Preparation of Alcohols Reduction of Fluorenone and Lucas test for Alcohols PDF

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Preparation of Alcohols: Reduction of Fluorenone and Lucas test for Alcohols

Partners: Victor, Zahra 04/05/2016 Methods and Background: The objective of this lab is to prepare 9-fluorenol by reducing 9-fluronone using sodium borohydride and methanol. The product is purified by the process of recrystallization and determined by its melting point. In accordance, the Lucas test is used to determine the differences in-between primary, secondary or tertiary alcohols. In the end, IR spectroscopy will be used to determine the different functional groups in expected product.

Figure 1

Reductions, and its counterpart oxidations, are common organic and inorganic chemistry reactions. The formal name for a redox reaction is "oxidation reduction reaction," and you can see that "redox" is just shorthand for the words reduction and oxidation. Thus, in a redox reaction, two things happen. Oxidation and reduction, these two have to happen together. One cannot have an oxidation reaction without a corresponding reduction reaction. Oxidation is the loss of electrons and reduction is the gain of electrons. Because electrons are negatively charged, an increase in electrons means a decrease in overall charge (the compound becomes more negatively charged). On the other hand, an atom that is oxidized has given up some of those negatively charged electrons, which will increase its overall charge (the compound becomes more positively charged). Notice that these definitions do not involve oxygen and hydrogen. Thus, redox reactions can occur with compounds that do not contain oxygen or hydrogen atoms. Carbon makes covalent bonds and therefore doesn’t formally gain or lose electrons. What happens is the electron density around carbon changes. If the carbon atom changes from more electronegative to least electronegative then the electron density around carbon increases and the reaction is reduced. Vice versa that is how you also get an oxidized reaction as well. The easy way to determine if a species is oxidized or reduced in an organic reaction is to calculate the

oxidation number of the carbon atom that is changing over the course of the reaction. If the oxidation state increase, then the reaction is oxidized and if the carbon atom is losing electrons than is the reaction is reduced. The oxidation state of the atom is a method of electron counting and considers the charge of the atom in an ionic bond state. An everyday example of a redox reaction that we are all familiar with is the process of rusting. Reducing agents are species that react and cause another molecule to become reduced, species that are oxidized over a course of a reaction. Many species can act as a reducing agent, but the three most common reducing agents are molecular hydrogen (H2), sodium borohydride (NaBH4), and lithium aluminum hydride (LiAlH4). Sodium borohydride is a good reducing agent. Although not as powerful as lithium aluminum hydride (LiAlH4), it is very effective for the reduction of aldehydes and ketones to alcohols. By itself, it will generally not reduce esters, carboxylic acids, or amides (although it will reduce acyl chlorides to alcohols).

*Notice the breaking of a C-O bond and replacing it with a C-H bond. This classifies the reaction is a reduction.

Lithium aluminum hydride (LiAlH4) is less reactive. Sodium borohydride is really useful for one thing: it will reduce aldehydes and ketones. In this sense it traverses one rung on the oxidation ladder. Rust is the flaky brown substance that forms on iron objects left exposed to the elements for too long, especially if the objects get wet. Rust doesn’t just form on the iron object, the iron actually turns into rust (rust is actually a form of oxidized iron).

Below are an example of a reduction mechanism and the use of the reducing agent of Sodium Borohydride

The mechanism of the reaction of sodium borohydride with aldehydes and ketones proceeds in two steps. In the first step, H(-) detaches from the BH4(-) and adds to the carbonyl carbon. This forms the C-H bond, and breaks the C-O bond, resulting in a new lone pair on the oxygen, which makes the oxygen negatively charged. In the second step, a proton from water (or an acid such as NH4Cl) is added to the alkoxide to make the alcohol. This is performed at the end of the reaction, a step referred to as the workup. The Lucas test for alcohols is a classification test to see which alcohol is primary, secondary, and or tertiary. It is almost similar to the silver nitrate test and it operates in the Sn1 mechanism aspect. It takes a mixture of ZnCl2 and HCl and increases the reactivity towards the alcohol. The rate determining step is the carbocation stability, so the reaction will occur the fastest if it were tertiary, then slowing down to secondary, and little to no reaction if the alcohol is primary. A positive test depends on the fact that the alcohol is soluble in the reagent, whereas the alkyl chloride is not; thus the formation of a second layer or an emulsion constitutes a positive test. Experimental Procedure: 5.04g of 9-fluorenone was to a 50 mL Erlenmeyer flask and add the calculated amount of methanol, which turned out to be 50.5 mL. Swirl and heat the flask until the fluorenone dissolves. Next, cool the solution to room temperature, weight the calculate amount of sodium borohydride (0.52g) and add it to the reaction and swirl the flask. Do not stopper the NaBH 4 because it creates H2 gas when mixed and the stopper will pop off. The reaction should become colorless after about 10 minutes, if not add more NaBH 4 and continue to swirl the mixture. We added 0.21g of more NaBH4. Before working up the reaction, take a TLC on a silica plate using 1:9 ethyl acetate : hexanes as your eluant to confirm that the reaction is complete. The TLC plate should contain three lanes: 1) pure 9-fluorenone; 2) pure 9-fluorenone + reaction mixture (co-spot); 3) reaction mixture.

Examine the TLC plate under UV light and circle the spots that are observed. Calculate R f values of 9-fluorenone and 9-fluorenol. With the reaction, add 3M sulfuric acid to the mixture and gently heat the flask for about 5-7 minutes. To minimize solvent loss, add a watch glass on top of the flask when heating. If for any mean the solvent isn’t dissolving add more methanol in (0.5-1mL portion). Then, remove from the heat and cool to room temperature and then soak the flask mixture into an ice bath until a solid precipitates forms, thus should roughly take 5-10 minutes. Next, filter the solid and wash thoroughly with the water, to see if the reaction is set, test it with a pH paper to ensure it is neutral. Dry the product attached to vacuum. Then the next step is to recrystallize the final product by adding hot methanol to the mixture and precipitates to form, once completes filter out the solid using vacuum filtration. Weight the final product and perform the melting point analysis, IR spectroscopy. For Lucas test, prepare three test tubes by adding 1 mL of the Lucas reagent to each. To one test tube, add ~5 drops of the tertiary alcohol. A positive test is indicated by the solution becoming cloudy. Note how quickly this occurs. Repeat the procedure for the secondary and primary alcohol. Compare the results of the test for primary, secondary, and tertiary alcohols.

Data Acquisition/Calculations: Table 1: Summary of Reaction Table Compound

Molecular Weight

mmol

Equivalents

180.19

D (g/mL) or Rxn Weight M or Volume (mmol/mL) (g or mL) n/a 5.0 g

9Fluorenone Sodium borohydride Methanol Sulfuric acid

27.75

1.0

37.83

n/a

0.52 g

13.87

0.5

32.04 98.08

0.792 g/mL 1.00 g/mL 3.00 mol/L

50.5 ml 18.5 ml

1248.65 55.50

45 2.0

Table 2: Lucas Test Observations

Tertiary

Changed fast, to a cloudy white color

Secondary

2 minutes, changed to a white smoky color

Primary

No reaction

Table 3: Melting point of 9-Fluorenol: 154 degrees Celsius

Melting Point (C) Table 4: IR Analysis of 9-Fluorenol: Wavenumber (cm-1) 738.42 2901.31 3334.66

Functional Group Aromatic ring unsat. C-H O-H

Positive test Appearance of a cloudy second layer or emulsion   

3o alcohols: immediate reaction 2o alcohols: 2 minutes 1o alcohols: no reaction

Percent yield of Products: The percent yield will help in determining the success of this experiment. The formula for calculating the percent yield as follows, Percent Yield = [(Actual Yield) / (Theoretical Yield)] x 100

Weight of the distillate (Actual Yield) = 3.69 g Theoretical Yield = (27.75 * 10-3 mol of 9-fluorenone) * (4 mol of 9-fluorenol / 4 mol of 9fluorenone) * (182.22 g of 9-fluorenol / 1 mol of 9-fluorenol) = 5.06 g 9-fluorenol Percent Yield: [(3.69 g) / (5.06 g)] x 100 = 73 %

( CONTINUED) Finding Rf values: 9-fluorenone (reagent— the left spot): 2.5 cm = 0.56 4.5 cm 9-fluorenol (product— the right spot): 1.2 cm = 0.27 4.5 cm

Conclusion; In conclusion, the purpose of this lab was to produce 9-Fluorenol by reducing 9-fluorenone using sodium borohydride. We then performed the Lucas test to classify the different types of alcohols, according to primary, tertiary, and or secondary. To distinguish the different types of functional groups, we then did IR spectroscopy and table 4 shows our results from the IR. Based on the results, the lab was successful. The yield of the final product was 73 %. Even though, the percent yield is not so close to 100%, it was expected because the mass of product us likely to go down after recrystallization because while the chilled solvent is saturated and should release some crystals, at least some of desired material will remain dissolved in the cold solvent and will be lost when the crystals and solvents are separated. Other possible error can be using too much solvent; inadequate cooling; inadequate warming; or excessive washing by solvent. Upon the testing of our reaction, we had to run a TLC to see if our reaction was progressing. We prepared a TLC plate and marked three spots on it, a product, reactant, and a co-spot. We then ran the TLC under in the solvent of 1:9 ethyl acetate: hexane, and observed these Rf values seen in our data and calculations section of this report. We can see that are co-spot and reagent moved up the TLC plate with Rf value of 2.5 cm, this let us know that our reaction was in progress. We also noticed that or product “drop” did not move up the tlc plate which was what we expected to

happen. The completion of the IR spectroscopy helped distinguished the many different functional groups in the expected product. As seen above in table 4, we identified an alcohol stretch at, 3334.66 cm-1, and we also found an aromatic ring stretch at 738.42 cm-1, so that also proved that our experiment was successful. The Lucas test was performed to see and classify the alcohols in the product. Since the test runs under a Sn2 environment, the rate determining step is the formation and stability of the carbocation. So the tertiary carbocation is the most preferred and stable for a formation of an alcohol and the stability will decrease if it were a secondary or and be even less stable if it is a primary alcohol. Based on our observations seen above in table 2, when we mixed the Lucas reagent and the reaction mixture, we got a quick cloudy, smoky color upon adding the reagent; this tells us that is was tertiary alcohol, which is consistent with a positive test of the Lucas test.

Reference Gilbert, John C., and Stephen F. Martin. Experimental Organic Chemistry. Cengage Learning, Massachusetts, 2011, 5th Ed, pp. 545, 584-586....