Chlorination of Guaiacol as a Model Compound of Benzene Nucleus of Lignin

Abstract

To make clear the mechanism of chlorination of lignin by an aqueous solution, chlorination of lignin and model compounds has been investigated since some years ago.In advance of the investigaton of the complex lignin-like models, guaiacol was investigated firstly, because it would be thought to show the fundamental principle of the chlorination reaction of the benzene nucleus.Part 1.A homogeneous aqueous solution of guaiacol (128 gr guaiacol/7l water) was treated with 7.5l of aqueous chlorine solution (molar ratio 1 : 1) for about 1/2 hr . at 0°C, a small portion of sodium solution was added to destroy the excess of remaining chlorine or hypochlorous acid if any, pH adjusted to nine with sodium hydroxide, sodium borohydride added in order to avoid the polymerization of quinoidal structure if any, acidified, the resinous products separated (I), and the whole was extracted continuously with chloroform (II) followed by ether (III).I and II were distilled in vacuo into four fractions respectively, each fraction was examined gas chromatographically, and the fractions containing similar components were combined.From the five fractions thus obtained the following compounds were separated and identified.1) Guaiacol : as 3.5-dinitro-benzoate, m. p. 137°-139°C.2) 5-Cbloroguaiacol : as 3.5-clinitro-benzoate, rn. p.170°-171.5°C.3) 4.5-dichloroguaiacol : White needles, m. p. 71°-73°C.The melting points of these three compounds are not depressed on admixture with the authentic samples.4) Trichloroguaiacol : Pale yellow needles, m. p. 103°-106°C.The position of chlorine atoms are not settled as yet.5) 3-3′-Dimethoxy 4-4′-dihydroxydiphenyl : White prisms, melting at 166°-167°C. Diacetyl derivative, melting at 200.5°-202°C. Dimethyl derivative (biveratrol), melting at 132°-133°C.Melting points of the diphenyl and its two derivatives agree well with the values reported in literatures.The diphenyl has one phenolic hydroxyl group per methoxyl group, according to the so-called Δe-method of Aulin-Erdtman.From III the following compounds were separated and identified.1) Pyrocatechol : White prisms, melting at 103.5°-104.5°C.2) 3-Chloropyrocatechol : The compound was isolated as mono carboxymethyl ether, melting at 122-123.5°C. undepressed on admixture with the authentic sample of 3-chlorocatechol monocarboxy methyl ether.The position of the carboxymethyl group of the monoether is not determined. Diether was not prepared, as it was found to be difficult to etherify both hydroxyl groups of 3-chloropyrocatechol by an.usual method.Part 2.0.48, 1.09, 3.15 and 4.23 moles of chlorine were brought into reaction with one mol of guaiacol, just as in the case of part I. Excess chlorine was removed by allylalcohol, and the solution was extracted with ether without being preceeded by any reduction with sodium borohydride.Pyrocatechol and 3-chloropyrocatechol were identified in the extract by paper chromatography using benzene : acetic acid : water =1 : 1 : 2 as mobile phase, confirming the direct formation of pyrocatechol structure by the chlorination of guaicol.Results so far obtained on the effects of aqueous chlorine water on the guaiacol nucleus are summarized as follows : 1) Methoxyl group is demethylated to give methanol in high yield.2) Isolation of pyrocatechol suggests that demethylation can occur without introduction of any chlorine atoms into the nucleus.3) One to three chlorine atoms are introduced without cleavage of methoxyl groups.4) It seems therefore to be probable that chlorination of the nucleus and the demethylation of methoxyl group are independent reactions.5) Guaiacol dimerizes by dehydrogenation.