Question

Data and Observations Record color and temperature changes that occurred during the reaction, as well as the weight and melting point of the product. Attach the IR spectrum to your report, and give the result of the 2,4-dinitrophenylhydrazine test. Calculations and Graphs Write a balanced equation for the reaction and calculate the per cent yield Conclusions 1. How many carbonyl groups in the benzil were reduced? Give the experimental evidence for your answer. (HINTS: Refer to your melting point, 2,4-dinitrophenylhydrazine test and IR spectrum. The discussion of IR spectroscopy in the text will help you interpret the spectrum) 2. What stereochemical form of the product did you get? Give your evidence. 3. Is the reaction exothermic or endothermic? Give your evidence. 4. Explain any color changes you observed when you added the sodium borohydride.

Answer 1

Balanced reaction:

C6H5-CO-CO-C6H5 + 2NaBH4 + 2H2O ----> C6H5-CH(OH)-CH(OH)-C6H5 + 2BH3 + 2NaOH

The percent yield of the product:

The molar mass of substrate (benzil) = 77 + 28 + 28 + 77 = 210 g/mol

Therefore, the moles of substrate = 0.5 g/(210 g/mol) = 0.002381 mol

The molar mass of product (hydrobenzoin) = 210+4 = 214 g/mol

The theoretical yield of product = 0.002381 mol * 214 g/mol = 0.5095 g

Hence, the percent yield of product = (0.35 g/0.5095 g)*100 = 68.7%