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Christian rukundo Chem 2161 Lab #3: Sodium Borohydride reduction (macro) Background In organic chemistry, oxidation reactions are generally defined as a net loss of hydrogen atoms or a net gain of oxygen atoms. Also, organic reductions are usually defined as a net gain of hydrogen atoms or a net loss of oxygen atoms. The most common reducing agents for carbonyl chemistry are lithium aluminum hydride (LiAlH4) and sodium borohydride (NaBH4). Both are excellent reducing agents, however LiAlH4 is stronger with the ability to reduce ketones, aldehydes, esters, and carboxylic acids, over NaBH4’s ability to only reduce ketones and aldehydes. Unfortunately, LiAlH4 is so strong that it reacts violently with water, making NaBH4 the preferred reducing agent. Sodium borohydride was discovered in 1943 by H. I. Schlesinger and H. C. Brown. Brown devoted his entire scientific career to this reagent, making it and other hydrides the most useful and versatile of reducing reagents. He received a Nobel Prize for his work. In this experiment,a solid diketone( yellow solution) dissolved in ethanol is reduced with solid sodium borohydride to a dialcohol (colorless solution) that crystallizes from the reaction mixture and is isolated by filtration, It will be easy to follow the progress of the reaction due to the difference in colors . Purpose To prepare the!diol,!hydrobenzoin!alcohol, by reducing!benzil!using sodium!borohydride Reagents/reaction

Mechanism

Procedure • Williamson, K.L.; Masters, K.M., 2017. Macroscale and Microscale Organic Experiments 7th ed., cengage. Page number 15

Observation After adding the 0.01g of sodium borohydride a yellow color was observed in the solution and fade away forming a clear solution. In addition to adding sodium borohydride to dissolve benzil no heat was used since the reducing agent already produce heat exothermically. But once the solution was clear and added water, Heat was required at this point of the reaction to help the water molecules break the boron-oxygen bonds to form alcohol products. Data yield=0.26g Melting point range=132-134°C! Conclusion This lab focused on the reduction of ketones to alcohols. Sodium borohydride was used as a reducing agent because of its mild reactivity compared to other hydrides as well as its selectivity. The mechanism shows that the negatively charged hydride on the BH4-!species attack the carbonyl carbon to eventually form a complex where the boron atom is!turns to 4 reduced carbonyl molecules at one time. This step is crucial to the process that comes next in which water is added to break apart the boron-oxygen bond of the complex to liberate up to 4 alcohols and!B(OH)4-. From the experiment, the yield was calculated to be 0.26 and based on the melting point of 132-134°C the product obtained was identified to be indeed a mess hydobenzoin.Overall, this experiment went well, and the conversion of a diketone dissolved in ethanol to a dialcohol using solid sodium borohydride in a reduction reaction was studied !Reference • Williamson, K.L.; Masters, K.M., 2017. Macroscale and Microscale Organic Experiments 7th ed., cengage.

Post lab question1 ! MW benzil = 210.23 g/mol!

100 mg benzil * (1mol/210.23g) * (1g/1E3 mg) * (1mol NaBH4/2mol benzil) = 0.24E-4 mol Note that each NaBH4 reduces 4 carbonyl. So 1 NaBH4 will reduce 2 benzil (each with 2 carbonyl) So 0.24 E-4 mol NaBH4 * (37.83g/1mol) =9 E-4gof NaBH4 or 9mg NaBH4 Question2 The hydrobenzene will have lesser absorption than benzil for the simple reason that benzil has the extending conjugation where as the hydrobenzoin does not have it. Because of the extending conjugation, the energy difference between highest occupied molecular orbital and lowest unoccupied molecular orbital is relatively lesser than hydrobenzion. For the same reason amount of energy required excite the electron from highest occupied molecular orbital to lowest unoccupied molecular orbital is relatively less. Since the energy and wave length of the light are inversely proportional to each other, Benzil will have longer wave length absorption than its counterpart i.e. hydrobenzoin Re duc tio n is the pro

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lu mi nu m hyd rid e, Li AL H4, and sod iu m bor ohy...