July 19, 2021
You will soon be rid of me.
What molecule am I?
Image of Pentachlorophenol 3D Image of Pentachlorophenol

Pentachlorophenol is an old-line pest control agent that came under regulatory pressure decades ago. Beginning in the 1930s, it was used as an insecticide (including for termite control), a defoliant for crops before they were harvested, a wood preservative (notably in telephone poles), and a biocide with numerous other agricultural and industrial uses.

Pentachlorophenol is made by chlorinating phenol at high temperature. In the commercial process, chlorination is incomplete, and the product contains even more toxic impurities such as dibenzo-p-dioxin derivatives. The compound’s odor is similar to that of benzene or phenol; it is sparingly soluble in water and slow to decompose in nature.

Pentachlorophenol’s pervasive toxicity and environmental threats (see the hazard information table) doomed it to severe use restrictions by the 1980s. Today, wood preservation is the only use permitted by the US Environmental Protection Agency. Earlier this year, the EPA proposed to ban it completely. The comment period for this proposal expired in May, and EPA is currently proceeding with canceling the registration

For additional information, see EPA’s information page on pentachlorophenol.

Pentachlorophenol hazard information

Hazard class*GHS code and hazard statement
Acute toxicity, oral, category 3H301—Toxic if swallowedChemical Safety Warning
Acute toxicity, dermal, category 3H311—Toxic  in contact with skinChemical Safety Warning
Skin corrosion/irritation, category 2H315—Causes skin irritationChemical Safety Warning
Serious eye damage/eye irritation, category 2AH319—Causes serious eye irritationChemical Safety Warning
Acute toxicity, inhalation, category 2H330—Fatal if inhaledChemical Safety Warning
Specific target organ toxicity, single exposure, respiratory tract irritation, category 3H335—May cause respiratory irritationChemical Safety Warning
Carcinogenicity, category 2H351—Suspected of causing cancerChemical Safety Warning
Short-term (acute) aquatic hazard, category 1H400—Very toxic to aquatic lifeChemical Safety Warning
Long-term (chronic) aquatic hazard, category 2H411—Toxic to aquatic life with long-lasting effectsChemical Safety Warning

*Globally Harmonized System (GHS) of Classification and Labeling of Chemicals. 
Explanation of pictograms. 

Molecule of the future

More than 50% of the world’s population is chronically infected with herpes viruses. One of the most common is herpes simplex virus (HSV), which attacks oral and genital areas. To date, no drugs have been developed to treat latent HSV, which recurs in ≈30% of those infected.

Molecule of the Future

Now, Innovative Molecules (Bad-Salzuflen, Germany) reports that a new small molecule, IM-2501, combats active and latent HSV in guinea pigs without inducing adverse side effects that plagued other drugs investigated for this purpose. The company’s CEO expects that IM-250 will begin Phase 1 clinical trials in late 2022 or early 2023.

1. (S)-2-[2′,5′-difluoro-(1,1′-biphenyl)-4-yl]-N-methyl-N-[4-methyl-5-(S-methylsulfon-imidoyl)thiazol-2-yl]acetamide.

Molecule of the Future

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fast facts

CAS Reg. No.87-86-5
Phenol, 2,3,4,5,6-pentachloro-
Empirical formulaC6HCl5O
Molar mass266.34 g/mol
AppearanceWhite crystals or powder
Melting point191 °C
Water solubility14 mg/L

MOTW update

Nitrous oxide (N2O) was the Molecule of the Week for January 14, 2013. It is the third-most-emitted climate change contributor, after carbon dioxide and methane, yet little has been done to curtail it. Now, pressure from government agencies and environmental groups is forcing the chemical industry worldwide to install equipment to abate N2O emissions.

MOTW update:
February 21, 2022

Pentachlorophenol is an old-line insecticide and wood preservative that is a “reasonably anticipated” human carcinogen and an environmental hazard.

After decades of restricting the use of pentachlorophenol, the US Environmental Protection Agency proposed a complete ban on it in early 2021. This month, EPA followed through on the ban, which will go into effect in 2024. Wood-treatment companies are permitted to use up existing stocks until 2027.

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