Good question. Lignin is resistent to acid hydrolisis, but not alkaline. Maybe the OH- is polar and H+ apolar can better explain how lignin degradation works.

Well, when you say lignin you need to know lignin has no defined struture and thus is considered amorphos. So, we don't know where the atoms are arranged for. And there are two types of lignin. Lignin for gymnosperm and for angiosperm... angiosperm lignin is from gymnosperm ancestors. In literature we find gymnosperm lignin is more resistent to alkali degradation then angiosperm ones. But both are resistent to acid medium. Idk how much the acid hydrolise can degradate both types of lignin. In acid condiction in a hydrolisis with H2SO4 72% we found most lignin is resistent (Klason lignin) and some is not (dissolved lignin in H2SO4). The dissolved lignin is like a few % of total insoluble lignin. If you are plaining to study lignin you need to know other substance are present in wood material. Such as pectine, extractives, mineral et.. The lignin is linked to hemicelluloses ...and hemicelluloses are linked to cellulose. So, lignin is like a cover on cellulose fibrils... We know cellulose is water soluble, but lignin is not soluble in water.... just became solube with the medium is alkaline, probleby OH ions break down the cross-linked bonds between the main products of lignin... look at my dissertation in Portuguese.. It might helps you.. https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwiH3Z7O4szxAhWlH7kGHazDCrMQFjAAegQIAxAD&url=http%3A%2F%2Frepositorio.unifap.br%2Fhandle%2F123456789%2F394&usg=AOvVaw0rkhcR5gS2H0ITephV_uGJ

Thanks for your precious experience: Rani P Ramachandran and Jefferson Bezerra Bezerra.

Best regards.

Interested

https://bioresources.cnr.ncsu.edu/resources/wood-based-lignin-reactions-important-to-the-biorefinery-and-pulp-and-paper-industries/

Dear Prof. Dr. Ahasan Ullah Khan,

Great. Thanks for your valuable data in the discussion. I appreciated it.

Best regards.

The pKa of phenolic groups in lignin is in the range of 9.4 - 10.6. Therefore, in alkaline conditions at elevated temperatures, the phenolic groups can dissociate into phenolate ions. As phenolate ions are formed, the conjugation in the molecule can result in formation of double bonds especially at Calpha - Cbeta position. Further, hydrolysis takes place at this position as a result of which water molecule gets added in this position. In acidic conditions, lignin is normally insoluble and resistant to acid hydrolysis as mentioned by Rani P Ramachandran

Please also refer to our publication for more details Article Towards a new understanding of the retro-aldol reaction for ...

Dear Dr. Ajinkya More,

Thanks for your valuable data in the discussion. I appreciated it.

Best regards.

Dear Dang T. Nguyen,

Please check the following documents.

With my best regards

About lignin dissolved in KOH aq. sol. and its precipitation from that sol. ― Lignin dissolved in KOH aq. sol. is likely present as the potassium salt of lignosulfonic acid; let us denote it as LigOK, while we may denote lignin as LigOH. We can then expect the concentration of dissolved KOH to change, accordingly: LigOH (s) + KOH (aq) → LigOK (aq) + H2O (l), what may be seen as a dehydroxylation reaction. Lignin precipitation from this solution can possibly be achieved by adding mineral acid or CO2. For the last case: LigOK (aq) + CO2 (aq) + H2O (l) → LigOH ↓(s) + KHCO3 (aq).