We did experiment to synthesize Triphenylmethanol using grignard reagent, which

Question

We did experiment to synthesize Triphenylmethanol using grignard reagent, which is

phenylmagnesium bromide shown above.

The questions are :

1-When Grignard reagent is prepared, what is the limiting reagent (show all calculations)?

please give me accurate numbers.

2-Why is it important to conduct this experiment under rigorously dry conditions?

3- How could the yield of this experiment be improved ?

Please try to explain your answers.

Thank you

We did experiment to synthesize Triphenylmethanol using grignard reagent, which is phenylmagnesium bromide shown above. The questions are : 1-When Grignard reagent is prepared, what is the limiting reagent (show all calculations)? please give me accurate numbers. 2-Why is it important to conduct this experiment under rigorously dry conditions? 3- How could the yield of this experiment be improved ?

Explanation / Answer

1)Write down the chemical equation :

C6H5MgBr + (C6H5)2CHO + H2O ---> (C6H5)3COH + Mg(Br)OH.

So, find the limiting reagent is either one of the grignard reagent or the ketone. The one that is left in the reaction jar among the reactants is the excess reagent.

Find out if the excess reagent is the ketone. Just treat the final mixture with sodium nitroprusside. If that gives a purple colour, then the ketone is there and the grignard reagent is in excess. Vice versa.

Then find out the weight of the product formed and that of the limiting reactant put in. Convert that into moles and then divide. That is the percent yield.

2)The Grignard reagent is a strong base and a strong nucleophile. As a base it will react with all protons that are more

acidic than those found on alkenes and alkanes. Thus, Grignard reagents react readily with water, alcohols, amines,

thiols, etc., to regenerate the alkane.

The starting material for preparing the Grignard reagent can contain no acidic protons. The reactants are apparatus

must all be completely dry; otherwise the reaction will not start.

In reactions involving Grignard reagents, it is important to exclude water and air, which rapidly destroy the reagent by protonolysis or oxidation. Since most Grignard reactions are conducted in anhydrous diethyl ether or tetrahydrofuran, side-reactions with air are limited by the protective blanket provided by solvent vapors. Small-scale or quantitative preparations should be conducted under nitrogen or argon atmospheres, using air-free techniques. Although the reagents still need to be dry, ultrasound can allow Grignard reagents to form in wet solvents by activating the magnesium such that it consumes the water

3)I would add the benzophenone soloution dropwise to an excess (perhaps 25% excess) of phenyl magnesium bromide in ether, then reflux until TLC analysis indicated all the benzophenone had been consumed.

Higher reaction temperatures are possible by using THF (tetrahydrofuran) as solvent, which should decrease reaction time and increase yield.

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