Ring Formation Mechanism of Adipic Acid

The mechanism of adipic acid ring formation: ① intramolecular dehydration condensation forms hexagonal cyclic anhydride. Under high temperature (≥ 280 ℃) or strong dehydrating agent (such as acetic anhydride), esterification reaction occurs between the two carboxyl groups of adipic acid to produce succinic anhydride. The infrared spectrum shows a characteristic absorption peak of acid anhydride at 1785cm ⁻¹, and X-ray diffraction confirms that the crystal structure is a symmetrical hexagonal ring. ② Dickman condensation generates a five membered cyclic intermediate. Diethyl adipate undergoes intramolecular Claisen condensation in toluene solution at 95 ℃ under the catalysis of sodium ethoxide (catalyst to ester molar ratio 1.8:1), forming 2-ethylcarbonylcyclopentanone. The NMR spectrum shows a shift of the methylene signal towards the lower field at δ=4.1ppm, confirming the cyclic structure. ③ Acid hydrolysis for ring opening and cyclization. 2-Ethoxycarbonylcyclopentanone is hydrolyzed in 1mol/L hydrochloric acid at 70 ℃ to first generate the intermediate of cyclopentanone carboxylic acid, followed by decarboxylation to form cyclopentanone. Gas chromatography analysis showed that when the reaction time exceeded 3 hours, the content of cyclopentanone polymerization products (such as dimers) increased by 12%. ④ Differences in alkaline hydrolysis pathways. When hydrolyzed with 1.5mol/L potassium hydroxide at 60 ℃, 2-ethoxycarbonylcyclopentanone is first saponified to form potassium cyclopentanone carboxylate, which is then acidified and decarboxylated to obtain cyclopentanone. Under this condition, the yield of the byproduct ring opening product (such as succinic acid) is 8.7% lower than that of acid hydrolysis. ⑤ High temperature decarboxylation directly forms a ring. When adipic acid is heated separately to 300 ℃, it generates cyclopentanone and CO ₂ through a synergistic decarboxylation mechanism. Thermogravimetric analysis shows that the theoretical release of CO ₂ corresponds to a weight loss rate of 34.5% (C ₂ H ₂ O ₄ → CO ₂+H ₂ O). ⑥ Catalyst regulates cyclization selectivity. Adding 0.5% zirconia can increase the yield of cyclopentanone to 62%, while adding an equal amount of alumina leads to a 25% increase in linear polyester by-products. XPS analysis shows that Zr ⁴+stabilizes the transition state intermediate through coordination.
⑦ The solvent effect is significant. In the polar solvent N, N-dimethylformamide, the activation energy of cyclization reaction decreases by 18 kJ/mol, but at the same time, the production of by-product succinic acid increases by 15%, reflecting the dual influence of solvent polarity on the reaction pathway.
⑧ Microwave assisted enhanced reaction. The use of microwave heating (2450MHz, power 300W) can shorten the cyclization time from 3 hours to 45 minutes and increase the yield by 9%, attributed to the selective excitation of molecular vibration by microwaves.
⑨ Structural verification of cyclization products. Single crystal X-ray diffraction shows that the C-O bond length in cyclopentanone molecules is 1.21 Å, which conforms to typical ketocarbonyl characteristics. The ring plane deviates from the ideal plane by 0.08 Å, reflecting the tension characteristics of a five membered ring.
⑩ Examples of industrial applications. A certain factory uses a fixed bed reactor with a supported palladium/attapulgite catalyst (metal loading of 0.8%) to prepare 1,2-cyclohexanedioctyl adipate by hydrogenation cyclization of dioctyl phthalate at 250 ℃ and 3MPa hydrogen pressure. The one-way conversion rate reaches 89% and the selectivity is 94%.
⑪ Side reaction control strategy. Adding 0.3% triphenyl phosphate can inhibit the polymerization of cyclopentanone, increasing the purity of the target product from 92% to 98.5%. In situ infrared monitoring shows that the inhibitor forms a reversible complex with the carbonyl group, blocking the polymerization chain reaction.
⑫ Green chemistry pathway. Using hydrogen peroxide as the oxidant and sodium tungstate sulfosalicylic acid as the catalytic system, selective epoxidation of cyclohexanone to generate ε - caprolactone was achieved under mild conditions (65 ℃, atmospheric pressure). The ring was further hydrolyzed and closed to obtain adipic acid with a total yield of 78%, and the byproduct was only water.