Nucleophilic Acyl Substitution: the Synthesis of Esters

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Nucleophilic Acyl Substitution: The Synthesis of Esters
Razon, Valjean Paulo Mella
Rico, Fatima Sarah Rogel
Institute of Chemistry, University of the Philippines, Diliman, Quezon City 1101 Philippines Department of Food and Nutrition, College of Home Economics, University of the Philippines, Diliman, Quezon City 1101 Philippines

Abstract
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The purpose of this experiment is to observe the synthesis of esters (also known as esterification), to understand the chemical processes that ester synthesis undergoes, and to know the optimum conditions needed for high yields. Esterification is a Nucleophilic Acyl Substitution Reaction wherein a carboxylic acid or a derivative of a carboxylic acid accepts a nucleophile which in turns substitutes for a leaving group. In esterification a leaving group connected to the carbonyl carbon is substituted by an alkoxide ion which forms the formula RCOOR’ which is an ester. For this particular experiment the esterification used was the Fischer esterification reaction which involves a carboxylic acid and an alcohol mixed in different volumes in the presence of an acidic catalyst H2SO4 and heated in a reflux set-up. The esters we produced are the following: pentan-2-yl acetate (banana flavor) with a yield of 83.95%, benzyl acetate (peach flavor) with a yield of 51.31%, and butan-2-yl methanoate (raspberry flavor) with a yield of 49.99%. The researchers were successful with synthesizing esters using the fischer esterification method. However they failed to purify the said esters via simple distillation due to the lack of time. -------------------------------------------------

Introduction
Esters are compounds classified as derivatives of oxoacids and alcohols (IUPAC,1997), however for the purpose of this formal report, esters refer to derivatives of carboxylic acids or carboxylic acid esters. (Gunawardena, 2009) Esters have closely identical functional groups with carboxylic acids which include an acyl group bonded to an oxygen which is bonded to another molecule the only difference is that carboxylic acids are bonded to hydrogen atoms (acyl group bonded to an –OH) while esters are bonded to an alkyl group(acyl bonded to a oxygen bonded to an alkyl group). (Refer to figures 1&2 for comparison)

Fig. 1 structure of a carboxylic acid ester

Fig. 2 structure of a carboxylic acid
Esters exist almost everywhere. They occur in nature in fats and are responsible for the aroma of many fruits. Industrially, these compounds are generally used as flavorings, fragrances, solvents, and plastics. The plastics in which are polyesters wherein the monomer possesses the ester functional group. Solvents are also great solvents for polar organic compounds, such as lacquers, printing ink, polystyrene and cement. Also, esters are also used in the pharmaceutical industry as extraction solvents. (Poss, 2008) Because of their widespread industrial use, the preparation of esters has become one of the important processes in many industries and due to their importance in the industry, one might ponder on how esters are prepared. The process of the preparation of esters is generally called esterification. Some kinds of esterification are the following: esterification of carboxylic acids or the Fischer esterification wherein a carboxylic acid is reacted with an alcohol in an acidic medium, alcoholysis of acyl chlorides and acid anhydrides, Alkylation of carboxylate salts, transesterification wherein an ester is changed into another via the reaction with an alcohol (the alkyl group of the ester is interchanged with the R’ group of the alcohol, carbonylation wherein an alkene undergoes hydroesterification in the presence of metal carbonyl catalysts, Addition of carboxylic acids to alkenes,the Pinner reaction wherein nitrile is reacted to an alcohol in the presence of an acid...
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