TRANSFORMATIONS IN
NONAQUEOUS
OF
LIGNIN
IN
NITRATION
MEDIA
N. N. Shorygina, and B. V. Lopatin
L.
L.
Sergeeva,
UDC 542.958.1 ~- 668,474
In all the reactions of lignin an important part is played by its "active" functional groups. These include, above all, benzyl alcohol and benzyl ether groups, which to a considerable extent determine such characteristic reactions for lignin as sulfitation in the sulfite kiering process of wood, the reaction with mercaptoaeetic acid, alkylation with alcohol at about 20 ~ and so on. The side chains of a considerable fraction of the structural units of lignin contain a primary hydroxy group in the 3-position relative to t:he aromatic nucleus. This is also responsible for some of the typical reactions for lignin, e.g., the formation of formaldehyde when lignin is heated with an inorganic acid. An extremely reactive group, determining many of the properties of lignin, is the free or etherified phenolic hydroxyl in the para position relative to the side chain. It was of interest to study the behavior of the above-listed "active" functional groups of lignin when the latter is treated with nitric acid. The complex and peculiar structure of lignin, the variability of its functional composition, and the strength of some of the links between structural elements make it impossible to break the lignin macromolecule down into separate fragments. To determine the changes occurring in separate structural units in the treatment of lignin with nitric acid we have studied the nitration of substances which can be regarded as models of the separate structural elements and contain the aboveindicated functional groups of lignin [1-4]. In these investigations as model compounds we have chosen derivatives of (4-hydroxy-3-methoxyphenyl)and (3,4-dimethoxyphenyl).-prepanes containing a benzyl alcohol, benzyl ether, or primary alcohol group. The nitration of the model compounds was conducted in absence of water in organic solvents so as to reduce the possibility of the occurrence of side reactions, particularly oxidation reactions. As nonaqueous solvents we used CCI 4 and dry ether. In the first case nitration occurred under heterogeneous conditions (because the model substances and nitric acid are sparingly soluble in CCI4). The amount of nitric acid was varied from 3 to 12 moles per mole of the substance being nitrated. In nitration with a solution of concentrated nitric acid in dry ether the reaction went in a homogeneous system. For a comparison of the behavior of the model compounds with that of lignin, in the present work we have nitrated lignin isolated by the hydrochloric acid process ("hydrochloric lignin") from fir under the same conditions as those used previously in the nitration of the model substances. In the nitration of hydrochloric lignin in CCI 4 the amountofnitric acid taken was based on 3, 6, 9, and 12 rnoles per mole of the phenylpropane unit of lignin, which we took to be 182 g. In some experiments after the addition of nitric acid in the course of 30 rain at 5 ~ to the CCI 4 suspension of lignin the nitrolignin was filtered off, and in others after the addition of the nitric acid stirring was continued further for 30 or 120 rain at room temperature, The nitration of lignin in dry ether was carried out with a 4 N solution of concentrated HNO 3 in the same solvent. The nitration of lignin in CCI 4 did net go uniformly: when the reddish-brown nitrolignin was ground, regions were met which had the color of the original lignin and did not contain nitrogen. Nonuniformity of nitration was most often observed when the reaction was conducted with 9 and 12 molecular proportions of nitric acid with insufficiently vigorous stirring. The nitration of hydrochloric lignin in dry ether had an autocatalytic character: the reaction mixture became slightly warm and the ether assumed a reddish-yellow color. All the nitrolignins obtained were analyzed for carbon, hydrogen, methoxy and earboxy groups, and total and ester nitrogen, in the CCI 4 or ether filtrate from the nitrolignins the content of 4,6-dinitroguaiacol was determined.
N. D. Zelinskii Institute of Organic C h e m i s t r y , A c a d e m y of Sciences of the USSR, T r a n s l a t e d f r o m I z v e s t i y a A k a d e m i i Nauk SSSR, Seriya K h i m i c h e s k a y a , No. 2, pp. 372-378, F e b r u a r y , 1967. Original a r t i c l e s u b m i t t e d October 19, 1964.
351
TABLE
i
Lignin : ethereal HNO
i: 20 1:20
3
Reaction time, rain
Yield of n i t r o lignin, 70 on lignin taken
Content, 70, of H
OCH 3 COOH
total N
Yield of 4,6dinitroguaiacol, ester % onwt. of lignin N taken
60 100.60 180 98.66 Original h y d r o c h l o r i c lignin
0.30 0.27
TAB LE 2 ! O
09
2 ~
CD ~-4 O
6 9 12 3 6 9 12 3 6 9 12 * Time
0 0 0 0 30 30 30 30 120 120 120 120
14-16 14 - 16 14-16 14-16 14-16 14-16 14-16 14-16
for the addition of HNO
i00.40 107.60 iii.00 120.00 103.00 ii0.00 115.00 118.00 120.00 116.00
0.88 0.39 0.40 0.19 0.83 0.31
112.00
0.28
114.00
0.16
3, 30 rain at 5 ~
The r e s u l t s obtained in the nitration of h y d r o c h l o r i c lignin in d r y e t h e r a r e given in Table 1, and for nitration in CC14 - in T a b l e 2. On the b a s i s of the data given in T a b l e s 1 and 2 calculations w e r e m a d e of the s e m i e m p i r i c a l " f o r m u l a s " of the s t r u c t u r a l units of nitrolignin showing the changes o c c u r r i n g in the functional composition of the lignin in its r e a c t i o n with nitric acid. We c o n s i d e r e d it p o s s i b l e to calculate such f o r m u l a s even though p a r t of the C6C3 s t r u c t u r e s underwent profound change, p r o b a b l y even with degradation of the a r o m a t i c nuclei. The a v e r a g e composition of the C6C3 s t r u c t u r a l e l e m e n t s of h y d r o c h l o r i c lignin is
3, [ Q )
CgH0.1sO2.~a(OCF a,~0.05~,COOH o.o17 The a v e r a g e composition of the C~C3 s t r u c t u r a l e l e m e n t s of the nitrolignin obtained by the nitration of h y d r o c h l o r i c lignin with 4 N e t h e r e a l ItNO 3 for a) 60 min and b) 180 rain is [ O~ ~ / NO2 ~ a) C~Hs.0703.15 (OCH3)o.o~ICOOH ]0.21 \--C--NO2/0.52
[ NO2 b) C~t]s.2aOs.29(OCHs)0.8, ( C 2 H ) 0 . 2 ~ \--C--NQ]o.ss The a v e r a g e compositions of the CsC a s t r u c t u r a l e l e m e n t s of the nitrolignins obtained in the nitration of h y d r o c h l o r i c lignin in COl 4 a r e as foIlows:
352
Without a standing period
No+
0++
--C-- NO++/+.st\ --O--NO++] o.o+
(o+
~ (
No++ ~ (
so++
,+
9M HNO~. C+Us.o+Os.+o(OCIt+)o,++\ CO0 H ]++.++t\--C--NO++/o.++\--o--mom)~.o+
,,.:z.,+.+u~o~. c+~+.,,,o+.+,+(o cH~)~ (co~/o.+++ ~-c--so++/o.+4 ~-o-~o++/o.+,. With a standing period of 30 rain
,,
0++
,,,+o+
\--C+NQ]o.++ ~--O--NO+/o,1+
O~ \ / NO~ \ / NO++ \ t COOH)o.a I--C--NQ)o.6~ t --O--NO2)o.r;
/
6M }IN03' CgHT.,oOaoo(0CHs
/
0++ ~
/
NO++ \
[
N%
9+:I//HNOs.C++Hs.se,O+,+a(OCHs)o.+o~CO0 H)o..t+..pt--C--N O,J o.,~c,\--0--N 0+].~.+o
With a standing period of 120 rain
( 2 :~t (
3M NHOa ' C9Hs.7~03.~o(OCH3)o,58 r ., +M .NO..c~
;
O~ ,,
r~ ,'
NO~
i ',
[
+oO+., ( o c H , ) o . + 4 c o o H ) o . g _ c _- ~ + O J o . + + _-.
O+ ~
/
f2MHNO3'C"II'320+~176 "
'
/
t OO (cO~H~
NO~
[
NO+
NO~, 'i
\--O--NOJo.++ ( NO~ \--O--NOd+.+I
~
~
]o46 \ - - C - - N O ~ / o . s s
[
NO~
\--O--NO+/oz,
From these "formulas" it will be seen that under all nitration conditions the number of OCH~ groups remaining in the nitrolignin is about 0.6 to each CGC 3 structural element, as compared with 0.95 in the original lignin. The amount of nitrogen determined by the Kjeldahl method less by the nitrate nitrogen determined by the Schultz-Tiemann method was taken to represent nitro groups entering the aromatic nuclei of lignin. The average number of these per C6C 3 unit was 0.38-0.70 in the nitrelignins obtained by nitration in CCI 4 and 0.52-0.56 in the nitrolignins obtained by nitration in ether. It follows that not every C6C 3 unit of the lignin is substituted by nitro, in the nitration of lignin in CCIr the esterification of the alcoholic hydroxyls of the lignin occurs with formation of nitric esters. The number of nitrate groups in the nitrolignin increases with increase in the amount of nitric acid taken for nitration. In nitration in ether, however, the formation of nitric esters was not observed. The nitration of lignin was accompanied by oxidation, which follows from the formation of carboxy groups in the nitrolignins. As a by-product in the nitration of lignin we isolated 4,6-dinitroguaiacol. The original hydrochloric lignin and some samples of nitrolignins were investigated spectroscopically. In Fig. i, we give the IR spectrum of hydrochloric lignin, and in Fig. 2 we give the IR spectra of nitrolignins prepared by the nitration of ligin in ether (i) and CCI 4 (2). The absorption band at about 1600 cn'l-I in the Ill spectra of the original lignin and of the nitrolignins indicates the presence of an aromatic ring, and the absorption band at 3450 cm -I is characteristic for OH groups. The intense absorption band at 1280 cm -i in tahe IR spectra of the original lignin and the nitrolignins is probably due to C-O vibrations in the grouping
O
--o--c.
In the IR spectra of the nitrolignins (Fig. 2) bands due to the symmetric and antisymmetric stretching vibrations of a nitro group attached to an aromatic ring are very distinct (vs 1340 cm -I, Uas 1530 cm-t). The IR spectrum of the nitrolignin obtained by the nitration of lignin in CCI 4 contains an absorption band at
353
,•1•0 i
~ I
I
I
$w
I
I
i
BSO0
I
ZOO0
I
I
t
~1
I
Iv-
I I l__J /000 000 dO0 p, cm -1
I ~ I ~ I
/800 /500 /~00 /ZOO
Fig. 1
9~. 1oo [
~ zob
f,i'~ J800
d800 3000 /600 /w
/~00
IZ00 /00g
" #00
B00
~),cm-1
Fig. 2 1642 cm -1 c h a r a c t e r i s t i c for the n i t r i c e s t e r group (-O-NO2) , whereas this is not p r e s e n t in the s p e c t r u m of the nitrolignin p r e p a r e d by the nitration of lignin in ether. This is in a c c o r d with the results of chemical analysis (Schultz-Tiemann method). Comparison of the r e s u l t s obtained in the nitration of lignin and in the nitration of the p r e v i o u s l y studied model compounds [derivatives of (4-hydroxy-3-methoxyphenyl)- and (3,4-dimethoxyphenyl)-prepanes] enables us to make some observations about the probable reactions o c c u r ring when lignin i s t r e a t e d with nitric acid. In the r e a c t i o n of lignin with nitric acid in the f i r s t place the nitro group enters the a r o m a t i c nuclei of lignin. In the s t r u c t u r a l units with a free phenolic hydroxyl in the para position relative to the side chain the nitro group e n t e r s the 5-position of the a r o m a t i c nucleus, but in the units with an etherified phenolic hydroxyl - in the 6-position. This follows from data obtained in the nitration of model compounds - in the 4 - h y d r o x y - 3 - m e t h o x y p h e n y l group the nitro group enters the 5-position, but in the 3,4-dimethoxyphenyl group it e n t e r s the 6-position. The fact that in the nitration we were unable to introduce m o r e than 0.7 of a nonester nitrogen atom per C6C3 s t r u c t u r a l unit can be explained on the view that in lignin some of the otherwise possible positions are either occupied or a r e difficultly a c c e s s i b l e as a r e s u l t of s t e r i c hindrance. In the nitration (in CC14) of ~ - e t h y l - 3 , 4 - d i m e t h o x y b e n z y l alcohol (I) and its methyl ether (II), as well as the e n t r y of a nitro group into the 6-position, the formation of a nitric e s t e r group o c c u r r e d by the esterification of the benzyl alcohol hydroxyl and by the t r a n s e s t e r i f i c a t i o n of the benzyl methyl ether, r e s pe ctive ly. RI i HCOR2 i
RI ] HCONO~ OiN
I
\
%/\ I OCIts OCHs OCH~ (III) RI=C2Hs; R~=H(I); R~=CHs(II). [
OCHa
I t is probable that the n i t r i c e s t e r groups f o r m e d in the nitration of lignin in CCI4 a r e mainly in the side chains in the oz-position relative to the a r o m a t i c ring (their formation m u s t be a s c r i b e d to the esterification of benzyl alcohol hydroxyls and the t r a n s e s t e r i f i c a t i o n of benzyl methyl ether groups in lignin). In the nitration of (i) and (II) with 12 m o l e c u l a r proportions of HNO3 in CC14 we obtained c~-ethyl-4,5-dimethoxy2,3-dinitrobenzyl alcohol nitrate (IV), in which as well as the nitric e s t e r group there are two nitric groups in the a r o m a t i c nucleus. The nitric e s t e r group of this compound shows a considerable r e s i s t a n c e to acid h y d r o l y s i s [ s e e s c h e m e (IV) on following page]. It m a y be supposed that with a considerable excess of nitric acid (nitration in CC14) in some units of lignin two nitro groups can enter the a r o m a t i c nucleus. M o r e o v e r , the introduction of the second nitro group into the nucleus should r a i s e the r e s i s t a n c e of the nitric e s t e r group to acid hydrolysis considerably.
354
R1
1~1
HCOR2
I
HCON02
QN
l OCII3 OCHs
I
02N
t
OC[t~
OCI[~
(~V) R~ -C2Ha; R2=It(I); R,.,=CII3(I[)
In the nitration of ~ - e t h y t - 4 - h y d r o • and ~ - e t h y l - 3 , 4 - d i m e t h o x y b e n z y l alcohols and their methyl ethers, as well as the entry ef a nitre group into the nucleus, the electrophilic replacement of the side chain by a nitro group occurred with formation, respectively, of 4,6-dinitroguaiacol and 4,5-dinitroveratrole. Moreover, the replacement ef the side chain by a nitro group occurred more readily when the guaiacol residue was present. On analogy- with the behavior of these model substances it may be supposed that in the nitration of lignin breakdown of the molecules occurs as a result of the eleetrophilic replacement of side chains by nitro groups in structural elements containing free or etherified benzyl alcohols groups. The elimination of a side chain will occur particularly readily in units with a free phenolic hydroxyl in the 4-position. The formation of 4,6-dinitrog~Jaiacol in the nitration of lignin is asseciated, in particular, with the presence of structural elements containing a benzyl alcohol er benzyl ether group together with a free phenolic hydroxyl in the 4-position of the aromatic nucleus. We have reported previously [3] that in the nitration of 3-(4-hydroxy-3-methoxyphenyl)-l-propanol (V) in dry ether a yield of up to 70% was obtained of the nitro-o-benzoquinone (V-i), i.e., demethylation subsequent oxidation o c c u r r e d . CH~0H
CH20II
CH~
CH2
CH~
CH~
r
i
% \ 0~H OCH~ (v)
/ 02N
with
\ II
O (v~)
0
On the basis of this it may be supposed that the breakdown of lignin molecules in nitration is possible also as a result of the cleavage of aromatic nuclei due to demethylation and the formation of nitro quinones, which under the further action of nitric acid will break down, probably with formation of acids. It should be noted that, irrespective of the relative amount of nitric acid, the reaction time, and the medittrn in which the nitration was carried out, the mtmber of OCH 3 groups remaining in the nitrolignin per C~C 3 structural unit was about 0.6, as compared with 0.95 in the original hydrochloric iignin, which indicates the demethylation of certain structural elements of the lignin. It is evident that such structural elements include units containing a free phenolic group in the para position relative to the side chain. In the reaction of lignin with nitric acid there may occur, depending on the conditions, various condensation processes determined by the presence of benzyl alcohol and benzyl ether groups. On entering the 6-positions of the aromatic nuclei of lignin, nitro groups lower the ability of benzyl alcohol groups to undergo condensation reactions, and in the substituted dibenzyl ethers formed they stabilize the ether link against acid and all,aline hydrolysis and the side chains against oxidation. The complexity of the interaction of lignin with nitric acid arises from the fact that the above-described reactions, which affect both the aromatic nuclei and the side chains, proceed simultaneously. EX 1~ ERI M E NTA Hydrochloric
lignin was prepared
from the wood
L
of newly felled fir by the method
described
in [5].
Nitration of Hydrochloric Lignin in CCI 4. 5gofligninand400nllofCCl4wereintr o_ duced into a three-necked flask fitted with stirrer, thermometer and dropping funnel. The CCI 4 suspension of lignin was cooled to 5 ~ and in the course of 30 rain the calculated amount of HNO 3 of sp.gr. 1.50 in 50 ml of CCI 4 was added dropwise with constant shaking of the dropping funnel. When the allotted period had ex-
355
pired, the nitrolignin was filtered off and t r a n s f e r r e d in small portions into 500 m l of ice water. The p r o d uct was again filtered off and washed until there was no longer an acid reaction in Congo Red. It was dried, f i r s t in air, and then to constant weight in a vacuum d e s i c c a t o r over P20~. Nitration of Hydrochloric Lignin in Dry Ether. A 4NsolutionofHNO3 ofsp.gr.l.50 in dry ether was poured onto 5 g of lignin, and the mixture was left at room t e m p e r a t u r e with periodic shaking for 60 o r 180 rain. The nitrolignin was filtered off and washed with ether, and it was t r a n s f e r r e d in s m a l l portions into 500 m l of ice water. It was again filtered off and washed until there was no longer an acid r e a c t i o n to Congo Red. It was dried, f i r s t in air, and then to constant weight in a vacuum d e s i c c a tor over P20~. Isolation of 4,6-Dinitroguaiacol f r o m t h e U s e d CC14 a n d E t h e r Reaction M e di a . The used solvent media in the above experiments were extracted with 2 N NaOH. The alkaline extracts w e r e acidified with dilute H2SO4 and extracted with benzene. The residue remaining after the r e moval of benzene was r e c r y s t a l l i z e d from alcohol and weighed. The total nitrogen content of the nitrolignin was determined by the Kjeldahl method. The amount of nitrate nitrogen was d e t e r m i n e d by the S c h u l t z - T i e m a n n method. The content of OCH3 groups was d e t e r mined by Biihn's method. The content of carboxy groups was determined by the calcium acetate method. IR s p e c t r a w e r e d e t e r m i n e d with a Nippon Bunke (Japan) DS 301 double-beam IR s p e c t r o p h o t o m e t e r on samples pelleted with KBr. CONCLUSIONS i. Hydrochloric lignin was nitrated in CCI 4 and in ether under the conditions used earlier for the nitration of model compounds derived from (4-hydroxy-3-methoxyphenyl)and (3,4-dimethexyphenyl)propanes. 2. The loss in methoxy groups observed in the nitration of lignin corresponds content of elementary units with a free hydroxy group in the lignin.
approximately
to the
3. The formation of nitric ester groups in lignin is effected when it is treated with a large excess of concentrated HNO 3 as a result of the esterification of mainly benzyl alcohol hydroxyls and the transesterification of etherified benzyl alcohol hydroxyls. 4. Possible routes of the degradation
of lignin in its nitration are indicated.
LITERATURE 1o
2. 3. 4. 5.
356
A. L. L. L. R.
CITED
A. Chuksanova, L. L. Sergeeva, and N. N. Shorygina, Izv. AN SSSR, Otd. khim. n., 195___99,2219. L. Sergeeva, N. N. Shorygina, and B. V. Lopatin, Izv. AN SSSR, Otd. khim. n., 1962, 1295. L. Sergeeva, N. N. Shorygina, and B. V. Lopatin, Izv. AN SSSR, Set. khim., 1964, 1254. L. Sergeeva and N. N Shorygina, Izv. AN SSSR, Set. khim., 196__.._~5,1630. Willst~tter and L. Kalb, B e t . , 5__55,2637 (1922).