Tania Cordova

DFT and ab-initio study on the mechanism of the gas-phase elimination kinetics of 1-chloro-3-methylbut-2-ene and 3-chloro-3-methylbut-1-ene and their isomerization

The mechanisms of the gas-phase elimination kinetics of 1-chloro-3-methylbut-2-ene and 3-chloro-3-methylbut-1-ene and their interconversion have been examined at MP2 and DFT levels of theory. These halide substrates yield isoprene and hydrogen chloride. The results MPW1PW91 calculations agree with the experimental kinetic parameters showing the elimination reaction occurs at greater rate for 1-chloro-3-methylbut-2-ene than that for the 3-chloro-3-methylbut-1-ene isomer. The mechanism for the molecular elimination of 1-chloro-3-methylbut-2-ene suggests proceeding through an uncommon six-membered cyclic transition state for alkyl halides in the gas phase, while 3-chloro-3-methylbut-1-ene eliminates through the usual four-membered cyclic transition state. The elongation and subsequent polarization of the C-Cl bond, in the direction of C^{δ+}…Cl^{δ-}, is rate determining step of these reactions. The isomerization of 1-chloro-3-methylbut-2-ene and 3-chloro-3-methylbut-1-ene was additionally studied. The 1-chloro-3-methylbut-2-ene converts to 3-chloro-3-methylbut-1-ene easier than the reverse reaction. This means that 1-chloro-3-methylbut-2-ene was found thermodynamically more stable than 3-chloro-3-methylbut-1-ene.

The thermal decomposition of 4-bromobutyric acid in the gas phase: A quantum chemical theory calculation

The gas-phase elimination kinetic of 4-bromobutyric acid to give butyrolactone, and hydrogen bromide was studied using Density Functional Theory DFT and Moller-Plesset Perturbation Theory of Second Order MP2 to investigate the more reasonable reaction mechanism. Good agreement of calculated activation parameters with the experimental values was obtained when using PBEPBE/6-31++Gd,p level of theory. Analysis of the calculated thermodynamic and kinetic parameters suggested the reaction mechanism is unimolecular, with involvement of the hydroxyl oxygen of the carboxylic moiety of the substrate assisting the exit of bromide in nucleophilic substitution. The alternate mechanism with the participation of the carbonyl oxygen in a slow step to give an intimate ion-pair intermediate was disregarded due to the high energy of activation. Bond order analysis shows the process is dominated by the breaking of the C-Br bond. The reaction can be described as unimolecular and moderately non-synchronous process.