A chemist performed a reaction between a phenylmagnesium bromide, (0.571 g) and

Question

A chemist performed a reaction between a phenylmagnesium bromide, (0.571 g) and benzophenone 0.4997 g. The acidified product of the reaction, ("product"), was extracted with other and after evaporating the solution 410 mg solid was obtained. This 410 mg solid was dissolved in 10 ml, hot 2-proponal. A crystalline solid formed and was filtered of 0.246 grams were obtained. Based on the data below, answer the following questions. a. Which compound is the limiting reagent in the reaction above and why? b. What is the percent recovery for the reaction? c. The literature melting point of the product is 260 degree C. The melting point of the solid obtained form 2-proponal was 230- 239 degree C. A TLC that was run on the final product showed two "spots". One spot had an Re of 0.2 and the other spot had an Rr of 0.75. Explain the TLC and give a plausible explanation of the results from the TLC and melting points. What could be done to correct these results?

Explanation / Answer

For the given grignard coupling reaction,

(a) moles phenylmagnesium bromide = 0.5371 g/181 g/mol = 0.00297 mol

moles benzophenone = 0.4997 g/182 g/mol = 0.0027 mol

as moles of benzophenone is lower, this is the limiting reactant

(b) Theoretical yield = 0.0027 mol x 260 g/mol = 0.702 g

percent recovery = 0.410 x 100/0.702 = 58.40%

(c) percent yield = 0.246 x 100/0.702 = 35.04%

(d) the solid obtained is impure and has unreacted benzophenone in it, whic lowers its melting point

The Rf of 0.2 is for the product alcohol and the Rf of 0.75 is for the benzophenone in the mixture in the given tlc

Recrystallization of the crude would yield a pure compound melting at the correct 260 oC.

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