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

 

Background& Methods

By the end of this this experiment our goal was to prepare 9-fluorenol by reducing 9-fluorenone using sodium borohydride. For this carbon-oxygen double bond specific reaction, we were assigned 1.20 grams of 9-fluorenone from our T.A. Using this value the amount of reagents used in the experiment were calculated. The reagents included were sodium borohydride, methanol, and sulfuric acid. After the reaction was complete, the final product we purified through recrystallization. To characterize the product most accurately melting point analysis and IR spectroscopy are used. Then, through the Lucas test alcohol formation was verified. The secondary product, 9-fluorenol, is compared to the primary and tertiary alcohol. After obtaining the final product IR, Lucas test, and melting point were taken to characterize it. Once completed the percent yield was calculated.

 

Within this experiment reduction, oxidation and oxidation state of carbon was used heavily. These reactions can be either organic or inorganic. In simple terms, reduction is the gain of electrons while oxidation is the loss of. This becomes a lot more complicated during the organic redox reactions. In actuality, the carbon that makes a covalent bond doesn’t gain or lose anything though the carbon atom faces a change in the electron density. Typically, the most electronegative atoms are nitrogen oxygen or halogens like Hydrogen. Once the oxidation number of the carbon atom is calculated it can be classified as either an oxidation or reduction reaction. In order to count electrons we use the oxidation station. It states the charge on the atom would be if all of the bonds were ionic. An increase in this number will result in oxidation, and a decrease in reduction. It takes a total of three steps to determine such:

  1. For every atom bound to carbon that is more electronegative than carbon add 1
  2. For every atom bound to carbon that is less electronegative than carbon subtract 1
  3. Every time carbon is bound to another carbon add 0.

The total sum results in the oxidation state of the carbon. In the reduction of 9-fluoreone to 9-fluorenol there is a change from +2 to 0 in carbon’s state.

 

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In the reduction of 9-fluoreone to 9-fluorenol a total of three reagents are used. The following are considered reducing agents: molecular hydrogen (H2), sodium borohydride (NaBH4), and lithium aluminum hydride (LiAlH4 or LAH). Though in this lab sodium borohydride is used and becomes oxidized over the course of the reaction. In order for a reducing agent to do its job it must react which causes another molecule to be reduced.

Catalytic Hydrogenations can be utilized in the in the reduction of other pi bonds like Ketones to alkanols, amines from imines, or nitro groups to amines. Sodium borohydride and lithium aluminium hydride are called metal hydrides. Sodium borohydride and lithium alumunium hydride operate in alternative mechanism to catalytic hydrogenation. The two i.e. sodium borohydride and lithium aluminium hydride assist in the reduction by simply acting the delivery agents of hydride to to electrophilic sites. Lithium aliminium hydride is more strong than sodium borohydride and can reduce functional groups like imines, aldehydes, ketones, carboxylic acids, esters amd amides. The less strong Sodium borohydride is common for reduction of imines, aldehydes, and ketones.

A crucial aspect of the metal hydrides chemistry lies in the selectivity acquired while utilizing these reagents rather than the catalytic hydrogenation. Of all the reactions, many of the reductions done using metal hydrides can also be effected by using H2 with a metal catalyst. Metal hydrides reduce pi bonds that are polar. Due to this fact they cannot be able to reduce carbon to carbon multiple bonds. However they may be a preferable alternative to reduce a C=O or C=N bond with the presence of an alkene or alkyne. Many forms of LAH and NaBH4 are found when 3 of the 4 hydrogen attached to the central B/Li are replaced by an alkyl. Cyano or alkoxy groups. Readjustments of these groups allows the reaction rate of LAH and NaBH4 to be increased or weakened in modification of the use and selectivity of the reagents.

Having completed the TLC and the IR spectroscopy we did the Lucas classification of test for alkanols. The test differentiates between the primary, secondary and tertiary alkanols. It is analogous to the silver nitrate test utilized in Lab 7 to differentiate between primary, secondary and tertiary alkyl halides. The analogy lies in the operation of SN1 necuophilic substitution mechanism. In the test there is a mixture of ZNCl2 and HCl which increase the reactivity of alcohols towards the acids. This is due to the strength of ZNCl2 as a Lewis acid and reacts with the lone pairs of electrons of the oxygen in alcohol. The ZNCl-OH then breaks down to give carbonication , which also reacts with chloride ion forming an alkyl chloride. What determines the rate is carbonation. This implies that the rate increase with increase carbocation stability. This shows that tertiary alkanols will have a higher rate of reaction and secondary and primary alcohols them being unreactive with the Lucas test. A positive test is shown by change of the solution from being clear to cloudy when alcohol is added. This is based on the fact that alkyl chloride is not soluble in the aqueous solution whereas the alcohol we started with was. We establish whether it is a secondary or a tertiary by just noting the rate at which it reacts. Tertiary alcohols react immediately whereas secondary alkanols may take 5-20 minutes to react. The Lucas test is ideal for the alkanols with six or fewer carbons. The alkanol we start with must be soluble in the Lucas reagent.

 

Methodology

Part 1: Synthesis of 9-fluorenol

We had been assigned 1.2 g of 9-fluorenone and then calculated the amount of the other reagents used in the lab today. After the calculation we poured 9-fluoronone into a 50 ml flask and calculated the value of methanol. We then dissolved the 9-fluorone by swirling and heating. The solution was put to cool down to the room temperature. Sodium borohydride was then added to the reaction flask. The flask was swirled vigorously to dissolve the reagent. The reaction flask was not closed. This is because it can be explosive incase hydrogen gas builds up in the bottle. The combination of occasional swirling and cooling was done for about 20 minutes within which the solution had cooled down to room temperature. However after the first about 10 minutes the solution was not yet colorless so we added 6 g of sodium borohydride and continued with the process. Before working up the solution we took a TLC. The ratio of 1 to 9 Ethyl acetate to hexanes was placed in the beaker. We drew a line on the TLC silica plate and three dots were placed on the plate. The three dots included the first which was pure 9-fluoronone , followed by the second which was 9- fluoronone and the mixture , finally the last was the mixture. The plate was then observed under UV light. We calculated the values of RF. When the TLC was finished, we worked up the reaction by adding up 3M of Sulphuric acid to the reaction mixture. The solid that was formed when we added the acid was reduced by heating and swirling the solution for about 5 to 7 minutes. A watch glass was also used to minimize solvent loss. The flask was then allowed to calmly cool at room temperature before transferring it to an ice water bath until it precipitated. The precipitate in solid form was then taken out and washed till it was neutral. A pH paper was used to confirm whether the pH was neutral. The solid was then let to dry. The solution started to re crystallize; the minimum amount of hot methanol was added until it dissolved. When the 9-fluoronone was dissolved the solution was taken out of heat and the solution let to cool. When I cooled to room temperature the contents were transferred to an ice bath. The final product was filtered, dried, weighed and the percentage yield calculated. Melting point and IR were then taken out. The product obtained was put in the capillaries; they were then dropped down to ensure the product was well packed. It was then transferred to the melting point apparatus. The IR machine was cleaned the sample placed on it for analysis.

 

Part II: Confirmation of alcohol formation by Lucas classification test.

Preparation of three test tubes was done by adding 1 ml of Lucas reagent to each tube. We added 3 ml of tertiary alcohol to one test tube. That test-tube was then shaken. The procedure was repeated for the other two secondary and primary alcohols. Our products were solid so we first needed to prepare a solution of 9-fluorenol in 3ml of ethanol before performing the test.

 

 

 

 

 

 

DATA ACQUISITION

 

Compound Molecular Weight (g/mol) d(g/mL) or M(mmol/mL) Rxn Weight or Volume (g or mL) mmol Equivalents
9-Fluorenone 180.10 n/a 1.5g 6.66 1.0
Sodium Borohydride 37.83 n/a 0.126g 3.33 0.5
Methanol 32.04 0.792 g/mL 12.12mL 299.7 45
Sulfuric Acid 98.08 1.00 g/mL

3.00 mol/L

4.4mL 13.32 2.0

 

Percent Yield

States the correct amount of the product that was made

Percent Yield = (Actual Yield/ Theoretical Yield) * 100

 

Percent Yield= (1.25g/1.20g) * 100= 104.2% of 9-Fluorenol

 

Discussion of Collected Data

The biggest problem with our collected data is that our percent yield is above 100 percent. Typically this is unlikely if the starting measured amount is less but may have occurred if the final product hadn’t dried enough or the initial measurement of 9-Fluorenone was greater than the 1.2 grams assigned by our T.A.

IR SPECTROSCOPY

Molecular Motion Theoretical Wavenumber (cm-1) Actual Wavenumber (cm-1)
O-H Stretch (Alcohol) 3400-3300 3306.53
Unsaturated C-H Stretch ~3020 3037.30
Aromatic Ring ~1450 1449.50

 

TLC

The reaction is complete when there is no more 9-Fluorenone in the reaction mixture.

 

LANE SOLVENT SOLUTE
1 2.7 mL 2.3 mL
2  
3 0.0 mL

 

Rf=distance spot moved/distance solvent moved

Rf=2.3/2.7= 0.852

Rf=0.0/2.7=0

ANALYSIS AND CLASSIFICATION TESTS

TESTS

   
Melting Point Analysis Ideal: 152-158°C

Actual: 152 °C

 
Lucas Test    
3° Alcohol Cloudy POSITIVE. The solution became cloudy rather quickly.
2° Alcohol Cloudy POSITIVE.  The solution became cloudy within a couple of minutes
1°Alcohol Clear NEGATIVE. The solution had no reaction therefore remained clear. No visible reaction.

 

 

CONCLUSION

By the end of this experiment we were able to conduct a complete reduction reaction of 9-Fluorenone to 9-Flurenol with the use of sodium borohydride. In the end we were able to produce 1.25 grams of 9-Fluorenol giving us a percent yield of 104.2% proving that our goal was achieved.  It is likely this value is high due to the fact that our product was not completely dry. Other possibilities for error may be directed to incorrect measurement of the original amount of 9-Fluorenone, 1.2 grams. Our melting point for the product was 152 °C, which is within the ideal range of 152-158°C.

 

Multiple tests were conducted post reaction completion such as the TLC test. In this experiment, the TLC test differentiates from other labs since it monitors the progress of the reaction. When there isn’t any 9-Fluorenone in the reaction mixture then the reaction is complete, or disappearance of the starting material. The Rf values of two spots on a TLC plate provided evidence of the identity of our compound, and the success of our fluorine reduction reaction. The more polar compound has a lower Rf value, and vice versa. Our TLC had a retention factor of 0.852 and 0.00. In short, this means that our reaction was complete since there is no more 9-Fluorenone in the reaction mixture.

 

The other test that was conducted was the Lucas classification test. In this test all 3 types of alcohols undergo 3 different reaction rates. It starts with tertiary then secondary, and finally primary alcohol reactions. Typically, primary alcohols don’t react with Lucas reagent at room temperature since it requires a very high temperature. During the SN1 mechanism the RDS is carbocation formation. In order for the test to be positive the solution will become cloudy when the solutions are added. If the solution remains clear in the time allotted then it results in a negative test, no reaction occurred.IR Spectroscopy confirmed that the experiment was conducted correctly by showing that 9-Flurenol was formed. An O-H stretch, alcohol, was seen at 3306.53 cm-1. An unsaturated C-H stretch was seen at 3037.30 cm-1 and an aromatic ring at 1449.50 cm-1.

 

Possible errors may have been made during the experimentation process. Again, our mass of 9-Flurenol was greater than the 1.20 grams of 9-Fluorenone that was used in the experiment. This was due to the excess amount of water that was retained by the product, 9-Flurenol. Other possible errors that may have occurred when our product was stuck on the vacuum. If our product had been thoroughly dried then a significant difference may have been seen in a loss of 9-Flurenol in our final weighing.

 Chemistry department