Aspirin Experiment

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Synthesis of Aspirin and Oil of Wintergreen
INTRODUCTION: Synthesis and use of organic compounds is an extremely important area of modern chemistry. Approximately half of all chemists work with organic chemicals. In everyday life, many if not most of the chemicals you come in contact with are organic chemicals. Examples include drugs, synthetic fabrics, paints, plastics, etc. Synthesis of Aspirin and Methyl Salicylate. The two compounds we will be preparing, aspirin (acetylsalicylic acid) and oil of wintergreen (methyl salicylate), are both organic esters. An ester is a compound that is formed when an acid (containing the COOH group) reacts with an alcohol (a compound containing an -OH group). O C R1 O H O

+
H

O C R2 R1 O R2

+
H

O H

acid

alcohol

ester

water

Here R1 and R 2 represent groups such as CH3 - or CH3 CH2 -. The reaction type shown above may be called a condensation reaction because the small molecule H 2 O is eliminated from the reactants while the remaining bits of the reactant condense together to give the main product. This reaction may also be called an esterification, since the product of the reaction is an ester, a compound containing the CO2 R group (see chapter 11 for definitions of acids, esters, and alcohols). Esters usually have pleasant, fruit-like odors and are the chemicals responsible for the odors and flavors of many fruits (oranges, bananas, pineapples) and flowers. In most cases, such natural products get their properties from a mixture of organic compounds. In this experiment you will prepare two esters of o-hydroxybenzoic acid, more commonly known as salicylic acid. One of the esters, acetylsalicylic acid, is aspirin, the common analgesic. We will synthesize aspirin by mixing salicylic acid with acetic anhydride. The second ester product is oil of wintergreen, or methyl salicylate, which we prepare by allowing salicylic acid to react with methyl alcohol. This compound, which has a familiar odor is used as a flavoring agent and in rubbing ointments. Both of these reactions are shown below. Preparation of acetylsalicylic acid H H C C H C H C C C C O O H O H

+

O H3C H3C C O C O H
H+

H H C C C H C C C C O O

O C O CH3
+

O C H3C O H

H

salicylic acid

acetic anhydride

acetylsalicylic acid (aspirin)

acetic acid

Preparation of methyl salicylate
H H C C H C H C C C C O O H O H
+

H O H H CH3
H+

H C C

C C C C H

O

H
+

O H H

O C O

CH3

salicylic acid

methanol

methyl salicylate (oil of wintergreen)

water

This experiment illustrates several properties of organic synthesis. While both product compounds in the experiment are esters of the same compound (salicylic acid), they are quite different in structure. Aspirin involves a reaction of the -OH group of salicylic acid, while methyl salicylate involves a reaction of the COOH group of salicylic acid. Organic chemistry is the broad field of studying the tremendous variety of such reactions of organic functional groups. Purification by Recrystallization. After preparing the aspirin, we will purify it. Most solids can be purified by recrystallization, at a cost of lower percent yield. Recrystallization is usually done by dissolving the substance in a suitable solvent, which is hot. If insoluble particles are present, the hot solution is filtered to remove them (we will skip this hot filtration step). The solution is allowed to cool slowly, and is eventually cooled in ice. The crystals that form slowly are more pure than the original solid. Characterization by Melting Point. A simple characterization technique that can be very useful in determining purity is melting point. It does not, however, tell much about the identities of the impurities. Pure materials usually have characteristic temperatures at which they melt, or a narrow temperature range (less than one degree) over which they melt. Impure compounds usually melt at a lower temperature, over a wider...
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