-hexyldecyl)
140
140
135
133
24
26
24
24
77.7
70.6
69.4
71.1
195/0.480
100/0.500
140/0.275
185/0.380
—
—
—
—
a0.3–0.9 mol% of Bu2SnO relative to alcohol was used as catalyst.
bIsolated yield of purified dialkyl carbonate.
TABLE 2
1H and 13C NMR Data of Mid-, Long-Chain and Guerbet Carbonates
Synthesized in the Present Work
Entrya
1H NMR
13C NMR
4a
4.09 (t, 4H, J = 6.7 Hz), 1.70–
1.60 (m, 4H), 1.45–1.20 (m,
22H), 0.85 (t, 6H, J = 6.9 Hz)
155.51, 68.08, 31.82, 29.25, 29.21,
28.75, 25.77, 22.69, 14.12
4b
4c
4d
4e
4f
4.03 (t, 4H, J = 6.7 Hz), 1.60–
1.54 (m, 4H), 1.30–1.19 (m,
24H), 0.80 (t, 6H, J = 6.9 Hz)
155.39, 67.86, 31.86, 29.48, 29.25,
28.72, 25.73, 22.65, 14.01
4.10 (t, 4H, J = 6.7 Hz), 1.70–
1.60 (m, 4H), 1.38–1.20 (m,
28H), 0.86 (t, 6H, J = 6.9 Hz)
155.53, 68.12, 31.96, 29.57, 29.37,
29.30, 28.76, 25.78, 22.75, 14.19
4.10 (t, 4H, J = 6.7 Hz), 1.70–
1.55 (m, 4H), 1.40–1.15 (m,
52H), 0.87 (t, 6H, J = 6.8 Hz)
155.53, 68.11, 32.90, 32.01, 29.77,
29.75, 29.64, 29.57, 29.52, 29.44,
29.31, 28.76, 25.78, 22.77, 14.19
4.11 (t, 4H, J = 6.7 Hz), 1.70–
1.55 (m, 4H), 1.40–1.15 (m,
60H), 0.87 (t, 6H, J = 6.8 Hz)
155.53, 68.12, 32.01, 29.77, 29.75,
29.72, 29.64, 29.57, 29.44, 29.31,
28.77, 25.79, 22.77, 14.20
5.40–5.29 (m, 4H, cis + trans),
4.08 (t, 4H, J = 6.7 Hz), 2.00–
1.85 (m, 8H), 1.63–1.55 (m,
4H), 1.27–1.23 (m, 44H), 0.85
(t, 6H, J = 6.9 Hz)
155.46, 130.44, 130.24, 129.98,
129.79, 68.03, 32.61, 31.92, 29.78,
29.73, 29.67, 29.53, 29.50, 29.40,
29.33, 29.21, 29.14, 29.05, 28.70,
27.22, 27.19, 25.71, 22.69, 14.11
4g
3.99 (d, 4H, J = 6.00 Hz),
1.60–1.50 (m, 2H), 1.35–1.23
(broad m, 16H), 0.87–0.83 (m,
12H)
155.78, 70.33, 38.91, 30.17, 28.91,
23.53, 22.98, 14.02, 10.89
4h
4i
4.01 (d, 4H, J = 5.95 Hz), 1.70–
1.57 (broad m, 2H), 1.34–1.23
(broad m, 32H), 0.86 (m, 12H)
155.80, 70.77, 37.50, 31.87, 31.06,
30.73, 29.66, 28.93, 26.69, 23.02,
22.70, 14.11, 14.06
4.01 (d, 4H, J = 5.94 Hz), 1.70–
1.60 (broad m, 2H), 1.28–1.24
(broad m, 48H), 0.86 (m, 12H)
155.82, 70.82, 37.53, 31.97, 31.88,
31.05, 30.01, 29.67, 29.63, 29.38,
26.73, 26.70, 22.74, 22.71, 14.14
aEntry numbers correspond to Scheme 1.
JAOCS, Vol. 81, no. 3 (2004)

SYNTHESIS OF DIALKYL CARBONATES
287
SCHEME 1
The column flow was 1.0 mL/min with helium head pressure of tion (125°C/0.48 mTorr) gave 78.0 g of 4b (entry 4b in Tables
15 psi (776 torr); split ratio of 75:1. Two different oven tempera- 1 and 2) as a colorless distillate (77.5%). Tables 1 and 2 con-
ture ramp programs were utilized: Program A, 100°C for 2 min, tain specific information on reaction conditions, product yields,
ramp 100–270°C at 20°C/min, hold 20 min at 270°C. Program and characterization data for each of the products prepared.
B, 140°C for 2 min, ramp 140–280°C at 20°C/min, hold 30 min
at 280°C. Injector and detector temperatures were set at 280°C.
RESULTS AND DISCUSSION
GC–MS. GC–MS analyses were conducted using a Hewlett-
Packard 5890 Series II Plus GC (HP-5MS column, 30 m × 0.25 Scheme 1 depicts the dialkyl carbonate interchange reaction
mm i.d.; Hewlett-Packard) coupled with a Hewlett-Packard sequence utilized to synthesize the linear and branched carbon-
5989B mass spectrometer using a mass range of 50–550 amu. ates, 4, prepared in this study. By using a modified procedure
EI was performed at 70 eV, whereas positive CI used methane of Shaikh and Sivaram (17), alcohol 1 was reacted with diethyl
as reagent gas. GC conditions: helium head pressure 3 psi (155 carbonate, 2, under neat conditions using dibutyltin oxide
torr); injector temperature set at 250°C; transfer line tempera- (0.3–0.9 mol% relative to alcohol 1) as catalyst. Although
ture set at 280°C. The two oven temperature ramp programs many catalysts are known to catalyze the dialkyl carbonate in-
described in the GC experimental section were utilized for terchange reaction (1,8), dibutyltin oxide was chosen in favor
GC–MS studies.
of other catalysts based on the work of Shaikh and Sivaram
Carbonate purification. Solid materials were recrystallized (17) that showed dibutyltin oxide to be an effective catalyst for
by dissolving material in warm acetone. Upon cooling to room the synthesis of dialkyl carbonates. Our reactions proceeded
temperature, white crystals formed that were collected by vac- smoothly, and although literature reports suggest GC analyses
uum filtration. Liquid materials were distilled in vacuo using a
Kügelrohr distillation apparatus.
Representative procedure for the synthesis of linear and
branched dialkyl carbonates. Synthesis of nonyl carbonate 4b
(17). In a three-necked round-bottomed flask equipped with a
magnetic stir bar and reflux condenser, a mixture of nonyl al-
cohol (93.6 g, 0.64 mol) and dibutyltin oxide (807.6 mg,
0.0032 mol, 0.5 mol% relative to alcohol) was heated to
80–95°C under a slight positive nitrogen pressure. When the
dibutyltin oxide catalyst dissolved in the alcohol, diethyl car-
bonate (37.9 g, 0.32 mol) was added dropwise over approxi-
mately 30 min. The flask temperature was then raised to 125°C
and refluxed 19 h. Reaction progress was monitored by taking
aliquots of the reaction mixture at various time points and ana-
lyzing them by GC. When deemed complete, the reaction mix-
ture was taken up in CH2Cl2 (100 mL) and washed with H2O
(3 × 50 mL). The combined CH2Cl2 extracts were dried
(MgSO4), filtered through a pad of Celite®, and concentrated
in vacuo on a rotary evaporator. The crude product was then
placed under vacuum overnight at 22°C. GC analyses of the re-
sulting crude pale yellow oil showed the following relative
amounts: starting alcohol (9%), mixed ethyl nonyl carbonate
(<1%), and desired nonyl carbonate (90%). The identification
FIG. 1. 400 MHz 1H NMR spectrum of 2-hexyldecyl carbonate, 4i, in
of each compound was verified by GC–MS. Kügelrohr distilla- CDCl3. Peak assignments are designated by the letters a, b, and c.
JAOCS, Vol. 81, no. 3 (2004)

288
J.A. KENAR ET AL.
FIG. 2. Assignment of the characteristic 13C NMR carbonate signals.
of smaller carbonates are problematic due to CO2 liberation
upon heating the carbonate (20), the reaction progress of the
carbonates in this study was conveniently monitored by GC ex-
cept for oleyl carbonate 4f (oven temperature program A was
used for carbonates 4a–c and 4g, whereas oven temperature
program B was used for 4d,e,h,i). Oleyl carbonate was prob-
lematic and gave poor GC results. The reasons for this are not
presently clear and may result either from its nonvolatility or
possible side reactions occurring at the unsaturated sites of the
molecule under the GC conditions tried.
During the early-time aliquots of the reactions, unsymmet-
rical carbonates, 3, were produced in conjunction with the de-
sired symmetrical carbonates, 4, in up to 1:1 relative amounts.
We did not isolate the unsymmetrical carbonates, but their
identities were confirmed by GC–MS analyses. As the reaction
progressed, 3 was subsequently converted into the desired car-
bonate, 4. Table 1 gives reaction conditions, b.p./m.p. data, and
isolated yields obtained for the various dialkyl carbonates syn-
thesized. As can be seen, the carbonates were obtained in yields
ranging from 69 to 80% after purification by distillation or re-
crystallization. IR spectroscopy of all the purified carbonates
showed two strong absorptions of interest at 1747 and 1250
cm−1, corresponding to the C=O and C–O stretching frequen-
cies, respectively.
NMR spectroscopy. The 1H and 13C NMR data for the car-
bonates synthesized are presented in Table 2 and Figure 2. The
1H NMR spectra of all the carbonate samples are generally simi-
lar, and the spectrum of 2-hexyldecyl carbonate, shown in Fig-
ure 1, is representative (Table 2, entry 4i). The main feature of
interest is the doublet at 4.01 ppm. This signal corresponds to the
α-methylene hydrogens (designated by “a” in Fig. 1) and is split
into a doublet by the adjacent β-methine hydrogen (designated
by “b” in Fig. 1). The corresponding α-methylene hydrogen sig-
nals in the linear carbonates 4a–f are located at approximately
FIG. 3. Positive CI (methane) and EI mass spectra of linear and branched
carbonates. (A) CI of decyl carbonate 4c; (B) CI of 2-butyloctyl carbon-
ate 4h; (C) EI of decyl carbonate 4c; (D) EI of 2-butyloctyl carbonate 4h.
the same chemical shift but are split into a triplet owing to the CH2 and CH3 peaks, and their integration corresponds to the ex-
presence of two adjacent β-methylene hydrogens (Table 2). The pected number of hydrogens. In addition to the signals observed
remaining hydrogen signals in the 1H NMR show the expected for the saturated carbonates, oleyl carbonate, 4f, also contained
JAOCS, Vol. 81, no. 3 (2004)

SYNTHESIS OF DIALKYL CARBONATES
289
TABLE 3
Positive CI and EI Mass Spectral Data of Mid-, Long-Chain, and Guerbet Carbonates
Synthesized in the Present Work
Entrya
PCI-MS
EI-MS
4a
327 ([M + C3H5]+, 10%), 315 ([M
+ C2H5]+, 15%), 287 (MH+, 100%),
203 (10%), 175 (25%), 159
287 (MH+, 2%), 175 (7%), 157 (1%)
129 (1%), 113 ([C8H17]+, 48%),
71 (100%)
(4%), 129 (11%), 113 (84%)
4b
4c
355 ([M + C3H5]+, 7%), 343 ([M +
C2H5]+, 14%), 315 (MH+, 66%),
217 (9%), 189 (23%), 173 (3%),
143, 11%), 127 (100%)
315 (MH+, 1%), 189 (8%), 171 (1%),
143 (1%), 127 ([C9H19]+, 52%)
71 (100%)
383 ([M + C3H5]+, 6%), 371 ([M +
C2H5]+, 12%), 343 (MH+, 61%),
231 (8%), 203 (17%), 187 (4%),
157 (14%), 141 (100%)
343 (MH+, 4%), 203 (9%), 185 (1%),
157 (1%), 141 ([C10H21]+, 69%),
85 (100%)
4d
4e
509 ([M − H]+, 6%), 241 (65%),
225 (100%)
511 (MH+, 1%), 287 (3%), 225
([C16H33]+, 49%), 85 (59%), 71
(74%), 57 (100%)
No molecular ion or adducts,
269 (61%), 253 (100%)
No molecular ion, 315 (2%),
253 (55%), 57 (100%)
4fb
4g
—
—
287 (MH+,19%), 285 ([M − H]+, 5%),
173 (21%), 159 (21%), 129 (11%),
113 (100%)
287 (MH+, 0.2%), 175 (1%), 129
(1%), 112 ([C8H16]+, 66%), 70 (83%),
57 (100%)
4h
4i
397 ([M − H]+, 1%), 229 (6%),
215 (2%), 185 (29%), 169 (100%)
399 (MH+, 0.2%), 231 (1%), 185
(1%), 168 ([C12H24]+, 71%), 111
(57%), 57 (100%)
510 (M+, 1%), 285 (5%), 271 (1%),
241 (32%), 225 (100%)
No molecular ion, 241 (1%), 224
(56%), 111 (50%), 57 (100%)
aEntry numbers correspond to Scheme 1.
bOleyl carbonate, 4f, was not amenable to GC–MS or DIP under the conditions tried. PCI, positive
CI; DIP, direct insertion probe.
signals at 5.29–5.40 ppm, corresponding to the hydrogens
attached to the olefinic carbons.
The shift of the α-carbons varies from about 68 ppm for lin-
ear alkyl chains to 70.3–70.8 ppm for branched alkyl moieties. It
is noteworthy that the α-carbon in long-chain di-Guerbet esters
(22), which corresponds to the α-carbon in the present com-
pounds, displays its shift slightly upfield (66.5–66.6 ppm) from
that of the present carbonates (23). The reason for the differences
in the shifts of the α-carbons in the present compounds vs. di-
Guerbet esters is likely the additional oxygen atom of the car-
bonates causing reduced electron density at the α-carbon of the
carbonates. The signals of the β-carbons are found at 25.7–25.8
ppm in the unbranched carbonates, but in the branched com-
pounds their shifts are observed at 37.5–38.9 ppm.
There also appears to be an effect of chain length, as the
shortest chain in the β-position (2-ethylhexyl carbonate) ap-
pears to cause an upfield shift (70.33 ppm) of the α-carbons’
signal compared to the longer chains (in 2-butyloctyl and 2-
hexyldecyl carbonate) with values around 70.8 ppm. A similar,
albeit reverse, chemical shift effect regarding chain length is
observed in the β-carbons, which are the branching sites in the
branched carbonates. For these β-carbons, the signal in the
The structure of the carbonates with assignments of charac-
teristic 13C signals is given in Table 2 and Figure 2. The 13
C
NMR spectra of the carbonates display the signal of the car-
bonate carbons at 155–156 ppm. These shifts are considerably
upfield from typical alkyl esters of FA, in which cases the sig-
nals of the ester carbons are usually in the 173–175 ppm range.
An electron-releasing effect of the additional oxygen atom
compensating for electron withdrawal of the carbonyl oxygen
has been discussed for the upfield shifts of carboxylic carbons
vs. keto carbons (21) and may be responsible for the further up-
field shift of the carbonate carbons vs. carboxylic carbons.
However, there appears to be a minor difference in the chemi-
cal shift of the carbonate carbons depending on the nature of
the alkyl chain. In the β-branched carbonates, the carbonate
carbon signal is observed at approximately 155.8 ppm, whereas
in the case of linear alkyl chains, the signal is found slightly
upfield at 155.4–155.5 ppm. The effect of branching is more
pronounced in the signals of the α- and β-carbons (see Fig. 2).
JAOCS, Vol. 81, no. 3 (2004)

290
J.A. KENAR ET AL.
Fragment ions at m/z 185 and 169 correspond to [C12H25O]+
and [C12H25]+, respectively. The prominent C2H5 adduct ob-
served in the linear carbonates (m/z 231 in Fig. 3A) was not ob-
served for any of the branched carbonate molecules.
short-chain ethyl group is observed at 38.91 ppm, but in the
case of the longer butyl and hexyl groups, the β-carbon signal
is shifted upfield to about 37.5 ppm.
MS. Only a few studies have examined the MS of carbonate
molecules, and these have focused mainly on short-chain car-
bonates (24,25). Both EI and methane positive CI GC–MS
were utilized to characterize the dialkyl carbonates prepared
here. The four mass spectra depicted in Figure 3 of decyl car-
bonate (linear) and 2-butyloctyl carbonate (branched) serve to
illustrate the important mass spectral features observed in the
CI and EI ionization modes. Methane CI, one of the most com-
monly used alternative ionization techniques, commonly gives
abundant product ions from the molecules that are indicative
of the M.W. Linear carbonates 4a–c were no exception, and, as
can be seen in the CI mass spectrum of decyl carbonate (Fig.
3A), intense fragment ions at m/z 343, 371, and 383 were ob-
served, corresponding to the protonated carbonate, the C2H5
adduct, and the C3H5 adduct, respectively. Interestingly, the
longer C16 linear carbonate, 4d, gave only a [M − H]+ adduct
at m/z 509, whereas the C18 linear carbonate, 4e, did not give
any ions corresponding to the M.W. Table 3 tabulates the dis-
tinctive EI and CI fragments for the compounds examined.
Branched carbonates, 4g–i, did not give any C2H5 or C3H5
adducts corresponding to the M.W., as can be seen in the CI
spectrum of 2-butyloctyl carbonate 4h (Fig. 3B). Instead, 2-
ethylhexyl carbonate 4g gave a relatively abundant MH+ mo-
lecular ion at m/z 287, 2-butyloctyl carbonate 4h gave a [M −
H]+ molecular ion at m/z 397, and 2-hexyldecyl carbonate 4i
gave a M+ molecular ion at m/z 510.
The linear and branched carbonates, 4a–c and 4g–i, respec-
tively, showed distinctive fragment ions due to cleavage around
the carbonate moiety, confirming its presence. For example, the
CI spectrum of decyl carbonate, Figure 3A, shows fragment
ions at m/z 203, 187, 157, and 141 corresponding to
[C11H23O3]+, [C11H23O2]+, [C10H21O]+, and [C10H21]+, re-
spectively. The fragment ion at m/z 203 [C11H23O3]+ is likely
derived from loss of the C10H21 alkyl group in conjunction with
a rearrangement of two protons, although a McLafferty-type
rearrangement of a protonated carbonate molecule is also plau-
sible. Loss of a C10H21O moiety with rearrangement of two
protons gives fragment ion m/z 187 [C11H23O2]+. This type of
double hydrogen rearrangement for small-chain carbonates has
been noted previously (22). The m/z 231 ion observed in the CI
of decyl carbonate (Fig. 3A) is likely a C2H5 adduct of frag-
ment ion m/z 202 ([C11H22O3]+, not observed). A C2H5 adduct
of this type was also observed in all three carbonates 4a–c.
The CI mass spectral characteristics for the branched car-
bonate series 4g–i are similar to that observed for 4a–c (Fig.
3B). The fragment ions observed for 2-butyloctyl carbonate 4h
at m/z 229, 215, 185, and 169 correspond to cleavage at the
same bond locations as observed for linear carbonates, 4a–c,
although the fragment mechanisms appear to be slightly differ-
ent. Fragment ion m/z 229 [C13H25O3]+ occurs from loss of a
C12H25 alkyl group without any hydrogen rearrangement, but
the fragment ion at m/z 215 [C13H27O2]+ likely results from
loss of a C12H25O moiety with rearrangement of two protons.
Generally, the spectra obtained using EI-MS gave similar
results. Many of the characteristic fragment ions observed,
however, were weaker in intensity, and some of the hydrogen
rearrangements seen in the CI spectra did not occur in the EI
spectra (Figs. 3C, 3D). The EI spectra also typically gave very
weak MH+ fragment ions, and in fact, no molecular ions were
observable for carbonates 4e and 4i. Finally, the EI mass spec-
tra of both the linear and branched carbonates gave more in-
tense fragmentation ions at the lower m/z ranges due to alkyl
chain fragmentation.
Dialkyl carbonates were synthesized from mid-, long-chain,
and Guerbet alcohols and characterized by MS and NMR. Both
EI and CI gave distinctive fragment ions useful in confirming
the identities of the carbonate structures.
ACKNOWLEDGMENTS
The authors thank Joneen McElligott for her help in the preparation
of these compounds and Dr. David Weisleder for collection of the
NMR data.
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[Received October 16, 2003; accepted January 16, 2004]
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