Lab Report Preparation of Alcohols Reduction of 9-Fluorenone and Lucas Test for Alcohols

Preparation of Alcohols: Reduction of 9-Fluorenone and Lucas Test for Alcohols Introduction The purpose of this experiment is to successfully synthesize 9-flourenol by reduction of 9-flourenone using sodium borohydride (see figure one). Next, 9-fluorenol is then purified by recrystallization to calculate the percent yield, and the alcohol formation is confirmed using the Lucas test. Finally, the product is analyzed using IR and NMR spectroscopies and characterized by melting point. In reduction [H] reactions, a carbon atom replaces a C-H bond with a covalent bond to an electronegative atom (e.g., oxygen or nitrogen), thus increasing the electron density. In addition, reactions where new C-H bonds are formed by addition of one or more hydrogen atoms to a functional group are considered reduction reactions. For reduction reactions, the oxidation number on the carbon atom decreases, in contrast to an oxidation reaction where the oxidation number increases. In this experiment, 9-flourenone to 9-fluorenol is a reduction reaction because the oxidation state of carbon goes from +2 to 0 (see figure two). To determine the oxidation state of a carbon atom, there are three rules as follows: for every atom bound to C that is more electronegative than C, add one; for every atom bound to C that is less electronegative than C, subtract one; for every time C is bound to another C, add zero. Finally, add the numbers together to obtain the oxidation state. Reducing agents are oxidized over the course of a reaction and in turn reduces another molecule. Commonly used reducing agents include, molecular hydrogen (H2), lithium aluminum hydride (LiAlH4), and sodium borohydride (NaBH4). For this experiment, we are using sodium borohydride as the reducing agent. Sodium borohydride is used because it works by a different mechanism of catalytic hydrogenation and is less potent than lithium aluminum hydride. In this experiment, Thin-Layer Chromatography (TLC) is used to monitor the reaction process to ensure that the reaction is complete by comparison of the Rf values of the first spot, reactant spot (pure 9-fluorenone), and the third spot, the spot of the reaction mixture. The second spot (middle spot) is the “co-spot”, in which consists of the both the reactant and the reaction mixture. By comparison of the reaction mixture (spot 3) and the reactant spot (spot 1), the Rf values must differ between the two spot for the reaction to be complete and both spots (1 and 3) must be a single spot. See figure three for an illustration of an incomplete versus an incomplete reaction. Similar to the silver nitrate test for alkyl halides, the Lucas test for alcohols allows us to distinguish between primary, secondary and tertiary alcohols via SN1 mechanism. Recall that with SN1 reactions, the rate of reactivity increases with increasing substitution (3°>2°>>>1°). Primary alcohols are unreactive with the Lucas test. In this experiment, 9-fluorenol, a secondary alcohol, is compared to other known primary, secondary and tertiary alcohols. The Lucas test uses a mixture of ZnCl2 and HCl in order to increase the reactivity between the alcohol and the acid to yield a carbocation, which is also the rate-determining step. The identification of a positive result is when the color of the alcohol solution changes from clear to cloudy upon addition of the Lucas reagent. Observations such as turbidity and rate at which the change occurred in order to distinguish between a tertiary and a secondary alcohol. Tertiary alcohols will react almost instantly, as opposed to secondary alcohols in which may take five to twenty minutes to react. In order an alcohol to be an appropriate testing sample, it must be soluble in the Lucas reagent and contain six carbons or less. Equations & Reactions !"#$%& ()*&+ Percent Yield: ! ,-*./*#)"%& ()*&+" ∗ 100% Retention Factor: '0 = +)1#%2"* #/%3*&&*+ 45 1$41#%2"* +)1#2%"* #/%3&&*+ 45 1.&3*2# = +! +" Figure 1. The image above is the reduction reaction of 9-fluorenone using sodium borohydride to produce 9-fluorenol. This image is from the UIC Organic Chemistry Lab Manual, pg. 127. Figure 2. This image is an illustration of the changes of the oxidation states of carbon in the reduction reaction. The molecule on the left is 9-flourenone and the molecule on the right is 9-flourenol. This image is from the UIC Organic Chemistry Lab Manual, pg. 134. Figure 3. This image is a comparison of two TLC plates in which one reaction is complete and the other is incomplete. This image is from the UIC Organic Chemistry Lab Manual, pg. 135. Procedure Reduction of 9-fluorenone Before beginning any experiment, safe lab attire, goggles and gloves are mandatory. To start, a sample of 9-fluourenone was measured with a graduated cylinder and added to an Erlenmeyer flask where it is dissolved in methanol using heat. Swirl the flask to ensure the 9fluorenone is completely dissolved and remove the heat to allow the flask to cool to room temperature. Sodium borohydride (NaBH4) is then added and the flask in one portion and is swirled vigorously for about fifteen minutes to dissolve the reagent. During this time, the yellow solution should turn colorless. Next, a TLC plate is prepared to ensure that the reaction is complete. To start, the TLC chamber contains a TLC solvent (1:9 ethyl acetate:hexanes) just so that it just cover the bottom of the beaker. The TLC plate is spotted with three spots from left to right: starting material (9flourenone), the co-spot, and the reaction material. Once the TLC plate has been spotted, it is out inside the TLC chamber with a watch glass covering the top of the beaker. The solvent is then allowed to elute up the TLC plate until it reaches about 1cm from the top where it is them removed from the TLC chamber and the solvent line is marked with a pencil. In order to see the spots on this TLC plate, it must be viewed under UV light, where you can then mark the spots with a pencil to calculate the Rf values of 9-fluorenone and 9-fluorenol. See figure three for an illustration of TLC plates for reactions that are complete and incomplete. Once the Rf values have been calculated, the reaction is quenched, or worked up, by adding 3M sulfuric acid (H2SO4) in parts to the reaction mixture using a graduated cylinder and gently swirled in between additions. Adding the sulfuric acid in parts prevents fizzing from the excess borohydride reacting with the acid. The flask is then clamped to a ring stand and a heating mantle is raised to heat the product a redissolve the precipitate. If necessary, more methanol can be added with heating until all of the solid has dissolved. Be sure to cover the flask with a watch glass to minimize solvent loss. Once the precipitate has been redissolved, the flask is removed from the heat, allowed to cool to room temperature and put on ice for about five to ten minutes until the solid precipitates. Next, the product is vacuum filtered and washed thoroughly with water. The purpose of washing with water is to remove the sulfuric acid (H2SO4). The 9fluorenol is washed with water until the pH of the water exiting the Buchner funnel is neutral (pH = 7) to ensure all acid has been removed from the product. Then, the product is dried using the vacuum. Once all of the acid has been removed from the product and is dried, the crude 9fluorenol is recrystallized using methanol as the recrystallizing solvent. To do this the crude 9fluorenol is added to a beaker and dissolved in a minimal amount of hot methanol using the ring stand and heating mantle. Once 9-fluorenol has been dissolved the solution is allowed to cool to room temperature and put on ice for crystals to form. If no crystals begin to form, then more solvent should be boiled off. The crystals are then filtered via vacuum filtration as suction is applied to remove any remaining solvent. After the crystals have been filtered and suction dried, the mass can be taken and recorded to calculate the percent yield. Finally, the last steps consist of the analysis of 9fluorenol using the Lucas test, IR and NMR spectroscopies. Analysis of 9-fluorenol Lucas Test For the Lucas test, we will be comparing our product, 9-fluorenol, a secondary alcohol, to two other alcohols: a tertiary alcohol, t-butanol, and a primary alcohol, 1-butanol. To start, three test tubes are obtained and about 1mL of each alcohol are added to their designated test tubes. Next, about 1mL of Lucas reagent was added to each test tube and mixed. Finally, observations are made and recorded based on the reactivity between the alcohols and the Lucas reagent. It is expected to have a negative result for the primary alcohol and positive results for the secondary and tertiary alcohols. IR Spectroscopy Once the Lucas test is complete, 9-fluorenol is then analyzed using IR spectroscopy. Before analyzing the product, a background scan must be taken first in order to tare the system. Once the background scan is complete, a small amount of 9-fluorenol is then added to the ATR optical sampler and the hammer is lowered to press the solid down to minimize air gaps. The spectrum is then taken, and the major peaks are analyzed. It is expected to see a major peak around 3000 cm-1 to represent the -OH functional group in the spectroscopy of 9-fluorenol and a peak around 1720 cm-1 to represent a ketone in the spectroscopy of 9-fluorenone. Results O" ! OH" 1 !. NaBH!4, CH!3OH! 2 . 3 M H!2SO4!! ! ! ! ! MW, g/mol d (g/mL) or M (mmol/mL) Rxn Weight or V (g or mL) mmol Equivalents 9-Fluorenone 180.19 - 6.73 g 37.3 mmol 1.00 NaBH4 37.83 - 0.707 g 18.7 mmol 0.5 CH3OH 32.04 0.792 g/mol 68.0 mL 1,680 mmol 45 Compound 1.00 g/mL 7.32 mL 74.6 mmol 3 mol/L Table 1. The table above are the details included in the reduction reaction, such as molecular weight, reaction weight, equivalents, etc. H2SO4 98.08 2.0 Figure 4. Calculations of mmol and equivalents for the starting material and reagents shown in table one. Figure 5. TLC plate. From left to right the spots are labeled 1-3. Spot 1 is the spot of our starting material, pure 9-fluorenone. Spot 2 is the “co-spot” that contains spots from both the starting material and the reaction mixture. Spot 3 is the spot of the reaction mixture. Figure 6. IR spectra of 9-fluorenone Figure 7. 1H NMR of 9-fluorenone Figure 8. IR spectra of 9-fluorenol. ! Figure 9. 1H NMR of 9-fluorenol. ! Table 2. This table shows to results of the Lucas test for the primary alcohol (1-butanol), secondary alcohol (9fluorenol), and tertiary alcohol (t-butanol). Data Analysis As shown in the calculations in figure four, using a percent yield of 71%, the actual yield of the product isolated, was calculated to be 2.42g. The theoretical yield of the product is 3.41g. To find the actual yield given the percent yield, the theoretical yield was calculated using the calculated data from table one using the mmol of our limiting reagent, sodium borohydride, and the molecular weight of 9-fluorenol. Then using the percent yield equation as shown in the equations section, it was used to find the actual yield of the product, 9-fluorenol. The results for the TLC plate that was carried out can be seen in figure five. The middle spot is the co-spot, the first spot is the starting material and the last spot is the reaction spot. As seen in the results, there is one spot for line one, and one spot for line three in which both spots have equal Rf values to their corresponding co-spot. This shows that the reaction is complete, and our product has been isolated. As expected the Lucas test, the primary alcohol tested negative while the secondary and tertiary alcohols tested positive (see table two). The tertiary alcohol proceeded at a significantly faster rate than the secondary alcohol. These results can be explained by the fact that the Lucas test proceeds via SN1 and the stability from most stable to least stable is as follows: tertiary alcohol > secondary alcohol > primary alcohol. For the IR spectroscopy of 9-fluorenone (figure six), as expected there is a carbonoxygen double bond around 1720 cm-1 to represent a ketone. There are also other major peaks around 1450 cm-1 - 1600 cm-1 that represent two aromatic compounds. These peaks for aromatic compounds can also be seen in the IR spectroscopy for 9-fluorenol (figure eight). However, for the spectroscopy of 9-fluorenol, there is a medium peak around 3300 cm-1 to represent the alcohol group. This -OH peak was expected to be stronger and broader. The measured melting point range of the product 9-fluorenol was 143°C-146°C. The literature melting point range is 152°C-155°C. The vast difference of the measured melting point versus the literature value in this experiment could be due to the fact that the sample is not pure enough and contains too many impurities in which melt at a lower temperature than compounds that are more pure, and do not have any impurities. Conclusion The goals of this experiment were to successfully synthesize 9-flourenol by reduction of 9-flourenone using sodium borohydride (see figure one). Next, 9-fluorenol was then purified by recrystallization to calculate the percent yield, and the alcohol formation is confirmed using the Lucas test. Finally, the product was analyzed using IR and NMR spectroscopies and characterized by melting point. In sum, the overall product of 9-fluorenol that was isolated was 2.42g with a 71% yield. Issues could have arisen during multiple points of this experiment in order to cause a lower percent yield and a lower than expected melting point. In this case, it seems as if the compound was not purified enough during the recrystallization process, thus the product contained too many impurities in which skewed the results. The TLC plate that was carried out in the middle of the experiment was successful in showing that the reaction that had taken place was complete. The IR spectroscopies for both the starting material and the product overall were to be expected with no major issues. The Lucas test results were also as expected given this test using an SN1 mechanism and will only react with secondary and tertiary alcohols, but not primary alcohols. Post-Lab Questions 1. (2.5 pts) For each of the reactions shown below, indicate whether it is a net reduction, an oxidation, or neither and calculate the change in oxidation number for any carbon being reduced or oxidized. 2. (2 pts) Using curved arrows to symbolize the flow of electrons, write the mechanism for the steps involved in the conversion of 9-fluorenone to 9-fluorenol. 3. (1.5 pts) After the reaction between sodium borohydride and the ketone is complete, the reaction mixture is treated with water and H2SO4 to produce the desired alcohol. Explain the reaction by clearly indicating the source of the hydrogen atom that ends up on the oxygen. Sodium borohydride is a reducing agent for carbonyl compounds such as ketones to produce an alcohol. A hydride ion from sodium borohydride acts as a nucleophile and attacks the carbon-oxygen double bond, thus oxygen loses the double bond to carbon and becomes an alcohol. In other words, the solvent or acid is the source of the hydrogen atom that ends up on the alcohol. 4. (1.5 pts) 9-fluorenone is colored (bright yellow color), but 9-fluorenol is not (white). What accounts for this difference? When the keto group of 9-fluorenone absorbs lights, it releases energy at visible region due to the transitions in wavelength of the light changes causing a color appearance. 9-fluorenone contains a alcohol group in which does not absorb light (colorless) at visible region due the lack of π-π* bonding electrons. 5. (1 pt) What functional groups can be reduced by a. lithium aluminum hydride, LiAlH4? Lithium aluminum hydride can reduce aldehydes, ketones, imines, carboxylic acids, esters and amides. b. sodium borohydride, NaBH4? Sodium borohydride can reduce aldehydes, ketones and imines. 6. (1.5 pts) Esters are normally unreactive toward sodium borohydride, but they react readily with lithium aluminum hydride to produce alcohols. a. Propose a rationale for the lack of reactivity of esters toward sodium borohydride. Sodium borohydride is a weak, stable and less reactive reducing agent that reduces polar π bonds. Selectivity is more than both lithium aluminum hydride and H2/[M]. Sodium borohydride can reduce aldehydes and ketones, but it cannot reduce esters. b. Propose a rationale for the greater reactivity of lithium aluminum hydride over that of sodium borohydride. Lithium aluminum hydride is a strong, unstable and reactive reducing agent that reduces polar π bonds. Selectivity is less than that of sodium borohydride but more than H2/[M], and can reduce aldehydes, ketones and esters. Sodium borohydride cannot reduce esters. c. Write the structures of the organic products that will be formed when methyl benzoate is treated with lithium aluminum hydride and the reaction mixture is worked up with aqueous acid.