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When the –OH group attaches itself to a carbon in the phenyl ring, the new molecule is known as a phenol, not an alcohol. See Figure 3.14. Phenols are very active and even stronger in their action than alcohols. Unfortunately for ease of recognition, phenols also end in ‘-ol’, so it is necessary to learn the major ones. These include carvacrol, eugenol, thymol – there are many; some are present as phenolic ethers which can be neurotoxic. The latter mostly end in ‘ole’, e.g. trans- and cis-anethole – trans-anethole is less toxic than cis-anethole. These are slightly more complicated and we will not go into their chemistry now. N.B. An exception to the ‘ole’ ending is the phenolic ether asarone, in carrot oil.

Powerful antiseptics and bactericides, phenols are stimulant both to the nervous and the immune systems but if present in significant amounts, the oil containing that particular phenol should be used with discretion, in low concentrations and for short periods of time. Phenols can be skin irritants if used incorrectly.

FIGURE 3.14: Phenol