Quantum Chemical Calculation Study On The Decarboxylation Mechanism Of Organic Carboxylic Acid

The decarboxylation of organic carboxylic acid is the key process for the removal of carboxylic acid in coal, petroleum, the generation of hydrocarbon in organic matter, and the pharmaceutical synthesis. In recent years, the decarboxylation reactions of organic carboxylic acid have been studied experimentally by many scientists, and a lot of results have been reported. However some experimental phenomena appearing in the process cannot be explained by general chemical rules, and the basic mechanisms involved in the reactions are still unclear, which limit the application of this kind of reaction. Using quantum chemical tools to study the devarboxylation mechanisms of fatty acids and their derivatives will be helpful in understanding the decarboxylation reaction process, interpretation of the experimental results, revealing the catalytic mechanism, and providing some theoretical guidance for their applications in petroleum acid removal, pharmaceutical etc.Five typical carboxylic acids and their derivatives have been selected as the research object in this paper, and the mechanisms of decarboxylation reactions have been studied using quantum chemistry calculation method. The main research contents and results are as follows:（1） Density Function Theory（DFT） was employed to study the mechanism about two kinds of decarboxylation（without any catalyst or catalyzed with H₂O） of four halogenated carboxylic acids and four carbonyl（cyano） carboxylic acids in aqueous solutions. The results showed that: H₂O showed some catalytic activity in the decarboxylation reaction of halogenated carboxylic acid, the decarboxylation rates are in the order:CCl₃COOH>CF₃COOH>CCl₃CH₂COOH>CF₃CH₂COOH; and H₂O were not conducive to the decarboxylation reaction of carbonyl acid. The decarboxylation without any catalyst for the three b-carbonyl（cyano） carboxylic acids were easier than the decarboxylation for the haloacetic acid acetic acid（or propionic） catalyzed with H₂O, which implied that the hydrogen bond existing in the transition state of the former played an important role in depressing the activation energy.（2） The mechanism for the decarboxylation of indole-2-formic acid, benzofuran-3-formic acid, benzothiophene-2-formic acid and thiophene-2-formic acid catalyzed separately with H₂O and H₃O⁺ were studied. The H₂O-catalyzed reaction is a three step one including the hydration of carboxyl group, the breaking of C-C bond and the decomposition of H₂CO₃. The decarboxylation catalyzed with H₂O was not effective due to their highest potentials. In the H₃O⁺-catalyzed reaction, firstly the H+ attacked the a-carbon or the oxygen atom of carbonyl, and then a nucleophilic addition of H₂O to the carbonyl happened, the next step was the ruption of C-C bond, finally the H₃CO₃⁺ decomposed. The energy barrier of the hydration of the carbonal group was the highest one for the whole process, which was much lower than the barrier catalysed with H₂O. The order of the decarboxylation rates for the four carboxylic acids catalysed with H₃O⁺ was: benzofuran-3-formic acid > thiophene-2- formic acid > benzothiophene-2-formic acid > indole-2-formic acid. The high catalytic activity of H₃O⁺ was mainly due to the participation of H+ which could effectively stabilize the transition state structure, and reduce the energy barrier.（3） The decarboxylation mechanisms of oxaloacetic acid（OAA） catalysed with protonated ethylenediamine（ENH+） or without any catalyst were studied using density functional theory. The decarboxylation mechanism of its dianion（OA₂^-） and monoanion（OA-） catalysed with protonated aliphatic diamines（ethylenediamine, 1,3-diaminopropane, 1,4-diaminobutane, 1,5-diaminopentane and 1,6-diaminohexane） were also studied. The results showed that the decarboxylation was a four-step reaction including nucleophilic attack by the amine on oxaloacetate（OAA, OA₂^-, OA^-） to form a carbinoamine intermediate, the dehydration of the carbinoamine to form an imine intermediate, the decarboxylation of the imine to form the corresponding pyruvate imine, and the hydrolysis of which to generate pyruvate and the amine catalyst. In the ENH+-catalysed decarboxylation of OAA, the energy barrier of the carbinoamine dehydration was 57.9 k J·mol^–1, much lower than that of the decarboxylation without any catalyst(99.8 k J·mol^–1), but was higher than those of the decarboxylation of OA₂^-and OA- catalysed by ENH+(OA₂^-: 39.2 k J·mol^–1, OA^-: 44.1 k J·mol^–1). In the decarboxylation of OA₂^- catalysed by the five kind of protonated diamines, the potential energy for the dehydraton of the carbinoamine was the highest one in the whole catalytic cycle, and the energy barrier was gradually high with the increasing of the distance between the two amino carbon atoms; and in the diamine-catalysed decarboxylation of OA^-, the potential energy of the dehydraton of the carbinoamine was the highest one for the whole catalytic cycle too, and the amine with two amino groups separated by 3-5 carbon atoms showed better catalytic activity. The research also showed that the function of the second amino group in the reaction was the proton donor and acceptor.（4） The decomposition mechanisms of maleic acid and fumaric acid in gas phase were studied. There were two reaction channels in the unimolecule decomposition of maleic acid, in the first channel, it dehydrated to form anhydride firstly, then maleic anhydride decoposited to form CO₂, CO and ethylene, the activation energy of the second step was 270.1 k J·mol^–1; in the second channel, it decarboxylated to produce CO₂ and acrylic acid, then acrylic acid continued to decompose to form CO₂ and ethylene, the activation energy of the the second step was 319.0 k J·mol^–1. Because the activation energy of the inverse reaction in channel 1 was much lower(79.9 k J·mol^–1), so at higher temperature, the reaction would be along with channel 2. The decomposition mechanism of fumaric acid in the gas phase was the same as channel 2 of maleic acid, and potential energy the second step was 297.1 k J·mol^–1, which was lower than maleic acid(319.0 k J·mol^–1).（5） Density functional theory was employed to study the decarboxylation mechanisms of unsubstituted b-propiolactone and 16 groups of propiolactone whose hydrogens on C（3） and C（4） atom were substituted by hydroxyl or methyl. The results showed that: The unimolecule（1）decarboxylation of b-propiolactone in the gas phase conditions was carried out with synergistic reaction process, and the reaction barrier of methyl or hydroxy substituted b-propiolactone was lower than that of the unsubstituted one;（2） The energy barrier for the decarboxylation of b-propiolactone with the C4 atom substituted was lower than those with the samne substituent on C3 atom, and the decarboxylation would be easier when the number of the substituents on C4 increased;（3） For isomers, the reaction for trans structure was easier than the cis one; The（4）energy barrier for b-propiolactone with hydroxyl substituent was lower than those with methyl substitutent at the same position, and the reaction would be easier.