EXPERIMENT 9 Organic Chemistry II Pahlavan Cherif Synthesis of Aspirin Esterification Materials Hot plate B chner funnel 125 mL Erlenmeyer flask 400 mL beaker Melting point capillaries Stirring rod Melting point apparatus acetic anhydride 95 ethanol Chemicals Salicylic acid 85 phosphoric acid Purpose To learn how to synthesis organic compound such as aspirin and determine the purity of aspirin INTRODUCTION Organic Chemistry is the study of compounds containing carbon There are millions of organic compounds known because carbon atom has the unique ability to bond to other carbon atoms to from large molecules In these compounds carbon may be bonded to other carbons via single bonds double bonds and or triple bonds Other elements such as oxygen and nitrogen are also oftentimes present To study the vast array of structures the compounds have been organized into families or functional groups according to common features Functional groups have characteristic properties and they control the reactivity of the molecule as a whole Two such functional groups are briefly discussed below Alcohols represent a class of organic compounds that contain the hydroxy group O H The OH group is characteristic of alcohols and the structure of any alcohol includes the C O H bond Carboxylic acids represent another class of organic molecules The characteristic functional group of a carboxylic acid is a double bonded oxygen and a hydroxy group bonded to the same carbon atom This can be represented as COOH Aspirin or acetyl salicylic acid belongs to a group of organic compounds known as esters Aspirin is an analgesic painkiller an antipyretic fever reducer and an anti inflammatory agent It is the premier drug for reducing fever a role for which it is uniquely suited As an anti inflammatory it has become the most widely effective treatment for arthritis Patients suffering from arthritis must take so much aspirin up to four grams per day that gastric problems may result For this reason aspirin is often combined with a buffering agent Bufferin is an example of such a preparation 1 Esters are the most common derivatives of carboxylic acids The general formula for an ester is RCOOR and its molecular structure is shown below R may be hydrogen or an alkyl group R we add a prime to distinguish groups that differ in structure from another may be an alkyl group but not a hydrogen atom O R C OR Esters are particularly widespread in nature contributing especially to the pleasant flavors and aromas of many flowers and fruits Esters are most commonly prepared by the reaction of a carboxylic acid with an alcohol in the presence of catalytic amounts of a strong inorganic acid such as H2SO4 This reaction is shown in equation 1 and is also known as esterification An esterification is always accompanied by the formation of water In the formation of an ester the hydroxyl group of the acid not of the alcohol is lost to form water O R O H C R R OH C OH Carboxylic acid 1 H2O OR Alcohol Ester Water The reaction is an equilibrium reaction thus it is said to be reversible Typically pseudo first order reaction conditions are used by reacting an excess of alcohol with the carboxylic acid which will drive the reaction to completion The limiting reactant therefore is the carboxylic acid Aspirin the salicylic ester of acetic acid is prepared by the reaction of salicylic acid with acetic acid as shown in equation 2 COOH COOH O H OH CH3COOH O C H2O CH3 alicylic acid Acetic acid Acetyl salicylic acid 2 Water In today s experiment we will be using acetic anhydride CH3CO 2O instead of acetic acid This reaction is shown in equation 3 Acid anhydride which is made by the removal of a water molecule from two acetic acid molecules reacts with alcohols in much the same way as acids to form the ester Drug manufacturers prepare aspirin from salicylic acid by this method which is a much better synthetic method than shown in 2 Using an excess of acetic anhydride has the advantage that the water that is produced by the reaction will react immediately with acetic anhydride This will shift the reaction to the right improving the yield of aspirin in the reaction Second unwanted side reactions are controlled well In both reactions a catalyst normally sulfuric acid or phosphoric acid is used to increase the reaction rate COOH OH COOH O H3C C O H3C H C O Salicylic acid Acetic anhydride O C CH3 CH3COOH O Acetyl salicylic acid 2 Acetic acid 3 Salicylic acid MW 138 12 g mol o mp 159 C Acetic Anhydride MW 102 09 g mol o mp 140 C Acetyl salicylic acid aspirin MW 180 15 g mol mp 128 137 oC Reaction mechanism Below is the detailed mechanism showing how phosphoric acid catalyzes the formation of acetylsalicylic acid Organic reactions seldom yield pure substances In many instances several reactions proceed simultaneously and mixtures of products result Likewise the reaction may be incomplete and a portion of the starting material will be present in the product It is for these reasons that one needs to purify the product Recrystallization is a method of purification based on solubility of substances Ideally it is the dissolution of impure crystals into a hot solution and upon cooling the collection of the pure component crystal s In today s experiment you will purify aspirin by this method You will also determine the melting point of aspirin Melting points are determined for several reasons If a compound is a known one the melting point will help to characterize the compound in hand The melting point is the temperature at which the solid coexists in equilibrium with the liquid at atmospheric pressure Most pure organic compounds have sharp melting points that is the process of melting occurs over a very narrow range of temperature Impurities have a dramatic influence on the melting points of organic compounds They typically lower the melting point and widen the melting point range For example while pure salicylic acid melts sharply at 157 159 oC a sample of wet salicylic acid water is the impurity may melt in the range 145 155 oC A melting point range larger than 3 oC is an indication that the sample is either impure or wet 3 Data Collection Determine the mass of the crude aspirin before and the mass recrystallization and then calculate your percent yield of the pure product Measure the melting point of your pure sample and compare the number you obtain to that found in the CRC Literature value Repeat if necessary Procedure I Preparation Dry a 125 mL Erlenmeyer flask