Organic Chemistry II  | Lecture | Laboratory 

Organic Chemistry Laboratory II
Synthesis of Banana Oil (Esterification)
Experiment Background

The Ester Functional Group
Esters are carbonyl-containing functional groups that are deriviatives of carboxylic acids.  The unique bonds of the ester functional group include the carbonyl group (C=O), an alkoxy substituent on the carbonyl carbon atom, and an alkyl or aryl group also bonded to the carbonyl carbon.

Esters can be prepared through a variety of methods, but the most common is the Fisher esterification method.  The Fisher esterification is an acid catalyzed reaction that is used to generate esters from carboxylic acids and alcohols.  The appropriate starting materials for preparing a specific ester can be determined by examining the structure of the ester.   The carboxylic acid starting material can be revealed by identifying the carbonyl group and the alkyl or aryl substituent directly bonded to that carbonyl group.  Attachment of a hydroxyl group (-OH) to the carbonyl carbon provides the appropriate carboxylic acid starting material.  The alkoxy group bonded to the carbonyl carbon of the ester is derived from the alcohol starting material.  Replacing the carbonyl carbon of the ester with a hydrogen atom (-H).  Some esters and the carboxylic acids and alcohols from which they are derived are given in table below.

Carboxylic Acid / Alcohol

The Fisher Esterification
The Fisher esterification is a classic organic reaction that involves the acid-catalyzed conversion of a carboxylic acid and alcohol to an ester functional group via a nucleophilic acyl substitution.  The Fisher esterification is a reverible reaction and often requires special reaction conditions to promote the formation of the ester product.

Mechanism of the Fisher Esterification
The Fisher esterification is a nucleophilic acyl substitution reaction.  Under acidic conditions, the carbonyl-group of the carboxylic acid becomes protonated.  This makes the carbonyl-carbon more electrophilic and reactive with the nucleophilic alcohol oxygen.  The oxygen atom of the alcohol forms a bond with the carbonyl carbon, forcing the carbon-oxygen pi bond to break.  Through a series of additional steps (outlined below) the hydroxyl group of the carboxylic acid is eventually lost to generate the ester, the substituted product and water.

Fisher Esterification Equilibria
Each step of the Fisher esterification is reversible at temperatures that are typically used to run the reaction.  The equilibrium constant for the reaction between acetic acid and isopentyl alcohol  to generate isopentyl acetate and water is ~4, and is represented by the expression:

Keq =          [isopentyl acetate] [H2O]
                 [acetic acid] [isopentyl alcohol]

The reaction is run under thermodynamic conditions, where equilibrium is achieved. La Chatelier’s principle predicts that the equilibrium of a chemical reaction will be restored (i.e, the Keq at a given temperature will be maintained) even when concentrations of reactants or products are altered throughout the reaction.  The theoretical yield of the ester product in the Fisher esterification, when equimolar quantities of acetic acid and isopentyl alcohol are used is only ~55%.  The yield of product can be enhanced by increasing the concentration of either the acid or the alcohol starting materials.  In this reaction, there is approximately 3 times more acetic acid than alcohol.  The higher concentration of acid increases the theoretical yield of the reaction.  While this strategy improves the yield of the reaction product, a consequence is that the product will be contaminated with starting material.  Therefore, the crude reaction mixture must be purified first by washing in the separatory funnel, then by distillation.