Question

Synthesis of banana oil

this is my discussion and below it is the professors comments. Can you correct it for me?

Discussion:

            The purpose of the experiment performed was to synthesize isopentyl acetate, or more commonly known as banana oil, from isopentyl alcohol and glacial acetic acid. Both liquids were combined and remained clear until the addition of concentrated sulfuric acid. The addition of an acid caused a reaction to occur that resulted in a polymer, which gave the mixture color. The liquid turned dark brown when the polymer formed. The polymer was removed via distillation.

            After creating the ester, the biggest challenge was separating it from all the other chemicals in the flask, which include water, acetic acid, sulfuric acid, and isopentyl alcohol. The isopentyl acetate is not soluble in water; so two layers are created in the separatory funnel. While the product will not dissolve in water, both acetic acid and sulfuric acid are. Since the acids are also slightly in the organic layer, sodium bicarbonate is used to convert the acids into a salt, which is soluble in water, as seen in Figure 2. The sodium bicarbonate reaction also produces carbonic acid, which is unstable and will break down into water and CO₂ as seen in Figure 3. The little amount of unreacted isopentyl alcohol is separated by removing the first ten drops of product during the distillation. This is possible because it has a lower boiling point then the ester that is produced.

            This whole reaction was first done and called the Fischer esterification. The general reaction scheme can be seen above. In the experiment, isopentyl alcohol and acetic acid react in the presence of sulfuric acid to form isopentyl acetate. This reaction mechanism can be seen below in Figure 5.

Figure 5 – Fischer esterification mechanism

In the reaction above, Acetic acid takes a proton from concentrated sulfuric acid. Then it reacts with isopropentyl alcohol. Delocalization of electrons and protons in later steps leads to elimination of water molecule to form final product.

            During the distillation, the temperature was recorded for the boiling point of isopentyl acetate. It was found to be at 129 degrees Celsius. This is low compared to the literature value of 142 degrees Celsius. This difference is believed to be due to the different typed of thermometers used to record the boiling point. The literature value most likely used a mercury thermometer, while the experiment performed used an alcohol thermometer. Alcohol thermometers are not as accurate as mercury thermometers, but have to be used due a law banning mercury thermometers in the state of New Jersey. Because of this, not much can be said about the purity of the substance obtained.

            While an NMR was not taking during the experiment, nor was an IR spectrum, attached is both an NMR example and an IR example of isopentyl acetate. On the NMR sheet, all hydrogen peaks are labeled on a drawing of the molecule, and all the information is written in the chart. On the IR sheet, peaks above 1500 cm^-1 are labeled for what they are believed to be. While the pictures were found from a reputable source, they were not from the experiment performed so one cannot be perfectly sure that the desired substance was obtained.

            The last topic for the reaction was the product loss. Most organic reactions are not perfect and have relatively low yields. The 42 percent yield was most likely due to side reactions occurring in the flask. Since there were several chemicals in the flask together, side reactions are bound to occur, creating a lower yield. For example, a polymer was formed which changed the color of the flask. This was not intended to happen so this reaction lowered the yield of the experiment.

professors comments.

You need to explain what substances should have been in the flask at the end of refluxing. Show a flow

diagram of the purification process. You did an extraction, a drying and a distillation.

Alcohol thermometers are quite accurate in the range they are designed to function. This type of

alcohol thermometer was not designed to work around 142 degrees C.

Treat the NMR and IR spectra as if they were obtained by you for your product. You should have tables

in your Results Section for IR, NMR and GC. You should discuss these results in more detail in the

discussion. What do we learn from each measurement.

Give more details of the many causes for obtaining less than 100% Yield.

Discuss whether this experiment is a good example of Green Chemistry. Does the Atom efficiency make

it a good candidate. Does the Reaction efficiency make it a good example of Green chemistry. What

about other things.

Answer 1

Discussion:

            The purpose of the experiment performed was to synthesize isopentyl acetate, or more commonly known as banana oil, from isopentyl alcohol and glacial acetic acid. Both liquids were combined and remained clear until the addition of concentrated sulfuric acid. The addition of an acid caused a reaction to occur that resulted in a polymer, which gave the mixture color. The liquid turned dark brown when the polymer formed. The polymer was removed via distillation.

            After creating the ester, the biggest challenge was separating it from all the other chemicals in the flask, which include water, acetic acid, sulfuric acid, and isopentyl alcohol.

The process of separation is described in the flow chart. As the isopentyl acetate is not soluble in water it can be separated from the aqueous soluble species (acetic acid, isopentyl alcohol, sulfuric acid). To neutralize the acidic species that remain in the flask, it was added saturated NaHCO₃ aliquots. In this way we deprotonate the acids and we favor their partition in the aqueous phase.

Figure 2.

The by-product of this neutralization with NaHCO₃ is CO₂, which is easily removed from the mixture as gas bubbles.

Figure 3.

The little amount of unreacted isopentyl alcohol is separated by distillation. In this process it was removed the first ten drops of the distill (head of distillation). This is possible because it has a lower boiling point then the ester that is produced. The temperature was recorded for the boiling point of isopentyl acetate. It was found to be at 129 °C. This is low compared to the literature value of 142 °C. The difference is believed to be due the use of an alcohol thermometer which has a limit range of use of 78 ºC. Even when mercury thermometers would be a better option in terms of work range of temperatures, they are banned in the state of New Jersey. The inability to measure the precise value of temperature should not be a problem in this case, while the liquid collected was obtained at a constant range of temperature.

This whole reaction was first done and called the Fischer esterification. The general reaction scheme can be seen above. In the experiment, isopentyl alcohol and acetic acid react in the presence of sulfuric acid to form isopentyl acetate. This reaction mechanism can be seen below in Figure 5.

Figure 5 – Fischer esterification mechanism

In the reaction above, Acetic acid takes a proton from concentrated sulfuric acid. Then it reacts with isopentyl alcohol. Delocalization of electrons and protons in later steps leads to elimination of water molecule to form final product.

While an NMR was not taking during the experiment, nor was an IR spectrum, attached is both an NMR example and an IR example of isopentyl acetate. On the NMR sheet, all hydrogen peaks are labeled on a drawing of the molecule, and all the information is written in the chart.

For the isopentyl acetate, some distinctive spectral characteristics appear in its NMR spectrum. The hydrogen corresponding to the OH group dissapears from the spectrum (compare with the spectrum of isopentyl alcohol). Besides this, the methylene hydrogens in C₄ (the one attached to the O) shift downfield by around 0.5 ppm. The downfield shift arises from the deshielding of these protons because the alcohol group was converted to a more electron-withdrawing group (ester). Another evidence of the formation of product is the presence of a singlet group below 2 ppm which corresponds to the methyl ester group (C₁) of the carboxyl group The remaining signal groups due to protons within the isobutyl group remain practicaly unchanged upon esterification of 3-methylbutanol.

For the IR spectra it is expected to appear a signal at aroun 1740 cm-1 corresponding to the C=O vibration. This signal would confirm the formation of the ester.

On the IR sheet, peaks above 1500 cm^-1 are labeled for what they are believed to be. While the pictures were found from a reputable source, they were not from the experiment performed so one cannot be perfectly sure that the desired substance was obtained.

Hree would go Results Section for IR, NMR and GC, but you did not provided the information. If you attach the pictures to this question I can discuss the result... I´ll be glad to help with this.

The last topic for the reaction was the product loss. Most organic reactions do not complete and have yields below of the 100%. The 42 % yield was most likely due to side reactions occurring in the flask. Since there were several chemicals in the flask together, side reactions are bound to occur, creating a lower yield. For example, a polymer was formed which changed the color of the flask. This was not intended to happen so this reaction lowered the yield of the experiment. Moreover during the extraction, drying and ditillation always there is some product mass loss. The more the steps of the purification, usually the lower the yield.

The reaction involved in this experiment (Fischer esterification) is an excelent example of Green Chemistry. At least in paper. If we consider the reaction of the isopentyl alcohol plus the acetic acid to get the isopentyl acetate we can calculate the atom economy as follows:

Replacing with values

The 87.8 % of atom economy indicates that most of the atoms in the reactants is incorporated into the products. The unique lost in the reaction is because of the formation of water, a small molecule (considered then a by-product). This result is in accordance with the Green Chemistry principle #2 with states that ¨Synthetic methods should be designed to maximize incorporation of all materials used in the process into the final product.¨

Related with Reaction mass efficiency this metric deals with the quality of a reaction in terms of the production of a desired product.

(At this point I do not know the actual values, you used of each reactant and the product formed)

Besides the metrics that could indicate the reaction or process as Green, if we analyse the waste produced and the several steps of separation the reaction is not a very good example of greeness. The sulfuric acid is not used as a catalyst but it is more than in excess. The use of this acid in a non catalytic amount produces the need to use a base as NaHCO3 in the work up (which is also waste at the end). All the water and extra organic solvents used are also residues in the end. The process could become greener if we use some solid acid catalyst to replace the H2SO4 we use. In fact H2SO4 is toxic because SO3 is evolved from the solution.

Hope this hints help you. If you want further help with NMR, IR and GC please add the spectrums.. I will be glad to help with it.