Semiquantitative Determination of Quinonoid Structures in Isolated Lignins by 31P Nuclear Magnetic Resonance

Article abstract of DOI:10.1021/jf9804802

A semiquantitative analytical method has been developed for the determination of the total quinonoid content (o-benzoquinones and p-benzoquinones) of soluble lignins. The method is based on detailed measurements and observations made with model o- and p-quinones, which in dry organic solvents were shown to form adducts with trimethyl phosphite in quantitative yield. These adducts gave ³¹P NMR signals for o- and p-quinones at around -46 and -2 ppm, respectively. In the presence of moisture and lignin the adducts from o-quinones were shown to be hydrolyzed to the open ring product, dimethylphenyl phosphate (-2 ppm), at an overall yield of ～70%. Similarly, the hydrolysis yields of p-quinone adducts with trimethyl phosphite were ～70%. Consequently, a number of important issues in relation to the use of trimethyl phosphite toward the quantitative analysis of quinonoid groups in lignins have been investigated, which permitted the development of an experimental semiquantitative protocol recommended for spectral acquisition.

Full text of DOI:10.1021/jf9804802

4628
J. Agric. Food Chem. 1998, 46, 4628−4634
Sem iqu a n tita tive Deter m in a tion of Qu in on oid Str u ctu r es in Isola ted
Lign in s by 31P Nu clea r Ma gn etic Reson a n ce
Dimitris S. Argyropoulos* and Liming Zhang
PAPRICAN and Department of Chemistry, Pulp and Paper Research Centre, McGill University,
3420 University Street, Montreal, Quebec, Canada H3A 2A7
A semiquantitative analytical method has been developed for the determination of the total quinonoid
content (o-benzoquinones and p-benzoquinones) of soluble lignins. The method is based on detailed
measurements and observations made with model o- and p-quinones, which in dry organic solvents
were shown to form adducts with trimethyl phosphite in quantitative yield. These adducts gave
31P NMR signals for o- and p-quinones at around -46 and -2 ppm, respectively. In the presence
of moisture and lignin the adducts from o-quinones were shown to be hydrolyzed to the open ring
product, dimethylphenyl phosphate (-2 ppm), at an overall yield of ∼70%. Similarly, the hydrolysis
yields of p-quinone adducts with trimethyl phosphite were ∼70%. Consequently, a number of
important issues in relation to the use of trimethyl phosphite toward the quantitative analysis of
quinonoid groups in lignins have been investigated, which permitted the development of an
experimental semiquantitative protocol recommended for spectral acquisition.
Keyw or d s: Adducts; analysis; determination; lignins; nuclear magnetic resonance (NMR);
phosphorus; quinones; spectroscopy
INTRODUCTION
method for determining such functional groups in
soluble lignins based on the ability of quinones to oxidize
hydrazine and release a stoichiometric amount of ni-
trogen. The number of quinone groups present in a
sample of Bjo¨rkman spruce lignin was thus determined
to be 0.26 wt % or 0.8/100 lignin C9 units. o-Quinone
detection has become possible for solid pulp samples
(Lebo et al., 1990; Argyropoulos et al., 1992, 1995;
Argyropoulos and Heitner, 1994) by oxyphosphorylation
followed by elemental analysis of the phosphorus con-
tent of the treated pulps. This gave an o-quinone
estimation of 0.7/100 C9 units for black spruce mechan-
ical pulp (Argyropoulos and Heitner, 1994) in agreement
with those of Imsgard et al. (1971) and Zakis et al.
(1987). Phosphorus elemental analysis on freshly oxy-
phosphorylated pulp samples, however, requires tedious
sample preparation, which introduces significant com-
plications.
The present work attempts to introduce a convenient
semiquantitative analytical method for the detection of
both o- and p-quinones in soluble lignins based on
quantitative 31P NMR. More specifically, this work
examines the potential of trimethyl phosphite toward
being a derivatizing reagent that can provide quantita-
tive information on quinonoid groups in soluble lignins.
Initially, we examined various salient features of the
reactions of o- and p- quinones with trimethyl phosphite
to fully comprehend the suitability of the proposed
derivatization system toward the quantitative analysis
of quinones. We then turned our attention to the
stability of the adducts, internal standard, and spin-
lattice relaxation considerations for model compounds
and lignins oxyphosphorylated with trimethyl phosphite
to arrive at a set of recommended spectral acquisition
conditions. Finally, we applied this technique to the
quantitative analysis of quinones in various lignins and
discuss the results in light of the actual lignin sample
and the method of preparation.
The color of paper has been an issue of major concern
in many research endeavors including those of the
pulping of wood, the bleaching of pulp, and the yellowing
of paper. Chromophoric lignin structures have long
been considered responsible for causing most of the color
in pulp and paper products. Despite numerous research
efforts, the state of our knowledge about the origin of
color in different wood pulps is still relatively limited.
Among the possible chromophoric structures suggested,
quinonoids are by far the most mentioned (Pew and
Connors, 1971; Leary, 1967; Imsgard et al., 1971).
Therefore, the detection and quantification of quino-
noids in pulp and soluble lignins is of considerable
significance from both a theoretical and a practical point
of view.
The formation of quinonoid structures in lignin during
photoyellowing has been qualitatively verified by model
compound (Zhang and Gellerstedt, 1994b), UV-visible
(Zhang and Gellerstedt, 1994a), fluorescence (Zhu et al.,
1995; Konya and Scaiano, 1994), and 31P NMR studies
(Lebo et al., 1990; Argyropoulos et al., 1992, 1995;
Argyropoulos and Heitner, 1994). Imsgard et al. (1971)
developed a colorimetric method, in which o-quinones
were reduced to the corresponding catechols followed
by analysis of the intensely colored, catechol-ferric
complexes in a DMSO/cellosolve mixture. About 0.7
mol/100 lignin C9 units of o-quinonoids was thus esti-
mated to be present in spruce milled wood lignin. An
important limitation of this technique, however, is the
fact that p-quinonoid chromophores cannot be detected.
Furthermore, the steps of quantitative reduction and
recovery of the reduced lignin samples are relatively
cumbersome. Zakis et al. (1987) have developed a
* Author to whom correspondence may be addressed [fax
(514) 398-8254; e-mail dimitria@shared1.lan.mcgill.ca].
10.1021/jf9804802 CCC: $15.00 © 1998 American Chemical Society
Published on Web 10/17/1998

Quinonoid Structures in Isolated Lignins
J. Agric. Food Chem., Vol. 46, No. 11, 1998 4629
F igu r e 1. Reaction of p-benzoquinones with trimethyl phosphite and 31P NMR spectra of compound 5. Its proton-coupled spectrum
gives seven lines (right).
EXPERIMENTAL PROCEDURES
Reaction of Lignin Samples with Trimethyl Phosphite. Prior
to analysis, all lignin samples were thoroughly dried in a
vacuum oven set at room temperature for at least 48 h and
then stored in a vacuum desiccator over Dryrite. Samples of
Compounds 1 and 9 are commercially available (Aldrich
Chemical Co.); all other quinone models were prepared in a
previous effort (Zhang and Gellerstedt, 1994).
lignin (50 mg) were accurately weighed in
a dry 2 mL
Mod el Com p ou n d Stu d ies. Internal Standard Solution.
Triphenyl phosphate (327.6 mg) and chromium acetylacetonate
(27.7 mg) were dissolved in dry dioxane, and the volume of
the solution was adjusted to 5 mL.
volumetric flask. The sample was then dissolved with ∼0.8
mL of DMF, 50 µL of trimethyl phosphite was then added,
and the mixture was capped and stirred in the dark for 24 h
at room temperature. Internal standard solution (100 µL con-
taining ∼1.3 × 10-3 mmol of triphenyl phosphate) was finally
added prior to dilution of the mixture to the mark with DMF.
31P NMR Spectroscopy. 31P NMR spectra were obtained on
a Varian XL300 Fourier transform (FT-NMR) spectrometer,
oprerating at a frequency of 121.5 MHZ. All chemical shifts
are relative to 85% phosphoric acid, which was used as
external standard. Quantitative spectra were acquired using
a flip angle of 45°, a pulse delay of 15 s, gated decoupling, and
a spectral window of 100 ppm (30 to -70 ppm).
Reactions of Model Compounds with Trimethyl Phosphite.
Approximately 20 µmol of a quinone model compound was
dissolved in the selected solvent [chloroform-d (CDCl3), diox-
ane, dimethylformamide (DFM)], alone or together with 50 mg
of dioxane lignin in a 2 mL vial. Trimethyl phosphite (50 µL)
was added to the vial, and the solution was stirred in the dark
at room temperature overnight. Internal standard solution
(100 µL) was then added to the vial. The whole mixture was
finally transferred to a 5 mm NMR tube together with 60 µL
of deuterated acetone for the purpose of providing the deute-
rium lock.
Lign in Isola tion a n d Rea ction s. Milled wood lignin was
prepared in accordance with the method of Bjorkman (1956).
Kraft lignin was isolated form spent liquor in accordance with
previously published procedures (J iang and Argyropoulos,
1994), whereas the treatment of the kraft lignin with dimeth-
yldioxirane was carried out in accordance with the procedure
described by Sun and Argyropoulos (1996). Finally, the oxygen
treatment of the kraft pulp and the isolation of the lignin was
carried out in accordance with the earlier effort described by
Sun and Argyropoulos (1995).
RESULTS AND DISCUSSION
Ch em ica l Rea ction s betw een Ben zoqu in on es
a n d Tr im et h yl P h osp h it e. Trimethyl phosphite is
known to react under ambient conditions with both o-
and p-quinones (Sun and Argyropoulos, 1995). p-
Quinones in the presence of trimethyl phosphite are
known to yield dimethylphenyl phosphates (Duthaler
et al., 1984), which are quite stable and can readily be
followed with 31P NMR.
In an effort to further understand these reactions and
to realize their quantitation potential, we used quanti-
tative 31P NMR in the presence of an internal standard
(triphenyl phosphate). It was thus found that the
various p-benzoquinones (Figure 1) formed the respec-
Lign in Qu in on e An a lyses. Internal Standard Solution.
Triphenyl phosphate (41.6 mg) and chromium acetylacetonate
(54.4 mg) were weighed in a dry 10 mL volumetric flask and
were dissolved with DMF to the mark. The sharp signal of
triphenyl phosphate at -16.13 ppm was thus used for accurate
quantification.

4630 J. Agric. Food Chem., Vol. 46, No. 11, 1998
Argyropoulos and Zhang
F igu r e 2. Reactions of o-benzoquinones with trimethyl phosphite and 31P NMR spectra of compound 11. Its proton-coupled
spectrum gives eight well-resolved and two very weak lines (right).
tive dimethyl phosphates in quantitative yield at room
temperature. These adducts give 31P NMR signals
between -2 and -3 ppm, (Figure 1), which allow the
quantification of the starting quinones by signal inte-
gration. When their 31P NMR spectra are recorded with
proton coupling, the seven signals in the spectrum can
clearly be seen (J ) 12 Hz), indicating that there are
two methoxy groups bonded to the central phosphorus
atom.
However, the reactions of o-benzoquinones with tri-
methyl phosphite are more complicated. A number of
reports have shown the first step in the reaction of an
o-quinone and trimethyl phosphite is the formation of
a pentoxyphosphorane, which gives 31P NMR signals at
∼ -46 ppm. In fact, Ramirez (1964) has obtained
several such phosphoranes in crystalline form which
were stable in the absence of moisture and air. This
was confirmed in our work because trimethyl phosphite
on reaction with the o-benzoquinones of Figure 2 gave
the expected cyclic oxyphosphorane adducts within
minutes at room temperature. These adducts were
found to be stable in chloroform solutions for several
weeks. For example, pure 11 was obtained by mixing
8 with trimethyl phosphite in chloroform followed by
solvent evaporation. The proton-coupled 31P NMR
spectrum of the pentoxyphosphorane shows 10 lines,
among which only 8 can clearly be observed to have a
coupling constant of 12.7 Hz, in accordance with the
structure of the cyclic pentoxyphosphorane as obtained
by Ramirez (1964).
In accordance with previous suggestions (Sun and
Argyropoulos, 1994), cyclic pentoxyphosphoranes were
found to be very sensitive to traces of water, usually
present in pulp and/or lignin samples. All samples of
o-quinone model compounds when treated with tri-
methyl phosphite in the presence of solvents containing
low levels of water or lignin showed the formation of
the cyclic oxyphosphorane as evidenced by the signal
at -46 ppm, which, however, quickly disappeared with
the simultaneous formation of a new signal at ∼ -2 ppm
(Figure 3). Konya and Scaiano (1994) have reported
that the conversion of the signal from -46 to -2 ppm
can be accomplished by adding acid. The proton-coupled
31P NMR spectrum of the newly formed signal was
found to be split into seven lines with a coupling
constant of 11.5 Hz, indicating that two methoxy groups
were still bonded to the phosphorus atom while the third
methoxy group was cleaved during the conversion. On
the basis of these observations, one may assume that
dimethylphenyl phosphate is formed during the conver-
sion. This is also supported by the work of Kirillova
and Kukhtin (1962), who have observed the formation
of dialkyl phosphate esters as the predominant products
of water addition to similar cyclic pentaoxyphospho-
ranes.

Quinonoid Structures in Isolated Lignins
J. Agric. Food Chem., Vol. 46, No. 11, 1998 4631
Ta ble 1. 31P NMR Ch em ica l Sh ift a n d Yield Da ta for a
Va r iety of Mod el Qu in on es Rea cted w ith Tr im eth yl
P h osp h ite
in high-purity
CDCl3
in DMF in the presence
of dioxane lignin
starting
compd
yield
%
31P
yield
%
product
δ
product NMR δ
1
2
3
7
8
4
5
6
10
11
-2.74 86.6
-2.33 84.8
-2.35 96.8
-46.08 92.0
-45.42 90.0
4
5
6
13
14
-2.32
-1.90
-1.92
-2.16
-2.33
-1.37
-1.78
52.5
64.6
73.8
68.7
63.8
9
12
-46.45 97.5
15
78.3
because the quinonoid content of most lignin samples
is extremely low and the reaction between lignin and
trimethyl phosphite requires at least 24 h. During this
period a very large signal (compared to the quinonoid
adducts) caused by these byproducts can be formed. The
large intensity of this signal causes proper phasing and
integration a rather cumbersome task. The signal at 3
ppm was due to trimethyl phosphate, which is an
oxidation product of trimethyl phosphite by air. In fact,
this is the signal that causes the major interference to
integration and phasing due to its proximity to the
signals derived from quinones (-1 to -3 ppm). The
formation of trimethyl phosphate may be minimized by
excluding the air in the system or by carrying out the
reaction at lower temperatures. Such effects are to be
examined in our future efforts. The identities of the
other two signals at 9 and 15 ppm have not been
determined, although their proton-coupled 31P NMR
spectra were both composed of seven lines (Figure 4),
indicating that there are two methoxy groups bonded
to the central phosphorus atoms in these structures.
F a ct or s Th a t In flu en ce t h e Qu a n t it a t ion of
Qu in on es. Reaction Yields. Table 1 shows the 31P
NMR chemical shift and yield data for a variety of model
quinones reacted with trimethyl phosphite. When the
reaction was carried out in high-purity CDCl3, the yields
were in the range of 85-97%. However, when the same
reactions were carried out in DMF in the presence of
dioxane lignin (50 mg), the yields of the expected
dimethylphenyl phosphates 4, 5, and 6 from p-quinones
1, 2, and 3 decreased to the range of 52-74%. Because
model quinone 3 is of the structure most likely to be
present in lignin, one may assume that lignin p-
quinones will yield 6 at ∼70%. (The low yield may be
entirely due to hydrolysis arising from H2O in DMF and
not from lignin itself.)
F igu r e 3. 31P NMR spectra demonstrating the conversion of
the cyclic pentoxyphosphorane 10 to dimethylphenyl phos-
phate 13. The signals at 3, 9, and 15 ppm arise from the
degradation of trimethyl phosphite.
Previous efforts (Lebo et al., 1990) have shown that
in the presence of water, pentoxyphosphoranes are
quantitatively converted to cyclic phosphate esters of
the type depicted by 16 (δ at 11.1; Lebo et al., 1990).
However, the proposed cyclic phosphate esters have only
one methoxy group bonded to the phosphorus atom,
which should show a characteristic quartet. Such a
signal was indeed detected at 13.9 ppm, in trace
amounts, when 12 was converted to 15 by the addition
of water (Figure 4). However, pentoxyphosphoranes 10
and 11 did not give rise to any such signals when
treated with water under the conditions used in this
effort. In contrast, the formation of dimethyphenyl
phosphates (13-15) was continuously observed as the
predominant products of this work, in agreement with
the accounts of Kirillova and Kukhtin (1962). The latter
group studied the formation of a similar cyclic phos-
phate by hydrolysis of its pentoxyphosphorane precur-
sor. Low temperatures (0 °C) were essential for isolat-
ing the cyclic phosphate at a maximum of 30% yield,
whereas the open ring dimethylphenyl phosphate (-2
ppm) was always the predominant product at a yield of
>70%.
The products of hydrolysis of pentoxyphosphoranes
10-12 can give two sets of isomers as shown in Figure
4. However, we found that 12 gave only one isomer of
15 with a clean characteristic septet in its proton-
coupled 31P spectrum (Figure 4). This may be due to
the steric effect of the neighboring tert-butyl group,
which favors the formation of only one of the isomers.
Compound 11, on the other hand, after hydrolysis gave
the two isomers of 14, which displayed two well-resolved
31P signals at -1.37 and - 2.33 ppm, respectively. The
31P NMR signals of the two isomers from compound 13
could not be resolved.
In accordance with the preceding discussion, o-qui-
nones 7, 8, and 9 initially formed the cyclic pentoxy-
phosphoranes 10, 11, and 12, displaying signals be-
tween -45.42 and -46.45 ppm. These were rapidly
converted to dimethylphenyl phosphates 13, 14, and 15
when the oxyphosphorylation was carried out in DMF
in the presence of dioxane lignin at an average overall
yield of ∼70%.
On the basis of the overall preceding discussion and
the data of Table 1, one may assume that quinones in
lignin will react with trimethyl phosphite to give 31P
NMR signals in the range -1.0 to -3.0 ppm at a yield
of ∼70%.
Trimethyl phosphite was also found to slowly react
with air and moisture, giving three characteristic
signals at 3, 9, and 15 ppm, respectively (see Figures 3
and 4). Such impurities cannot be avoided because they
spontaneously form within trimethyl phosphite. These
signals did not cause significant interference during our
model compound study. They are, however, of greater
concern when lignin samples are examined. This is
Relaxation Time Considerations. Our efforts to estab-
lish a quantitative analytical method necessitated the
need to measure the spin-lattice relaxation times (T1)
of the 31P NMR signals that are to be eventually

4632 J. Agric. Food Chem., Vol. 46, No. 11, 1998
Argyropoulos and Zhang
F igu r e 4. Formation of dimethylphenyl phosphates by hydrolysis of pentoxyphosphoranes and the 31P NMR spectra showing
trace amounts of cyclic phosphate ester 16 derived from 12.
Ta ble 2. 31P Sp in -La ttice Rela xa tion Tim e (T1) Va lu es
concentrations, such acquisition protocols will be unac-
for Ad d u cts of Qu in on es a n d Tr im eth yl P h osp h ite in th e
ceptable. Kasler and Tierney (1978) have demonstrated
P r esen ce a n d Absen ce of Ch r om iu m Acetyla ceton a te
that quantitative 31P NMR spectra of organic com-
spin-lattice relaxation time (T1) (s)
pounds may be obtained in the presence of a compound
bearing a paramagnetic metal center [chromium(III)
acetylacetonate; Cr(acac)3], and this approach has suc-
cessfully been applied in our earlier lignin work (Argy-
ropoulos, 1994; Granata and Argyropoulos, 1995).
in the absence
of Cr(acac)3
in the presence
of Cr(acac)3
structure
5
10
13
2.7
6.5
2.9
4.2
1.5
3.0
1.6
2.1
The resulting relaxation data in the presence of Cr3+
for the model compounds of this work are also shown
in Table 2. All T1 values are seen to significantly
decrease, thus reducing the time needed for quantitative
measurements. The cyclic pentoxyphospholane (10) was
found to have the longest T1. However, it is the final
products, dimethylphenyl phosphates (5 and 13), that
are to be analyzed for quantification. Both of these
compounds were found to have shorter T1 values than
that of the internal standard, triphenyl phosphate. In
the presence of Cr3+ triphenyl phosphate showed a T1
of 2.1 s. Therefore, a relaxation delay of 15 s was
selected for quantification purposes. This should be
sufficiently long, considering the fact that quinones in
lignin are bound on a polymer which moves more slowly
and therefore is of shorter T1 than those of the examined
model compounds. Furthermore, a pulse flip angle of
45° would further ensure that conditions of complete
relaxation are met prior to the next pulse.
triphenyl phosphate
integrated. This is because to obtain accurate signal
areas under Fourier transform NMR conditions, suf-
ficient delay time between pulses should be used. In
our earlier work (Argyropoulos et al., 1993), the phos-
phorus-31 spin-lattice relaxation times for a variety of
model tricoordinated phoshites were determined and
found to range between 5 and 10 s. However, we had
no relaxation information for pentacoordinated phos-
phorus compounds similar to those examined in this
work. As such, the spin-lattice relaxation times for
selected model quinones derivatized with trimethyl
phosphite and that of the internal standard (triphenyl
phosphate) were measured. As shown in Table 2, the
T1 of such systems may vary between 3 and 7s. Such
high T1 values are not unusual in 31P NMR spectroscopy
(Dale and Hobbs, 1971). Therefore, a delay time of at
least 40 s needs to be applied between pulses if
quantitative claims are to be made from the obtained
spectra. Under conditions of relatively low quinone
Quantification of Quinones in Lignin. The experi-
mental conditions employed for the quantification of
quinones in lignin were chosen according to the data

Quinonoid Structures in Isolated Lignins
J. Agric. Food Chem., Vol. 46, No. 11, 1998 4633
Ta ble 3. Tota l Qu in on e Con ten t of Differ en t Lign in
Sa m p les Mea su r ed in Th is Wor k
tively when it reacts with trimethyl phosphite in dry
organic solvent. In the presence of moisture or lignin,
the initially formed pentoxyphosphorane hydrolyzes to
give dimethylphenyl phosphate in a yield of ∼70%.
Dimethylphenyl phosphate shows a unique signal at
∼ -2 ppm in the 31P NMR spectrum, which allows the
semiquantitative estimation of the total quinone content
of lignin samples.
lignin sample
black spruce MWL
softwood solubilized kraft lignin
indulin (commercial kraft lignin)
dimethyldioxirane-treated hardwood
residual kraft lignin
quinone content (mmol/g)
0.020
0.029
0.023
0.24
oxygen-treated kraft lignin
0.055
LITERATURE CITED
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16 (5) J 139.
where A0 is the area integration for signal from qui-
nones at -1 to -3 ppm and A1 is the area integration
for the internal standard.
A number of lignin samples were analyzed for their
quinone content using the procedure developed in this
work. The accumulated data are given in Table 3.
Assuming that the repeat unit in milled wood lignin
(MWL) has a molecular weight of 194, the quinone
content of black spruce MWL obtained in this work (0.02
mmol/g) is equal to 0.4/100 C9 units, which is somewhat
lower than the results obtained in other previous studies
(Imsgard et al., 1971; Argyropoulos and Heitner, 1994).
Furthermore, oxygen delignification was found to in-
crease the quinone content within kraft lignin, possibly
via demethylation reactions as recently demonstrated
by Asgari and Argyropoulos (1997). Finally, a treat-
ment of a hardwood (aspen) solubilized kraft lignin with
dimethyldioxirane resulted in a dramatic increase in its
quinone, as expected from its actual reactivity with
lignin model compounds (Argyropoulos et al., 1996).
Con clu sion s. Trimethyl phosphite reacts with p-
benzoquinones to form dimethylphenyl phosphate in
high yield in dry organic solvent at room temperature.
If moisture or lignin is present in the reaction mixture,
the yield of dimethylphenyl phosphate will be ∼70%.
o-Benzoquinone forms pentoxyphosphorane quantita-
Pew, J . C.; Connors, W. J . Color of Coniferous Lignin. Tappi
1971, 54 (2), 245.

4634 J. Agric. Food Chem., Vol. 46, No. 11, 1998
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Received for review May 11, 1998. Revised manuscript received
August 17, 1998. Accepted August 26, 1998.
J F9804802