Research kinetics of the monochloroacetic acid esterification

Abstract

Esterification of carboxylic acids has vast academic applications in organic synthesis and finds industrial uses as well; for instance, reactions involving esterification of monochloroacetic acid with alcohols are widespread in the pharmaceutical industry. Traditionally esterification has been performed using liquid mineral acid catalysts, such as H₂SO₄, H₃PO₄, HF, etc. Since reactivity of esters is higher than that of carboxylic acids, they are more applicable for organic synthesis. Particularly, high reactivity of C‒Cl bonds in monochloroacetic and 3-chloropropionic acids is used in many syntheses. Monochloroacetic and 3-chloropropionic acid esters are widely used as alkylation agents for nucleophilic atoms, such as Sulfur, Nitrogen, and Oxygen, at standard conditions.

The purpose of this work was to investigate patterns of the esterification reaction between monochloroacetic acid and butan-1-ol in the presence of concentrated sulfuric acid.

Materials and methods. Starting reagents were purchased from Sigma-Aldrich. Boiling points were determined using conventional distillation at atmospheric pressure. Bruker Alpha spectrophotometer was used to obtain IR spectra in the wavenumber range of 7500–400 cm^-1 using a technique of placing a liquid onto the film. Agilent 7890B gas chromatography system coupled with Agilent 5977B mass spectrometry detector was used for the separation of the compounds, while NIST14 library was used for compound identification by mass spectra.

Results. In the investigation of the esterification reaction, samples were drawn from the reactive medium at 20, 40, 90, 120, 160, 200, 240, 360 minutes of the reaction and analyzed by gas chromatography. Analysis of the chromatograms showed that monochloroacetic acid migrates into the aqueous phase completely, and it is not observed in the organic phase. Mainly two peaks were present on chromatograms, which were identified by mass spectra as butan-1-ol and butyl monochloroacetate. Reaction rate constants were evaluated using a second-order kinetic model following the graphical method, which uses the slope of the best-fit line. The activation energy (Е_act.) was calculated using the plot of ln k the inverse of absolute temperature, and the obtained value was in agreement with the literature data available for esterification reactions.

Conclusions. Mainly two peaks were observed on the chromatograms, which correspond to butan-1-ol and butyl monochloroacetate, as identified by mass spectra. Based on the obtained results, it may be established that temperature increase from 100 ℃ to 120 ℃ leads to the decrease of the reaction rate constant, and the reaction progresses by one order faster. The activation energy (Е_act.) of the esterification reaction equals 165 kJ/mol. The analysis of a mass spectrum of butyl monochloroacetate revealed that butyl monochloroacetate molecules are fragmented into monochloroacetic acid fragments, (CH₃CO)⁺ ions, butyl radical particles, and (Cl-CH₂CO)⁺ ions. IR spectrum contains absorption bands characteristic to esters, specifically: νC = О, С-С-О, CH₃, CH₂, O-CH₂-C, C-Cl.