Commercially Important Chlorinated Phenols: History
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Phenols are compounds having a hydroxyl group attached to an aromatic ring such as a benzene or naphthalene ring. Several phenols that also possess one or more chlorine substituents attached to the aromatic ring have significant commercial importance, and these are the subject of this entry.

  • selective chlorination
  • phenols
  • sulphuryl chloride
  • sulphur-containing catalysts
  • aromatic electrophilic substitution reactions
  • monochlorination
  • double chlorination
  • para/ortho-isomer ratio

1.  Commercial Applications of Chlorinated Phenols

As a class of compounds, chlorinated phenols are of substantial commercial interest. Already by 1936, 2,3,4,5,6-pentachlorophenol was being used as a wood preservative in the US; in addition, it was involved in the preparation of paints, adhesives, ropes, and insulation [1]. 2,4-Dichlorophenoxyacetic acid (2,4-D), manufacture of which involves 2,4-dichlorophenol, was first synthesized in 1941 and has been commercially produced in many parts of the world since the 1950s[2]. Subsequently, the range of applications of chlorinated phenols has expanded and they are currently useful both as synthetic intermediates for more complicated products and as end products themselves. The range of areas of application includes antiseptics, herbicides, pesticides, and dyes[3]. For example, 2,4-D and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) were the first herbicides produced commercially for weed control[4] and 2,4-D is still in widespread use in 2021[5]. In addition, 2,4,5-trichlorophenol acts as a leather and wood fungicide[6], 2,3,4,5,6-pentachlorophenol is used as an insecticide[7], 4-chloro-3,5-dimethylphenol is used as a household and hospital disinfectant, 4-chloro-2-methylphenol is used as a herbicide, and 4-chloro-3-methylphenol is used as an antiseptic and preservative [8][9][10]. Figure 1 shows some of the most common industrial products containing chlorinated phenolic components.

Figure 1. Common industrial products containing chlorinated phenol components.

2. Synthesis of Chlorinated Phenols

Chlorinated phenols are almost always manufactured by direct chlorination of the parent phenols. Many approaches have been attempted for such chlorinations[12][13][14][15][17][16][18][19][20][21][22][23], including use of a range of different chlorinating agents, but either chlorine gas or sulfuryl chloride is usually used in commercial processes. Chlorination using chlorine is not very regioselective for many phenols, and several important products require a pure para-chloro derivative. A review in 2021 [24] has reported on the development of various sulfur-containing catalysts and in particular poly(alkylene sulfide)s that render chlorination reactions of several simple phenols highly selective with sulfuryl chloride as the reagent[25][26][27][28][29][30][31][32][33][34].

This entry is adapted from the peer-reviewed paper 10.3390/org2030012


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