the reduction of benzophenone using sodium borohydride

This paper is based on an investigation into the benzophenone reduction process utilizing sodium borohydride. It is divided into rationally presented sections that assist the reader understand how the inquiry flowed. The principle underlying the reduction of the ketone with the mentioned metal hydride is briefly summarized in the introduction step. The list of materials and tools used to carry out the experiment has been provided. The experimental procedure used has also been laid out systematically such that any other interested party can perform the same research and end up with the same results. The results conducted in this laboratory which is in the form of charts have been presented in the appendix section. The discussion of the result has been done to justify the outcome of the experiment. The discussion section equally takes into account other questions which were asked in the lab report manual. in this section, lithium aluminum hydride was discussed in depth using some typical examples of reduction reactions that it can perform. The conclusion sums up the overall outcome of the experiment.

Introduction

In ketones, the carbon-oxygen bond is polarized. The charge distribution is polarized. The unsaturation factor as a result of the double bond is a principal consideration when rationalizing the thermal reactions of such bonds. An excellent example of such a ketone is the benzophenone. In these types of compounds, the reaction starts by a nucleophile which attacks the carbon atom in the carbonyl group which has a partial positive charge. The nucleophilic attack ends up producing an oxygen anion at the center point. The progress at this stage often leads to the production of a proton which is yielded at the product. The source of the proton could be present in the reaction medium. The chemical reaction shown below demonstrates a general reaction process of ketones when attacked with a nucleophile.



This reaction represents the possible chemical processes that a ketone can go through. An excellent example of the operation demonstrated above is a reaction of sodium borohydride and benzophenone. The equation is shown below:

The ultraviolet irradiation of the ketone results in a photochemical reaction coming from electronically excited state and not a grounded state. In this case, the sodium borohydride reduces the benzophenone to alcohol, benzo phenol. The same reduction process can be achieved when lithium aluminum hydrides are used. In fact, the latter is more reactive than the former. It can react with the less reactive ketones. This lab exercise is based on sodium borohydride, but a theoretical background of lithium aluminum hydride will be previewed based on the literature information.

Apparatus and Reagents used

3x 250ml conical flask

250ml round-bottomed flask

2x 250ml beaker

Stopwatch

Filter funnel and filter paper

Cork ring

25ml beaker

Measuring cylinders (10ml and 100ml)

250ml separating funnel

Rotary evaporator

Ethanol

Sodium borohydride

2M hydrochloric acid

Diethyl ether

Anhydrous magnesium or sodium sulfate

Experimental Procedures

1g, 0.005mol benzophenone was added into the 250ml conical flask while continuously shaking until a complete dissolution occurs. Using a separate beaker, a 250mg of sodium borohydride was dissolved in 2ml of water. The solution was slightly warmed in a warm tap to accelerate the process of dissolution. A mixture of 80ml water with 20ml, 2M hydrochloric acid was prepared in a 250ml beaker. The mixture was then placed in an ice bath. The stopwatch was tested to ascertain its functionality. The sodium borohydride solution was then added at once to the benzophenone solution using the conical flask provided. Timing was then started using the stopwatch to let the process proceed for a period of ten minutes. The reaction was quenched by precipitating out the reaction product by adding the cold-water mixture. The contents of the conical flask were transferred into the separating funnel. It was then washed using a drop of diethyl ether. The aqueous layer was run into a clean beaker. The organic layer of ether was collected in a conical separating funnel. The aqueous layer was replaced by the separating funnel. It was then extracted using the other fresh portion of 50ml diethyl ether. The applied aqueous layer was then discarded and combined with ether layer using a separating funnel. The layer of ether was then washed using 50ml distilled water. The washing was then removed, and the organic layer was then transferred to a 250ml conical flask. The organic layer was then dried using anhydrous magnesium sulfate. The drying agent was then filtered out using a fluted filter paper. The filtrate was directly collected using a round bottom flask. The ether solvent was removed using rotary evaporation at a reduced pressure. The IR spectra of the initial material were recorded and the crude products from the different reduction times used. The frequencies were compared and contrasted based on the optimum time required to complete reduction according to the reaction conditions utilized.

Results

The results of this lab report were in the form of images and had been taken to the appendices section.

Discussion

This experiment was meant to carry out the reduction of benzophenone to diphenylmethanol using sodium borohydride. The process involved recrystallization process in an organic solvent and the product was also tested to evaluate their IR values. The process was carried out at different time intervals to determine the optimum time needed to complete the entire process. The TLC process could readily be employed in this method to facilitate the process. The results were obtained quickly in this reaction since plates are formed while the reaction is going on and the presence of UV light is vital in visualizing it. The end of the reaction can be quickly noted due to the disappearance of the reactant (substrate) and the product formation as a result of the differences in Rf. The removal and appearance of the spots differently indicate the proceeds of the reaction. The reaction has to come to an end in the span of 5-10 minutes. However, if this does not happen, it means that some of the substrates are not there in sufficient supply. Therefore, an extra amount of the reagent can be added to address the problem. The TLC plates clearly indicate the absence of the ketone. This is an indication that it has been consumed by the other reagent and that a reaction has proceeded to the end.

Response to questions

In the experiment conducted, there are other possible metal hydrides that can be used in reducing the carbonyl compounds to primary alcohols. An example is lithium aluminum hydride. This hydride can be used in lowering aldehydes and esters as well. Below is a set of reactions that occur when this compound is reacted with butanal (aldehyde) and ethyl ethanoate (ester).

CH3CH2CH2CHO CH3CH2CH2CH2O

Butanal Butan-1-ol

Butan-1-or is a primary alcohol formed when the butanal is reduced by LiAlH4.

The structure may be drawn as:



Butanol

When esters react with lithium aluminum hydride, they are entirely reduced to the two alcohols.

The equation below gives a general demonstration of what happens when an ester reacts with LiAlH4.



Therefore, methanol is one of the alcohols formed. The other alcohol depends on the ester involved in the process. In the case at hand, the following products are formed:

Methanol

CH2OH

Propanol

CH3CH2CH2OH

Though metal hydrides are potent reagents in reducing aldehydes to alcohol, they cannot be used for lowering alkenes to alkanes. For performing this reaction, a hydrogenation process is always employed. In this case, hydrogen and a catalyst, often palladium is used. Where hydrogen is used alone, its temperature and pressure must be increased. Typically, the pressure of about 1500 Psi is usually required. Therefore, to convert cyclohexene to cyclohexane shown below, these conditions can be utilized.



To convert alkynes to cis-alkenes, catalytic hydrogenation is also carried out. In this case, the catalyst required is one formed by combining palladium and carbon. This catalyst is referred to as Lindlar’s catalyst. It catalyzes the conversion of alkynes to cis-alkenes. Therefore, the equation below can be achieved using these conditions.



Conclusion

In conclusion, this experiment was conducted successfully. It was meant to demonstrate the ability of a ketone to be reduced with sodium borohydride. The TLC plates were used in the disappearance of the benzophenone as a result of reduction using sodium borohydride. The other section of this experiment was meant to give a theory of the possible reducing agents which can be used in place of sodium borohydride. In fact, from the literature, lithium aluminum hydride is an active reducing agent than sodium borohydride.



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Appendices