Esters derive from the reaction between a carboxylic acid and an alcohol (Figure 1).
Figure 1. The General Reaction Equation of Ester Formation From A Carboxylic Acid & An Alcohol. Carboxylic acids contain the functional group –COOH whereas in ester’s the hydrogen is replaced with an R denoting any alkyl or aryl group; -RCOOR’. Esters with low-molecular weights are commonly used as components in the flavor’s and odors of many fruits & fragrances’ as well as to enhance foods & beverages. A food technologist may utilize an ester or many esters to produce characteristic fragrances & flavors in foods (Table 1).
Table 1. Flavor/Odor’s of Esters
Although esters are fairly ubiquitous, they are not used in the production of perfumes that are applied to the body. In the presence of perspiration esters, are generally unstable and tend to break down forming carboxylic acids, which characteristically have unpleasant odors.
Esters can be prepared by the reaction of a carboxylic acid and an alcohol in the presence of a catalyst such as concentrated hydrogen chloride, sulfuric acid, or the acid form of an ion exchange resin (Figure 2).
Figure 2. Preparation of ester via ion exchange resin
The Lewis or Brønsted Acid – Catalyzed esterification is also know as Fischer Esterification which a carboxylic acid reacts with an alcohol to yield an ester; typically in equilibrium. The position of equilibrium can be influenced or shifted by adding more of the acid or alcohol depending on the intended direction. The Fischer Esterification mechanism involves a series of completely reversible steps: an initial protonation of the carboxyl group, attack by the nucleophilic hydroxyl, a proton transfer, and a loss of water followed by a loss of the catalyzing proton to yield the ester, broken into three stages (Figure 3, 5 & 6): 1). Stage 1: The protonation of the carboxyl group’s carbonyl oxygen yields a resonance – stabilized intermediate oxonium or carbenium...