The Esterification Process
Aspirin can be made by using a process called esterification. Esterification occurs when a carboxylic acid and an alcohol combine in a reaction to produce an ester. This reaction can be used to synthesize aspirin from salicylic acid. In the lab, the carboxylic acid alcohol mixture is heated in the presence of H2SO4, sulfuric acid, which acts as a catalyst. During the reaction process, a molecule of water splits off and the remaining carboxylic acid and alcohol fragments become attached producing an ester.
General Reaction of Esterification
Esterification of Aspirin Usng Acetic Acid
What We Are Doing In The Lab
You start by weighing out a sample of salicylic acid into an Erlenmeyer flask. To the sample you will add acetic anhydride. We are using acetic anhydride instead of acetic acid because acetic anhydride has a faster reaction time. The catalyst concentrated H2SO4 is added to speed up the reaction. The flask containing this solution is then heated in a boiling water bath for about 15 minutes. This process is called esterifcation.
Esterification of Aspirin In Our Lab Using Acetic Anhydride
The flask is then removed and allowed to cool. Ice cold distilled water is slowly added to the flask to decompose any unreacted acetic annhydride.
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Decomposition of Unreacted Acetic Anhydride
Ice cold distilled water is added to the flask again. The flask is then chilled in an ice-water bath for about 10 minutes until crystallization of the aspirin is complete. The aspirin crystals are collected on a Buchner funnel and washed with additional ice cold distilled water. You are using ice cold distilled water instead of room temperature distilled water because aspirin is insoluble in cold water and you would not be dissolving any of your aspirin product. The acetic acid and H2SO4 are water soluble, in any temperature water, and can be removed by washing the aspirin with the chilled water. Salicylic acid is only slightly soluble in water and any unreacted salicylic acid cannot be removed completely in the washing process. Once the aspirin crystals are purified they are allowed to dry. They are then weighed and tested for purity.
Why We Use Ice Cold Distilled Water
1) Aspirin is insoluble in cold water.
2) Acetic anhydride decomposes to water soluble acetic acid.
3) H2SO4, sulfuric acid, is soluble in water.
4) During the washing process impurities (acetic acid and H2SO4, sulfuric acid) are removed.
Testing For Impurities
We use iron III chloride to determine the purity of our aspirin. Iron III chloride combines with the phenol group to form a purple complex. If salicylic acid is present (impurity) the product will turn purple when FeCl3 is added, because salicylic acid is a phenol.
Purple color when FeCl3 added – impure product, salicylic acid present. Unreacted salicylic acid is present.
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No color change, appearance of yellow color, or a faint purple tinge – pure product, no unreacted salicylic acid present.
Salicylic acid (from Latin salix, willow tree, from the bark of which the substance used to be obtained) is a monohydroxybenzoic acid, a type of phenolic acid and a beta hydroxy acid. This colorless crystalline organic acid is widely used in organic synthesis and functions as a plant hormone. It is derived from the metabolism of salicin. In addition to being a compound that is chemically similar to but not identical to the active component of aspirin (acetylsalicylic acid), it is probably best known for its use in anti-acne treatments. The salts and esters of salicylic acid are known as salicylates.
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Salicylic acid has the formula C6H4(OH)COOH, where the OH group is ortho to the carboxyl group. It is also known as 2-hydroxybenzenecarboxcylic acid. It is poorly soluble in water (0.2 g/100 ml H2O at 20 °C).[2] Aspirin (acetylsalicylic acid or ASA) can be prepared by the esterification of the phenolic hydroxyl group of salicylic acid with the acetate ion from acetic anhydride or acetic chloride.
Salicylic acid is biosynthesized from the amino acid phenylalanine. In Arabidopsis thaliana it can also be synthesized via a phenylalanine-independent pathway.
Sodium salicylate is commercially prepared by treating sodium phenolate (the sodium salt of phenol) with carbon dioxide at high pressure (100 atm) and high temperature (390K) -a method known as the Kolbe-Schmitt reaction. Acidification of the product with sulfuric acid gives salicylic acid:
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It can also be prepared by the hydrolysis of Aspirin (acetylsalicylic acid)[6] or methyl salicylate (Oil of Wintergreen) with a strong acid or base.
Fischer esterification or Fischer–Speier esterification is a special type of esterification by refluxing a carboxylic acid and an alcohol in the presence of an acid catalyst. The reaction was first described by Emil Fischer and Arthur Speier in 1895.[1] Most carboxylic acids are suitable for the reaction, but the alcohol should generally be a primary or secondary alkyl. Tertiary alcohols are prone to elimination, and phenols are usually too unreactive to give useful yields. Commonly used catalysts for a Fischer esterification include sulfuric acid, tosic acid, and Lewis acids such as scandium(III) triflate. For more valuable or sensitive substrates (for example, biomaterials), dicyclohexylcarbodiimide is often used. The reaction is often carried out without a solvent (particularly when a large reagent excess (e.g. of MeOH is used) or in a non-polar solvent (e.g. toluene) to facilitate the Dean-Stark method. Typical reaction times vary from 1–10 hours at temperatures of 60-110°C.
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Direct acylations of alcohols with carboxylic acids is preferred over acylations with anhydrides (poor atom economy) or acid chlorides (moisture sensitive).
The main disadvantage of direct acylation is the unfavorable chemical equilibrium that must be remedied e.g. by a large excess of one of the reagents, or by the removal of water (for example by Dean-Stark distillation, the use of molecular sieves, or the use of a stoichiometric quantity of concentrated sulfuric acid).
In one study [2] it is found that Tetrabutylammonium tribromide (TBATB) is a very effective catalyst. For example the acylation of 3-phenylpropanol with glacial acetic acid and TBATB at reflux generates the ester in 15 minutes in a 95% chemical yield without the need to remove water. It is believed that hydrobromic acid released by TBATB protonates the alcohol over the acid making the carboxylate the actual nucleophile in a reversal of the standard esterification mechanism.
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Sources of Experimental Error
As in all experiments, experimental error is inevitably present, yielding inaccurate results. The complexity of this experiment only increases the possibility of the entrance of an unforseenable and/or uncontrollable variable. This section aims to present the sources and implications of these variables.
During the course of the experiment, it was possible that the thermometer inaccurately measured the melting range of the aspirin, being non-digital in operation. Exact values and decimal points could not be visually obtained; however, it is believed that the mercury thermometer was fairly functional and accurate. The melttemp apparatus, however, limited accuracy and precision of the melting range made it very difficult to control the pace of heating of the sample to small increments of 1-2°C per minute.
Secondly, the digital scale was also a potential source of error, as it was extremely sensitive and results were easily disturbed by small fluctuations such as the flow of air in the lab. The scale was also neither calibrated nor standardized. The masses of the products or reactants are subject to such uncontrollable fluctuations. As the amounts of reactants and aspirin synthesized were relatively small, even minor fluctuations can translate into multiple percentages in error.
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Thirdly, impurities in the reactants or the aspirin could not be isolated, controlled, or eliminated apart from qualitatively during recrystallization. The lab in which the experiment was performed nor the cupboard in which the product was dried are unlikely to be sterile, therefore impurities could have entered the reaction or reactants at any time, especially since the containers used in the experiment were largely left open. Both acetic anhydride and acetylsalicylic acid decompose in humid air, which reduced percentage yield. Another possible source of contamination was the lab equipments used in this experiment. Although all the lab equipments were previously rinsed with water and dried, it is possible that chemical residue from previous experiments were present during the course of the experiment.
Fourthly, because sulfuric acid was used instead of phosphoric acid as a catalyst to synthesize aspirin, the percentage yield was quite low. This occurs because sulfuric acid reacts more strongly with the organic molecules in the reaction than phosphoric acid. However, it was still used because the synthesis of asprin entailed the absorbtion of water. Phosphoric acid does not absorb water, however sulfuric acid does. This property is called hygroscopy, and extends to dessicators such as calcium chloride and, to a lesser extent, silica, which was used to dry the aspirin prior to testing the melting point of the crude product.
