Synthesis and characterization of picryl cellulose

Synthesis and characterization of picryl cellulose MICHAELJ. STRAUSS,' REUBENTORRES,JOHNPHELAN,ANDYCRAFT,BRUCEPITNER,A N D DEANENASON Department of Chemistry, University of Vermont, Burlington, VT 05401, U.S.A.

YVONCARIGNAN U.S. Army Armament Research and Development Command, Dover, NJ 07801, U.S.A. AND

JULIANM. DUSTA N D ERWINBUNCEL Chemistry Department, Queen's University, Kit~gstot~, Ont., Canada K7L 3N6 Received January 13, 1987

Can. J. Chem. Downloaded from www.nrcresearchpress.com by 204.85.2.245 on 08/03/15 For personal use only.

This paper is dedicated ro Professor John T. Edward MICHAELJ. STRAUSS,REUBENTORRES,JOHNPHELAN,ANDYCRAFT,BRUCEPITNER,DEANENASON,YVONCARIGNAN, JULIANM. DUST, and ERWINBUNCEL.Can. J. Chem. 65, 1891 (1987). 2,4,6-Trinitrophenyl cellulose (picryl cellulose) was synthesized by SNAr displacement of chloride from picryl chloride by sodium cellulosate. The cellulosate was prepared in situ from microcrystalline cellulose and sodium methoxide. Depending upon the procedure used, two products with different degrees of picrylation resulted; one contained one picryl ring per ca. 6.5 glucosyl units (PC-6), while the other had ca. one picryl ring per 12 (PC-12). These picryl ethers were characterized by several independent methods: 400-MHz 'H nuclear magnetic resonance spectroscopy of the DMSO-soluble material, temperaturedependent mass spectrometry (ion pyrograms), and differential scanning calorimetry (DSC). 'The nuclear magnetic resonance spectrumdisplays peaks in the low field region (8-9.5 ppm) assigned to the picryl rings; these resonances are well separated from those for possible alternative compounds such as unreacted picryl chloride, picric acid, or 2,4,6-trinitroanisole. It is suggested that the various picryl resonances arise primarily from different microenvironments and (or) conformational preferences of the polymer chain rather than from different substitution sites. Substitution at a primary C-6 position of the glucosyl moiety is favoured. DSC shows that while cellulose undergoes an endothermic decomposition between 320 and 350°C, picryl cellulose exhibits an exothermic decomposition at ca. 230°C. This exotherm is accompanied by the liberation of picric acid, as confirmed by nuclear magnetic resonance; mass spectral results indicate concurrent chain cleavage to yield smaller polysaccharides. A mechanism for initiation of pyrolytic decomposition is proposed, in which loss of picric acid is accompanied by rupture of a glycosidic bond with concomitant formation of glucosan and enolic end groups on the remaining fragments. MICHAELJ. STRAUSS,REUBENTORRES,JOHNPHELAN,ANDYCRAFT,BRUCEPITNER,DEANENASON,YVONCARIGNAN, JULIANM. DUSTet ERWINBUNCEL.Can. J. Chem. 65, 1891 (1987). On a rCalisC la synthese de la trinitro-2,4,6 phCnyl cellulose (picryle de cellulose) en prockdant a une substitution SNArdu chlorure de picryle par le cellulosate de sodium. On a prCparC le cellulosate en faisant rCagir in siru de la cellulose microcristalline avec du methylate de sodium. Suivant la mCthode utiliske, il se forme deux produits comportant des degrks differents de picrylation: un produit qui contient un groupement picryle par 6,5 unitCs glucosyles (PC-6) et un autre qui contient un groupement picryle par 12 unites glucosyles (PC-12). On a caractCrisC ces Cthers picryles par plusieurs mkthodes indtpendantes dont la spectroscopie de rksonance magnCtique nuclkaire du 'H a 400 MHz du produit soluble dans le DMSO, la spectromktrie de masse tempCrature variable (pyrogrammes ioniques) et la calorimktrie a balayage diffkrentiel (CBD). Les spectres rmn prCsentent des bandes dans la rCgion des bas champs (8-9,5 ppm) qui ont CtC attribuCs aux picryles; ces raies sont bien sCparCesde celles qui pourraient provenir d'autres composCs comme le chlorure de picryle qui n'aurait pas rCagi, de l'acide picrique ou du trinitro-2,4,6 anisole. On suggkre que les diverses raies dues au picryles proviennent principalement de divers microenvironnements et (ou) de prkfirences conformationnelles des chaines de polymeres plut6t que de divers sites de substitution. La substitution au niveau du carbone primaire en position C-6 de la portion glucosyle est favorisee. La CBD permet de dkmontrer que la cellulose subit une dCcomposition endothermique entre 320 et 350°C et que le picryle de cellulose donne lieu a une dCcomposition endothermique environ 230°C. Cette dCcomposition exothermique est accompagnCe d'une liberation d'acide picrique et on a confirm6 cette conclusion par rmn; le donnCe de la spectromktrie de masse indiquent qu'il se produit une clivage concomitant qui fournit des polysaccharides plus petits. On propose un mkcanisme pour l'initiation de la dCcomposition pyrolytique dans lequel la perte d'aide picrique est accompagnke par la rupture d'une liaison glycosidique avec la formation concomitante de glucosane et de groupements Cnoliques en bout de chaine sur les fragments qui restent. [Traduit par la revue]

The interaction of polynitroaromatic and heteroaromatic compounds with electron-rich donors results in the formation of electron donor-acceptor (1) and (or) anionic Meisenheimer type a-complexes (2). The a-complexes, particularly the adducts arising from novel electron-deficient substrates, have been the subject of considerable recent interest (3-6). In this context, the reaction of polynitroaryl ether derivatives of cellulose with nucleophiles could form an interesting system of study and part of our ongoing program of research. ' ~ u t h o rto whom correspondence may be directed. Printed in Canada i Imprimc au Canada

During the past few decades there have been several attempts to prepare and characterize the polynitroaryl ethers of cellulose (7, 8). Nitration of cellulose phenyl ether resulted in partial nitration of the hydroxyl as well as the phenyl groups (7). Avny et al. attempted the preparation of picryl cellulose (PC) directly from cellulose membrane (cellophane), as well as from cotton fabric, by reacting the corresponding sodium cellulosates with picryl chloride in benzene, but the material obtained was only partially characterized (8). The results of this work were interpreted on the basis of the highly coloured species, assumed to be an adduct formulated as 1, that was isolated and neutralized with

1892

CAN J. CHEM. VOL. 65, 1987


acetic acid. Two alternative neutralization routes were proposed, leading either to the cellulose and picryl chloride starting materials or to the desired picryl ether of cellulose. It was suggested that neutralization favoured reversion of adduct 1 to the starting materials, due to steric hindrance between the picryl and glucopyranoside rings in the picryl cellulose (8). It is known that the kinetically preferred site of attack by nucleophiles on polynitroaromatic substrates such as picryl chloride or 2,4,6-trinitroanisole occurs meta to the substituent in the 1-position (9, 10). In the present system such a process would give rise to adduct 2, which could only regenerate

S 2

1

starting material upon addition of acid. Intervention of the 1,3 adduct and its subsequent acid decomposition would result in significantly lower yield of picryl cellulose. Since sodium ~ e l l u l o s a tis e ~expected to be more soluble and a better nucleophile in dimethyl sulfoxide (DMSO) than in benzene, the picrylation of cellulose in DMSO should offer advantages (1 1). We report here our results on the synthesis of picryl cellulose in DMSO and the full characterization of the product.

Results and discussion Synthesis of picryl celluloses, PC-6 and PC-12 In this work. various procedures were investigated in the preparation of picryl cellulose. Two representative procedures, which led to the successful preparation of products that were subsequently characterized, are described, while one procedure that did not yield desired product is also described. In all preparations, sodium cellulosate was prepared in situ from microcrystalline cellulose by treating a sample of the desiccated cellulose first with DMSO, to swell it thoroughly with the solvent, and then with freshly prepared sodium methoxide. The resulting cellulosate was washed with dry DMSO to remove excess methoxide and, in the first procedure, reacted with a solution of picryl chloride in DMSO to yield PC- 12 after work-up. In the second procedure, picryl chloride was added as a powdered solid to the sodium cellulosate in DMSO; this process afforded PC-6, a more highly picrylated material. In either case, the picryl cellulose was well washed with a variety of solvents, followed by Soxhlet extraction of the material for 4 days and drying under vacuum. This extensive washing and extraction procedure militates against the presence of any adsorbed or entrained picryl chloride, or picrate. In both methods of preparation addition of the picryl chloride, whether in solution or as a solid, was relatively rapid. In one of the attempted preparations, a concentrated solution of picryl chloride in DMSO was allowed to stand for 30 min prior to addition to the cellulosate suspension. The picryl chloride solution became very hot over this period of time and in this

he exact nature of the sodium cellulosate is not well defined. We consider sodium cellulosate to be the intermediate material derived by treatment of microcrystalline cellulose with base. In this material an eauilibrium concentration of oxv-anion or activated hvdroxvl sites eiists for subsequent reaction with picryl chloride to give the covalently bound picryl cellulose. ->

(ppm)

The 'Hnrnr spectrum of picryl cellulose (PC-6) in DMSOd6.Inset (a), aromatic region intensified (6 7-10); inset (b), peaks due to picryl moieties of the picryl cellulose and of picrate ion. FIG. I .

d

d

preparation no picryl ring was incorporated into the product. These observations are in accord with the known reactivity of picryl chloride in DMSO without added nucleophile (12). The preferred method for the synthesis of picryl cellulose in terms of yield of product and degree of picrylation (PC-6) involved rapid introduction of finely powdered picryl chloride directly into the stirred solution of sodium cellulosate in DMSO.

Characterization of picryl celluloses, PC-6 and PC-12 1 . Physical properties, elemental analysis, and hydrolytic cleavage The picryl celluloses synthesized in this work are tan powders when dry. PC-6 and PC- 12 are extremely hygroscropic and hydrate rapidly, i.e., the weight of both materials in air increases over time. Hydration is complete within 24 h, at which point the weight increase levels off at 7.3%. The hydrated products are bright yellow in colour, in contrast to the tan anhydrous powder. The process is probably accompanied by some hydrolysis (vide irfra). Elemental analysis (C, H, N) clearly differentiates the two picryl cellulose products. The nitrogen content and number of glucosyl units per picryl ring for PC-6 and PC- 12 are given in the experimental section. Based on the requirement of 3 nitrogens per picryl ring, the PC-6 contains the larger number of picryl rings per glucosyl residue. When finely powdered picryl cellulose is rapidly stirred in distilled water, partial dissolution occurs and the pH slowly drops while the solution turns bright yellow. An absorption maximum is noted in the ultraviolet-visible spectrum of the aqueous solution at 352 nm, corresponding to the absorption band for picric acid (13). Picric acid adsorbed on the surface of cellulose might be expected to be released instantly into aqueous solution. It is, therefore, significant that the absorption assigned to picric acid attained a maximum value only after 30 min at room temperature. The formation of picric acid is indicative of a hydrolytic cleavage. This hydrolysis could occur either via an SNAr the picry' ring (pathway a) Or via an S ~ 2 - t y p e attack of water On C-6 (pathway b; Scheme 2 . Evidence from ' H nuclear magnetic resonance An 'H nrnr spectrum of picryl cellulose in (CD3),S0 can be

STRAUSS ET AL

picric acid


cellulose

obtained using a saturated solution prepared by sonication. The 'H nmr spectrum of PC-12 was identical to that of PC-6, although not as intense. Resonances for the glucosyl residues appear in the upfield region of the spectrum (Fig. 1) from 2.0 to 5.4ppm; these are broad and not particularly well resolved, presumably due to conformational changes and slow tumbling of the cellulose chains in solution. Resonances due to the aromatic picryl moieties of picryl cellulose, as well as free picrate, appear in the downfield region of the spectrum. The expanded spectrum of Fig. 1 shows three relatively broad peaks located at 9.055,9.015, and 8.919 ppm, as well as a very sharp singlet at 8.582 ppm. The sharp singlet at 7.524ppm is due to p-dibromobenzene (DBB), used as an internal standard. The sharp signal at 8.582 pprn results from the two equivalent protons of picrate anion, PicO-. 'The peaks centered around 9.00ppm result from picryl ether moieties covalently bonded to the cellulose chain. It is significant to note that the singlet at 8.582 ppm, which is assigned to picrate anion, appears at exactly the same chemical shift as authentic picric acid in (CD3),S0. This compound is known to be fully ionized in DMSO (1 1). If picrate were adsorbed on the surface of the cellulose or if some free and some adsorbed picrate existed in rapid equilibrium, an averaged signal would be observed at a position different from that for picrate in (CD3)?S0 alone (14). Similarly, the signals centered at 9.00 pprn are not due to contamination from trace amounts of unreacted picryl chloride or from 2,4,6-trinitroanisole (P-OMe). When picryl chloride or P-OMe was added to the PC sample, the spectra obtained showed sharp singlets for picryl chloride at 9.234 and for P-OMe at 9.101 pprn respectively. These signals do not overlap with those for picryl cellulose. The signals at 9.055-8.919ppm do not shift from their previously recorded chemical shifts, as would occur if significant equilibration occurred between the free and bound picryl groups. Also, the signals for picryl chloride and P-OMe in (CD3)2S0appear at the same chemical shift in the presence or absence of PC. Clearly, the signals situated at 9.055, 9.015, and 8.919ppm are assignable to picryl ring(s) covalently bonded to the cellulose polymer. There are three possible sites where bonding of the picryl ring to the glucosyl moiety might occur, namely C-2, C-3, and C-6. It is tempting to assume that the resonances

centered around 9.00 pprn result from bonding at these different sites. However, the primary hydroxyl function at C-6 is expected to be the preferred nucleophilic moiety in attack on picryl chloride (15- 17), in part because the C-2 and C-3 sites are much more hindered, and also because of the preferential deprotonation of the primary C-6 hydroxyl. Thus, for reasons of both steric hindrance and relative acidity, it is likely that SNAr displacement yields picryl moieties bonded to the oxygen on C-6. Conformational preferences in the chain would be expected to produce different microenvironments for a C-6 picryl ether, and this could give rise to the three separate picryl signals that are observed. The "effective concentration" of the picryl ether moieties is 0.015 2 0.005 M, as determined by comparison of the integral for the resonances centered around 9.00 pprn with the integral for the protons of the DBB standard at 7.524 ppm. The concentration of picryl ether moieties is much higher than expected from the elemental analysis data. Two possible explanations can be considered to account for the discrepancy between the "effective concentration" of picryl units as determined by nmr, and the concentration calculated from elemental analysis data and the derived empirical formula. First, it could be argued that a range of lower molecular weight chains has preferentially dissolved in the (CD3),S0 solvent and that such lower molecular weight material contains more highly picrylated regions than the bulk material. In this case the 'H nmr spectrum may not be representative of the bulk polymer, which presumably consists of a distribution of various picryl cellulose chain lengths. In short, the process of dissolution has actually been a process of fractionation on the basis of molecular weight. The second possible explanation is that the observed solubility is directly related to the degree of picrylation. In that regard, it has been reported that increased derivatization, by reaction of the hydroxyl groups, leads to increased solubility (17, 18). As well, in the present work we have found that ca. 70 wt.% of PC-6 dissolves in DMSO whereas ca. 39 wt.% of PC-12 is soluble. Thus it is found that the bulk material that is most highly picrylated is also the most soluble in DMSO. It can be concluded that in the case of PC-6 the 'H nmr spectrum obtained on the DMSO-soluble material is certainly representative of the bulk picryl cellulose. The situation in the case of PC-12 is not


izm

-

CAN. J. CHEM. VOL. 65, 1987

PIC WTI 5.80 mg SCAN RATE1 10.00 deg/min

CURVE B ( P I C - 1 2 )

CURVE C (PIC-6) /

TEKPERATURE (C)

CSC

FIG.2. Differential scanning calorimetry for picryl cellulose (PC-6 and PC-12) and cellulose. The cellulose energy axis has been rescaled to allow comparison of transition temperatures.

as well defined, although it appears that any fractionation that may have occurred on dissolution in DMSO is not extensive. Also in accord with these observations is the ratio of integrals " of the picryl ether absorption to the total glucosyl proton absorption found upfield. Based upon the bulk material, which contained 6.5 glucosyl units per picryl ring (PC-6), this integral ratio is predicted to be ca. 1:29. The measured value is 1 : 15, a further indication that preferential solubilization of more highly picrylated material in (CD3)?S0 has occurred. It should be emphasized that the residual solid material that is insoluble in DMSO still contains picryl groups. Thus the infrared spectrum of the residual solid (PC-6 and PC-12), after removal of DMSO, thorough washing with diethyl ether, removal of the ether, and vacuum drying of the solid, displays ir bands at 1535 and 1340 cm-'. characteristic of the asvmmetric and symmetric stretching vibrations of the attached aromatic nitro groups. Clearly, both the DMSO-soluble and insoluble materials are Dicrvlated. ~ o m ~ l e k e n t a to r ythe nrnr studies described above, further information was sought through differential scanning calorimetry and mass spectral analysis. Since these methods involve heating to high temperatures, it seemed desirable to monitor by 'H nrnr any changes that may occur on heating the picryl cellulose. It is found that in a standard s a m ~ l e(PC-6). .. ,, the ratio of the integral sum of the picryl ether moieties compared to the picrate anion integral is ca. 9:l. The picrate anion may result from hydrolysis in the DMSO solution caused by small amounts of water bound in the polymer or by adventitious water in the commercial (CD3)$30. This ratio was almost reversed (1:7) when the dry picryl cellulose was preheated to 230°C in an argon atmosphere before being dissolved in (CD3),S0 for spectral analysis. It is concluded that hydrolysis does not account for this increase in picrate anion concentration but, rather, pyrolytic ~

cleavage occurs in the dry solid. The differential scanning calorimetry and mass spectral studies described below confirm this conclusion.

3. Differential scanning calorimetly (DSC) The thermal decomposition of cellulose is a complex process, which can be monitored by differential scanning calorimetry. The differential thermal analysis results have been previously found to be dependent on the temperature range studied, the period of heating, and the heating rate, as well as on the physical and chemical properties of the cellulose (19-22). For these reasons, a control DSC curve of the thermal behaviour of the microcrystalline cellulose that was the precursor of both PC-6 and PC-12 was determined (curve A, Fig. 2). The endothermic maximum, which occurs at 340°C, represents an initial depolymerization (which involves cracking and dehydration), closely followed by the competing reactions of decomposition, volatilization, polymerization, and aromatization (19, 20). The large endotherm (AH = -278 cal g-') is indicative of the predominant depolymerization and volatilization processes; at about 350°C these reactions either cease or are masked by the competitive exothermic reactions. The latter subside and little further heat is evolved in the temperature region studied ( I 00-400°C). While microcrystalline cellulose and picryl cellulose show similarities in their thermal decomposition, it is clear that the latter displays thermal behaviour characteristic of C-6 substituted cellulose ethers. Upon cursory examination both PC-12 and PC-6 exhibit two thermal transitions in their DSC curves (Fig. 2; curves B and C). In the case of PC-12 (curve B), the first is a shallow exotherm that occurs between 230 and 235°C and is not present in the precursor cellulose (curve A). This initial exotherm is the dominant feature of the DSC trace for PC-6 (curve C). Note that the ratio of evolved heats for PC-6 corn-

STRAUSS ET AL.

WT8

5.75 .g

SCAN RATE8

"


"

10.00 &g/mln

PEAK FRCk

2M. 31 TOa 257.18

PEAK FROMI 2SE i O a 343.69 DNSETn 203. 4 5 CAL/GRAHI 4 8 . 2 5

MINI

TEMPERATURE (C)

312.77

DSC

FIG. 3. Differential scanning calorimetry for PC-6. The curves represent cycled heating and recooling: Curve 1, 25-260°C; Curve 2, sample from curve 1 cooled and then heated, 25-380°C; Curve 3, sample from curve 2 cooled and then heated, 25-380°C.

pared toPC-12 (AHPC-6/AHPC-12 = -40.3 cal g-'1-9.2 cal g-') is 4.4 and parallels the picryl content of the samples. Hence the picryl moiety is directly involved in the exothermic reaction. The broad and diffuse endotherm with maximum at 350°C (curve B) forms the secondary feature of the DSC plot for PC-12. This endothermic peak corresponds to the same type of decomposition processes that occur in the microcrystalline cellulose sample (curve A). It is significant to note the trend to larger exotherms at 230-235°C and diminished endotherms at 350°C as a function of the increase in the degree of picrylation (cellulose to PC-6). The interdependence of the endothermic and exothermic features of the DSC trace is clarified in the cycling experiment shown for PC-6 in Fig. 3. First the sample was heated past the point at which the initial exothermic reaction has ceased, i.e., 280°C (curve 1, Fig. 3). The sample was permitted to return to room temperature and was then reheated; the DSC plot (curve 2, Fig. 3) exhibits no exotherm at 230°C, but a significantly broadened exothermic peak commencing at 259°C and centred at 313°C is observed. The diminished endotherms displayed by PC-12, and even more so by PC-6 (Fig. 2), are thus the result of a competing exothermic reaction revealed in this cycling experiment. The heat evolved in this exothermic process is absorbed in the endothermic decomposition of the cellulose chain in PC-6 and PC-12. Note that only in the absence of the 230°C exotherm is the corresponding 3 13°C exothermic maximum observable. The disappearance of the 230°C exotherm is indicative of picryl ether bond scission coupled with another process. In this regard, it is important that the sample colour changed markedly at 250°C, in a control experiment conducted in a melting point capillary, going from tan to dark brown. Coincident with the colour change, water and a yellow oil were observed to condense on the unheated upper portions of the capillary. The water no doubt arises from the known loss of adsorbed water that

occurs at ca. 80-100°C (20) and from the dehydration processes that accompany depolymerization of the cellulose chain. Thermal decomposition in a sublimation apparatus leads to the isolation of picric acid as crystals on the cold finger and thus identifies the yellow oil noted in the melting point capillary experiment. Furthermore, in a recent study of the thermal behaviour of the C-6 substituted cellulose ethers, namely cellulose p-toluenesulfonate, tritylate, and phosphate, Jain et al. found an initial small endotherm for the tritylate and phosphate (157 and 160"C, respectively), which they attributed to cleavage of the ethers (22). In contrast, cellulose tosylate exhibited an exothermic maximum at 175°C (22). The unique behaviour of the C-6 cellulose tosylate was ascribed to a simultaneous endothermic scission of thep-toluenesulfonyl group, as the acid, and induced decomposition of the cellulose chain. By analogy, rupture of the C-6 picryl ether bond in PC- 12 and PC-6 would be expected to result in the release of picric acid, accompanied by an exothermic cellulose decomposition. In view of these results, the relationship of the large exotherm and the broadened cellulose decomposition endotherm of PC-6 and PC-12 is straightforward. The exotherm (230°C) represents concurrent endothermic picryl ether scission and highly exothermic acid-induced cellulose decomposition. This decomposition pathway may be visualized as in Scheme 2. 4. Mass spectral analysis and ion pyrograms Further clarification of mechanistic details of the exo- and endothermic pyrolytic reactions of cellulose and picryl cellulose can be obtained by carrying out the pyrolyses directly in the solid probe of a mass spectrometer. Ion pyrograms showing approximate amounts of both primary and secondary products of pyrolysis can be used to identify fragments evolved during the various cleavage processes. While accurate control and

CAN. J. CHEM. VOL. 65, 1987


HOPic

dehydration and further decomposition

dehydration and further decomposition SCHEME 2

IHTEH 588884.

I. Temp

(OC

1

S c a n No.

1 13

37 73 85

94 110 139 164

nax c a . 3 6 5 ' ~

RIC

1

1

,

1

1

1

158 7138

1

1

1

1

288 l01m

1

1

1

2% 12:s

388 l51m

358 17:m

488 SCFH 28188 TINE

FIG.4. Electron impact mass spectral map (total ion plot) for microcrystalline cellulose from 200 to 40OoC.

measurement of the exact temperature at the point of sample pyrolysis is difficult, the overall qualitative picture is clear and informative. The total ion spectrum map (ion pyrogram) for microcrystalline cellulose is shown in Fig. 4. Maximum ion intensity is achieved at ca. 365"C, with significant ion production from 350 to 400°C. Based upon the DSC evidence, this behaviour must correspond to the endothermic decomposition processes of cracking and dehydration. The electron impact (70 eV) mass spectrum, measured at the onset of significant ion production at 350°C, is shown in Fig. 5. The parent ion of 1,4: 3 ,6-dianhydro-P-D-glucopyranose is found at m/e 144. Although this ion is of low relative intensity

(1.04%), it is significant in that anhydroglucoses (pyranose or furanose analogs of glucosan, for example) generally do not yield observable ions of m/e larger than 98. Other significant, albeit low intensity, fragments occur at m / e 126 (0.74%) and m/e 98 (5.28%). The former may represent either the ion of 5-hydroxymethyl-2-furfural or of 2-furylhydroxymethyl ketone, whereas the latter arises primarily from fragmentation of the furfural(23-25). The characteristic intense peaks at m / e 73 and 60 derive from anhydroglucose parent ion fragmentation (23). As in the case of cellulose, the ion pyrogram of PC-6 (Fig. 6) shows maximum ion production at relatively high temperatures (ca. 386°C) but, in contrast, an initial ion-producing process commences at approximately 250" and reaches a maximum at

-

STRAUSS ET AL.

69.1

5t.t I

,

I

I I

I

es.1 1

81.1


r . . . . , . . . - - . - C L . . . . r 1 - - - . I . . . . r . . . . I . d . . r . . . . + . . . . t . . . .

63.1

FIG.5. Electron impact mass spectrum of microcrystalline cellulose at 350°C.

Temp ( O C ~

200 220

2: 245 250 260 270 280 290 300 31 0 320

. .

max c a

;?:

2 8 0 ~ ~

Scan No. 1

31 56 73 88 103 121 156 181 199 216 228 251 27 5 301

max.

RIC

380 448 ':70

p r o c1 e s s 1

*_c

ca. 386'~

1

A 1

FIG.6. Electron impact mass spectral map (total ion plot) for picryl cellulose (PC-6) ca. 280°C. Two temperature-dependent processes appear to exist, each associated with a maximum in ion production (Fig. 6). A mass spectrum of PC-6 measured at ca. 280°C displays two

prominent peaks at mle 229 (46%) and m l e 199 (4.0%), which are unique to the picrylated product (Fig. 7). The m l e 229 can be readily ascribed to the picric acid radical cation formed by cleavage from the cellulose chain. Loss of NO would give rise to


C A N . J. CHEM. VOL. 65, 1987

FIG. 7 . The electron impact mass spectrum of picryl cellulose (PC-6) at 280°C.

the m l e 199 ion:

The temperature-dependent behaviour of PC-6 and the pyrolytic product ions of the picryl ether can be correlated. Thus Fig. 8 shows the ion pyrogram of m l e 229, 199, 144, 126,97, 73, and 60 ions as a function of temperature. As can be seen, maximum intensity of the m l e 229 ion correlates well with ion production at ca. 280°C, as does the intensity of m l e 199, which confirms the relationship between the two peaks. On the other hand, maximum intensity for the m / e 144, 1 2 6 , 9 7 , 7 3 , and 60 ions corresponds to a probe temperature of ca. 386°C. Similar and confirmatory observations can be made from chemical ionization generated mass spectral maps and ion intensities. Clearly, the decomposition pathways for PC-6 involve the picryl ether moieties. Picric acid is lost from PC-6 at a lower temperature than that at which decomposition is noted in the precursor microcrystalline cellulose. ~ u r t h e r the , two temperaturelion production maxima for PC-6 are in accord with the two regions of decomposition detected by DSC. In fact, the presence of the picryl ring in the cellulose chain promotes the cellulose cleavage, as in the mechanism proposed in Scheme 2. Thus, presuming a relatively random distribution such that approximately one picryl ring is bonded per 6-12 glucosyl residues, fragmentation to release picric acid would lead to smaller oligosaccharides with much greater volatility and mobility. These oligosaccharides would be expected to undergo even more facile pyrolytic rupture than the intact cellulose chain.

Conclusions Two picryl celluloses were prepared, PC-6 and PC- 12, differing only in the number of picryl rings incorporated into the cellulose chain. Elemental analysis, hydrolytic cleavage behaviour, 'H nmr analysis in DMSO-d6, differential scanning calorimetry, as well as mass spectrometry, identify these products as picryl ethers, distinct from cellulose containing adsorbed picryl chloride, picric acid, or trinitroanisole. The different analytical methods are complementary. Pyrolytic behaviour noted by DSC was confirmed by mass spectrometry. Evolution of picric acid was detected by mass spectrometry, and picric acid was isolated in a control experiment from the thermal decomposition of PC-6 and was also observed in 'H nmr experiments (as picrate anion). The 'H nmr provides evidence that the picryl ether moieties are most likely attached at C-6 of the glucosyl residues. The picryl groups noted in the 'H nmr of the DMSO-soluble material derived from PC-6 or PC-12 are also present, according to ir data, in the DMSO-insoluble residure. It is concluded that the 'H nmr spectral data are representative of the bulk picryl celluloses. The thermal decomposition of the cellulose C-6 tosylate noted in the recent literature (22) is analogous to the thermal behaviour of picryl cellulose and further identifies PC-6 and PC-12 as C-6 cellulose ethers, in accord with the 'H nmr evidence. Experimental Preparation of picryl cellulose, PC-6 Microcrystalline cellulose was dried over P2OS at 50°C (2Torr; 1Torr = 133.3 Pa) for 5 h. A sample of the desiccated cellulose (9.5 g) was added to 200 mL of dry DMSO and stirred for 20 min. To the stirred mixture was added 140 mL of freshly prepared 4 M sodium methoxide in methanol and the mixture was allowed to stir for another 25 min before it was rapidly filtered. The filtered solid was kept covered with dry DMSO and the cake was broken up with a spatula into granular chunks and washed with copious amounts of dry DMSO (5 separate 100mL washes with dry DMSO). This washing procedure was performed to remove any excess sodium methoxide. The washed material

STRAUSS ET AL.

MASS


For temperature - s.:an NO. correlation, see Fi::ure 7 .

1% 5: Be

I

288 6: 48

258 8: 28

388 18:BB

358 11:48

488 13: 28

450 15:88

I

500 SCAN 16:40 TIME

FIG. 8. Electron impact mass spectral map (total ion plot, and masses 60, 73, 97, 126, 144, 199, and 229) for PC-6 from 200 to 400°C. was added to 300 mL of freshly distilled dry DMSO in a 1-L Erlenmeyer flask and stirred rapidly with a magnetic stirrer until the solid became finely divided. To this rapidly stirred mixture 2 5 g of dry powdered picryl chloride was added in small portions. During the rapid addition, another 200 mL of dry DMSO was introduced into the flask since the dark red mixture had gelled. After stirring for 7 h at room temperature the gel became less viscous and appeared dark orangeyellow. At this point the reaction was quenched by pouring the mixture into 2 L of anhydrous ether. The yellow precipitate was filtered off and washed with three successive 100 mL portions of dry acetone, followed by two successive 100 mL portions of dry ethanol. At the end of this process the ethanol filtrate was colourless. The resulting yellow powder was extracted with methanol for 4 days using a Soxhlet extractor. The purified solid was transferred to a drying pistol and dried

for 2 days over PzO5 at 100°C (2 Torr). The total yield of pale tan powder was 7.5 g. The material obtained is very hygroscopic. It is insoluble in water, methanol, and other organic solvents but slightly soluble in DMSO. Analysis of the tan powder shows C 41.44, H 4.85, and N 3.31%, which, basing calculations on the amount of cellulose bonded to one picryl ring (3 nitrogens), yields an empirical formula of C43H61N3039. This corresponds to about 6.5 glucosyl moieties per picryl ring. Preparation of picryl cellulose, PC-12 This preparation was carried out using the same quantities as in the preparation of PC-6, but the picryl chloride was dissolved in DMSO and then added to the stirred sodium cellulosate in DMSO. The product so obtained incorporated less nitrogen. It analyzed for C 42.26, H 5.49,

1900

CAN. J. CHEM. \IOL. 65, 1987

N 1.93%, which corresponds to about 12 glucosyl residues per picryl ring.


Solubility studies Weighed quantities of PC-6 and PC-12 were placed into test tubes fitted with septum caps. The transfer of material was done in an N2-filled dry box. The sample weights were determined by difference (PC-6: 14.02 mg; PC-12: 93.51 mg). DMSO (800 pL) was injected into each tube. After sonication and centrifugation the supernatant DMSO solution was removed and replaced with diethyl ether by means of a syringe. The samples were repeatedly subjected to ether treatment followed by centrifugation. Finally, the sample was dried at reduced pressure ( < I Torr) in a drying pistol and the dried residues were weighed. Of the PC-6, 4.14mg remained, while of PC-12, 57.34 mg remained. The weight percent of material that had dissolved is therefore PC-6, 70.5%; PC-12, 38.7% 'H nuclear magnetic resonance spectroscopy All nmr spectra were recorded on an AM 400-MHz Bruker specprobe. Standard Wilmad trometer equipped with a standard ' H / ' ~ c PP-507 nmr tubes were used. Samples were sonicated using a Bransonic ultrasonic bath. Typically, 10-25 mg of the cellulose derivative was placed in a septum-covered small volumetric flask. (CD3)?S0(800 pL) was injected into the nitrogen-flushed vessel and the suspension was sonicated in an ultrasonic bath for 2-3 h. The resultant mixture was allowed to settle and the transparent supernatant solution, usually yellow or orange in colour, was carefully withdrawn through the septum via syringe. (Separate centrifugation of a control sample of the supernatant showed that it contained no visible insoluble material.) The contents of the syringe were injected into a dry, septum-covered, nitrogen-flushednmr tube containing 2 pL of stock p-dibromobenzene (DBB) solution and the spectrum was taken. The DMSO-d5 resonance centered at 2.500 ppm was used as the internal shift standard. Typically, the instrument was adjusted as follows: spin rate, 25 rps; relaxation delay, 3 s ; acquisition time, 2.982 s ; pulse width, 5 ps; pulse angle, 45"; temperature, ambient (20 + 2°C). For most experiments, 128 transients/FID sufficed to obtain the spectra reported herein. Differential scanning calorimetry The DSC experiments were performed using a Perkin Elmer DSC-4 instrument equipped with the Thermal Analysis Data Station (TADS). The instrument was calibrated with an indium standard prior to use. Each sample was encapsulated in an aluminum pan. The encapsulated sample was placed in the sample holder and an empty aluminum pan was placed in the reference holder. Samples were heated at a rate of 10"C/min over the ranges shown in the various plots. In all scans nitrogen was used as purge gas. When the scan was completed, onset temperatures and energy changes were determined with the aid of the TADS system. For example, temperature limits for an on-screen peak of interest were input to the TADS. AH values and temperatures of maxima or minima were then obtained. Mass spectroscopy All spectra were obtained on a Finnegan 4610 quadrupole mass spectrometer at 70 eV and were calibrated with perfluorotributylamine. ScientificInstrument Services S-95A glass capillaries were used for the solid probe. Approximately 100 pg of each sample was placed in the capillary and secured with a plug of clean glass wool. The heating rate was controlled manually and the probe temperature was read directly from the digital display of the instrument. Chemical ionization spectra emvloved methane. The heating rate for the electron impact spectra was approximately 10 Tt 3"C/min between 200 and 400°C. The scan time was 2.9 s with a 0.05-s hold between scans. CI spectra employed a heating rate of A

,

approximately 30"C/min and a scan time of 1.45 s with a 0 . 0 5 s hold time.

Acknowledgements The authors thank the Army Armament Research and Development Command in Dover, New Jersey (M.J.S.), and the Natural Sciences and Engineering Research Council of Canada (E.B.) for support of this research. 1. R. FOSTER. Organic charge transfer complexes. Academic Press, London. 1969. 2. (a) E. BUNCEL,M. R. CRAMPTON, M. J. STRAUSS, and F. TERRIER. Electron deficient aromatic- and heteroaromatic-base interactions. Elsevier, Amsterdam. 1984; (b) F. TERRIER. Chem. The chemistry of amino, nitro Rev. 82,77 (1982); (c) E. BUNCEL. and nitroso compounds. Suppl. F, Edited by S. Patai, Wiley, London. 1982. S. COHEN,and E. BUNCEL. 3. R. A. RENFROW, M. J. STRAUSS, Aust. J. Chem. 36, 1843 (1983). 4. F. TERRIER, J. C. HALLE,M. P. SIMONNIN, and M. J. POUET.J. Org. Chem. 49, 4363 (1984). S. K. MURARKA, and A. R. NORRIS. Can. J. Chem. 5. E. BUNCEL, 62, 534 (1984). 6. (a) M. J. STRAUSS, R. A. RENFROW, and E. BUNCEL.J. Am. R. A. RENFROW, Chem. Soc. 105,2473 (1983); (b) E. BUNCEL, and M. J. STRAUSS. Can. J. Chem. 61, 1690 (1983); (c) J. Org. Chem. 52, 488 (1987). J. Polym. Sci. 30, 537 (1958). 7. Z. A. ROGOVIN. 8. Y. AVNY,R. RAHMAN, and A. ZILKHA. J. Macromol. Sci. Chem. A6, 177 (1972). 9. (a) B. GIBSON and M. R. CRAMPTON. J. Chem. Soc. Perkin Trans. M. A. EL GHARIANI, and H. 2,648 (1979); (b) M. R. CRAMPTON, A. KHAN.Tetrahedron, 28, 3299 (1972). J. Am. Chem. Soc. 87,5495 (1965); (b) J. Am. 10. (a) K. L. SERVIS. Chem. Soc. 89, 1508 (1967). 11. E. BUNCEL and H. WILSON.In Advances in physical chemistry. Vol. 14. Edited by V. Gold. Academic Press, London. 1977. pp. 133-202. 12. (a) J. HIRSTand K. U. RAHMAN. J. Chem. Soc. Perkin Trans. 2, 21 19 (1973); (b) M. E. C. BIFFINand D. B. PAUL.Aust. J. Chem. 27, 777 (1974). 13. J. G. GRASSELLI and W. M. RITCHEY(Editors). CRC atlas of spectral and physical properties of organic compounds. Vol. IV. 2nd ed. CRC Press, Cleveland, OH. 1980. p. 97. 14. L. ANDOand G. A. WEBB.Theory of NMR parameters. Academic Press, New York. 1983. p. 76. and 0 . HERNANDEZ. Tetrahedron Lett. 3, 219 15. S. K. CHAUDHRY (1978). and P. LAVALLEE. Can. J. Chem. 53,2975 (1975). 16. S. HANESSIAN 17. M. D. NICHOLSON and D. C. JOHNSON. Cellul. Chem. Technol. 11, 349 (1977). 18. T. J. BAKER, L. R. SCHROEDER, and D. C. JOHNSON. Carbohydr. Res. 67, C4 (1974). 19. R. C. MACKENZIE (Editor). Differential thermal analysis. Vol. 2. Academic Press, New York. 1972. 20. M. KILZERand A. BROIDO.Pyrodynamics, 2, 151 (1965). 21. F. A. WODLEY. J. Appl. Polym. Sci. 15, 835 (1971). 22. R. K. JAIN,K. LAL,and H. L. BHATNAGAR. Thermochim. Acta, 97,99 (1986). 23. W. E. FRANKLIN. Anal. Chem. 51, 992 (1979). 24. D. GARDNER. J. Chem. Soc. (C), 1473 (1966). 25. F. SHAFIZADEH, R. H. FURNEAUX, T. T. STEVENSON, andT. G. COCHRAN. Carbohydr. Res. 61, 519 (1978).