Theoretical study on the esterification of methanol with acetic acid and acid halides.

Abstract

Esters are a unique class of organic compounds whose production and reactivity are examined in chemical industries and research laboratories throughout the world on a daily basis. An ongoing advancement in esterification reactions is applied in biofuel production from natural and waste organic sources. The roles of esters in daily and industrial activity are enormous, which includes: perfumery, glues, solvents in chemical reactions, dye production, cosmetics removal and functional groups in the production of pharmaceutical drugs. Dating back to the 1890s, Fischer esterification remained a crucial process through which esters are formed by coupling carboxylic acid with alcohol in the presence of an acid. For the past three decades, focus on esterification in the presence of solid acids as well as eco-friendly catalysts have been developed to enhance this process between carboxylic acids and alcohols. Important derivatives of esters are acid anhydrides and acid halides. They produce esters when reacted with alcohol either in presence or absence of catalyst. A number of mechanisms involved in ester formation have been proposed and kinetics obtained via experiments while little attention has been paid to exploiting mechanistic aspects at the molecular level. Herein, a thorough investigation on ester formation from acetic acid and acid halides (acetyl fluoride, chloride, bromide and iodide) reacting with methanol is done using density functional methods. In the first step, efforts were made to examine the feasibility of an uncatalyzed model for this reaction in gas and solvent (methanol only). Before this could be achieved, a comprehensive investigation was done to assign a suitable basis set in which defTZVP was upheld for being sufficient and efficient for all atoms involved in this project at the B3LYP and M06-2X levels of theory. A notable conclusion from the basis sets investigation is that a broad basis set is required to span all halogen atoms. Esterification of these substrates to yield methyl acetate was modelled to have occurred in a concerted manner through three different cyclic transition states. The one-step 6-membered mechanism gives lower activation energies for XAc with X = Cl, Br and I. On the other hand, the two-step 6-membered concerted model gave the lowest activation barriers for XAc with X = OH and F. The calculated thermodynamic parameters gave free energy barriers of 35.4 and 21.9 kcal mol-1 for acetic acid and acetyl chloride reaction with methanol respectively. This observation is in excellent agreement with experimental values of about 34 and 20 kcal mol-1 from literature. The esterification reaction of acetic acid and its halide analogues with methanol was also studied in the presence of an acid catalyst using the M06-2X hybrid density functional and the defTZVP basis set. The reaction was modelled as a one-step concerted 6-membered cyclic transition state. An activation of 19.77 kcal mol-1 which is in reasonable agreement with experimental value was obtained. The simple one step concerted 6-membered ring mechanism provides a suitable description of the acid-catalyzed esterification reaction. Esterification of acid halides (X = Cl, Br and I) with methanol in the presence of a hydrogen ion also produced values confirming their spontaneous reactivity while acetyl fluoride vary with a high solution phase free energy of 9 kcal mol-1. The outcome of this research has provided a rational molecular level understanding on the concerted mechanism of esterification via 6-membered ring transition states.