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Published on March 17, 2014

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2 | P a g e AUGUSTINE ADU – KNUST THE PREPARATION OF A COMMERCIALLY IMPORTANT ESTER AIMS AND OBJECTIVES: To be equipped with the preparation of ester. To determine the percentage yield of the product formed. INTRODUCTION Esters have the general formula This class of compounds is widely distributed in nature and responsible for the characteristic flavors and fragrances of flowers and fruits. Simple esters tend to have very pleasant odors such as banana, peach, or apple. In some cases, they may even have a minty odor. Although esters tend to be very fragrant, they are rarely used in perfumes or scents, as they are not stable to perspiration. Upon contact with sweat, esters hydrolyse to carboxylic acids and alcohols. Unlike esters, these compounds (particularly carboxylic acids) are generally foul smelling. Many esters have the characteristic property of a pleasant odor. As such, they are used as artificial flavors and perfumes. The flavors from products such as peppermint gum or banana-flavored ice cream most likely originate from organic compounds known as esters. Not all esters have odors, but most volatile esters do. An example of an odorless ester would be vegetable oil. Esters are formed by the process of dehydration synthesis using an alcohol with an acid. . The name of the ester is formed from the alcohol and acid used in their making. The reaction between an organic acid and an alcohol is shown below: RCOOH + R*OH ←→ H2O + RCOOR* These R and R* represent organic groups such as hydrocarbons. (parts of molecules made out of hydrogen and carbon) The -OH group from the acid combines with the -H from the alcohol producing

3 | P a g e AUGUSTINE ADU – KNUST water molecules. The R*-O- group from the alcohol then attaches to the carbon on the acid forming the ester. The reaction is catalyzed by adding some concentrated sulfuric acid, H2SO4 Concentrated sulfuric acid is a strong dehydrating agent, and helps the reaction by removing the water molecules as they are formed. Salicylic acid contains both the organic acid group (-COOH) as well as the phenolic group (-OH), salicylic acid is capable of undergoing two separate esterification reactions, depending on whether it is behaving as an acid (through the –COOH) or as an alcohol analog (through the –OH) anditisthecombination ofthesetwogroupsthatleadstotheharshnessofsalicylicacidonthedigestivetract The salicylic acid acts as an alcohol when reacted with acetic acid. CHEMICALS AND EQUIPMENT  Acetic acid anhydride  Concentrated sulpuric acid  Salicylic acid  Cold water  Water bath  50ml measuring cylinder  25ml measuring cylinder  Stirring rod  Spatula  Electronic balance  Vacuum pump. PROCEDURE 4.0g of salicylic acid is weighed and put into a 250ml erlenmyer flask. It is the missed with 8ml of acetic anhydride. 7 drops of concentrated sulphuric acid was added and missed thoroughly by swirling. A whitish solution is observed. The flask and its content is heated on boiling water for 20minutes. A light yellowish solution is observed. It is then removed after 20minutes and slowly 25ml of iced water is added

4 | P a g e AUGUSTINE ADU – KNUST to form the precipitate. After the precipitate is formed it is filtered using a vacuum. Wash the precipitate with cold water and dry in air. Weigh it when it is dried. CALCULATIONS Mass of filter paper = 0.958g Mass of filter paper and the precipitate form = 5.381g Mass of product = 4.423 C7H6O3 + C4H6O3 → C9H8O4 + C2H4O2 Salicylic acid Acetic anhydride Acetylsalicylic acid m (C7H6O3) = 4g v(C4H6O3) =8ml M (C9H8O4) = (12 x 9) +(8 x 1) + (16 x 4) = 180g/mol M (C7H6O3) = (12 x 7) + (6 x 1) + (16 x 3) =138g/mol n (C7H6O3) = m(C7H6O3 )/M(C7H6O3 ) = 4/138 = 0.0290mol n(C7H6O3) = n(C9H8O4) in terms of mole ratio Hence, n(C9H8O4) = 0.0290 mol m(C9H8O4) = n(C9H8O4) x M(C9H8O4) m(C9H8O4) = 0.00290 x 180 = 5.22g Therefore theoretical yield of the product = 5.22g Percentage yield =( weight of product / theoretical yield) x 100 Percentage yield = (4.42 / 5.22) x 100 = 84.67%

5 | P a g e AUGUSTINE ADU – KNUST LABORATORY QUESTIONS. 1. Suggest two practical methods whereby the yield of ester based on the acid might be increased. (i) Small esters are formed faster than bigger ones ,gently heat a mixture of ethanoic acid and ethanol in the presence of concentrated sulphuric acid, and distil off the ester as soon as it is formed . (ii) Larger esters tend to form more slowly. In these cases, it may be necessary to heat the reaction mixture under reflux for some time to produce an equilibrium mixture. 2.If the oxygen atom in the alcohol is labelled as 18 o, the water that is formed contains non e of the label. What does this say about the mechanism? This means that the 18 O was not involved in the formation of the water but rather formed the ester. 3. Write the equation for the preparation of acetyl salicylic acid. C7H6O3 + C4H6O3→ C9H8O4 + C2H4O2 Salicylic acid Acetic anhydride Acetylsalicylic acidAcetic acid. 4. Oil of wintergreen is a close relative of aspirin. Its correct name is methyl salicylate, how would you prepare it? Methyl salicylate can be produced by esterifying salicylic acid with methanol 1) 0.2 grams salicylic acid was dissolved in 2.0 mL of methanol in a 5 mL round bottom flask. 2) 4-5 drops concentrated H2SO4 was added 3) Few boiling chipswas also added to promote smooth boiling.

6 | P a g e AUGUSTINE ADU – KNUST 4) Reflux for 40 minutes. 5) After 40 minutes, product is removed and isolated. DISCUSSION The hydroxyl functional group (R-OH) on the salicylic acid reacts with part of the acetic anhydride to produce an ester in the acetylsalicylic acid product in a reaction is known as an esterification. The other part of the acetic anhydride reacts to produce the byproduct acetic acid. Sulfuric acid is used here as a catalyst and is not part of the reaction product but serves to allow the reaction to proceed to completion more easily. To make a change appropriate, energy had to be applied to the sample so that the reactant molecules will collide with sufficient force to break their existing bonds and form the bonds of the product. Too much energy (heat) will not allow the products to form. This was the reason why the heating was done in a water bath. Cold water was added to facilitate the formation of the crystalline. PRECAUTIONS  Safety eyewear approved by your institution must be worn at all times while you are in the laboratory, whether or not you are working on an experiment.  The concentrated sulfuric acid used is very hazardous; severely corrosive to eyes, skin, and other tissue, care must be taken when handling it.  The organic acids and alcohols are flammable. Use great care around flames. Do not heat directly with a burner, but use a water bath.  Never smell a compound by putting it directly under your nose. Instead, hold the compound at least eight inches from your face with one hand, and use your other hand to gently waft the vapors toward your nose.

7 | P a g e AUGUSTINE ADU – KNUST ERROR ANALYSIS  After the addition of iced water to the hot content, it was not swirled which lead to the delay in formation of the crystals.  The precipitate did not form as expected for the reagent (sulphuric acid) might lose its concentrative power.  The storage for drying the precipitate was not good enough. For instance the blowing air might lose some of the precipitate there by affecting the mass.  The precipitate was left in the mother liquor for too long which might contaminate it. CONCLUSION At the end of the experiment the aims were achieve. It could be deduced that the theoretical yield is 5.22g and the percentage yield is 84.67%. This means that 84.67% of the mass precipitated out of the total mass of the sample which was theoretically calculated. REFERENCE  Chemistry Laboratory Manual(III) and (IV) KNUST page39  Organic Chemistry, Seventh Edition. By Francis A Carey. Pages 1065-1068  Stuart RK. Platelet function studies in human beings receiving 300 mg of aspirin per day. J.Lab.Clin.Med. 1970; 75:463-71.  Whitten, K. W., R. E. Davis, M. L. Peck, G. G. Stanley (2007). Chemistry. 8th ed. USA: Thomson Brooks/Cole, p. 947.


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