Question

Please i am writing up a practical report for the ;Reduction of benzophenone to corresponding alcohol(diphenylmethanol)using sodium borohydride,and the need the following;

1.Please state possible observations you are likely to get from these experiment

2.State possible errors and improvements

3.And then conclusions.

Thanks

Answer 1

Dear friend

The two most common reducing agents used to reduce carbonyl compounds to alcohols are sodium borohydride (NaBH4) and lithium aluminum hydride (LiAlH4). Both of these reagents are capable of transferring hydride ions to aldehydes and ketones to form complexes, which can then be hydrolyzed to the corresponding alcohols. If an aldehyde is used in the reaction, a primary alcohol is produced; if a ketone is used, the product is a secondary alcohol.

Sodium borohydride is a weaker reducing agent than lithium aluminum hydride. Reductions using the former may be carried out in aqueous or alcoholic solutions, while those involving the latter require the use of an inert solvent (e.g., tetrahydrofuran).

In this experiment you will compare the purity of crude diphenylmethanol with that of recrystallized diphenylmethanol by spotting both samples on a single TLC plate. A sample of benzophenone will also be spotted on the same plate so that you can determine whether any of this starting material is present in either the crude or recrystallized product. Visualization will be achieved using an iodine tank.

Note: As an additional check on the purity of your product, an infrared spectrum will be run on the crude product, the recrystallized product, and the benzophenone starting material.

Use a Pasteur pipette to add the aqueous solution of sodium borohydride one drop at a time to the solution of benzophenone. Swirl the reaction mixture between the addition of each drop in order to disperse any cloudiness. Do not add more sodium borohydride until the cloudiness caused by the previous drop has disappeared.

Decompose the excess sodium borohydride by slowly adding the slurry of crystals and solvent to a mixture of 30 g of crushed ice and 3 mL of concentrated hydrochloric acid in a 250 mL beaker. (CAUTION: Prepare the latter by adding the concentrated hydrochloric acid to the crushed ice, not vice versa. Do this step in a fume hood. Wear gloves and protect your eyes.)

Recrystallize the diphenylmethanol using hexane as the solvent. (Hint: About 25-30 mL of solvent will be required, use 50o C water bath to warm solvent.)

Safety
Benzophenone is harmful if swallowed, inhaled or absorbed through the skin. Flammable.

Methanol is harmful to the lungs, skin, eyes and other organs. Poisonous if swallowed. Highly flammable. Use in a fume hood.

Sodium borohydride is toxic if ingested. Avoid contact with skin and take precautions against inhaling its dust.

Diphenylmethanol is an irritant and is poisonous when ingested.

Concentrated hydrochloric acid is extremely corrosive to the skin and eyes. Its vapour is irritating to the eyes, lungs and skin. Wear gloves and eye protection. Use only in a fume hood.

Hexane is highly flammable. Its vapour is irritating and can have a narcotic effect.

Chloroform (trichloromethane) is poisonous if swallowed. Its vapour is an anaesthetic and causes nausea, headaches, vomiting and unconsciousness.

Ethyl acetate is harmful if swallowed. Prolonged exposure to its vapour can cause corneal cloudiness and anaemia. Highly flammable.

Ligroin (or petroleum ether bp. 60-80o C) is harmful if inhaled or swallowed. Can cause skin irritation and exposure may produce a burning sensation, headache and vomiting. Very flammable!

Iodine causes internal irritation if swallowed. Its vapour is harmful to the respiratory system. Contact with the skin or eyes is dangerous.

Thank you.

Edited:

Errors:

1. Sodium Borohydride is moderately unstable to water (hygroscopic nature), so protect from moisture while weighing.

2. Sodium borohydride reacts vigorously with acid with the evolution of hydrogen gas, so do not expose to acid. Use the NaBH4 immediately after weighing.

3. While adding it to methanol (CH3OH) it starts to give nacsent hydrogen, if we do not close flask after the addition of boro hydride this nascent hydroge escapes from reaction mixture resulting poor yield of the product.

Conclusions:
This reaction of benzophenone with sodium borohydride to form diphenylmethanol is an example of a sodium borohydride reduction of a ketone to an alcohol. The reaction was relatively easy to run and provided a fairly good yield of the desired product.
The reaction appeared to completely convert most, if not all, of the benzophenone to the desired diphenylmethanol. This conclusion is based on:
1. The absence of a carbonyl absorption and the appearance of strong OH absorption bands in the infrared spectrum of the purified product.

2. The melting point (64 – 66° C) of the obtained product compares favorably with the literature mp value (65 - 67° C) of the expected diphenylmethanol.

3. According to the above equation for the borohydride reduction of benzophenone, there is an excess of borohydride in this reaction; consequently, benzophenone is the limiting reagent (LR) and it is used to calculate the theoretical and actual % yields.

4. While the yield of desired product was relatively good in this experiment, the yield could have possibly been increased by the addition of slightly more water to force more of the product out of solution during the crystallization procedure; however, this may have caused more impurities to co-precipitate with the product and decreased its purity slightly. Overall, the yield and purity of product from this experiment is completely satisfactory for a reaction of this type.