Fischer Esterification

An ester was synthesized during an organic reaction and identified by IR spectroscopy and boiling point. Acetic acid was added to 4-methyl-2-pentanol, which was catalyzed by sulfuric acid. This produced the desired ester and water. After the ester was isolated a percent yield of 55.1% was calculated from the 0.872 g of ester recovered. This quantitative error was most likely due to product getting stuck in the apparatus. The boiling point of the ester was 143° C, only one degree off from the theoretical boiling point of the ester 1,3-dimethylbutyl, 144 ° C. The values of the major peaks on the IR spectrum also corresponded to the theoretical values of the ester 1,3-dimethylbutyl. Therefore, it was concluded that this was the ester formed and the experiment was a success, because even though there was error, this was only in the amount of the ester, not in its identity. Introduction

Fischer Esterification is the simplest means of synthesizing an ester and requires the reaction of a carboxylic acid and an alcohol. The general reaction of Fischer esterification is, CH3CO2H + ROH ↔ CH3CO2R + H2O (reaction 1)

An alcohol (ROH in reaction 1) is a functional group containing a hydrogen, oxygen bond. Carboxylic acids (CH3CO2H in reaction 1) are characterized by a carbon, oxygen double bond, with one alcohol group and an alkyl or aromatic side chain. Esters (CH3CO2R in reaction 1) are identified by a carbon, oxygen double bond and an oxygen alkyl or aromatic group. Carboxylic esters often have pleasant odors and are used in foods and beverages to create flavors mimicking fruit. Depending on the ester synthesized smells such as banana, pineapple, or orange can be obtained. Reaction one is a double displacement reaction. The carboxylic acid donates its hydrogen atom to the to the alcohol group, which accounts for water as a product. The ester is formed by the carboxylic acid gaining the alkyl side chain of the alcohol. A catalysis is added to the reaction in order to accelerate the process. The catalysis used in reaction 1 is sulfuric acid, H2SO4. The reaction also occurs in reflux, meaning it is boiled at a high temperature, also accelerating the process.

The reaction of a carboxylic acid and an alcohol, like in reaction 1, is in equilibrium. A reaction in equilibrium means the net change of the products and reactants is zero. The equilibrium is dynamic because the reaction is still occurring but the amount of reactants and products is no longer changing. In order to create more ester, the reaction must be shifted to the right, the product’s side. The reaction can be shifted to the right most easily by adding more reactants, specifically an excess of the carboxylic acid. The reaction will adjust to the right because of added reactant by creating more products, i.e. more ester. Considering that the reaction has an excess of the carboxylic acid, the alcohol will be the limiting reactant. The amount of ester yielded will depend upon the amount of alcohol in the reaction. The alcohol being used in this specific Fischer esterification is 4-methyl-2-pentanol.

The specific reaction is, CH3CO2H + CH3CH(CH3)CH2CH(OH)CH3↔CH3CO2(CH2)3CH + H2O (reaction 2)  Sulfuric acid is used as a catalysis in reaction 2, similar to reaction 1. The reaction occurs by the carboxylic acid donating a proton in the form of hydrogen to the alcohol. The oxygen of the carboxylic acid is now able to bond with the alkyl group of the alcohol creating the ester. The reaction will occur in reflux, meaning it will be boiled at a high temperature for a significant amount of time. Reflux gives the ability to isolate the ester from the other components of the reaction and therefore purifies the ester. The water is removed from the ester with the aid of sodium bicarbonate. The addition of sodium bicarbonate, which is soluble in water, will separate the aqueous layer from the organic layer and the aqueous can then be removed with a pipet. If necessary the centrifuge can also be used to further separate the two layers. A final means of drying the ester product is the addition of granular sodium sulfate.

The purity and identity of the product can be determined through the use of smell, IR spectroscopy and melting point determination. A general idea of what the ester is can be obtained by smell. Esters can have specific smells and if the scent can be determined, one may have an idea of the ester was created. IR spectroscopy will identify the bonds of the functional groups. The ester can be considered pure if there are no additional peaks on the IR spectrum. For example, if a peak corresponding to an alcohol group appeared on the IR spectrum it would mean the ester is not completely pure. Melting point can also be used to confirm the identity and how pure the final product is. Comparison of the experimental melting point and the literature value allows for determination of how close the experimental value is to the literature value. If the experimental value is within the range of the literature value, the product can be considered pure. Procedure:

A 10 mL round-bottom flask was weighed both before and after approximately 1.5 mL of the given alcohol, 4-methyl-2-pentanol, was added. 3 mL of glacial acetic acid, one boiling chip, and 2-3 drops of concentrated sulfuric acid were added to the flask in that order. The reflux apparatus was assembled, the solution poured in, and the assembly placed in the heating mantle for approximately one hour. At this time, an IR of the starting material was taken. After the reaction mixture was cooled, about 3 mL of 5% aqueous sodium bicarbonate was added while stirring. The reaction mixture was then transferred to a centrifuge tube and shaken vigorously. After the mixture settled into two layers, the lower organic aqueous layer was removed with a pipet. The reaction mixture was then centrifuged before repeating the previous step two additional times. A microspatula full of granular sodium sulfate was added. The percent yield was calculated, an IR spectrum was taken, and the boiling point was calculated of the ester that formed.

A Fischer esterification was performed, meaning an ester was synthesized from a reaction between a carboxylic acid and an alcohol. The alcohol used was 4-methyl-2pentanol and the resulting ester was (1,3-dimethylbutyl)acetate. The ester was successfully formed from the reaction and therefore the objective of the lab, to synthesize an ester, was met.

The ester was formed in a reflux apparatus. By placing the reaction in reflux (boiling at a high temperature), the reaction is speed up and the ester was formed quicker. The reaction needed to spend a certain amount of time in reflux to fully complete the formation of (1,3-dimethylbutyl)acetate. If the reaction was not in reflux long enough there would be residual alcohol that would show up in the final product and the maximum amount of ester would not be formed. The amount of product formed is determined by the amount of alcohol used, as it is the limiting reagent.

Ester and water are both products of the esterification. In order to measure the theoretical yield, of the ester, the water must be removed from the product. Water is able to be removed by utilizing the solubility of sodium bicarbonate. Sodium bicarbonate is polar; therefore it is soluble in water. The addition of the water soluble sodium bicarbonate causes the product to separate into an aqueous layer and an organic layer. The separation of the water from the ester allows for removal of the water with a Pasteur pipet. The aqueous layer was on the bottom while the organic layer was on top, meaning the organic layer was less dense than the aqueous layer.

In order to confirm the purity of the final product and to confirm that the reaction was in reflux long enough, IR spectrums were taken of the final product and the boiling point of the experimental product was compared to literature values. IR spectrums were taken of acetic acid and 4-methyl-2pentanol, the reactants, as well as the product, (1,3-dimethylbutyl)acetate. The IR spectrum of the ester confirms the purity and identity of the product. The spectrum contains no alcohol peaks, confirming that the reaction was in reflux long enough and the reaction went to completion. The spectrum has the correct peaks corresponding to the carbon, oxygen double bond, as well as the peaks for the sp3 hybridized carbon, oxygen bonds. The boiling point of the formed ester was found to be 143℃. The literature value of the boiling point is 144℃. Comparison of the literature and experimental value of the boiling point confirm the identity of the ester and its purity. The boiling point signifies that all the impurities of the ester were removed and the final product was pure (1,3-dimethylbutyl)acetate.

The theoretical yield was calculated to be 1.584 g and the experimental yield was 0.872 g. The percent yield of the product was 55.1%. Loss of product can be contributed to residual product being left in the reflux apparatus, test tubes, or spatulas. In order to minimize, product being left in test tubes, the final product was dried with sodium sulfate but if not enough was used or if not enough time was allowed for drying, some product would be left in test tubes, contributing to lost product. Error can also be found in the initial or final weighing’s of the materials. The scales being used can have inconsistencies which will give inaccurate amounts of the reactants or products.