Kinetics of 3,4-Dihydro-2H-3-phenyl-1,3-benzoxazine Synthesis from Mannich Base and Formaldehyde

To study oxazine ring formation in benzoxazine synthesis from phenol, primary amine and formaldehyde, reactions between 2-phenylaminomethylphenol (Mannich base) and formaldehyde in homogeneous dioxane/water solution are researched. Reaction orders, rate constants at different temperatures, and activation energy are calculated, the process of reaction is proposed, and the influence of acid and base is discussed. The reaction can occur at low temperature and generate the product 3,4-dihydro-2H-3-phenyl-1,3-benzoxazine. No intermediate 2-(N-hydroxymethyl-N-phenylamino)methylphenols are observed. The concentration of a Mannich base has little effect on the reaction rate, while the reaction is first-order to formaldehyde. Based on chemical properties of formaldehyde in aqueous solution, the reaction process includes the following three steps: degradation of poly(oxymethylene) glycols to methylene glycol, dehydration of methylene glycol to formaldehyde molecule, and the reaction between a Mannich base and formaldehyde molecule to benzoxazine. Dehydration of methylene glycols to formaldehyde molecules is the rate-controlling step.