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Reduction of Benzophenone with Sodium Borohydride and the Use of Thin Layer Chromatography to Follow the Progress of Reaction...

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Experiment: Reduction of Benzophenone with Sodium Borohydride and the Use of Thin Layer Chromatography to Follow the Progress of Reaction Date: 01/10/18 Introduction: This experiment involves a reduction reaction, turning Benzophenone to an alcohol, using sodium borohydride as a reducing agent. After obtaining the crude product, the sample is to be purified and the melting point range is to be determined so that the purity of the sample can be known. Sodium Borohydride (NaBH4) and lithium aluminium hydride (LiAH4) are the most common reducing agents with carbonyl compounds. Sodium borohydride is a weaker reducing agent and can only reduce ketones and aldehydes down to alcohols and will have no effects on carboxylic acids. The double bond in the ketone, benzophenone in this case, can resonate so there is a slight negative charge on the oxygen atom and a slight positive charge on the carbon atom. This makes the ketone more vulnerable to be reduced by the hydride.

The first step in the reaction involves the electrons in the C-H bond attacking the positive carbon atom on the benzophenone. One of the bonds in the double O=C bond breaks and the electrons go onto the oxygen atom. The oxygen atom will then have a negative charge, which is cancelled out by the positively charged Na+ atom. In the second step of the reaction, an acid or water is added in the reaction work-up for the purpose of quenching the reaction. It deactivates any unreacted reagents. It provides the hydrogen atoms need to turn the molecule into an alcohol. The negative charge on the oxygen attacks the hydrogen atom in the acid. A bond is the formed between the O and H atom, creating an alcohol functional group in the molecule. Diphenylmethanol is formed as a result of the reaction.

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Starting the experiment, the reduction reaction time is unknown. This is common problem when working in research situations. There are many solutions to overcome this difficulty, one of which involves the use of Thin Layer Chromatography. Thin Layer Chromatography (TLC) separates non-volatile mixtures of components and allows the identification of the components. TLC is performed on a sheet of plastic, glass or thick aluminium foil which is coated with a thin layer of silica, or a different absorbent material. This is known as the stationary phase. TLC also involves a mobile phase which is a solvent or solvent mixture which is drawn up the TCL plate using capillary action. Samples of the mixture being tested are put on the plate and as the solution moves up the plate, separation of the components occurs. The spots left on the TLC plate after this procedure can be observed under a UV lamp. This occurs because components of different polarities move up the plate at different rates. Silica gel is very polar and the mobile phase is non-polar. The non-polar molecules move with the mobile phase and travel up the plate and the polar molecules are left behind. Thus, the further up the spot is on the TLC plate, the less polar the component is. Similarly, the lower spots on the TLC are more polar. The end product of the reaction in the experiment is diphenylmethanol, which is an alcohol. Alcohol is a polar substance so it is expected to remain near the bottom of the silica plate. To analyse the results of the procedure, the retention factor, Rf must be measured. It is the ratio of the distance travelled by the centre of a spot of analyte to the distance travelled by the mobile phase from the origin. Rf value =

distance moved by spot distance moved by solvent

Ideally, the Rf value of an analyte in Thin Layer Chromatography should be equal to the R value used in column chromatography. When using Thin Layer Chromatography for the identification of the completion of a chemical reaction, the technique first needs to be validated. It is important to note that it is only possible to follow the course of the reaction if the starting and ending material have different retention factors. Samples of the starting compound and of the end product are separated using TLC and the retention factors are calculated. Now, if another compound is tested and has the same retention factor as one or both of the samples, we can tell how far along the reaction it has progressed. Then, during the chemical reaction, samples from the mixture can be taken at various times and put on a TLC to separate the components. Comparing the retention factor of the components of the mixture, it can be observed whether the reaction contains any of the starting components or any end products. If the spots have the same retardation factor as the spots of the end product, then the reaction is complete. To check the purity of the product, the melting point range can be tested. The closer this range is to the literature range of the substance, the more pure it is. An impure sample will usually have a lower melting point. For diphenylmethanol, the literature melting point range is between 68-69oC.

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Procedure: First, the Thin Layer Chromatography(TLC) technique was validated. This made it possible to tell whether the reaction in the experiment has progressed completely. First, the TLC tank was prepared. A small wide-necked jar was lined with filter paper, leaving a narrow space so that the TLC plate can be viewed. The mobile phase was poured into the jar so that it reached a height of about 5-7 cm in the jar after the filter paper is completely saturated with it. The lid was put on the jar and kept on unless the plates were being put in or taken out of the tank. This is to decrease the evaporation of the volatile solvents in the mobile phase. The samples being separated (starting and end product) were loaded onto the silica plates. On a TLC silica plate, a line was drawn very lightly with pencil about 1cm from the bottom edge of the plate. This line acted as the origin. The places on the line where the samples were to be loaded were labelled: (S) for the starting material and (P) for the product. These samples were put on the plate using a clean capillary tube for each sample and they were spaced apart from each other. The plate was put in the TLC tank to stand as close to being vertical as possible. The solvent was made sure not to pass the line of origin on the plate. This would diffuse the chemicals on the plate and ruin the process of separation. The solvent was allowed to travel up the silica plate, separating the components of the two mixtures, bringing the less polar chemicals with it. Before the solvent reached the top of the plate, about 2-3 mm, the plate was taken out of the tank and the solvent front line was marked. The plate was then left to dry. Once the solvent had evaporated off the sheet, the plate was viewed under a UV lamp. Gloves were worn to prevent damage to skin from the UV rays. The spots on the plate were marked with a pencil. The lengths from the spots to the origin and the length from the solvent front and the origin were measured. The retention factor of each constituent of each product was calculated. Starting the reaction procedure, 728mg of benzophenone was weighed and transferred to a 50ml conical flask where it was dissolved into 10cm2 of ethanol, by swirling the flask. In a small beaker, 168mg of sodium borohydride was dissolved into 3cm3 of cold water. This solution was added drop-wise into the benzophenone solution using a Pasteur pipette. The solution was swirled constantly until all the borohydride solution was added. The mixture was left to sit at room temperature, stirring occasionally. Because the length of the reaction is unknown, a sample was taken from the mixture at various times of the reaction and separated using TLC. A sample was taken after 10 mins, and for every 15 minutes after that until it was found that the reaction was complete. Along with the sample of the reaction mixture, the starting material and end product were also put on each TLC plate that went through the separation process. For each sample taken, the retention factor was calculated and recorded. This was compared to the retention factors of the starting and end products to determine what stage the reaction was in. When the sample from the reaction gave a retardation factor with a value close to that of the end product, the reaction was known to be complete. When the reaction was over, 2cm2 of concentrated hydrochloric acid was added to 20cm2 of icewater in a 100cm3 beaker in a fume hood. The reaction mixture was then added into this solution slowly. The mixture was left in the fume hood for a couple of minutes until the solution stopped 3

fizzing. After this, the solution was filtered through Buchner funnel for about ten minutes. A small portion of the crude product left from the filtration process was retained and the rest was put into a beaker. To recrystallise the product, just enough petroleum ether was added to the beaker so that the crystals were just dissolved. This solution was then filtered again by vacuum filtration through a Buchner funnel. The crystals collected were washed with a cold solvent to remove any soluble impurities in the diphenylmethanol crystals. The yield of the product was weighed and recorded and the percentage yield was calculated. To check the purity of the compound formed, the melting point of the crude product and the melting point of the recrystallised product was measured. Experimental Results: Length from the origin 0 mins

10 mins

20 mins

35 mins

Starting material

28

22

23

20

Reaction mixture

———

22, 11

23, 10

10

End product

12

10

12

9

Solvent front

70

60

65

58

0 mins

10min

20mins

35mins

Starting material

0.4

0.36

0.354

0.345

Reaction mixture

————

0.36, 0.183

0.35, 0.15

0.172

End Product

0.17

0.16

0.184

0.155

Retention factors:

After thirty five minutes, the retention factor of the reaction mixture was the same as the retention factor for the end product. This meant that they both contained the same compounds and that the reaction progress was complete. The final end product of the entire experiment was a fine white powder. The melting point range of the crude product was measured to be 65-67oC and the melting point range of the recrystallised purified product was measured to be 67-68oC. Calculation and Discussion: The final, recrystallised product formed was a fine white powder. The borohydride was used in excess to ensure the complete reduction of the carboxyl group in the benzophenone. So the benzophenone was the limiting reactant in the reaction. Calculation of theoretical yield: 0.728g of benzophenone were used → 0.728g / 182g mol-1 = 4x10-3 mol 4x10-3 moles of benzophenone produces 4x10-3 moles of diphenylmethanol 4

4x10-3 moles of diphenylmethanol = (184 g mol-1)x(4x10-3 moles) = 0.736g → Theoretical yield is 0.736g The weight of the end product was weighed out to be 0.68g. → Percentage yield = (actual yield / theoretical yield) x 100 = (0.68g / 0.736g) x100 = 92.39 % For every TLC that was prepared, the distance from the solvent front to the origin was measured along with the distance moved by the compound. The Rf value was calculated using the following : Rf = distance moved by compound / distance moved by the solvent front. During the TLC process, some plates had uneven spots when viewed under a UV lamp. Some of the spots spread out a lot as they moved up the plate. This is the result of the solution sample being too concentrated or too much of the sample was loaded onto the plate.

Conclusion: The time when the reduction reaction was known to be complete was found out by the use of Thin Layer Chromatography. A sample of the reaction mixture was taken every ten to fifteen minutes and put through the TLC process. A retention factor was calculated. When the retention factor was the same as that of the end product, the reaction is known to be complete. Using sodium borohydride as a reducing agent, benzophenone was reduced from a ketone to an alcohol. NaBH4 added a hydrogen to the carbocation in the benzophenone molecule and forced one of the bonds in the double oxygen bond to be broken and remain as an electron pair on the O atom. The hydrochloric acid is used to quench the solution. It provides the hydrogen atoms necessary to add onto the oxygen atoms to form an alcohol functional group. The theoretical yield of the product was calculated to be 0.736g. From this and from the experiment, the percentage yield was calculated to be 92.39%.

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