Question

2. Why should all of the glassware and the magnesium used for the preparation of the Grignard reagent be any 3. What is the role of the anhydrous calcium chloride used in this experiment? 4. Calculate the theoretical yield (in g) of diphenylmethanol in this experiment. Show your work.

Answer 1

Answer 2) In Grignard reagents, RMgX, the carbon bonded to Mg atom carries δ-charge and hence it has significant carbanion character. Since carbanions are among the strongest bases, they can abstract H+ ion from traces of water i.e, they can react with even trace amount of water to give the corresponding hydrocarbon, RH.

RMgX + H2O = RH + Mg(OH)X

Hence when dealing with Grignard reagents, all glasswares must be free of traces of water.

And magnesium too reacts with water to give magnesium hydroxide which would become useless for the preparation of RMgX.

Mg + 2 H2O = Mg(OH)2 + H2

Answer 3) Anhydrous calcium chloride is a hygroscopic substance i.e, it absorbs moisture from its immediate surroundings and forms CaCl2.2H2O

Hence Anhydrous calcium chloride is employed in the preparation of Grignard reagents to keep the reaction environment moisture free and avoid their accidental reaction with water.

Answer4)

Hence theoretical yield of diphenylmethanol is 2.70 g.

O MO BY OH Diphenyl methanol It is clean from the above reaction sequence, Mg, bromobenzene e benzaldehyde are all required in equimolar proportions. Now, from the data given No of modes of Mg - mass mags = 0.4 = 0.0167 mot Molar mass 24 No. of moles of bromobenzene = 207 = 0.0172 mol 157 No of moles of benzaldehyde - 1.56 -0.0147 mol 106 OH clearly, benzaldehyde is available in least amount & therefore acts as 'Limiting reagent'. Now, I mot ho gives I mol O o so moles of diphenyl methanol fome expected = 0.0147 mol OR Theoretical yield of diphenylmethanol = nxM -0.0147 X1844 = 2.70 g