A reply to the note by Nimz (1981) on the occurrence of benzyl non-cyclic ether bonds in lignin.
Introductory Remarks
None of the evidence that Nimz (1981) has assembled for the frequent occurrence of benzyl noncyclic aryl ethers or the absence of benzyl non-cyclic alkyl ethers in lignin is irrefutable and most of it can be readily accounted for by alternative explanations. I summarise the main points he makes as follows:
I. The lignols isolated by Freudenberg and Friedmann (1960) are benzyl aryl ethers since they give monodinitrophenyl ethers and the dinitrophenyl ethers of their hydrogenated (1) and acetylated (2) derivatives have the expected 1312 nmr spectra (Nimz and Liidemann 1974).
1I. Vanillyl alcohol and its quinone methide are poor models for lignin p-hydroxybenzyl alcohols or quinone methides and hence their behaviour may be anomalous.
IlL In dilute aqueous solutions (pH 5.4) phenolic addition to quinone methides is preferred to addition of alcohols.
IV. The increase in phenolic hydroxyl groups following mild acid treatment of milled wood lignin is attributable to the hydrolysis of benzyl aryl ethers that were also present in the original lignin in the wood.
V. The formation of lignols on mild wood hydrolysis results, at least in part, from benzyl aryl ether hydrolysis.
VI. The presence of benzyl non-cyclic aryl ethers and absence of benzyl non-cyclic alkyl ethers is confirmed by 13C nrnr spectroscopy of milled wood lignins.
VII. The benzyl alkyl ether isolated by Sakakibara et al. ( 1978) is an artifact. VIII. Benzyl non-cyclic alkyl ether formation during 'aging' of lignin will be negligible since the 'mobility' of water would somehow give it an advantage over aliphatic hydroxy groups even if the latter are permanently close to the p-hydroxybenzyl alcohols.
Some of these points (II, III) are misleading or incorrect; others (V, VII, VIII and, to a lesser extent IV) are obviously inconclusive, and the two discussed below (I, VI) only superficially support Nimz's argument. Evidence from the Structure of Synthetic Lignols or DHP Lignin (I) There are disquieting weaknesses in the derivatisation procedures used (Freudenberg and Friedmann 1960) and the 13C nmr data presented (Nimz and Ltidemann 1974) for the dinitrophenyl ethers (1) and ( 2) are difficult to interpret, but I agree with Nimz that the evidence is in agreement with the conclusion that the derivatives isolated were