Phenol, synthesis chlorobenzene process

The addition-elimination mechanism for nucleophilic aromatic substitution requires strong electron-withdrawing substituents on the aromatic ring. Under extreme conditions, however, unactivated halobenzenes react with strong bases. For example, a commercial synthesis of phenol (the Dow process ) involves treatment of chlorobenzene with sodium hydroxide and a small amount of water in a pressurized reactor at 350 °C ... 

Liittringhaus et al. 68) have isolated many interesting substances from the byproducts ( 8%) of the Bayer process. The mechanism has been fully clarified and shown to be an aryne route. When chlorobenzene or diphenylether are treated with sodium phenyl, the products are ortho metalated derivatives and benzyne. These give the same products which are formed in the industrial phenol synthesis. The most interesting compounds are 2- and 4-hydroxy biphenyl, 2,6-diphenyl- and 2,4-diphenyl-phenol l). For similar syntheses see 69). 

Nucleophilic substitution of aryl halogen atoms requires significant energy input. Thus, in the Dow process for the synthesis of phenol from chlorobenzene, the chlorine atom is only successfully hydrolysed by aqueous sodium hydroxide at 300 °C under pressure. Displacement by ammonia is achieved at 200 °C over copper(I) oxide and conversion to benzonitrile occurs using copper(I) cyanide in boiling dimethylfor-mamide, HCONMe. ... 

An apparent exception to the generalization about the lack of reactivity of aryl halides to nucleophilic substitution is an early industrial process for the synthesis of phenol from chlorobenzene. When heated at 300°C under high pressure with aqueous NaOH, chlorobenzene is converted to sodium phenoxide. Neutralization of this salt with aqueous acid gives phenol. 

Phenol was manufactured for many years by the Dow process, in which chlorobenzene reacts with NaOH at high temperature and pressure (Section 16.S). Now, however, an alternative synthesis from isopropylbenzene, commonly called... 

The base or nucleophile in the Dow phenol process is aqueous NaOH that needs to be heated to 300° C, which is only possible at a pressure of 200 bar. This conconction is only just capable of deprotonating chlorobenzene to benzyne or of adding to the latter subsequently. Thus, there is no analogous synthesis of diaryl ethers from sodium phenolates and haloben-zenes. And as expected, the product resulting from the Dow process did not contain more than 15% of diphenyl ether. 

A certain amount of phenol, as well as the cresols, is obtained from coal tar (Sec. 12.4). Most of it (probably over 90%) is synthesized. One of the synthetic processes used is the fusion of sodium benzenesulfonate with alkali (Sec. 30.12) another is the Dow process, in which chlorobenzene is allowed to react with aqueous sodium hydroxide at a temperature of about 360°. Like the synthesis of aniline from chlorobenzene (Sec. 22.7), this second reaction involves nucleophilic substitution under conditions that are not generally employed in the laboratory (Sec. 25.4). 

For many years, phenol was manufactured b the Dow process, in which chlorobenzene reacts with NaOH at high temperature and pressure (Section 16.9). Now, however, an alternative synthesis from isopropylbenzene (cumene) is used. Cumene reacts with air at high temperature by a radical mechanism to form cumene hydroperoxide, which is converted into phenol and acetone by treatment with acid. This is a particularly efficient process because two valuable chemicals are prepared at the same time. 

Although the by-produpts in each step of this process are advantageously employed in some phase of the synthesis, it is obvious, from the description below, that relatively large quantities of materials (4.3 lb per lb phenol) have to be handled. The process of the Dow Company, which accounts for a substantial percentage of domestic production, starts from chlorobenzene, hydrolyzing it under high pressure in an aqueous alkaline solution with copper as catalyst. 

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