Preparation of cellulose and cellulose derivative azo compounds

Maha IbrahimCellulose and Paper Department, National Research Center, El-Tahrir St., Dokki, Cairo, Egypt (e-mail:mwakleed@hotmail.com)

Received 20 September 2001; accepted in revised form 18 February 2002

Key words: Cellulose azo compound, Cellulose sulfate, Diazotization, Methyl cellulose

Abstract

Wood pulp and cotton linter are the most common sources of cellulose for industrial use. Methyl cellulose (MC)and cellulose sulfate (CS) were prepared using bleached wood pulp and cotton linter. Coloured azo compoundswere also prepared from coupling cellulose, wood pulp, MC and CS with aromatic diazonium salt. The presenceof electron-releasing or withdrawing substituents affected the electrophilic substitution reaction. The producedazo compounds were characterized by FT-IR methodology, as well as mass spectrometry, in which the functionalgroups and the ion fragments of the products were analyzed.

Introduction

Cellulose is the most abundant organic chemical onthe face of the earth. It is generally accepted that cel-lulose is a linear condensation polymer consisting ofD-anhydroglucopyranose units joined together by�-1,4-glycosidic bonds (Purves 1954; Kennedy andWhite 1983; Han and Rowell 1997). Every bond in-volves the potentional aldehyde group of one glucoseand a hydroxyl group of another. The involvement ofthe hydroxyl groups in hydrogen bonding, as well asdispersion forces determined by the proximity ofneighbouring atoms, impart a different reactivity tothe three hydroxyl groups available for chemical re-actions (Hebeish 1994). The essential features of thecellulose molecule, to which most of its chemicalproperties may be related, are the primary and sec-ondary alcohol groups in each monomer unit and theglycosidic bonds (Nevell and Zeronian 1985). Theprimary and secondary alcohol groups in cellulosereact in the same way as in simple substances of sim-ilar chemical constitution.

It is well known that the cellulose units repeatthemselves throughout the entire length of the cellu-lose molecule and the only reactive function it pos-sesses is provided by the hydroxyl groups (Hebeish1994).

The most common sources of cellulose for indus-trial use are wood pulp and cotton linter (Nevell andZeronian 1985). A novel method for preparation ofcellulose bearing aromatic amino groups, by reactingcotton cellulose with 2,4-dichloro-6-(p-nitroanilino)-s-triazine, was described by Hebeish et al. (1978). Inanother study (Hebeish et al. 1979), chemically mod-ified cotton fabric samples with different amounts ofaromatic amino groups were reacted with tetrakis (hy-droxymethyl) phosphonium chloride (THPC). More-over, pre-treatments of cellulose, namely desizing,scouring, bleaching and mercerization, are prerequi-sites for interaction of cellulose with dyestuffs andfinishing agents (Hebeish 1994).

Dyed cellulosic fabrics have been produced by thediazotization and coupling of selected celluloseamino esters and ethers. Vigo and co-workers (1965)have modified cotton cellulose by reacting it with o-aminobenzoic acid to form the ester on cellulose (cel-lulose o-aminobenzoate). They found that celluloseo-aminobenzoate may be treated with dilute aqueousacid and nitrite solutions at room temperature to formdiazonium salts of cotton cellulose. The equation(Vigo et al. 1965) for this transformation is:

337Cellulose 9: 337–349, 2002.© 2002 Kluwer Academic Publishers. Printed in the Netherlands.

Since the reaction of diazotization is characteristicof primary amines, it is clear from the above equa-tion that the presence of the amino group in o-ami-nobenzoic acid is responsible for the diazotization re-action of the cellulose o-aminobenzoate forming thestable diazonium salt. However, the present work wasundertaken with the primary objective to produce cel-lulose azo compounds by activating the cellulose tobe reacted like phenols in the coupling reaction withdiazonium salts. Cellulose was modified by reactingit with dimethyl sulfate and sulfuric acid to form me-thyl cellulose (MC) and cellulose sulfate (CS), thenforming cellulose and cellulose derivative azo com-pounds by direct coupling to the azo group (-N=N-)present in the diazonium salts of aromatic amino andaromatic nitro compounds. The equations for thesetransformations are:

Experimental

Chemicals

Wood pulp (�-cellulose = 82.7%) and cotton linter(�-cellulose = 91.3%) were used as a source of cel-lulose to prepare methyl cellulose and cellulose sul-fate, respectively. Dimethyl sulphate (BDH) and ace-tone (ADWIC), used for the preparation of MC, aswell as sulphuric acid (AR, s.d. fine-chem Ltd.) andn-butyl alcohol (ADWIC), used for the preparation ofCS, were used as received. The following chemicalswere obtained and used without further purification:cellulose (Merck), aniline (Riedel-de Haën), 4-nitroa-niline (BDH), 1,4-phenylenediamine (Merck), aceticanhydride (Merck), glacial acetic acid (ADWIC), so-

dium nitrite purified LR (s.d. fine-chem Ltd.), hydro-chloric acid (Cambrian Chemicals) and sodium hy-droxide (Fluka).

Preparation of methyl cellulose

Bleached wood pulp, 1 g, was mercerized using 40%sodium hydroxide solution, at 5% consistency, for 1h at 20 °C. The excess sodium hydroxide was thenremoved by pressing to a liquor ratio of 1:3 (cellulo-se:sodium hydroxide solution). Dimethyl sulphate(DMS) was used in a ratio of 1:3 (cellulose:DMS).The reagent was added dropwise and the reactionmixture was allowed to proceed at 50 °C for 2 h innonaqueous media using acetone as diluent in a ratioof cellulose:acetone 1:9. At the end of the reaction,the material was neutralized with 10% acetic acid,filtered in a G2 sintered glass crucible, and washedwith acetone. The MC obtained was dried in a vac-uum oven at 50 °C.

Preparation of cellulose sulfate

Cotton linter was used to prepare CS, to which con-centrated sulphuric acid was added in 10 times the dryweight of the cotton linter in the presence of n-bu-tanol. The reaction mixture was allowed to proceedat 20 °C for 30 min. At the end of the reaction, thematerial was filtered in G2 sintered glass crucible andwashed with a mixture of water and butanol (1:3).The CS obtained was dried in a vacuum oven at 40°C.

Diazotization

Three g of the aromatic amines, namely aniline, 4-ni-troaniline or 1,4-phenylenediamine, was mixed with25 mL of water and 7 mL of concentrated hydrochlo-ric acid in a 50–100 mL flask. The obtained clear so-lution was cooled to 0 °C in an ice-salt bath.

Sodium nitrite, 2.3 g, was dissolved in 4–5 mL ofwater in a separate test tube and added dropwise,while cooling and shaking, to the cooled aromaticamine solution. The temperature of the mixtureshould never rise above 5–7 °C. The reaction wastested with strach-iodide indicator paper.

The flask containing the almost fully clear solutionof diazonium salt was kept on ice until use.

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Coupling reaction

The reaction of diazo compound with cellulose orcellulose derivatives is known as the coupling reac-tion. This reaction was effected in an alkaline me-dium, where cellulose, MC and CS were mercerizedin dilute alkaline solution. An equal volume of diazo-nium salt solution was then added to each of them.The products of the coupling reaction, azo com-pounds, are quite stable and are usually coloured.

Characterization

Infrared spectroscopyA JASCO FT/IR-300 E spectrophotometer (Fouriertransform infrared) instrument was used for the char-acterization of the molecular composition of the cou-pling products by analyzing the characteristic vibra-tions of functional groups.

Mass spectrometryA Finnigan SSQ 7000 instrument was used for thecharacterization of the molecular ion fragments of theproduced coupling compounds.

Results and discussion

A series of cellulose and cellulose derivatives cou-pling with aromatic amino compounds can be pre-pared by a diazotized reaction. The first step was thepreparation of diazonium salts from aniline, 4-nitroa-niline and 1,4-phenylenediamine. The second stepwas coupling of cellulose, MC and CS with the pre-pared diazonium salts in a dilute alkaline medium toprepare azo compounds. The couplings are illustratedin Schemes 1–4.

In a dry round-bottom flask attached to a refluxwater-condenser, 10 mL of a mixture of equal vol-

Scheme 1. Coupling with aniline.

Scheme 2. Coupling with 4-nitroaniline.

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umes of acetic anhydride and glacial acetic acid wasadded to 5 g of 1,4-phenylenediamine. The mixturewas boiled gently for 30 min on a sand bath. Thenthe hot liquid was poured into 100 g of crushed icewhile stirring. The acetanilide rapidly crystallised,

was then filtered at the pump and washed well withwater, drained, and dried.

All forms of cellulose, wood pulp and couplingcompounds were characterizied. Figures 1 and 2 showthe FT-IR spectra of both cellulose and wood pulp,and their azo compounds I and II resulting from cou-

Scheme 3. Coupling with 1,4-phenylenediamine.

Scheme 4. Reaction with acetanilide.

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pling with aniline. The peaks at 1170–1040 and 3400cm−1 in Figure 1 (a and b) are due to the ether link-age and the hydroxyl group of the cellulose (Hwang

and Chen 1993). The peak at 1577 cm−1 in Figure 2(a and b) is due to the −N=N− moiety of the azocompounds I and II. The bands at 1585 and 1309

Figure 1. FT-IR spectra of (a) cellulose; and (b) wood pulp.

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Figure 2. FT-IR spectra of (a) cellulose azobenzene (compound I); and (b) wood pulp azobenzene (compound II).

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cm−1, as can be seen in Figure 3, may be attributed tothe symmetric and symmetric axial deformations ofthe NO2 (Covolan et al. 1997) presence in the cellu-lose azo compound III. Furthermore, it can be seen

that these bands are relatively similar to those bandsobserved in the 4-nitroaniline spectrum (Figure 4).

Figure 5 shows the FT-IR spectra of azo com-pounds IV, V and VI. The peaks at 1600–1500 and

Figure 3. FT-IR spectrum of cellulose azonitrobenzene (compound III).

Figure 4. FT-IR spectrum of 4-nitroaniline.

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Figure 5. FT-IR spectra of (a) cellulose azoaminobenzene (compound IV); (b) methyl cellulose azoaminobenzene (compound V); and (c)cellulose sulfate azoaminobenzene (compound VI).

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Figure 6. Mass spectra of: (a) cellulose azoaminobenzene (compound IV); (b) methyl cellulose azoaminobenzene (compound V); and (c)cellulose sulfate azoaminobenzene (compound VI).

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833 cm−1 are due to C=C for aromatic ring and N-Hbending out-of-plane in spectra a, b and c, while thepeak at 3353 cm−1 is due to N-H stretching of aminein spectra b and c. The peaks at 1149–1060 and 3353cm−1 in spectrum a are connected to the ether linkageof the cellulose, and �N-H and �O-H (Covolan et al.1997). The peaks at 1240 and 1240–1140 cm−1 inFigure 5 (spectra b and c) are due to C-O of OCH3

for methyl cellulose and S=O for sulfonate in cellu-lose sulfate.

The mass spectra, Figure 6a–c, of cellulose azoam-inobenzene (IV), methyl cellulose azoaminobenzene(V), and cellulose sulfate azoaminobenzene (VI) clar-ified the fact that cellulose and cellulose derivativesare coupled directly to the azo group (−N=N−) of thediazonium salt, in which the presence of ion frag-ments of m/z 92 and 120 represents the formation ofp-aminobenzenediazonium salt before coupling withcellulose, MC and CS. The presence of ion fragmentsof m/z 162, 204, and 322, respectively (Figure 6a–c),represents a fragment of one molecule of cellulose,

MC and CS, respectively. These fragments can berepresented according to the following:

Moreover, reaction of acetic anhydride and glacialacetic acid with 1,4-phenylenediamine results in ac-etanilide compound VII. This can be indicated fromthe mass spectrum of compound VII, in which itsmolecular weight is identified to be 192 (Figure 7).

Figure 7. Mass spectrum of acetanilide (compound VII).

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Figure 8 (a and b) shows the FT-IR spectra of theproduced products VIII and IX resulting from the re-action of the acetanilide with sodium nitrite and cel-lulose or MC. The frequencies at about 3300 and 835cm−1 in compounds VIII and IX refer to �N-H forsecondary amine and N-H bending out-of-plane.Peaks at 1517 and 1317 cm−1 are due to the N=Ogroup, while the peak at 1662 cm−1 is due to the C=Ogroup and the peak at 1255 cm−1 is due to -COC from-COCH3. Besides, the peak at 1008 cm−1 is due tothe ether linkage of the cellulose.

The mass spectra for the above-mentioned com-pounds are shown in Figure 9 (a and b), in which afragment ion peak of 192 for the acetanilide VII ispresent.

For the six azo compounds the yield percentageshave been calculated and are listed in Table 1.

The coupling reaction of a diazo compound is anelectrophilic substitution reaction, in which the elec-trophile is the cation of the diazonium salt (Nekrasov1978), and hence the base-strengthening substituentsare the ones that activate an aromatic ring towardelectrophilic substitution, while the base-weakeningsubstituents are the ones that deactivate an aromaticring toward electrophilic substitution.

For cellulose azonitrobenzene (III) and celluloseazoaminobenzene (IV), in which cellulose was cou-pled to both 4-nitroaniline and 1,4-phenylenediamine,respectively, one can notice that coupling with 1,4-phenylenediamine results in a higher yield percentage(Table 1). This can be attributed to the substituentspresent in both aromatic rings, i.e., −NH2 and −NO2.The electron-releasing substituent, −NH2, tends todisperse the positive charge on the diazonium ion, andthus stabilizes the cation of the diazonium salt. Be-sides, the electron withdrawing substituent, −NO2,tends to intensify the positive charge, and thus desta-bilizes the cation present.

From another point of view, one can consider thatan electron-releasing group pushes electrons towardnitrogen and makes the fourth pair more available forsharing, whereas an electron-withdrawing grouphelps to pull electrons away from nitrogen and thusmakes the fourth pair less available for sharing.

Table 1. Yield percentages of the produced azo compounds.

Azo compound Colour Yield (%)

Ar-N=N-cellulose (I) Brown 178.7

Ar-N=N-wood pulp (II) Brown 181.5

O2N-Ar-N=N-cellulose (III) Green 187.3

H2N-Ar-N=N-cellulose (IV) Black 251.3

H2N-Ar-N=N-MC (V) Black 154.6

H2N-Ar-N=N-CS (VI) Black 171.8

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Figure 8. FT-IR spectra of (a) cellulose nitroso (compound VIII); and (b) methyl cellulose nitroso (compound IX).

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Conclusions

In this work, cellulose, wood pulp, MC and CS werecoupled to diazonium salts. The produced azo com-pounds are coloured. The coupling reaction of a diazocompound is an electrophilic substitution reaction, inwhich the electron-releasing or withdrawing substitu-ent present in the aromatic ring affected the stabilityof the cation of the diazonium salt. FT-IR and massspectrometry are used to characterize the producedazo compounds.

References

Covolan V.L., Innocentini Mei L.H. and Rossi C.L. 1997. Chemi-cal modifications on polystyrene latex: preparation and charac-terization for use in immunological applications. Polym. Adv.Technol. 8: 44–50.

Han J.S. and Rowell J.S. Chemical composition of fibers. In: Row-ell R.M., Young R.A. and Rowell J.K. (eds), Paper and Com-posites from Agro-Based Resources. Lewis Publishers, NewYork, Chapter 5, p. 85.

Purves C.B. 1954. In: Ott E., Spurlin H.M. and Grafflin M.W. (eds),Cellulose and Cellulose Derivatives, Part I. 2nd edn. Inter-science Publishers, New York, pp. 29–98.

Kennedy J.F. and White C.A. 1983. Bioactive Carbohydrates. EllisHorwood Ltd., Chichester, UK, p. 149.

Hebeish A. 1994. Development in Textile Chemistry and ChemicalTechnoloy (Special Contribution). 2nd edn. Academy of Scien-tific Research and Technology, Cairo, Egypt, p. 149.

Hebeish A., Waly A., Morsi A.Z. and El-Rafie M.H. 1978. Prepa-ration of chemically modified cotton via introduction of aro-matic amino groups. J. Appl. Polym. Sci. 22: 2713–2716.

Hebeish A., Waly A. and El-Kashouti M. 1979. Durable flame re-sistance via reaction of cotton cellulose bearing aromatic aminogroups with tetrakis (hydroxymethyl) phosphonium chloride. J.Appl. Polym. Sci. 23: 1803–1810.

Hwang M.C. and Chen K.M. 1993. The removal of color from ef-fluents using polyamide-epichlorohydrin-cellulose polymer. I.Preparation and use in direct dye removal. J. Appl. Polym. Sci.48: 299–311.

Nevell T.P. and Zeronian S.H. Cellulose chemistry fundamentals.In: Cellulose Chemistry and Its Applications, Chapter 1. EllisHorwood Ltd., Chichester, UK, p. 17.

Vigo T.L., Wade R.H. and Welch C.M. 1965. Preparation and prop-erties of cotton cellulose diazonium salts. Textile Res. J. 35:1009–1013.

Figure 9. Mass spectra of (a) cellulose nitroso (compound VIII); and (b) methyl cellulose nitroso (compound IX).

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Nekrasov V.V. 1978. Practical Organic Chemistry. Mir Publishers,Moscow.