Practical 13 Reduction of Benzophenone: Synthesis of Diphenylmethanol PDF

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Practical 13 Reduction of Benzophenone: Synthesis of Diphenylmethanol
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Practical 13 − Reduction of Benzophenone: Synthesis of Diphenylmethanol Tabulate your data as follows: Mass of benzophenone (g)

2.2 g

Molar Mass (MM) of benzophenone (g mol-1)

182.21 g/mol

Mass of NaBH4 (g)

0.4 g

Molar Mass (MM) of NaBH4 (g mol-1)

37.83 g/mol

Mass of clock glass (g)

2.11 g

Mass of clock glass with crystals (g)

4.02 g

Mass of crystals (g)

1.91 g

Molar Mass (MM) of diphenylmethanol (g mol-1)

184.238 g/mol

Melting point range (°C)

66-68ºC

Answer the following questions with workings, calculations, and units. 1) How many moles of benzophenone were used? 𝑔𝑟𝑎𝑚𝑠

Moles = 𝑚𝑜𝑙𝑎𝑟 𝑚𝑎𝑠𝑠 2.2 𝑔

Moles = 182.21 𝑔/𝑚𝑜𝑙 = 0.012 moles of benzophenone

2) How many moles of NaBH4 were used? NaBH4 is a source of hydride ions. How many moles of hydride were provided by the amount used? 0.4 𝑔

Moles = 36.83 𝑔/𝑚𝑜𝑙 = 0.01 moles of NaBH4 As there are 4 hydride ions in every molecule of sodium borohydride 0.01 x 4 = 0.04 moles of hydride

3) What is the limiting reagent? Justify your answer. The balanced equation for the reduction of benzophenone by sodium borohydride 4(C₆H₅)₂CO + NaBH4 → 4(C₆H₅)₂CHOH The ratio between the moles of benzophenone and sodium borohydride is 4:1. This means that because there are 0.012 moles of benzophenone present in this reaction, the benzophenone only needs 0.003 moles of sodium borohydride to react with as, 0.012 moles/4 = 0.003 moles. However, there are 0.01 moles of sodium borohydride present in the reaction meaning that it is in excess, and benzophenone is the limited reagent.

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Also, because there are 0.01 moles of sodium borohydride, it needs 0.04 moles of benzophenone, as 0.01 x 4 =0.04. There are only 0.012 moles of benzophenone present proving that benzophenone is the limiting reagent.

4) What is the theoretical yield of diphenylmethanol in grams? Theoretical yield = 0.012 moles of benzophenone x 184.24 g/mol (Molar mass of diphenylmethanol) = 2.2 grams

5) Calculate the actual percent yield of the reaction. We use the mass of the diphenylmethanol crystals formed and divide it by the theoretical yield of diphenylmethanol that we calculated in the above question and multiple it by 100. 1.91𝑔 100 𝑥 = 86.8% 2.2𝑔 1 5) Explain why the yield is lower than 100%. There are many reasons for this. •

It could be due to loss of sample of diphenylmethanol during recovery, i.e., improper handling of products, losing product during transfer into clock glasses, conical flasks, filtration apparatus

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Incomplete reaction due to unfavorable reactions conditions, i.e., insufficient time for recrystallization after adding petroleum ether, insufficient energy for substances to react

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There may have been an excess amount of petroleum ether used to dissolve the mixture, which would have disturbed the recrystallization.

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Improper use of apparatus, i.e., not cleaning apparatus fully, allowing substances to mix

6) What is the literature melting point of diphenylmethanol? 65-67ºC 7) How does the melting point of your sample compare with the literature data? Explain any discrepancies observed. We recorded a melting point of 68 degrees Celsius, which is a degree higher than the literature melting point of diphenylmethanol. This could be due to using excess sample or poor placement of the sample on the capillary tube or in the melting point apparatus. Heating the diphenylmethanol too fast may also causes discrepancies for the melting point.

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