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1. Basic Identification Information
| Parameter | English Specification/Value |
|---|---|
| IUPAC Name | 4-Hydroxybenzoic acid (also: p-Hydroxybenzoic acid) |
| Common Name(s) | 4-Hydroxybenzoic acid; PHBA; p-Hydroxybenzoic acid; Benzoic acid, 4-hydroxy-; Protocatechuic acid (historical misnomer) |
| CAS Registry Number | 99-96-7 |
| EC/EINECS Number | 202-804-9 |
| Molecular Formula | C₇H₆O₃ |
| Molecular Weight | 138.12 g/mol |
| Chemical Class | Aromatic carboxylic acid; hydroxybenzoic acid derivative; preservative precursor; organic intermediate |
| Related Compound | 3-Hydroxybenzoic acid (CAS 99-06-9) (meta-substituted, lower solubility); 2-Hydroxybenzoic acid (Salicylic acid, CAS 69-72-7) (ortho-substituted, keratolytic/antiseptic properties); Parabens (methyl/ethyl/propyl/butyl esters of PHBA, primary preservatives) |
2. Physicochemical Properties
| Property Category | Specific Parameter | Specification/Value |
|---|---|---|
| Structural Features | Functional Groups | Carboxyl group (-COOH) at C1; hydroxyl group (-OH) at C4 (para-substitution, enables intramolecular hydrogen bonding; pKa ≈ 4.5 for -COOH, pKa ≈ 9.3 for -OH) |
| Physical Appearance | State & Color | White to off-white crystalline powder; odorless; bitter taste (taste threshold: 0.05 g/L) |
| Melting Point | 214–217 °C (lit.; sublimes under vacuum at 180 °C) | |
| Boiling Point | Decomposes before boiling (decomposes above 336 °C to form phenol and CO₂) | |
| Density (25 °C) | 1.46 g/cm³ (crystalline form) |
