Lab 8 Reduction PDF

Preview

Summary

Lab 8...

Description

Vanessa Nichols CHEM 233 04/15/2021 Maxim Radzhabov Lab 8: Preparation of Alcohols: Reduction of Fluorenone and Lucas Test for Alcohols Methods and Backgrounds

Figure 1: Mechanism of 9-Fluorenone to 9-Fluorenol.

The purpose of the laboratory was to use 9-fluorenol by reducing 9-fluorenone through reduction using sodium borohydride and methanol. Through the process of recrystallization, the product was purified, and the melting point was determined. The reaction process was analyzed through thin layer of chromatography (TLC), established reagents that are necessary in order for the percent yield and reaction of synthesis, and products were observed by melting point characterization. Melting point characterization was analyzed through IR and NMR spectroscopy and Lucas classification. The Lucas test was initiated to confirm the formation of alcohols in the product. Compared to different alcohols, the secondary alcohol (9-flourenol) was then analyzed against other alcohols to distinguish between primary, secondary and tertiary alcohols. Once the Lucas classification test was done, percent yield was gathered from the calculation of synthesis reactions. Redox reactions, also classified as reduction and oxidation reactions, are common organic and inorganic chemistry reactions. If the electron density around carbon increases, the reaction is a reduction. Such usually occurs when a caron that is attached to a more electronegative atom is swapped with a carbon attached to a less electronegative atom. In semi-similar way, the reaction is oxidation if the electron density around carbon decreases. This will occur when a carbon that is attached to a less electronegative atom is swapped with a carbon that is attached to a more electronegative atom. The oxidation number of the carbon number, which is changing during the reaction, needs to be determined to tell whether an organic reaction is a reduction or oxidation. Fundamentally, the oxidation state of an atom is a method of electron counting that considers what the charge on the atom would be if all bonds were ionic. If the oxidation state of carbon increases over the course of the reaction, the reaction is an oxidation. If the oxidation state of carbon decreases over the course of the reaction, the reaction is a reduction. Calculation of oxidation states can be seen in the reaction of 9-fluorenone to 9-fluorenol (Figure 2). Since the carbon changes from +2 to 0, the reaction is a reduction reaction because the oxidation state of carbon decreases.

Figure 2: Reduction Reaction of 9-Fluorenone to 9-Fluorenol.

For reduction reactions, a reducing agent is required which are species that react and make another molecule become reduced. There are many common reducing agents in organic chemistry but the most common are H2, NaBH4, and LiAlH4. Sodium borohydride is the reducing agent that is used in the reduction of 9-fluorenone (Figure 3). NaBH4 essentially reduces polar pi bonds and attacks the ketone which makes the polar pi bond less strong. As a result, the reduction will turn into an alcohol and such reaction is used when producing 9-fluorenol. Figure 4 shows such reaction as the reduction of ketones into alcohols is occurring. First, from the complex, the hydrogen of borohydride breaks off and begins to attack the carbonyl carbon of the ketone. NaBH4 acts as a source of nucleophilic hydride ion which goes to the electrophilic carbon of the carbonyl group on the ketone. Next, the solvent (MeOH) acts as proton (H+) source to neutralize the alkoxide intermediate. The reaction is complete when it is fully neutralized.

Figure 3: Common Reducing Agents in Organic Chemistry.

Figure 4: Reduction of Ketones into Alcohols – Mechanism.

To monitor the reduction reaction progress, thin-layer chromatography (TLC) is used. As well, TLC is used in the laboratory to find conditions to separate a mixture and confirm the identify of a compound that is isolated. From this, an estimation can be shown pertaining to when the reaction is finished by the disappearance of 9-fluorenone on the TLC plate (Figure 5). This was done by spotting the TLC plate with 3 spots as one contained a pure sample of 9fluorenone, another with a co-spot that has a reaction mixture and 9-fluorenone, and one with just the reaction mixture. The TLC plate was then eluted to identify the progress of the reaction. Considering there is a difference in polarity between the starting material and product, Rf values

of the spots located on the TLC plate can be observed which will give insight on the progress of the reaction.

Figure 5: Thin-Layer Chromatography (TLC) Plates Used to Monitor the Reaction Progress Of 9Fluorenone. Lane 1 contains pure 9-fluorenone, lane 2 contains a co-spot (9-fluorenone + reaction mixture overlapped), and lane 3 has a reaction mixture.

Lucas classification testing was the final test that was conducted in the lab as it tests and analyzes for alcohol groups that are present. The test classifies whether an alcohol is primary, secondary, or tertiary alcohols. It is driven by an SN1 nucleophilic substitution mechanism as the rate determination step (RDS) forms a carbocation that results in an increased rate and interaction pertaining to the stability of a carbocation. The Lucas reagent, ZnCl2 and HCl, has a SN1 substitution reaction that forms an insoluble alkyl chloride mixture that increases the reactivity of the alcohol in the direction of the acid (Figure 6). ZnCl2 reacts with a lone pair of electrons of the oxygen of the alcohol. Following this, ZnCl-OH (positive charge) is formed after ZnCl2 is attacked from the oxygen in the alcohol group. A carbocation is then obtained once ZnCl-OH dissociates and the carbocation reacts with the Cl ion to produce an alkyl chloride product. The alkyl chloride results in the mixture to turn cloudy since it is insoluble.

Figure 6: Lucas Test Mechanism. SN1 nucleophilic substitution mechanism of ZnCl2 and HCl by the use of the Lucas test.

Tertiary alcohols will be identified if the reaction proceeds the fastest and the rate is further followed by an alcohol that is secondary which will produce a cloudy color. When no reaction is reactive to the solutions with the Lucas test, it will be a primary alcohol. If clear liquid suddenly turns cloudy, a positive test will be verified because of alkyl chloride forming which is not soluble in water. IR and NMR spectroscopy is completed after the Lucas test is done to confirm functional groups that are in the alcohol product. Experimental Procedures Part I: Synthesis of 9-Fluorenol Reduction Reaction Procedure

To a 50 mL Erlenmeyer flask that has the calculated volume of methanol, a weight of 9fluorenone was added. The flask was swirled gently and warmed until all the 9-fluorenone diminished. The solution was then cooled down to room temperature. Before the sodium borohydride was added to the reaction flask immediately in one portion, the calculated amount was weighed, and the flask was vigorously swirled to make sure the reagent had dissolved. When the reagent dissolved, it was made sure to not place the stopper on the flask with NaBH4 because hydrogen gas was evolving and had been building up in the flask. For 20 minutes, the reactions sat at room temperature as it was swirled occasionally, and the reaction should have turned colorless during that time span. If the reaction was not colorless after 10 minutes, a small amount of sodium borohydride was placed in as it should have been no more than half the amount that was initially put in. The reaction mixture was then swirled again. Then, before reaction was worked up, a TLC was taken and placed on a silica plate. 1:9 ethyl acetate:hexanes was used as the eluant which verified if the reaction was fully completed. The TLC plate had 3 lanes: a pure 9-fluorenone, a pure 9-fluorenone + reaction mixture, and a reaction mixture. Under UV light, the TLC plate was observed, and spots that were seen were circled. Following this, the Rf values of 9-fluorenone and 9-fluorenol were calculated. Using 200 mL of water, the solid was filtered and washed well to test the water coming out of the Büchner funnel after washing was done using pH paper. The product was dried and recrystallized as a minimum amount of hot methanol was used. Then, 10-20 mL of methanol was heated, and the hot solvent was placed to the product until it diminished. After the 9-fluorenol dissolved, the solution was taken off the heat and cooled to room temperature in an ice bath followed by the final product being filtered and dry attached to vacuum. The final product was weighed, and the percent yield was calculated before tests were performed. Finally, the products were characterized by melting point analysis and IR and NMR spectroscopies. Regarding NMR, 100 mg was dissolved in 0.5 mL CDCl3 and placed into the NMR tube. Part II: Confirmation of Alcohol Formation by Lucas Classification Test Lucas Test Procedure 3 test tubes were taken, and 1 mL of the Lucas reagent was added to all. In one test tube, 5 drops of the tertiary alcohol were added. If the solution turned cloudy, the test was positive. It was noted how fast that happened. The test tube was then shaken for 5 minutes if the solution did not immediately become cloudy. The result was a negative test if the solutions did not become cloudy in that duration of time. Then, the procedure was repeated for the secondary and primary alcohol. If the product was a solid, a solution of 9-fluorenol in 0.3 mL of ethanol was prepared before the test was completed. It was made sure that 1 small spatula that is 9-fluorenol was added to the ethanol and that it fully dissolved. For the primary, secondary, and tertiary alcohols, the results were compared. Chemical Table Chemical Name

9-Fluorenone

Chemical Formula C13H8O

Chemical Structure

Molar mass (g/mol) 182.2 g/mol

Physical state Solid

Sodium Borohydride

NaBH4

37.8 g/mol

Solid

Methanol

CH3OH

32.04 g/mol

Liquid

9-Fluorenol

C13H10O

180.2 g/mol

Solid

Ethanol

C2H5OH

46.1 g/mol

Liquid

Zinc Chloride

ZnCl2

136.3 g/mol

Liquid

Sulfuric Acid

H2SO4

98.1 g/mol

Liquid

HCl

36.5 g/mol

Liquid

Hydrochloric Acid Data Acquisition

Table 1: Summary of Reaction Table Compound

MW, g/mol

d (g/mL) or

Rxn Weight or V (g

M

or mL)

9-Fluorenone

180.19

(mmol/mL) -

NaBH4

37.83

-

mmol

Equivalents

6.74 g

37.4

1.00

0.704 g

18.7

0.500

CH3OH

32.04

0.792 g/mol

53.6 g

1683.0

45.0

H2SO4

98.08

1.00 g/mL 3

7.30 g

74.8

2.00

mol/L 9-Fluorenone 674433568 ) = 6.74 g 108 1 mol 1000 mmol × mmol = ( 6.74 ¿ × = 37.4 mmol 180.19 g 1mol NaBH4 1mol 37.83 g × Rxn weight = ( 18.6 mmol ¿ × = 0.704 g 1000 mmol 1 mol mmol = ( 37.4 mmol ¿ ×(0.500) = 18.7 mmol CH3OH 32.04 g 1 mol Rxn weight = ( ) = 53.6 g 1mol x¿ 1674.0 mmol ¿ × 1000 mmol 45.0 ) = 1683.0 mmol Mmol = ( 37.4 mmol ¿ ׿ H2SO4 1 mol 98.08 g × = 7.30 g Rxn weight = ( 74.4 mmol ¿ × 1000 mmol 1 mol mmol = ( 37.4 mmol ¿ ×(2.00) = 74.8 mmol Rxn weight = (

(

)(

)

(

)(

(

(

)

)

)(

)

Percent Yield Reaction yield = 87%

Table 2: TLC Rf Value Calculated. Yield, g Lucas Test (positive + or negative -)

5.906 g +



Theoretical Yield of 9-fluorenol = ( 1 mol 1 mol 182.22 g 6.74 g ¿ x =6.789 g of 9−fluorenol x x 180.9 g 1 mol 1 mol Actual Yield of 9-fluorenol = (.87) x (6.789) = 5.906 g

(



)(

)(

)

Table 3: TLC Rf Values. Rf values for starting material (9-fluorene) and product based on TLC. Compound

Distance Traveled By

Distance Traveled

Retention Factor (Rf)

9-Fluorenone

Solute 2.3

By Solvent 5.1

0.45

Co-Spot: 9-Fluorenone +

2.3 and 1.5

5.1

0.45 and 0.29

1.5

5.1

0.29

Reaction Mixture 9-Fluorenol 

9-fluorenone

( 2.3 5.1 ) 

= 0.45

Co-spot:

( 2.3 5.1 )

= 0.45

1.5 5.1 ) = 0.29 ¿ 

9-fluorenol (

1.5 ¿ = 0.29 5.1

Table 4: Prediction of Lucas Test for Three Alcohols.

-Primary alcohol

-Secondary alcohol

-Tertiary alcohol

-Solution will be clear

-The Lucas test is positive

-Solution will be cloudy

indicating a negative result as

because secondary alcohols react

indicating a positive Lucas

primary alcohols do not react

with the Lucas reagent.

test as tertiary alcohols hold

with the Lucas reagent.

-After several minutes, turbidity

the most reactivity with the

forms

Lucas reagent.

-More slow reaction compared to tertiary alcohol. Melting Point Analysis: 

Measured melting point of (9-Fluorenol) = 142 – 144 °C



Measured melting point of (9-Fluorenol) - reported literature value = 152 – 155 °C



The melting point of the 9-fluorenol is lower than the reported literature value melting point as it is can contain small impurities in it which can result in the solid to have a lower melting point. Small impurities in the crystal interrupts energy.

Figure 7: IR and 1H NMR of 9-Fluorenone.

Figure 8: IR and 1H NMR of 9-Fluorenol.

Conclusion The purpose of the laboratory was to use 9-fluorenol by reducing 9-fluorenone through reduction using sodium borohydride and methanol. Through the process of recrystallization, the product was purified, and the melting point was determined. The reaction process was analyzed through thin layer of chromatography (TLC), established reagents that are necessary in order for the percent yield and reaction of synthesis, and products were observed by melting point characterization. Melting point characterization was analyzed through IR and NMR spectroscopy and Lucas classification. The Lucas test was initiated to confirm the formation of alcohols in the product. Compared to different alcohols, the secondary alcohol (9-flourenol) was then analyzed against other alcohols to distinguish between primary, secondary and tertiary alcohols. The given percent yield of the final product was 87% which is not far from 100% but it could be closer. Regardless, the rate of reaction was successful with the percent yield being 87% and out of the 6.789 g calculated theoretical yield, 5.906 g was gathered from the reduction reaction. For the percent that was missing, this means that a small number of impurities were in the solid. Regarding impurities being found, the melting point was critical for additional confirmation and results showed that the melting point (142 – 144 °C) was much lower than the reported literature value (152 – 155 °C) which means that the product was not pure. The given percentage could have also been the result of the product losing mass as crystals are lost after recrystallization is completed in the procedure. It is possible that cooling or heating the product was not executed properly or even too much solvent could have been used. With testing the reaction, a TLC plate was run to confirm if the reaction was progressing or not. After analyzation, the TLC plates revealed calculated retention factor values of 0.45 for 9fluorenone and 0.29 for 9-fluorenol. Such Rf values indicates that 9-fluorenol has a higher polarity compared to 9-fluorenone because it has a lower retention rate (9-fluorenol). The co-spot and reagent traveled up the TLC plate as the Rf value was calculated to be 2.3 cm which demonstrates that the reaction was successfully in progress. When analyzing IR spectroscopy, it was noted that an alkene was produced at around 1600 cm-1, an alcohol was depicted at around 3300 cm-1, and a sp2 C-H bond was shown at roughly at 3000 cm-1. When looking at 1H NMR, one peak was seen at 6.0 ppm which indicates an alcohol. Another peak was seen around 6.5 ppm which indicates an alkene. As well, there was an aromatic ring present as peaks were shown approximately between 7 and 8 ppm. The IR spectroscopy and 1H NMR assisted in confirming the different functional groups that were in the expected product. The Lucas test was utilized to test and analyze for any alcohol groups that were present. It was seen that there was a reaction that took place between the secondary alcohol and the Lucas reagent, but there was no interaction seen with the primary alcohol and the Lucas reagent. The primary alcohol showed a clear color which verifies it to be a negative test. The secondary alcohol produced a solution that was cloudy. With the Lucas reagent, there was a positive test regarding secondary and tertiary alcohols. Overall, all of the objectives were accomplished and were successful. Post-Lab Questions 1. (2 pt, 0.5 pt each) 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. (2.0 pts) Explain the purpose of adding dilute (3 M) and not concentrated sulfuric acid to the reaction during work up. -The purpose of adding dilute (3 M) and not concentrated sulfuric acid to the reaction during work up is because concentrated sulfuric acid is an oxidizing agent and does not have the ability to withdraw positive hydrogen ions. A reducing agent is needed for the reaction during work up. Diluted sulfuric acid essentially reduces oxidation in the solution which results in positive hydrogen ions being formed. For this, sulfuric acid during work up will form positive hydrogen ions as it is quenches reagents that unused and left behind. 3. (1.5 pts) 9-fluorenone is colored (bright yellow color) but 9-fluorenol is not (white). What accounts for this difference? -9-fluorenol is not colored bright yellow primarily because it has an alcohol group that does not have the ability to absorb light. It cannot absorb light as there are no pi bonds which results in a white color. With 9-fluorenone, a bright yellow color is produced because it has a carbonyl group which is able to absorb light. More specifically, the group absorbs light in the UV-visible region (400 nm) which is a bright yellow color. 4. (0.5 pts) Balance the following sodium borohydride reduction equation below

5. (1.0 pt, 0.5 pt each) Calculate oxidation states of carbon atoms with *

6. (2.0 pts) Calculate the theoretical amount of NaBH4 needed to convert 500 mg of A to B. Show your work. (0.5 pt) Draw the structure of possible by-product of this transformation. 1 mg = 0.001 g Mass of A reacted = 500 mg 0.500 g Molar mass of A -Mass of C: 12 g/mol (16 C) x (12 g/mol) = 192 g/mol -Mass of H: 1 g/mol (14 H) x (1 g/mol) = 14 g/mol -Mass of O: 16 g/mol (4 O) x (16 g/mol) = 64 g/mol -Molar mass of A: (192) + (64) + (14) = 270 g/mol -Molar mass of NaBH4 = 37.8 g/mol -1 mol A = 1 mol NaBH4 270 g A = 37.8 g NaBH4 37.8 = 0.14 g NaBH4 1 g A = 270 0.500 g A = (0.500) x (0.14) = 0.07 g NaBH4 So, 70 mg of NaBH4 is needed to convert 500 mg of A to B.