p-Hydroxybenzaldehyde

I. Product Overview

p-Hydroxybenzaldehyde (4-hydroxybenzaldehyde, CAS 123-08-0, C7H6O2, MW 122.12) is an important aromatic aldehyde compound widely used as a key intermediate in pharmaceuticals, pesticides, fragrances, and liquid crystal materials.

II. Manufacturing Methods

1. Phenol-Based Routes (Multiple Methods)

• Reimer-Tiemann Reaction: Phenol reacts with chloroform in alkaline aqueous solution at 60 to 100 deg C for 2 to 4 hours, producing both p-hydroxybenzaldehyde and o-hydroxybenzaldehyde (salicylaldehyde). Total yield about 50 percent, with p-isomer yield at only 17 percent maximum. This process is primarily used for salicylaldehyde production, with p-hydroxybenzaldehyde as a byproduct. Disadvantages: low conversion and yield, large amounts of tar byproduct, excess chloroform required, unreacted phenol difficult to recover, product separation and purification challenging
• Gattermann Reaction: Phenol reacts with HCN in the presence of AlCl3 with dry HCl gas, decomposed in ice water to yield p-hydroxybenzaldehyde. Using zinc cyanide instead of HCN yields nearly theoretical amounts. Disadvantages: high toxicity of cyanides requiring strict operational control; anhydrous conditions demand expensive equipment; salicylaldehyde co-production complicates separation
• Phenol-Glyoxylic Acid Route: Uses phenol and glyoxylic acid via condensation and oxidation. Relatively clean process
• Phenol-Formaldehyde Route: Phenol and formaldehyde via condensation and oxidation. Raw materials easily available but yield is low

2. p-Nitrotoluene Route

• Oxidation-Reduction Step: p-Nitrotoluene is simultaneously oxidized and reduced using sodium polysulfide to produce p-aminobenzaldehyde. At 80 to 85 deg C, sodium polysulfide aqueous solution is added dropwise over 2 to 3 hours. Product is isolated by steam distillation and ether extraction with conversion and yield both exceeding 90 percent
• Diazotization and Hydrolysis Step: p-Aminobenzaldehyde is treated with 40 percent sulfuric acid, then 30 percent sodium nitrite solution is added at 0 to 3 deg C for about 30 minutes to form diazonium salt. The solution is hydrolyzed at 80 to 85 deg C in the presence of sulfuric acid for about 30 minutes, then extracted, purified, and dried. Yield exceeds 90 percent
• Advantages: Raw materials are inexpensive
• Disadvantages: Long process route, large equipment; intermediate p-aminobenzaldehyde is toxic; diazotization requires low temperature (0 to 3 deg C) demanding expensive refrigeration

3. p-Cresol Catalytic Oxidation Method (Mainstream Development Direction)

• Process: p-Cresol, sodium hydroxide, and methanol are charged into a stainless steel autoclave. After complete dissolution, cobalt acetate catalyst is added. The vessel is sealed, heated to 55 deg C, and oxygen is introduced maintaining 1.5 MPa pressure for 8 to 10 hours
• Reaction Control: Oxygen feed rate strictly controlled; coil cooling system maintains temperature around 60 deg C. After reaction, methanol solvent is recovered by distillation, water is added, and hydrochloric acid is used for salting out. The solid is filtered and dried to yield product with purity above 98 percent
• Advantages: Short process route, abundant raw materials, high product purity (above 98 percent), environmentally friendly
• Technical Requirements: Requires efficient catalysts (cobalt-based, copper-based, etc.); strict temperature and pressure control; higher equipment requirements
• Development Status: Deeply researched by Japan, USA, and Germany in the 1980s; industrialized in China (Jiangsu, Shanghai, Dalian) by the late 1980s and early 1990s

4. Phenol-Chloral Route

• Phenol reacts with chloral via condensation and hydrolysis to produce p-hydroxybenzaldehyde
• Process is relatively simple but yield is low and cost is high

III. Core Advantages

• Key Pharmaceutical Intermediate: Essential raw material for synthesizing amoxicillin, TMP (trimethoprim), p-hydroxyglycine, gastrodin, and many other important drugs
• Fragrance Synthesis Precursor: Core precursor for synthesizing vanillin, ethyl vanillin, heliotropin, syringaldehyde, anisaldehyde, and raspberry ketone
• Cross-Industry Applications: Spans pharmaceuticals, pesticides, fragrances, liquid crystals, photographic materials, and electroplating industries
• Mature p-Cresol Oxidation Technology: Commercially produced in China with product purity exceeding 98 percent and environmentally friendly process
• Export Competitiveness: Lower production costs in China provide strong competitive advantage in international markets

IV. Applications

1. Pharmaceutical Industry (Core Application)

• Amoxicillin: p-Hydroxybenzaldehyde is a key intermediate in amoxicillin synthesis
• Trimethoprim (TMP): Used in the synthesis of 3,4,5-trimethoxybenzaldehyde for antibacterial synergy agents
• p-Hydroxyglycine: Important amino acid pharmaceutical intermediate
• Cefadroxil: Cephalosporin antibiotic synthesis intermediate
• Gastrodin (Artificial Tianma): Neuroprotective drug synthesis
• Farrerol: Natural active ingredient synthesis
• Esmolol: Short-acting beta-blocker synthesis

2. Fragrance Industry

• Vanillin: p-Hydroxybenzaldehyde is the core precursor for vanillin synthesis via methylation
• Ethyl Vanillin: Synthesized via ethylation
• Heliotropin: Used in high-grade fragrance formulations
• Syringaldehyde: For lilac-type fragrance synthesis
• Anisaldehyde: For anise-type fragrance synthesis
• Raspberry Ketone: For fruit-flavored fragrance synthesis

3. Pesticide Industry

• Bromoxynil: Selective herbicide
• Chloroxuron: Herbicide
• Fungicide Intermediates: Raw material for various agricultural fungicides

4. Liquid Crystal Materials

• Used in liquid crystal intermediate synthesis, an important chemical raw material in the LCD display industry chain

5. Other Applications

• Photographic Emulsifier: Raw material for photographic chemicals
• Nickel Plating Brightener: Used in electroplating to improve coating luster
• Polymer Raw Material: Specialty polymer material synthesis

V. Physical and Chemical Properties

• Molecular Formula: C7H6O2
• Molecular Weight: 122.12
• CAS Number: 123-08-0
• Appearance: Light yellow or white crystalline powder with faint aromatic odor
• Melting Point: 112 to 116 deg C
• Boiling Point: 246.6 deg C
• Flash Point: 101.3 deg C
• Density: 1.129 g/cm3 (30/4 deg C)
• Refractive Index: 1.57055 (130 deg C)
• Solubility: Freely soluble in ethanol, ether, acetone, ethyl acetate, and DMF; slightly soluble in water (solubility 1.388g/100ml at 30.5 deg C); soluble in benzene (solubility 3.68g/ml at 65 deg C)

VI. Safety Information

• Hazard Code: Xi (Irritant)
• Hazard Class: 36/37/38
• Safety Class: 26-36
• Safety Terms: Avoid contact with skin and eyes; in case of eye contact, rinse immediately with plenty of water and seek medical advice; wear suitable protective clothing
• Risk Terms: Irritating to eyes, respiratory system, and skin

VII. Market Overview

• Downstream Drivers: Growing demand for amoxicillin and other antibiotics, expansion of LCD display industry chain, stable demand from fragrance sector
• Technology Trends: p-Cresol catalytic oxidation method gradually replacing traditional phenol and p-nitrotoluene routes due to environmental advantages and high purity
• Global Supply Chain: China is a major producer and exporter with abundant raw material supply (p-cresol, phenol)
• Price Competitiveness: Lower production costs via p-cresol method provide competitive export pricing