28 November 2012 Experiment 15: Fischer Esterification and Combinatorial Chemistry Phill Rasnick Introduction Esterification reactions, which produce esters, have proven to have great importance in organic chemistry. Esters are characterized by a carbonyl with an adjacent ether linkage. Acting as hydrogen-bond acceptors, esters can participate in hydrogen bonding to other molecules. There are several methods in which which esters can be synthesized. The most common ester syntheses occur through reactions with carboxylic acids. One mechanism in which a carboxylic acid is converted into an ester takes place through SN2 reaction of a carboxylate anion with a primary alkyl halide. Acid-catalyzed nucleophilic acyl substitution can also form an ester from a carboxylic acid and alcohol; this process is known as Fischer esterification. Esters can also be formed by alcoholysis where acid chlorides react with alcohols in the presence of base to neutralize the HCl formed. Acid anhydrides can also form esters also by alcoholysis. Additionally, esters can be converted in organic reactions to form other types of molecules. For example, esters can be converted to carboxylic acid by hydrolysis; in a basic solution this process is known as saponification because it is a method in which soap is synthesized through boiling fat with base in order to hydrolyze the ester linkages within the fats. Esters can also be converted to amines through aminolysis, and to alcohols by both reduction and through a Grignard reaction. 1 Esters are very abundant in nature. Naturally occurring fats are often composed by esterification of glycerol. 2 These fatty acids are generally in the form of triglycerides. Experimentally these triglycerides can be formed by a transesterification reaction between the methyl esters of the fatty acids and glycerol in the presence of sodium hydroxide. Cholesterol is another type of lipid which is transported in the body as esterified cholesterol. 3 The esterified
2 cholesterol is transported in vesicles called chylomicrons which transport dietary lipids and are composed of triglycerides, phospholipids, cholesterol, and proteins. Esters are also an important component of fragrances. For example, sex pheromones are composed with ester linkages. 4 Additionally, ester bonds are a very important component of deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) due to phosphodiester bonds. 5 Phosphodiester bonds are responsible for attaching the 3 carbon of the sugar deoxyribose or ribose to the 5 carbon of another sugar molecule creating the backbone of the DNA or RNA. Esters have also been shown to have medicinal value. Polyester dendritic systems have proven to be of value in drug delivery. 6 Polyester dendritic systems are large macromolecules composed of numerous monomer units. These polyesters are good candidates for drug delivery due to their properties being water soluble and nontoxic. In addition to its medicinal use as a drug delivery system, esters also have industrial implications. They can be used in the formation of certain plastics. Polyester plastics often compose flexible and rigid foams, rubber-like materials, lacquers, coatings, and films. 7 Polyesters are also used in producing capacitors as dielectrics. 8 In this experiment, an ester is synthesized by method of a Fischer esterification reaction which occurs between a known carboxylic acid and an unknown alcohol. Synthesis of an ester through Fischer esterification takes place by a nucleophilic acyl substitution, catalyzed by the presence of strong acid as depicted by scheme 1.
3 Scheme 1: Electron pushing mechanism for Fischer esterification In the first step of the reaction, the chosen carboxylic acid, propionic acid, is protonated by sulfuric acid. This step activates the carboxylic acid allowing the carbonyl to be a good electrophile. Following the protonation of propionic acid, the acid then forms a bond with the oxygen of the alcohol, creating a tetrahedral intermediate; this bonding occurs by nucleophilic attack by the alcohol on the acyl group of the carboxylic acid. Proton transfer is then responsible for transferring a proton from the positively charged oxygen of the alcohol group to one of the hydroxyl groups forming a better leaving group. Deprotonation of the other hydroxyl group forms a double bond between the oxygen and carbon, creating an ester and expelling water as a leaving group.
4 The purpose of the experiment was to synthesize an ester by way of Fischer esterification between a known carboxylic acid and an unknown alcohol. This reaction proceeds by acid catalyzed nucleophilic attack by the alcohol. The ether layer is then washed and the ester product is isolated. The synthesized ester is then analyzed by fragrance, 60MHz 1H NMR, IR, GC, and GC-MS to identify the ester produced and the composition of the unknown alcohol. Experimental In a 5ml microscale round-bottom flask, the unknown alcohol(0.650 ml) is added followed by carboxylic acid(6 mmol) and a micro stir-bar. The unknown alcohols labeled A-D could be any of the following; 1-butanol, isobutanol, isoamyl alcohol, or neopentyl alcohol. The carboxylic acid chosen to be used in the reaction was propionic acid, and the alcohol used was unknown alcohol A. 5 drops of concentrated sulfuric acid was then added; an air condenser was attached, and the solution was gently refluxed with stirring for one hour. After refluxing, the reaction mixture as allowed to cool to room temperature. Upon reaching room temperature, ether (2mL) is added to the round bottom flask. The contents of the flask were then transferred to a separatory funnel using a Pasteur pipette. The flask was then further washed with ether (4 ml) and the contents were also added to the separatory funnel. The ether layer was then extracted four times first by aqueous sodium bicarbonate(5%, 5 ml), and then again with distilled water(5 ml); the ether layer will remain on top. After extracting, the ether layer is dried over sodium sulfate for 10 minutes and decanted into another flask. The ether layer was then evaporated under a stream of nitrogen. The resulting ester product is then analyzed by means of 60MHz 1H NMR, IR, GC, and GC-MS. On completion of the reaction, the ester had a rather fruity fragrance with 1 H NMR (60MHz, CDCl3) δ (ppm) 4.183-3.976 (t, 2H), 2.510-2.140 (q, 2H), 1.897-0.488 (multiplet,
5 13H); IR (ATR) υmax (cm -1 ) 2960.6, 2875.7, 1736.4, 1181.8; GC (propanoic acid, butyl ester, 40ºC to 275ºC at 10ºC per min) RT 8.56; GC-MS (propanoic acid, butyl ester, 40ºC to 275ºC at 10ºC per min) RT 8.85, m/z 130. Results and Discussion An ester was synthesized by Fischer esterification between propionic acid and an unknown alcohol (alcohol A). The product ester was noted to give off a fruity scent and had a calculated percent yield of 67%. To test the success of the reaction and to analyze the composition of the product to identify the unknown alcohol, 60 MHz 1 H NMR as well as IR, GC, and GC-MS data were collected on the product. 1 H-NMR, using chloroform-d as the solvent, proved to be a beneficial method in helping to determine the final ester product (Figure 1, supplemental information). A triplet at a ppm of 4.183-3.976 represents the two Hc protons on the carbon adjacent to the oxygen of the ester. This triplet is caused by the two Hd protons of the adjacent alkyl carbon. The presence of this triplet enables the possible alcohol precursors to be narrowed down. It is evident that isobutanol and neopentyl alcohol are not possible precursors as these would produce a splitting patterns of a doublet and singlet respectively for the Hc protons, rather than a triplet. The next downfield peak is a quartet in the region of 2.51-2.14 which corresponds to the two Hb protons on the carbon adjacent to the carbonyl group. The quartet splitting pattern reassures propionic acid as the starting carboxylic acid because the pattern is caused by the three alkyl Ha protons of the adjacent carbon. The large multiplet from 1.897-0.488 is caused by the alkyl protons Ha,d,e,f. The integral value for this peak is unusually high with a value of 13 making it hard to identify the actual ester product. The high integral value may have been caused by either contamination or
6 error when integrating the peaks. Ideally, 400MHz 1 H-NMR could be used to further analyze the peaks and components of the multiplet which could enable better identification of the product. While the NMR proved to be helpful in analyzing the composition of the product, the NMR also indicates that there is no carboxylic acid starting material present indicated by the lack of a peak within the 11-12 ppm range which would pertain to the alcohol proton of the acid. IR data is further used to analyze the sample (Figure 2, Supplemental Information). Peaks at wavenumber 2960.6 and 2875.7 indicate the presence of alkyl groups in the product. The presence of the carbonyl group is indicated by the large peak at 1736.4 and the large peak at the wavenumber 1181.8 is designated to the C-O bond of the ester. In addition to these peaks, the lack of large broad peaks in the region of 2500-3600 indicates that there is no presence of carboxylic acid or alcohol precursors in the ester product. GC and GC-MS also proved to be useful instruments in analyzing the ester product. GC data shows one significant peak with a retention time of 8.56 (Table 1, Figure 3, supplemental information). Component RT Area m/z %composition Propanoic acid, 8.56 min 20076 130.07 100% Butyl ester Table 1: GC, GC-MS data for propanoic acid, butyl ester The sole peak beyond the dichloromethane peaks indicate that the product is composed of only one component, thus implying the purity of the ester product. Mass spectroscopy proved to be the most useful tool in analyzing the composition of the final ester product (Figure 4-9,
7 supplemental information). Mass spectroscopy allows for the sample to be bombarded by an electron beam creating radical cations which can then be sent through a magnetic field to a detector which analyzes the different masses of the fragment ions (Figure 5,8, 9, supplemental information). The fragmentation pattern is then matched using a library of samples to identify the product (Figure 6, supplemental information). The parent peak corresponds to the unfragmented cation radical and is the peak at the largest m/z value. The parent peak is located at a m/z value of 130 meaning that the molecular weight of the unfragmented product is roughly 130 g/mol; this peak is relatively small due to the unstable properties of the ester. The base peak is the largest peak which is assigned to 100% intensity. For the synthesized product, the base peak is found at a m/z value of 57. The peak at this m/z is most likely caused by an ethyl-co radicle or a butyl radicle (Figure 1) Figure 1: Base peak ion fragment for GC-MS of propanoic acid, butyl ester
8 The experimental MS data was then matched to a library of data, positively identifying the ester product to be propionic acid, butyl ester (figure 7, supplemental information). By knowing the composition of the starting carboxylic acid (propionic acid) and the final ester product (Butyl Propionate), the alcohol reactant can be determined. By observing the structures, it is apparent that the unknown alcohol used, alcohol A, is identified to be 1-butanol. The properties of esters make them very abundant in nature and give them a valuable role in medicine as drug delivery systems, and as precursors to many commonly used plastics. In nature, ester linkages have proven to be very important from the formation of fatty acids and other lipids to the phosphodiester linkages forming the backbone of DNA and RNA molecules. This experiment has shown one of the more simple ways of creating esters through Fischer esterification of carboxylic acids and alcohols in the presence of acid catalyst. Using NMR, IR, GC, and GC-MS, the purity of the sample as well as the composition of the product was able to be analyzed. The data was not only able to identify the final product, but the purity was also analyzed proving this technique to be valuable and ease of synthesizing and isolating pure esters.
9 References 1 McMurry, J.Organic Chemistry, 8 th ed.: Brooks/Cole, 2012. 815-843, 1092-1093. 2 Mattson, F.H.; Volpenhein, R.A. Rearrangement of Glyceride Fatty Acids During Digestion and Absorption. Journal of Biological Chemistry. 1962. 237. 53-55. 3 Deykin, Daniel; Goodman, DeWitt. The Hydrolysis of Long-Chain Fatty Acid Esters of Cholesterol wit Rat Liver Enzymes. Journal of Biological Chemistry.1962. 237. 3649-3656. 4 Jewett, D.M.; Matsumura, F.; Coppel, H.C. Sex Pheromone Specificity in the Pine Sawflies: Interchange of Acid Moieties in an Ester. Science. 1976. 192. 51-53. 5 Olivera, B.M.; Lehman, I.R. Linkage of Polynucleotides Through Phosphodiester Bonds by an Enzyme from Escherichia Coli. Proc Natl Acad Sci U S A. 1967. 57(5). 1426 1433. 6 Padilla De Jesu s, O.L.; Ihre, H.R.; Gagne, L.; Fre chet, J.; Szoka, F.C. Polyester Dendritic Systems for Drug Delivery Applications: InVitro and In Vivo Evaluation. Bioconjugate Chem. 2002, 13, 453 461 7 Holtschmidt, H. Polyester Plastics Stabilized with Carbodimides. United States Patent Office. 1965. 3,193,524. 8 Frame, N. Capacitance Switch. United States Patent. 1983. 4,373,124. Supplemental Information 60 MHz 1 H-NMR, IR, GC, GC-MS