4
Reactivity of Lignin in Electrophilic
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Displacement Reactions
Κ. V. SARKANEN, BERNT ERICSSON, and JURO SUZUKI Department of Chemical Engineering, University of Washington, Seattle, Wash. 98105
Estimating the reactivity of various aromatic nuclei in lignin involves studies to determine the protodedeuteration rates of certain selectively deuterated model compounds. The combined effects of hydroxyl, methoxyl, and side -chain substituents are approximately additive in guaiacyl models while syringyl models show significant deviation from the predicted behavior. The reactivity of the position meta to methoxyl groups is higher while those of the ortho and para positions are lower than anticipated on the basis of combined substituent effects.
Lignin macromolecules undergo electrophilic displacement reactions easily owing to the reactivity of the aromatic nuclei that are activated by several electron-releasing substituents. These reactions include or dinary aromatic substitutions involving the displacement of a hydrogen from the ring as well as side-chain displacements. The latter reactions have been shown to occur during chlorination (3, 4, 16) and nitration (7) and are particularly facilitated if the side chain contains a carbinol group adjacent to the ring. The reaction scheme below illustrates both types of displacement reactions. Some of the most important electrophilic displacement reactions of lignins are listed in Table I with the reactive species indicated. In general, the complexity of the products from these reactions makes structural evaluation extremely difficult. Therefore, an approach using model compounds related to lignin appears desirable. A typical lignin C6C3 unit (see reaction scheme below) contains a side chain at position 1, a methoxyl at position 3, and a phenolic hydroxyl or phenol ether group at position 4. Another methoxyl group or a condensed 38 In Lignin Structure and Reactions; Marton, J.; Advances in Chemistry; American Chemical Society: Washington, DC, 1966.
4.
SARKANEN E T AL.
Electrophilic
Displacement
39
I
H+
OR
OR
Ε
H
OMe
OMe OR'
OR'
OR
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Aromatic Substitution +.
E+
OMe OR' R, R' = Η or alkyl
OR
OR
OMe
OMe +ÔR'
OR'
Decarbinolation side chain may be linked to position 5. Alternatively, no methoxyl groups may be attached to the aromatic nucleus. T o what degree, then, is it possible to predict the reactivities of the unsubstituted aromatic sites towards various electrophilic displacement reactions? This question could be answered more easily if we knew that the C C units conformed with the principle of additivity. This principle can be formulated as follows. If the introduction of each of two substituents alters the free energy of activation at a particular position by amounts χ and jy, the presence of both substituents would alter the free energy of activation by an amount (x + y). If this relationship holds, it allows one to predict the reactivities of the individual positions in disubstituted benzene derivatives from the rate data obtained for the corresponding monosubstituted ones. The partial rate factor for a given position of a 6
3
Table I.
Electrophilic Displacement Reactions of Lignin Reactive Species
Halogenation and halodecarbinolation Nitration, nitrosation, nitrodecarbinolation Hydroxylation Hydrogen exchange Various condensations Mercuration Diazonium coupling
2
+
CI,, C1+, Br , B r , etc. N0
+ 2
+
, Ν Ο , NO , N 0 2
δ
2
RC0 H 3
H A (DA) R C Hg (OAc),,Hg (NO,) Ar N + +
2
In Lignin Structure and Reactions; Marton, J.; Advances in Chemistry; American Chemical Society: Washington, DC, 1966.
2
4
40
LIGNIN STRUCTURE AND REACTIONS
disubstituted compound would equal the product of the rate factors for the two monosubstituted compounds. B y extending this principle further, the partial rate factors for the three positions of a trisubstituted aromatic compound can be predicted; each partial rate factor in this case is obtained as the product of three individual partial rate factors. According to the définition, the partial rate factors express the ratio of the rate constant of a particular substitution reaction at a given site to the rate constant of a single site in an unsubstituted benzene nucleus. T o illustrate, this principle would predict that the partial rate factor for position 6 in 4-hydroxy-3-methoxytoluene is equal to the product / X / X 0
H
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W
0
M
e
P
Me
/o The validity of the additivity principle has been tested so far in relatively few cases, but it has been notably successful in predicting correctly the rates of halogenation (12) and hydrogen exchange (9) reactions of polymethylbenzenes.
Protododeuteration. A previous paper (6) pointed out that a protododeuteration reaction has several advantages in determining partial rate factors. T o use this method, deuterium is introduced into a particular position in the aromatic ring, and the rate of back-exchange to protium is determined in an aqueous mineral acid solution of suitable strength. The rate determinations are conveniently carried out by infrared spectrophotometry. In other substitution reactions, such as halogenation, the reactivity determinations are necessarily based on the quantitative analysis of product mixtures. The results can often be dubious, especially if individual site reactivities show a wide spread. Protodedeuteration is not subject to this limitation because the reactivities of individual positions are determined in separate experiments. Under suitable conditions, even dideuterated derivatives can be used for kinetic studies as long as the deuterium substituents do not occupy adjacent sites. Ericsson et aL (6) showed that estimating the reactivities of positions 4 and 6 in general is feasible from a single kinetic run on 4, 6dideuteroguaiacol (G ,e). The dedeuteration process proceeds as follows: 4
B y using a mathematical treatment formally analogous to that of Spurlin (19) for the substitution of cellulose derivatives, the following equations are derived for the mole fractions of individual deuterated guaiacols as a function of time:
In Lignin Structure and Reactions; Marton, J.; Advances in Chemistry; American Chemical Society: Washington, DC, 1966.
4.
SARKANEN ET AL.
Electrophilic
41
Displacement
G ,6 = W G = e- ^ V V -(k+k)t G = e~\ 4 6 4
k l
4
A
l
cp
6
1 +
tf-VV
In this particular case, it is convenient to determine £ + k% from the 4
rate of disappearance of the G ,6 species, and £ from the decay of the G 4
6
6
species after the reaction has proceeded to the point where the concen tration of the G
4 i 6
species is insignificant.
Other approaches are, of course,
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Specifically deuterated derivatives of phenols can be prepared con veniently by using appropriate combinations of acid- and base-catalyzed exchange reactions.
Treating with N a O D (or N a O H ) causes exclusive
exchange in positions ortho and para to the phenolic group while D P 0 3
deuterates all available aromatic sites.
4
As an example, the conversion of
4-hydroxy-3-methoxytoluene to 5-monodeutero- and 2,6-dideuteroderivatives is illustrated below. Me
I
D
! OMe OH
Me NaOD ν
OMe HO Me .D3PO4
D
j
D
D
Me !
D
NaOH D
I OH
OMe
OMe OH
Protodedeuteration reaction has, however, some limitations. It can not be used to study compounds subject to secondary condensation in acidic medium, such as certain benzyl alcohol and benzyl ether derivatives. Moreover, the partial rate factors depend greatly on the medium used be cause different acids show different degrees of selectivity. For instance, the reported partial rate factors for the para position of toluene (f ) range from 170 (in sulfuric acid at 65°C.) (13) to 4000 (in hydrogen bromide) (22). Me
P
In Lignin Structure and Reactions; Marton, J.; Advances in Chemistry; American Chemical Society: Washington, DC, 1966.
42
lignin
structure
and
reactions
Results and Discussion
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Partial Rate Factors for Protodedeuteration in Perchloric Acid Solution. A l l lignin model compounds studied contained hy droxyl, methoxyl, and methyl groups as the only substituent groups. Table II summarizes what the authors believe are the most reliable partial rate factors for these substituents in perchloric acid solution. A l l partial rate factors are based on determinations in 57% perchloric acid solutions. The partial rate factors for ortho and para positions of methyl and methoxyl groups are based on later work by Suzuki (21). The partial rate factors for phenol were calculated from data on 2, 4, 6-trideuterophenol (6) using the value 9.1 Χ 1 0 hr." for monodeuterobenzene by Suzuki (21) and assum ing 1:3.2 ortho to para ratio. -5
1
Comparing the partial rate factors in perchloric acid with those obtained in other mineral acids, it is interesting to note that they are, in general, slightly lower than those observed for sulfuric acid solutions. For instance, the partial rate factors in sulfuric acid for the ortho, meta, and para positions of toluene are, under comparable conditions, 250, 5, and 250, respectively, (5) while the corresponding values for anisole in the same medium have been reported to be 2.3 Χ 10 , 0.25 and 5.5 X 10* (18). This suggests that perchloric acid medium is probably slightly less selec tive in protodedeuteration than sulfuric acid. 4
Deuterated Model Compounds and Protodedeuteration Rate Measurements. Protodedeuteration rate studies were outlined for 2,4,6-trideuterophenol and 2,4,6-anisole, 3,5-dideutero- and 4,6-dideuteroguaiacols, 3,5-dideuteroveratrole, 5-deutero-4-hydroxy-3-methoxytoluene, and 2,6-dideutero-4-hydroxy-3-methoxytoluene (6). Using appropriate combinations of acid- and base-catalyzed deuterium exchange reactions, the following deuteroderivatives were prepared in this study; 3,5-dideuteroand 4-deutero-2,6-dimethoxyphenols, 4,6-dideutero- and 5-deutero-l,2,3trimethoxybenzenes, and 2,6-dideutero-4-hydroxy-3,5-dimethoxytoluene.
Table II. Substituent CH OCHOH 3
Partial Rate Factors for Protodedeuteration in Perchloric Acid Solution f
f
0
m
82 1.4 χ 10 8.5 X 103 4
2.5
f
P
80 5.5 Χ 10 2.6 Χ 10
4
4
As a rule, 36% perchloric acid at 2 5 ° C . was found to be the most con venient medium for protodedeuteration rate studies. Again, infrared spectrophotometric determinations on samples recovered from the protodedeu teration medium after appropriate reaction times were used to determine the rate constants.
In Lignin Structure and Reactions; Marton, J.; Advances in Chemistry; American Chemical Society: Washington, DC, 1966.
4.
sarkanen
et
Electrophilic Displacement
al.
43
Table III summarizes the observed rate constants, some of which were determined in 57% perchloric acid solution. It is interesting to note that the ortho and para positions of phenol are slightly less reactive towards protodedeuteration than those of anisole. In most electrophilic displacement sections the reverse order is observed.
Table III.
Summary of Protodedeuteration Rate Constants Ri 6V,
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5,3
3
Rate Constants, k X lO' , hrr Λ
kl 6.04 12400ί 2.73 \1460 — 15.9 — / 7.47 \34305801.27
RA
H
H
OH
H"
H
OMe
OH
H
Me H Me
OMe OMe OMe
OH OH OH
H OMe OMe
H
H
OMe
H*
H H
OMe OMe
OMe OMe
H OMe
S
e
h — 0.20 31011 3590 18000 — e
1901610
h 1.93 770— — — — 2.47 1130— —
h 1.93 7700.87 4702.73 — — 2.47 1130190—
— 0.63 88086 3590 18000— 5801610
- Rate constants determined in 57% perchloric acid; all others determined in 36% perchloric acid. o/p ratios assumed to be the same as in guaiacol. Experimental error likely to be somewhat larger than in other measurements because of the exceptionally fast rate. b
c
In order to determine the applicability of the additivity principle for poly substituted models, the rate data in Table III have been expressed in terms of partial rate factors of individual substituent groups in Table I V . The partial rate factors for polysubstituted compounds are calculated on the same basis as those of monosubstituted ones. For instance, the partial rate factors in guaiacol refer to the factors by which the rate constants of phenol are modified in positions 3, 4, 5, and 6 by introducing a methoxyl group in position 2. Likewise, the partial rate factors for methoxyl in 4-hydroxy-3,5-dimethoxytoluene indicate how the protodedeuteration rates of 4-hydroxy-3-methoxytoluene at positions 2 and 6 are modified by inserting a methoxyl group at position 5. If the protodedeuteration rates were governed strictly by the principle of additivity, the partial rate factors would be independent of the degree,of substitution, except for the rate factors for ortho positions flanked by two substituent groups. Examples of such factors a r e / for 4-hydroxy-3,5dimethoxytoluene and / for the position 2 of 4-hydroxy-3-methoxyM e 0
0
M e
G
In Lignin Structure and Reactions; Marton, J.; Advances in Chemistry; American Chemical Society: Washington, DC, 1966.
LIGNIN STRUCTURE A N D REACTIONS
44 toluene.
In these cases enhanced steric hindrance effects should lower the
partial rate factors. The data in Table I V demonstrate both reasonable agreement and substantial deviations from the principle of additivity discussed in detail below.
Table IV. Comparison of Partial Rate Factors Between Compound Used for Détermination
Substituent Group Downloaded by NANYANG TECH UNIV LIB on November 6, 2014 | http://pubs.acs.org Publication Date: January 1, 1966 | doi: 10.1021/ba-1966-0059.ch004
—OMe
HClO
Anisole" Guaiacol Veratrole 2,6-Dimethoxyphenol 1,2,3-Trimethoxybenzene 4-hydroxy-3,5-dimethoxytoluene c
h
SI 36 57 57 36 36 36
—OH
Phenol Guaiacol
57 57 36
—Me
Toluene"
57
4-hydroxy-3-methoxytoluene
36
4-hydroxy-3,5-dimethoxytoluene
36
α 6 c d
%
Data by Suzuki {21). Value given by Satchell for sulfuric acid solution (/
