Di-tert-butylphenol

Commercial Di-tert-butylphenol is supplied predominantly as the 2,4-di-tert-butylphenol isomer, CAS 96-76-4, with the molecular formula C14H22O and a molecular weight of 206.33 g mol⁻¹. Without an isomer prefix, the term di-tert-butylphenol is ambiguous; in stabilizer-intermediate commerce it usually denotes the 2,4-isomer because the 2,6-isomer is normally traded under its explicit isomer name. The 2,4-isomer is a crystalline solid at ambient temperature, with a melting range of 53–56 °C, a boiling range of 263–265 °C at 101.3 kPa, and a flash point of 115 °C measured by ASTM D93 closed cup. It is soluble in toluene, ethanol, acetone, and hexane, and sparingly soluble in water at neutral pH. The melt density is approximately 0.907 g cm⁻³ at 60 °C; flake bulk density varies with particle size distribution and is commonly reported between 0.45 g cm⁻³ and 0.60 g cm⁻³.

Generic product codes distinguish flake, pastille, and molten delivery forms. A representative technical-grade flake designation is 2,4-DTBP-99, where the numeric suffix denotes a minimum assay of 99.0% by gas chromatography; additional packaging suffixes are supplier-specific. Product selection based solely on assay is insufficient for phosphite ester synthesis, because residual 2-tert-butylphenol and 2,4,6-tri-tert-butylphenol alter reaction stoichiometry, color, and the hydrolytic stability of the finished organophosphite. In phosphite production, the monoalkylphenol content is therefore controlled by a separate specification rather than by assay alone.

Catalytic alkylation of phenol with isobutylene is carried out over a sulfonic acid ion-exchange resin in a fixed-bed or batch loop reactor. The crude alkylate contains residual phenol, 2-tert-butylphenol, 2,4-di-tert-butylphenol, 2,6-di-tert-butylphenol, and 2,4,6-tri-tert-butylphenol. The 2,4-isomer is recovered by vacuum rectification; published data for specific catalyst selectivities in production-scale loops is limited, but operating temperature is typically maintained below 120 °C to limit olefin oligomerization and color-body formation. The separation of the 2,4- and 2,6-isomers is a narrow boiling-point split and is usually performed in a structured-packed column with 20–30 theoretical stages.

What Distinguishes 2,4-Di-tert-Butylphenol from 2,6-Di-tert-Butylphenol in Steric Hindrance?

In the 2,4-isomer, one ortho position and the para position relative to the phenolic hydroxyl carry tert-butyl groups, leaving the second ortho site available for electrophilic substitution. In the 2,6-isomer, both ortho positions are blocked and the para position is unsubstituted. This structural difference controls the product’s industrial chemistry. The 2,4-isomer reacts with phosphorus trichloride to form tris(2,4-di-tert-butylphenyl) phosphite, whereas the 2,6-isomer is more commonly alkylated at the para position or bridged through propionate ester chemistry to produce high-molecular-weight hindered phenolic antioxidants.

The steric environment of the hydroxyl group is also different. In the 2,6-isomer, two ortho tert-butyl groups shield the hydroxyl and hinder dimerization and oxidative coupling of the phenoxy radical. In the 2,4-isomer, the single ortho tert-butyl group provides less complete shielding, which reduces the compound’s performance as a direct chain-breaking antioxidant but improves its selectivity as an intermediate for phosphorus-based stabilizers. Published pKa data for this specific isomer pair in aqueous ethanol are limited; the apparent pKa difference is generally reported as less than 0.5 units when measured under identical solvent conditions, indicating that steric effects outweigh electronic effects in directing downstream reactions.

At the production scale, the isomer separation step is a significant cost driver. The boiling point difference between the 2,4- and 2,6-isomers is approximately 10 °C, which requires a structured-packed vacuum distillation column with 20–30 theoretical stages and a reflux ratio above 3:1 when a high-purity split is required. A thin-film evaporator with a heated surface of 0.5–2 m² operating at 1–5 kPa is commonly used to recover the 2,4-isomer from alkylate bottoms before finishing.

Specification Framework for Commercial 2,4-Di-tert-Butylphenol

Table 1 summarizes the specification profile for a technical-grade flake intended for organophosphite synthesis. The values are supplier-neutral and represent typical commercial thresholds rather than ISO or ASTM consensus specifications.

Moisture is a particularly stringent parameter for phosphite synthesis. The esterification reaction consumes water through hydrolysis of phosphorus trichloride and partially substituted phosphites, leading to acidic species that must be neutralized. A water limit of ≤0.10 wt% is therefore common, and incoming flake is often re-certified after transport in humid environments. In integrated plants, flake storage silos are operated with dry-air purging at −20 °C dew point; in non-integrated supply chains, the material is packed in nitrogen-blanketed polyethylene-lined paper bags with a moisture barrier.

Color control is also operationally important: APHA color above 50 Pt-Co in a 10% toluene solution often correlates with oxidative degradation products that carry into the final organophosphite and shift the melt-flow stability of the compounded resin during multiple extrusions. Therefore, color is not merely cosmetic but functions as an indirect oxidation-state marker in process control.

Across flake production lines, the most persistent bottleneck is not synthesis conversion but the finishing of a low-melting solid under humid ambient conditions. Moisture pickup above 60% relative humidity is sufficient to raise surface water above 0.10 wt% in unwrapped flake within 8–12 h; this water reacts with phosphorus trichloride during phosphite ester synthesis, generating acidic hydrolysis by-products and increasing the load on downstream neutralization. The use of nitrogen-blanketed storage and conveying at 20–30% RH is therefore standard in plants producing tris(2,4-di-tert-butylphenyl) phosphite.

In a typical batch esterification vessel, 2,4-DTBP is dissolved in toluene or xylene and reacted with phosphorus trichloride at 60–80 °C, with hydrogen chloride removed by an alkaline scrubber. Because the solid feed must be moisture-free and free of lumps, a lump breaker and a gravimetric screw feeder with ±1% feed accuracy are used when feeding flakes. The flakes are not typically pre-melted if the esterification reactor is equipped with sufficient agitation; a 2,4-DTBP melt feed at 60–65 °C is preferred for larger lines using mass-flow metering. For molten deliveries, heated tank containers are held at 60–65 °C under nitrogen; the product is pumped through steam-traced lines and acceptable temperature excursion is ±2 °C to avoid both solidification and thermal color shift.

Downstream, the derived phosphite is let down into polyolefin compounds at 0.05–0.25 phr. Processing stabilization is evaluated by multiple-pass extrusion in a co-rotating twin-screw extruder with an L/D ratio of 44:1 and screw speeds between 200–600 min⁻¹. Melt-flow retention is measured according to ISO 1133-1:2022 after 1 and 5 extrusion passes; the phenolic intermediate itself is not added directly to the resin for this purpose. Tensile yield retention after oven aging at 100 °C is evaluated by ASTM D638-14 for compounds containing the derived phosphite.

When the Para Position Is Blocked by a tert-Butyl Group

Compared with p-tert-butylphenol, the additional tert-butyl group in 2,4-di-tert-butylphenol raises the molecular weight from 150.22 g mol⁻¹ to 206.33 g mol⁻¹ and suppresses condensation with formaldehyde into phenolic resole or novolac matrices. p-tert-Butylphenol reacts readily at both ortho positions to produce resin-building methylol intermediates; the 2,4-isomer has only one available ortho site and exhibits reduced aqueous alkaline solubility, which shifts its use from phenolic resins to stabilizer intermediates.

Table 2 compares the main related hindered phenols. The difference in melting point is often the first practical discriminator during storage and handling: 2,4-DTBP flakes soften near 53 °C, whereas p-tert-butylphenol remains solid until approximately 98 °C, and 2,6-DTBP melts below 40 °C. This difference determines packaging type, warehouse stacking limits, and the need for climate-controlled storage in warm regions.

Against BHT, the 2,4-isomer is not a direct finished antioxidant. BHT is a chain-breaking primary antioxidant with a para methyl group and two ortho tert-butyl groups, and is cleared for direct food-contact use under FDA 21 CFR 172.115. The 2,4-isomer lacks the para methyl group and has only one ortho tert-butyl group, so its phenoxy radical is less persistent and its migration kinetics are different. Its industrial value is mainly as an intermediate for the secondary antioxidant tris(2,4-di-tert-butylphenyl) phosphite, which is added to polyolefins at 0.05–0.25 phr to decompose hydroperoxides during extrusion. Multiple-pass extrusion studies according to ISO 1133-1:2022 and ASTM D1238 are used to evaluate the resulting melt-flow retention; the phosphite itself is not the same as the phenolic intermediate.

The 2,6-isomer is also converted via Michael addition to methyl acrylate to yield methyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, which is then transesterified with stearyl alcohol or pentaerythritol to produce high-molecular-weight primary antioxidants. That sequence is sterically accessible because the para position in 2,6-DTBP is unsubstituted; in 2,4-DTBP the para position is blocked, so the analogous sequence is not used commercially at the same scale.

Under current industrial hygiene and storage practice, 2,4-di-tert-butylphenol is handled as a combustible organic solid. Storage should be below 50 °C and above 0 °C in a dry, nitrogen-blanketed area. Avoid contact with strong oxidizing agents, concentrated nitric acid, and strong bases; exothermic reactions can occur with oxidizing media. For moisture-sensitive downstream chemistry, pre-drying under vacuum at 40–50 °C is required if the water content exceeds 0.10 wt%. A supplier-stated shelf life of 12 months applies to sealed, nitrogen-blanketed flake; product that has been exposed to repeated ambient humidity should be retested for water before use. From a toxicological perspective, the product is not a direct food-contact substance and is not included in FDA 21 CFR 172.115; that clearance applies to BHT. The derived organophosphite may be considered in food-contact polymers under regional positive lists, but migration testing of the finished article is required under EU Regulation 10/2011 or equivalent national legislation. Under EU REACH, the substance must be registered for the relevant tonnage band; product-specific compliance statements should be obtained from the supplier. Where flake is ground or pneumatically conveyed, combustible dust precautions according to NFPA 652 should be evaluated; published minimum ignition energy data for this product is limited. Inhalation and dermal exposure limits are not harmonized globally; a current safety data sheet and the relevant workplace exposure framework should be consulted for the specific isomer and grade. Published data for specific melt processing configurations is limited, so commissioning trials should verify dusting, melt viscosity, and color stability under site-specific humidity and conveying conditions.