The brief introduction of Methyl benzoate

Introdcution

Methyl benzoate [MB; C6H5COOCH3; molecular weight (MW), 136.15 g/mol] is a volatile ester that exists naturally as a metabolite in plants. It was isolated from freshwater fern Salvinia molesta for the first time about 10 years ago. It is also the most abundant fragrant compound in bee-pollinated snapdragon flowers. It is formed by the condensation of methanol and benzoic acid. It is a colorless to slightly yellow liquid that is insoluble with water, but miscible with most organic solvents.

Natural source

Methyl benzoate is found in allspice. Methyl benzoate is present in various flower oils, banana, cherry, pimento berry, ceriman (Monstera deliciosa), clove bud and stem, mustard, coffee, black tea, dill, starfruit and cherimoya (Annona cherimola).

Reaction property

Reaction between two methyl benzoate molecules cannot occur because it has no α-hydrogen atoms. Therefore, methyl benzoate is mixed with a base, and methyl propanoate is then slowly added. The reaction converts methyl propanoate to an ester enolate anion. The enolate of methyl propanoate reacts preferentially with methyl benzoate for two reasons. Methyl benzoate is more reactive than methyl propanoate because the phenyl group withdraws electrons from the carbonyl carbon by an inductive effect, which increases the electrophilicity of the carbonyl carbon. In addition, the concentration of the ester enolate anion of methyl propanoate in the mixture is much smaller than that of methyl benzoate.

Uses

Methyl benzoate is used in flavourings. It is one of many compounds that is attractive to males of various species of orchid bees, who apparently gather the chemical to synthesize pheromones; it is commonly used as bait to attract and collect these bees for study.

S-Adenosyl-l-methionine: benzoic acid carboxyl methyltransferase (BAMT) is responsible for the biosynthesis of MB in snapdragon flowers. BAMT catalyzes the transfer of the methyl group of S-adenosyl-l-methionine to the carboxyl group of benzoic acid to generate MB. In addition, MB is emitted from insect-damaged rice plants. Because of its fruity smell, it is used in perfumery and as a pesticide to attract orchard bee. Recent studies have demonstrated toxic effects of MB against various insect pests including D. suzukii, H. halys, and Bemisia tabaci. Other natural benzyl esters, such as ethyl benzoate (EB) and synthetic benzoate, such as vinyl benzoate (VB), have been shown to have insecticidal effects on some insect pests.

Safety

Based on the existing data, methyl benzoate does not present a safety concern for skin sensitization under the current, declared levels of use[3]. The chemical structure of this material indicates that it could possibly react with skin proteins with little to no reaction under physiological conditions. In a murine local lymph node assay, methyl benzoate was found to be negative up to maximum tested concentration of 100% which resulted in Stimulation Index (SI) of 2.98. In guinea pigs, open epicutaneous tests and Freund's complete adjuvant tests with methyl benzoate did not present reactions indicative of sensitization. In a human maximization test, no skin sensitization reactions were observed with 4% or 2760 μg/cm2 methyl benzoate in petrolatum. Based on the weight of evidence from structural analysis as well as animal and human studies, methyl benzoate does not present a safety concern for skin sensitization under the current, declared levels of use.

References

[1] Heeyoun Bunch. “Evaluating cytotoxicity of methyl benzoate in vitro.” Heliyon (2020): e03351.

[2] Ouellette, R. J. and J. Rawn. “22 – Condensation Reactions of Carbonyl Compounds." Organic Chemistry (2014): 747-801.

[3] A.M. Api . “RIFM fragrance ingredient safety assessment, methyl benzoate, CAS Registry Number 93-58-3.” Food and Chemical Toxicology 122 (2018): Pages S218-S224.

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