Profile of Salicylic Acid

Jun 1, 2011 | Contact Author | By: Mike J. Fevola, Johnson & Johnson
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Title: Profile of Salicylic Acid
salicylic acidx activex anti-acnex 2-hydroxybenzoic acidx anti-dandruffx
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Keywords: salicylic acid | active | anti-acne | 2-hydroxybenzoic acid | anti-dandruff

Abstract: 2-Hydroxybenzoic acid, or salicylic acid (SA), is among the most important medicinal compounds ever discovered. Since ancient times, humans have benefited from the therapeutic effects of naturally occurring SA precursors, which readily metabolize to SA and provide analgesic, anti- inflammatory and antipyretic benefits.

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M Fevola, Profile of Salicylic Acid, Cosm & Toil 126(7) 482 (2011)

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2-Hydroxybenzoic acid, or salicylic acid (SA), is among the most important medicinal compounds ever discovered. Since ancient times, humans have benefited from the therapeutic effects of naturally occurring SA precursors, which readily metabolize to SA and provide analgesic, anti- inflammatory and antipyretic benefits. Today, synthetic SA continues to play an important role as an active ingredient in over-the-counter (OTC) topical drug products and as a precursor to other drug compounds such as aspirin (acetylsalicylic acid), methyl salicylate and the sunscreen actives homosalate and octisalate. SA is also a critical starting material for the synthesis of certain fragrance ingredients, including benzyl salicylate and isoamyl salicylate.

Chemistry

SA, shown in Figure 1, is the ortho isomer of monohydroxybenzoic acid. It is commonly referred to as a beta hydroxy acid due to the location of its hydroxyl (–OH) group, which is two carbons away from the carboxylic acid (–COOH) group. SA is a relatively small molecule with the chemical formula C7H6O3, corresponding to a molecular weight of 138.12 g/mol. It is more acidic than most organic acids; for example, the pKa of SA is 3.0 whereas the pKa of the unsubstituted benzoic acid is 4.2. This increased acid strength is due to the location of the electron-withdrawing –OH group on its aromatic ring, which promotes dissociation of the –COOH group at lower pH values.

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Table 1. Summary of US OTC monograph indications for SA as an active ingredient

Table 1. Summary of US OTC monograph indications for SA  as an active ingredient

Table 1 summarizes the FDA-approved indications for SA in OTC drug products in the United States.

Figure 1. Salicylic acid and its naturally occurring precursors, salicin and methyl salicylate

Figure 1. Salicylic acid and its naturally occurring precursors, salicin and methyl salicylate

As previously shown in Figure 1, examples of such naturally occurring SA precursors include the glucoside of salicyl alcohol, known as salicin, and the methyl ester of SA, methyl salicylate.

Figure 2. Synthesis of SA via the Kolbe-Schmitt reaction

Figure 2. Synthesis of SA via the Kolbe-Schmitt reaction

SA is synthesized commercially via the Kolbe-Schmitt reaction, shown in Figure 2.

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