Formation mechanism of p-methylacetophenone from citral via a tert-alkoxy radical intermediate.
The aim of this study was to clarify the formation mechanism of a potent off-odorant, p-methylacetophenone, from citral under acidic aqueous conditions. An acidic aqueous solution (pH 3.0) containing 10 mg/L of citral was stored at 40 degrees C for 2 weeks. Among the compounds detected in the stored citral solution, 4-(2-hydroxy-2-propyl)benzaldehyde was identified for the first time as a degradation product from citral. The formation of p-methylacetophenone and 4-(2-hydroxy-2-propyl)benzaldehyde behaved the same when antioxidants were added to the citral solution. In addition, both compounds were formed by the Fe(2+)-induced decomposition of 8-hydroperoxy-p-cymene, another compound identified in the stored citral solution. These results suggested that both p-methylacetophenone and 4-(2-hydroxy-2-propyl)benzaldehyde can be formed via the same radical intermediate [p-CH3C6H4C(CH3)2O*] that can be derived from the O-O bond homolysis of 8-hydroperoxy-p-cymene. On the other hand, the degradation of 8-hydroperoxy-p-cymene without Fe2+ under acidic aqueous conditions did not yield p-methylacetophenone and 4-(2-hydroxy-2-propyl)benzaldehyde, but the degradation of citral without Fe2+ did. Therefore, other than the decomposition of 8-hydroperoxy-p-cymene, a mechanism to generate the tert-alkoxy radical intermediate was proposed for the formation of p-methylacetophenone and 4-(2-hydroxy-2-propyl)benzaldehyde in the citral solution.
Ueno
Toshio
T
Material Research and Development Laboratories, Ogawa & Company, Ltd., 15-7 Chidori, Urayasushi, Chiba 279-0032, Japan. ueno.toshio@ogawa.net
Masuda
Hideki
H
Ho
Chi-Tang
CT
D000098:Acetophenones / Q000138:chemical synthesis*; D000080462:Acyclic Monoterpenes; D000975:Antioxidants / Q000494:pharmacology; D006861:Hydrogen Peroxide / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D039821:Monoterpenes / Q000737:chemistry*; D009812:Odorants / Q000032:analysis*; D010084:Oxidation-Reduction; D014867:Water
D000098:Acetophenones; D000080462:Acyclic Monoterpenes; D000975:Antioxidants; D039821:Monoterpenes; D014867:Water; C492488:p-methylacetophenone; D006861:Hydrogen Peroxide; C007076:citral