Diphenyl Ether CAS： 101-84-8

Diphenyl ether, also known as phenyl ether, is a colorless oily liquid or crystal with a faint geranium-like aroma. It has a relative density of 1.086, a melting point of 26–28 °C and a boiling point of 259 °C. It is insoluble in water, inorganic acid solutions and alkaline solutions, but soluble in ethanol, diethyl ether, benzene and glacial acetic acid. It is prepared by heating sodium phenoxide or potassium phenoxide with chlorobenzene or bromobenzene in the presence of copper powder catalyst.

Chemically, diphenyl ether is stable, resistant to discoloration and has a long-lasting fragrance. It is widely used as a synthetic fixative in soap fragrances. It is listed as GRAS by FEMA. Its usage limits specified by FEMA (mg/kg) are: 7.5 for soft candy and 2.5 for non-alcoholic beverages. According to GB 2760-2001, it can be used for citrus preservation at a dosage of 3.0 g/kg, with the residual amount not exceeding 12 mg/kg.

In terms of chemical reactivity, diphenyl ether is relatively stable in aqueous amine or alkaline solutions, while its ether bond tends to break upon heating to form phenol. For instance, when heated under reflux with sodium or potassium metal in pyridine, over 90% of diphenyl ether converts into phenol. Grignard reagents can also cleave the ether bond, and the reaction can proceed at room temperature with anhydrous cobalt chloride as the catalyst. It does not decompose when heated to 250 °C with hydrogen iodide, and shows good stability against chromium oxide (CrO₃, Cr₂O₃) in acetic acid. It reacts with concentrated nitric acid to form 4,4'-dinitrodiphenyl ether.

Diphenyl ether serves as one of the key components of organic high-temperature heat transfer fluids. The mixture of diphenyl ether and biphenyl at a mass ratio of 73.5:26.5 is called Dowtherm A, with an operating temperature range of 232–399 °C in the vapor phase and 21–399 °C in the liquid phase, and the working temperature shall not exceed 400 °C. The blend of diphenyl ether and methylbiphenyl, named Dowtherm LF, features a low freezing point and remains usable at -30 °C, making it a low-temperature, low-viscosity heat transfer medium.

With an aroma reminiscent of geranium and pelargonium, plus sweet orange and narcissus notes, diphenyl ether is also referred to as crystalline geranium oil or artificial geranium oil in the fragrance industry. Thanks to its good alkali resistance, it is commonly formulated into soap fragrances and used as a substitute for natural geranium oil for floral cosmetic fragrances, deodorants and other products. It acts as an intermediate in organic synthesis for the fragrance industry, synthetic resin production and other organic synthesis processes. It can be applied as a constant-temperature heat transfer medium and a stationary phase for gas chromatography (maximum operating temperature: 100 °C, solvent: acetone). When used together with diphenylmethane, it delivers a lighter scent, contributing green notes to rose, geranium, hyacinth, fern, new-mown hay and sandalwood fragrances for soaps, detergents and other daily commodities.

For production, diphenyl ether is obtained via the condensation reaction of chlorobenzene and phenol in caustic alkali solution using copper as catalyst. Potassium hydroxide, phenol and chlorobenzene are mixed at a molar ratio of 1:1.4:1.06 together with copper powder, followed by stirring and heating for condensation. After the reaction, the mixture is acidified, and the oily diphenyl ether layer is separated and purified by vacuum distillation to obtain the finished product. The reaction can also be carried out in sodium hydroxide solution. Another industrial source of diphenyl ether is the by-product from the hydrolysis of chlorobenzene for phenol production. Around 10% of chlorobenzene is converted into diphenyl ether during alkaline hydrolysis, and the proportion can reach 20% in certain processes. The product is then isolated by extraction and refining.