Epichlorohydrin coupling reactions with wood

Transcription

1 Wood Science and Technology 28 (1994) Springer-Verlag 1994 Epichlorohydrin coupling reactions with wood Part 1. Reaction with biologicallyactive alcohols R. M. Rowell, G. C. Chen Summary Properties of wood can be improved by reacting chemicals with hydroxyl groups of wood cell wall polymers. To achieve this improvement in wood properties, bioactive compounds containing hydroxyl groups, such as pentachlorophenol, 3,5-dimethyl phenol, and 2-naphthol, can be reacted with epichlorohydrin to give corresponding glycidyl ethers. The new epoxide formed during this reaction can be used to bond bioactive compounds to wood. This bonding may result in improved wood properties. The objective of this study was to develop a simple procedure for synthesizing glycidyl ethers. The alcohol was reacted with epichlorohydrin and a catalyst and monitored by thin-layer chromatography. Shortwave ultraviolet light was used to detect spots. Resulting products were analyzed for carbon, hydrogen, and in one case, chlorine. Reaction of pentachlorophenol with epichlorohydrin formed only one enantiomeric glycidyl ether, whereas reaction of 3,s-dimethyl phenol with epichlorohydrin led to two enantiomeric glycidyl ethers in a 1 to 3 ratio. Reaction of 2-naphthol with epichlorohydrin also led to two enantoimeric glycidyl ethers in equal amounts. In future research, these glycidyl ethers will be reacted with wood, and their toxicity to wood-destroying fungi in bonded form will be determined. 371 Introduction For several years, the U.S. Department of Agriculture, Forest Service, Forest Products Laboratory has had a research program focused on changing the properties of wood by reacting chemicals with the hydroxyl groups of wood cell wall polymers. Most of this research has been directed to improving wood properties such as dimensional stability, biological restistance, and restistance to ultraviolet radiation (Rowell 1984 a). The effectiveness of the chemicals that become bonded to the wood cell wall in improving wood properties is based on the location and distribution of the reacted chemicals in the cell wall polymers and is not due to the properties of the reacting chemical. The improvement in biological restistance of wood by cell wall modification is not due to toxicity of the reacting chemical but rather to a lowering of the fiber saturation point of the substrate below a moisture level necessary for attack and to an alteration of the chemical and molecular configuration of the substrate to inhibit enzymatic reactions. Biological attack can also be stopped by bonding toxic chemicals to wood cell wall polymers that either remain toxic in the bound form or slowly release the toxic chemical into the wood structure (Rowell 1984b, Chen et al. 1990). We have used reactive epoxide (Rowell and Ellis Received 14 January 1993 USDA Forest Service Forest Products Laboratory One Gifford Pinchot Drive Madison, WI

2 ), isocyanate (Rowell and Ellis 1981), isothiocyanate (Chen 1992 a), or sulfonyl chloride (Chen 1992b) functional groups to bond potentially toxic chemicals to wood. By using a difunctional epoxide or isocyanate, it is possible to react one functional group with a chemical that has a reactive hydroxyl, leaving the second functional group to react with a hydroxyl on wood cell wall polymers (Ellis and Rowell 1984). This technology allows a hydroxyl that contains a biocide or fire retardant to be bonded to wood. The weakness in the reactive difunctional approach is that in many cases, once one functional group has reacted, the second group becomes much less reactive. This can be overcome by using a monofunctional group that generates a new functional group during the reaction with the hydroxyl group. Epichlorohydrin is capable of undergoing this type of reaction. In this case, epichlorohydrin reacts with an alcohol generating an ether-bonded chlorohydrin (Fig. 1). The chlorohydrin can then undergo an internal epoxidation giving a new epoxide functional group by eleminating hydrochloric acid. The newly formed epoxide containing the chemical that will be bonded to the wood can then react with a cell wall hydroxyl group. To test this reaction scheme, pentachlorophenol was chosen as the biocide alcohol because its toxicity to wood-destroying fungi is well known. Using this procedure, it should be possible to bind pentachlorophenol to wood and determine if its toxicity is retained or lost because of bonding. The reaction between epichlorohydrin (I) and pentachlorophenol (II) to form the pentachlorophenyl glycidyl ether (IV) (Fig. 2) was first investigated by Alquist and Slagh (1940). They dissolved (II) in water containing sodium hydroxide, added (I), and heated the mixture at 60 to 70 C for 1 h. Upon cooling, (IV) was separated out and recrystallized from ethyl alcohol. We tried to repeat this procedure but found a mixture of products, as shown by thin-layer chroma- Fig. 1. General reaction scheme of epichlorohydrin with alcohol Fig. 2. Reaction ofepichlorohydrin with pentachlorophenol

3 tography (TLC). The reaction mixture contained five compounds; the major one was unreacted (II). Saayman (1974) used a similar procedure to produce the epoxide derivative of phenol, but that procedure did not produce the desired epoxide derivative using pentachlorophenol. Another procedure by Zyl et al. (1954) with the phenol in dioxane also produced a mixture of products. The major product seemed to be a polymeric material that did not move on TLC. A similar procedure by McKelvey et al. (1959) also showed that the major product of the reaction was a polymer. After all published procedures to form the pentachlorophenyl glycidyl ether had failed, we decided to develop our own procedure for synthesizing this type of compound. The purpose of this research was to develop a simple procedure for synthesizing glycidyl ethers. In future research, these glycidyl ethers will be reacted with wood and their toxicity to wood-destroying fungi in the bonded form will be determined. The procedure was developed using pentachlorophenol and then expanded to include other potential biocide alcohols such as 3,5-dimethyl phenol and 2-naphthol. 373 Materials and methods Synthesis of pentachlorophenyl glycidyl ether (IV) Pentachlorophenol (30 g, 113 mmoles) was dissolved in epichlorohydrin (100 ml, 1,279 mmoles) containing catalyst triethylamine (1 ml, 7 mmoles). The mixture was refluxed for 15 min, and then the excess epichlorohydrin was distilled off. Upon cooling, the mixture so lidified. The solid was dissolved in hot acetone and crystallized upon cooling. Recrystallization from hot acetone gave 25 g (68% yield), mp 119 C of pentachlorophenyl glycidyl ether. Spots were detected by shortwave ultraviolet light and also charrring after spraying with 10% sulfuric acid in ethanol. Samples of the compound (IV) were analyzed for carbon, hydrogen, and chlorine. Theoretical analysis for the chlorohydrin (III) C 9H 6O 2Cl 6 would be C, 30.12%; H, 1.56%; and Cl, 54.99% and for the epoxide (IV) C 9H 5O 2Cl 5, C, 33.53%; H, 1.56%; and Cl, 54.99%. Actual analysis was C, 33.71%, H, 1.64%; and Cl, 54.87%. This shows that the compound isolated was the glycidyl ether(iv). Synthesis of 3,5-dimethyl phenyl glycidyl ethers (VI) and (VII) Epichlorohydrin 31.2 ml, 400 mmoles) and catalyst triethylamine (0.33 ml, 2.4 mmoles) were added to 3,5-dimethyl phenol (9.76 g, 80 mmoles). The solution was refluxed for 1 h. Thin-layer plates (precoated silica gel sheets with fluorescence indicator F-254; chloroform) showed a com plete conversion of dimethyl phenol to two enantiomeric 3,5-dimethyl phenyl glycidyl ethers, (IV) and (VII), as visualized by shortwave ultraviolet light. The mass of the fast-moving enan tiomer was only one-third the mass (as evidenced by the size of the spots) of the slow-moving enantiomer. The Rf values of the two enantiomers, ethers (VI) and (VII), were 0.52 and 0.38, respectively. After the reaction, excess epichlorohydrin was removed under vacuum ( Pa (25 mm Hg)) in a rotary evaporator at 50 C to give a light-brown liquid (22.32 g). Part of the liquid (9.77 g) was distilled under vacuum at Pa (5 mm Hg) to yield four boiling fractions (Table 1 and Fig. 3). Samples of Fractions B and C were analyzed for carbon and hydrogen. Theoretical analysis for the epoxide (VI) or (VII) C 11H 14O 2 would be C, 74.13% and H, 7.92% and for chlorohydrin (V) C 11H 15O 2Cl, C, 61.53% and H, 7.04%. Actual analysis for Fraction B was C, 64.57% and H, 7.50% and for Fraction C, C, 68.21% and H, 7.34%. As shown by NMR, Fraction C contained a small amount of impurity, which contributed to much lower values of carbon and hydrogen than the theoretical values.

4 Table 1. Boiling fractions of 3,5-dimethyl phenyl glycidyl ethers (VI) and (VII) 374 Fig. 3. Reaction of epichlorohydrin with 3,5-dimethyl phenol (Dmp) Synthesis of 2-naphthyl glycidyl ethers (IX) and (X) Epichlorohydrin (11) (39 ml, 500 mmoles) and catalyst triethylamine (0.4 ml, 3 mmoles) were added to 2-naphthol (14.4 g, 100 mmoles). The solution was refluxed for 1 h. The reactions were monitored by TLC. Thin-layer plates (precoated silica gel sheets with a fluorescence indi cator F-254; chloroform as solvent) showed that the products contained two enantiomeric glyci dyl ethers (IX) and (X) (R f 0.46 and 0.74) in equal amounts as visualized by shortwave ultraviolet light. After the reaction, excess epichlorohydrin was removed under vacuum at Pa (25 mm Hg) in a rotary evaporator at 50 C. The product was a light-brown liquid (31.5 g). Part of the crude product (13.2 g) was distilled under vacuum at Pa (5 mm Hg) to yield one boiling fraction and a solid (Table 2 and Fig. 4). Attempts to crystallize the solid (Fraction B) from organic solvents including acetone, benzene, toluene, dioxane, chloroform, and tetrahydrofuran were unsuccessful. Samples of Fraction B after trituration with heptane were analyzed for carbon and hydrogen. Theoretical analysis for the epoxide (IX) or (X) would be C, 77.98% and H, 6.04% and for chlorohydrin (VIII) C 13H 14O 2Cl, C, 65.69% and H, 5.94%. Actual analysis was C, 71.66% and H, 6.13%. As shown by NMR, Fraction B contained a small amount of impurity, which contributed to much lower values for carbon and hydrogen than the theoretical values.

5 Table 2. Boiling fraction and solid ofenantiomeric glycidyl ethers (IX) and (X) 375 Fig. 4. Reaction of epichlorohydrin with 2-naphthol (Nap) Results and discussion By using a very simple procedure, pentachlorophenol was reacted in an excess of epichlorohydrin with a small amount of triethylamine as catalyst. By distilling off the excess epichlorohydrin, the reaction between pentachlorophenol, and epichlorohydrin was driven to completion. No pentachlorophenol was left in the reaction mixture, and the initial yield, before recrystallization, was more than 85%. The reaction of pentachlorophenol with epichlorohydrin formed only one enantiomeric glycidyl ehter. Thin-layer chromatography (precoated silica gel sheets with a fluorescence indicator F-254) using chloroform/hexane (l/l, v/v) showed (IV) R f 0.4 ((II) R f 0.33) as only one spot. Infrared spectrum of pentachlorophenyl glycidyl ether (IV) showed the characteristic oxirane absorptions. The absorptions at 866 and 900 cm -1 are attributed to asymmetric C-O-Cstretching vibration, at 1,256 cm -l to symmetric C-O-Cstretching vibration, and at 3,000 to 3,040 cm -1 to stretching of terminal epoxides (Colthup et al. 1964). Proton nuclear magnetic resonance (NMR) (chemical shifts in ppm, d-chloroform) showed the following characteristic protons: 4.28, 4.05 (d, 2 H, epoxy CH), 3.48 (s, 1 H, epoxy CH), 2.93, and 2.72 (d, 2 H, CH 2OPenta). To determine if this procedure could be used to make other glycidyl ethers, 3,5-dimethyl phenyl glycidyl ethers and 2-naphthyl glycidyl ethers were also synthesized. For 3,5-dimethyl phenyl glycidyl ethers, thin-layer plates showed that boiling Fractions B and C were two enantiomeric glycidyl ethers, (VI) and (VII). Fraction D contained only one enantiomer as well as residues. No attempts were made to separate the two enantiomers. Infrared spectra of 3,5-dimethyl phenyl glycidyl ether (VI) or (VII) showed the characteristic oxirane absorptions. The absorptions at 836 and 905 cm -1 are attributed to asymmetric C-O-Cstretching vibrations and

6 376 at 1,280 cm -1 to symmetric C-O-Cstretching vibrations (Colthup 1964). The intensities of these absorptions were moderate to weak. Proton NMR (ppm) in d-chloroform of Fraction C showed the following characteristic protons: 6.16 (s, 2 H, aromatic), 3.75 (m, 2 H, epoxy CH), 3.25 (m, 1 H, epoxy CH), 2.79, 2.62 (m, 2 H, CH 2ODmp), and 2.25 (s, 6H, CH 3). Protons derived from triethylamine hydrochloride in trace amounts and other byproducts in small amounts were also present. Infrared spectrum of 2-naphthyl glycidyl ethers (IX) and (X) obtained from the products triturated with heptane showed the characteristic oxirane absorptions. The absorptions at 838 and 900 cm -1 are attributed to asymmetric C- O- Cstretching vibrations and at 1,256 cm -1 to symmetric C-O-Cstretching vibrations (Colthup et al. 1964). The intensity of these absorptions was moderate to weak. Proton NMR (chemical shifts in ppm; d-chloroform) of Fraction B after triturating with heptane showed the following characteristic protons: 7.72 (m, 3 H, aromatic), 7.40, 7.14 (m, 4H, aromatic), 4.16, 4.00, (m, 2H, epoxy CH), 3.39 (m, lh, epoxy CH), 2.91, and 2.77 (m, 2 H, CH 2ONap). Protons of triethylamine hydrochloride in trace amounts and other byproducts in small amounts were also present. The two enantiomeric 3,5-dimethyl phenyl glycidyl ethers (IV) and (VII) were in a 1 to 3 ratio as visualized by shortwave ultraviolet ligth. The two enantiomeric 2-naphthyl glycidyl ethers (IX) and (X) were in equal amounts. The glycidyl ethers derived from 3,5-dimethyl phenol or 2 naphthol could not be separated by vacuum distillation. Concluding remarks Reaction of pentachlorophenol with epichlorohydrin formed only one enantiomeric glycidyl ether, whereas reaction of 3,5-dimethyl phenol or 2-naphthol with epichlorohydrin led to two enantiomeric glycidyl ethers. The results of this study have provided a method for making potentially bioactive epoxides that can be reacted with hydroxyl groups on wood cell wall polymers. Research is presently underway to react these bioactive epoxides with wood and determine their effectiveness in resisting attack by wood-destroying fungi. References on recycled paper