Lignin (19-13-7) Physical and Chemical Properties
Lignin
Heterogeneous, plant-derived aromatic biopolymer supplied as an amorphous powder used as a renewable feedstock and performance additive in industrial formulations.
| CAS Number | 19-13-7 |
| Family | Lignins (aromatic biopolymers) |
| Typical Form | Amorphous powder or granular solid |
| Common Grades | EP |
Lignin is a heterogeneous, high–molecular‑weight aromatic biopolymer belonging to the class of phenylpropanoid-derived structural polymers. Structurally it is an irregular, three‑dimensional network formed primarily by oxidative radical coupling of three monolignol precursors (coniferyl, p‑coumaryl and sinapyl alcohols) that produce a mixture of phenylpropane units connected by C–O and C–C linkages (notably β–O–4, β–5 and β–β motifs). The polymer backbone is rich in substituted phenyl rings, aliphatic side‑chains and oxygenated functionalities (phenolic OH, methoxy groups and occasional carbonyls), which give lignin a predominantly aromatic electronic character with localized electron density on phenolic oxygens.
At the bulk level lignin exhibits strong polarity and amphiphilic behavior due to multiple polar functional groups (phenolic and aliphatic hydroxyls, carboxylates and sulfonates in derivatized forms) alongside hydrophobic aromatic domains. This results in low intrinsic solubility in nonpolar solvents and increased solubility in alkaline media or in the presence of ionic counterions for sulfonated derivatives. Lignin is chemically recalcitrant to simple hydrolysis but is susceptible to oxidative depolymerization and alkaline fragmentation; thermal treatment tends to produce char and small volatile phenolics rather than clean volatilization. Industrial and biological relevance is substantial: lignin provides mechanical strength and water transport control in vascular plants and is a major co‑product of pulping and biorefining processes, where controlled depolymerization is a key valorization challenge.
Common commercial grades reported for this substance include: EP.
Molecular Overview
Molecular Weight and Composition
The supplied molecular descriptor corresponds to a defined low–molecular‑weight, derivatized species with formula \(\ce{C18H13N3Na2O8S2}\). The computed molecular weight is \(509.4\ \mathrm{g}\,\mathrm{mol}^{-1}\). Exact and monoisotopic masses are reported as \(508.99394529\ \mathrm{Da}\).
Note: commercial and research material labelled as "lignin" frequently refers to heterogeneous macromolecular fractions whose average molecular weights and compositions vary widely with biomass source and extraction method; many analytical or reagent records instead represent small model compounds or derivatized, water‑soluble lignin fractions.
Charge, Polarity, and LogP
The computed formal charge for the shown derivative is neutral (overall charge 0 when counterions are included), but the structure contains two sulfonate anions balanced by sodium cations, producing a salt with high ionic character. Topological polar surface area (TPSA) is \(205\ \mathrm{\AA}^2\), and the molecule has multiple hydrogen bond acceptors (10) and hydrogen bond donors (2), consistent with strong hydrophilicity for the derivatized form. Rotatable bond count is 3 and heavy atom count is 33, indicating a compact, polar small molecule rather than a polymeric network.
No experimentally established value for partition coefficient (logP) is available in the current data context.
Biochemical Classification
In biological and industrial contexts, lignin is classified as a phenylpropanoid‑derived aromatic polymer (a structural biopolymer of plant secondary cell walls). Functionally it is an amorphous, irregular polymer formed from radical coupling of monolignols; heterogeneous substitution and branching result in variable solubility and reactivity. Analytical samples and reagent grades may be represented by sulfonated or otherwise derivatized low‑molecular‑weight analogues to increase water solubility and to simplify characterization.
Chemical Behavior
Stability and Degradation
Lignin is chemically robust under neutral conditions; its network of carbon–carbon and aryl–ether bonds confers resistance to simple hydrolytic cleavage. Thermal decomposition primarily yields char and complex mixtures of phenolic volatiles; the material crosslinks and carbonizes rather than exhibiting a sharp boiling point. Oxidative treatments (chemical oxidants, oxygenases such as lignin‑peroxidase and laccase in biological systems) cleave aromatic rings and ether linkages and are effective routes to depolymerization. Under strongly alkaline conditions many lignin fractions become soluble and undergo fragmentation, whereas concentrated acid promotes condensation reactions that increase apparent molecular weight and decrease solubility.
Hydrolysis and Transformations
Hydrolytic cleavage of lignin is relatively slow; however, alkaline hydrolysis and sulfite/sulfonation processes (used industrially to produce lignosulfonates) increase solubility by introducing anionic sulfonate groups. Depolymerization strategies typically target β–O–4 ether linkages via reductive, oxidative, catalytic or hydrolytic routes to yield phenolic monomers and oligomers. Oxidative cleavage can convert methoxy and phenolic substituents to carboxylic acids or quinone‑type structures. Derivatization (acetylation, sulfonation, methylation) is commonly used to alter solubility, reactivity and analytical behavior.
Biological Role
Functional Role and Pathways
Lignin functions as a major structural polymer in vascular plants, providing mechanical strength, hydrophobicity of cell walls and resistance to microbial attack. Biosynthetically, lignin is formed via the phenylpropanoid pathway producing monolignols (p‑coumaryl, coniferyl and sinapyl alcohols) which are enzymatically oxidized to radicals and then polymerize by radical coupling. The variability of monolignol incorporation and the random nature of radical coupling yield a heterogeneous polymer tailored to species, tissue type and developmental stage.
Physiological and Cellular Context
Within plant secondary cell walls lignin is deposited in the matrix surrounding cellulose microfibrils and hemicelluloses, contributing to rigidity and reducing cell wall permeability. Polymerization is mediated by oxidative enzymes (peroxidases and laccases) that generate phenolic radicals; localized polymer growth and crosslinking result in complex three‑dimensional architectures. Biologically, lignification modulates water transport (by making xylem elements hydrophobic), structural integrity and defense against pathogens. Microbial and enzymatic systems capable of lignin modification are of interest for biomass valorization.
Identifiers and Synonyms
Registry Numbers and Codes
- CAS number: 19-13-7
- Molecular formula: \(\ce{C18H13N3Na2O8S2}\)
- Molecular weight: \(509.4\ \mathrm{g}\,\mathrm{mol}^{-1}\)
- Exact mass / Monoisotopic mass: \(508.99394529\ \mathrm{Da}\)
- Topological polar surface area (TPSA): \(205\ \mathrm{\AA}^2\)
- Hydrogen bond donors: 2; hydrogen bond acceptors: 10
- Rotatable bond count: 3
- Formal charge: 0
- IUPAC name (computed): disodium;4-acetamido-5-hydroxy-6-phenyldiazenylnaphthalene-1,7-disulfonate
- InChI:
InChI=1S/C18H15N3O8S2.2Na/c1-10(22)19-13-7-8-14(30(24,25)26)12-9-15(31(27,28)29)17(18(23)16(12)13)21-20-11-5-3-2-4-6-11;;/h2-9,23H,1H3,(H,19,22)(H,24,25,26)(H,27,28,29);;/q;2*+1/p-2 - InChIKey:
RLLNZXVKBYGKIP-UHFFFAOYSA-L - SMILES:
CC(=O)NC1=C2C(=C(C=C1)S(=O)(=O)[O-])C=C(C(=C2O)N=NC3=CC=CC=C3)S(=O)(=O)[O-].[Na+].[Na+]
Synonyms and Biological Names
Reported synonyms and deposited names include: - Lignin - LIGNIN - Lignins - disodium;4-acetamido-5-hydroxy-6-phenyldiazenylnaphthalene-1,7-disulfonate - Disodium 4-(acetylamino)-5-hydroxy-6-(phenylazo)naphthalene-1,7-disulphonate
(Several depositor-supplied and legacy synonyms exist that reflect derivatized, fractionated or analytical forms; nomenclature varies with method of preparation.)
Safety and Handling Overview
Lignin and its derivatives are generally of low acute hazard for many formulations; aggregated industrial submissions report that most commercial lignin materials do not meet criteria for classification under common hazard categories. Nevertheless, physical form and impurities influence risk: dusts from powdered lignin can present inhalation and dust‑explosion hazards; certain derivatized or contaminated fractions may contain low levels of hazardous impurities.
Standard industrial hygiene and handling precautions are recommended: - Use appropriate personal protective equipment (gloves, eye protection and lab coat); for dusty materials, respiratory protection and local exhaust ventilation are advised. - Avoid formation and accumulation of airborne dust; implement dust control and housekeeping. - Store in a cool, dry, well‑ventilated area in sealed containers to limit moisture uptake and microbial growth; protect light‑sensitive derivatives from prolonged exposure to strong light if applicable. - For spills and waste, collect solid material by dry methods where safe; avoid dispersal and follow local waste disposal regulations.
For detailed hazard, transport and regulatory information, users should refer to the product‑specific Safety Data Sheet (SDS) and applicable local legislation.
Handling and Storage of Biochemical Materials
When handling biological or biomass‑derived lignin fractions in research or production contexts, additional considerations include control of microbial contamination (store dry or refrigerated as appropriate), stability monitoring (oxidative discoloration over time), and segregation from strong oxidants or reducing agents that could alter polymer properties. Quality control typically monitors moisture content, ash/inorganic residue, and solubility in designated solvents for the intended application. For critical applications (e.g., food additives or pharma excipients), select material grades that meet the relevant pharmacopoeial or regulatory quality expectations.
