The purpose of this experiment is to derive Isopentyl acetate (banana oil) from the reaction of an alcohol and a carboxylic acid in a process called Fischer esterification (Fischer-Speier esterification). The name, banana oil, is due to banana odour released by formation of Isopentyl acetate. The alcohol and the carboxylic acid used in this experiment are Isopentyl alcohol and Glacial Acetic Acid. Both the overall reaction of Fischer esterification and the specific Fischer esterification reaction that was carried out in this lab is shown in figure 1. It worth mentioning that the by-product of such reaction is water. In organic chemistry, the reaction in which water molecule is released is known as condensation reaction (Hornback, 2006). Figure1 is drawn using Chem Sketch software. a)
Figure 1: Typical Fischer esterification reaction (a) in comparison to the specific esterification reaction used in this lab (b)
The reaction can also be categorized in terms of its mechanism as nucleophilic substitution reaction. The alcohol which contains a hydroxyl group (OH-) has a partial negative charge compared to the alkyl group bound to it which is Isopropyl in this lab. The reason is that oxygen has higher electronegativity compared to carbon. So, there is more attraction to the lone pair electrons around oxygen which makes the hydroxyl group partially negative. Analyzing the structure of the glacial acetic acid, we notice that the hydroxyl group and the oxygen on the carbonyl group will draw electrons towards themselves to a higher extent compared to the carbon which is bound to the both functional groups. Accordingly, the carbon will have a partial positive charge compared to the two functional groups mentioned. In nucleophilic substitution reaction, the electrons of the partially negative hydroxyl group of the alcohol (nucleophile) attacks the partially positive carbon on the acid (electrophile). Eventually, the nucleophile donates its electrons to the electrophile to form a bond. Meanwhile, a leaving group (hydroxyl group of the acid in this case) departs from the acid.
A main characteristic of Fischer esterification reaction is the need for an acid to act as a catalyst in the presence of heat to speed up the reaction. A catalyst is a substance that increase the rate of a chemical reaction by reducing the energy required to by-pass the intermediate step. The catalyst used for Fischer esterification is normally a very acidic organic acid such as toluenesulfonic acid (TsOH) or a strong mineral acid such as concentrated sulphuric acid (H2SO4) (Organic Chemistry Lab Manual CKCH 143, fall 2012). For the purpose of this lab, concentrated sulphuric acid is selected as the catalyst. In addition, there are the option of using cation exchange resins like Dowex which provide the same functionality in terms of reaction rate but are easier to be extracted at the end of the reaction. Using sulphuric acid, we had to use a great deal of base (5% aqueous sodium bicarbonate) to neutralize the acid-catalyzed process. Furthermore, there are strong acids such as trifluoroacetic acid (CFCO2H) and formic acid (HCO2H) which drive the Fischer esterification reaction forward without the need of a catalyst.
Since the reactants and products are in equilibrium, the equilibrium constant shown by Kc determine the progress of the reaction. LeChatelier’s principle is used to drive the reaction forward and increase the yield of the reaction. The equilibrium constant provided for the reaction of either primary or secondary reactant is nearly 4 (Organic Chemistry Lab Manual, CKCH 143, fall 2012). The best plausible yield using equal molar ratio of the reactants is suggested to be 67% of the theoretical yield. There are multiple approaches to boost the yield of the experiment which are proposed based on Le Chatelier’s principle. For the sake of this lab, the preferred method to increase the amount of isopropyl acetate is to use...
References: Hornback, J. M. (2006). Organic Chemistry,2nd Edition., California: Thomson Brooks, p. 858.
Ryerson University. (2012). Organic chemistry laboratory manual CKCH 143.
Weast, Robert C., ed. (1975) CRC Handbook of Chemistry and Physics: A ready-reference book of chemical and physical data (56th ed.). Cleveland, OH: CRC Press, p. C-81, C-86, C-222.
Relative error = (Corrected RI - Literature RI) / Literature RI
Isopropyl acetate --> (1.4008-1.4003) / 1.4003 = 0.000357 ~ 0.00036
Isopropyl alcohol --> (1.4046-1.4053) / 1.4053 = 0.000498 ~ 0.00050
Please join StudyMode to read the full document