What is Methyl Salicylate?

Methyl salicylate (MeSA), commonly known as wintergreen oil, is a colorless or pale yellow oily liquid with a wintergreen aroma. It has a sweet and spicy taste, a melting point of -8.6°C, and a boiling point of 218-224°C (partially decomposing at boiling). It shows a purple color when tested with ferric chloride and is easily soluble in organic solvents such as ethanol, ether, and acetic acid.

MeSA is a β-hydroxy acid, derived from its precursor compound SA through the isochorismate (IC) and phenylalanine ammonia-lyase (PAL) pathways. Initially, MeSA was isolated from Gaultheria procumbens (Cahours 1843) but is widely present in the plant kingdom. In some cases, its concentration can produce the characteristic “wintergreen aroma” detectable by the human nose.

Where Does Methyl Salicylate Come From?

Methyl salicylate is produced by many types of plants and is also synthetically manufactured. It is an organic ester naturally produced by many plant species, particularly wintergreen plants. In 1843, the compound was first extracted and isolated from Gaultheria procumbens plant material. Methyl salicylate can be extracted from the leaves of the Ericaceae family (Gaultheria procumbens) and the bark of the Betulaceae family (Betula lenta).

Which plants produce methyl salicylate? The formation of methyl salicylate involves increased levels under biotic stress, particularly during pathogen infection, where it plays a critical role in inducing resistance. Methyl salicylate functions by converting into the plant hormone salicylic acid. Due to its volatility, methyl salicylate can spread through the air to distal parts of the same plant or even to neighboring plants, acting as an interplant communication mechanism, warning surrounding plants of danger. Some plants release methyl salicylate when damaged by herbivorous insects, where it may serve as a cue to help recruit predators.

Some plants produce large amounts of methyl salicylate, potentially involved in direct defense against herbivores or pathogens. Examples include certain species of the Ericaceae family, such as Gaultheria procumbens, wintergreen, or eastern teaberry; species of the Betulaceae family, particularly those in the subgenus Betula, like black birch; all species of the Rosaceae family’s Spiraea genus, also known as meadowsweet; and species of the Polygalaceae family’s Polygala genus. Methyl salicylate can also be a component of floral scent, particularly in plants that rely on night-pollinating insects, such as moths, beetles, and (nocturnal) bees.

Although less common, methyl salicylate is also found in small quantities in plants like cloves, apples, and strawberries.

How Is Methyl Salicylate Synthesized?

Natural Wintergreen Oil

Natural wintergreen oil can be obtained by steam distilling the leaves of the wintergreen plant. Additionally, oil of sweet birch, distilled from the bark of sweet birch, has a composition similar to wintergreen oil and is often commercially mixed and referred to as “natural wintergreen oil.”

Synthesis of methyl salicylate

After the reaction is complete, pure product can be obtained through processes such as vacuum distillation or extraction and washing.

How to Make Methyl Salicylate?

Weigh 6.90 g (0.05 mol) of salicylic acid and add it to a three-neck flask. Add a certain amount of methanol in a ratio of (1:5~1:25) and a certain amount of catalyst, then heat and reflux for several hours. After refluxing, cool to room temperature, replace the reflux apparatus with a distillation apparatus, and recover unreacted methanol. Prepare a saturated sodium bicarbonate solution. After the solution has cooled, slowly add the saturated sodium bicarbonate solution with constant stirring until no more bubbles form. Finally, extract with ethyl acetate, place the oil layer in a stoppered conical flask, and dry with excess anhydrous magnesium sulfate for 12 hours. Filter, distill, and obtain a pale yellow oily liquid, which is the product.

What are the common methods to extract methyl salicylate from natural sources?

The main method for extracting methyl salicylate from plants is steam distillation.What plants produce methyl salicylate?

Wintergreen (Gaultheria procumbens), birch, sweet birch (Betula lenta), etc. Although steam distillation is the primary method for extracting methyl salicylate from natural sources, other techniques have been explored, although they are less commonly used:

Solvent Extraction

Organic solvents can be used to extract methyl salicylate from plant materials. However, this method requires careful selection and removal of the solvent to ensure product purity.

Supercritical Fluid Extraction (SFE)

This technique uses carbon dioxide under high pressure and temperature to extract the desired compound. While it can be highly efficient, it requires specialized equipment and conditions.

Steps for Extracting Wintergreen Oil (Methyl Salicylate)

Weigh 30 g of pre-harvested, washed, and dried wintergreen leaves, cut them into small pieces, and place them in a filter paper cylinder. Cover the top with absorbent cotton, then place it in the extractor. Take 150 mL of 95% ethanol in a round-bottom flask, heat to siphon and soak for extraction. After about 3 hours, when the solution in the extractor becomes very light, stop heating immediately after the reflux liquid in the extractor has just siphoned down.

Modify the apparatus into a distillation setup, add two pieces of boiling stones, and distill to recover most of the ethanol from the extract. Then switch to a steam distillation apparatus for steam distillation until no more oil droplets appear (or the distillate no longer changes the color of FeCl3 solution).

Place the distillate in a separatory funnel, shake gently, and let it stand until the oil droplets almost settle at the bottom. Separate the crude product. Saturate the aqueous layer with sodium chloride, then extract with ether (3 × 30 mL). Combine the crude product and ether extracts, distill off the ether, and recover. Then switch to an air condenser and electric heating mantle, and collect the fraction at 220-224°C, 3.0 g, nD20 = 1.5390. Take a small amount of the fraction, add 1-2 drops of FeCl3 solution, and observe an immediate blue-gray color, or drop the fraction onto filter paper, and under ultraviolet fluorescence, observe a yellow-green fluorescence.

References:

[1]Wang Bing, Wu Hongmei, Liu Peng, et al. Study on the synthesis process of wintergreen oil[J]. Tianjin Chemical Industry, 2016, 30(3):11-14. DOI:10.3969/j.issn.1008-1267.2016.03.004.

[2]https://en.wikipedia.org/wiki/Methyl_salicylate

[3]Wan Fuxian, Jiang Lin, Yin Hongzong, et al. Extraction of wintergreen oil and synthesis of acetylsalicylic acid[J]. Laboratory Science, 2015, 18(6): 36-38. DOI: 10.3969/j.issn.1672-4305.2015.06.011.

[4]Chen Weimin. Preparation and application of holly and holly oil[J]. Oral Care Products Industry, 2012(3):58-59. DOI:10.3969/j.issn.2095-3607.2012.03.020.

[5]https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/methyl-salicylate

[6]https://www.sciencedirect.com/science/article/abs/pii/B9780080552323621580

[7]Singewar K, Fladung M, Robischon M. Methyl salicylate as a signaling compound that contributes to forest ecosystem stability[J]. Trees, 2021, 35: 1755-1769.