Prelab As usual, write out balanced chemical reaction, determine theoretical yie
ID: 564490 • Letter: P
Question
Prelab As usual, write out balanced chemical reaction, determine theoretical yield and prepare a table of the physical properties of the reagents. Answer the following questions. (1) Why must the flasks be dry? What product will be produced if H-O is present? (2) What is the solid that forms after the addition of benzaldehyde to the Grignard/ether solution? (3) What is the reason for adding extra H SO, if solid Mg is present? What is the reaction? (4) Briefly, what are the steps necessary to recrystallize your product from heptane?Explanation / Answer
Preparation of diphenylmethanol
Reaction,
PhCHO + PhMgBr + H2O ---> (Ph)2CHOH + MgBr(OH)
(1) Grignard reagent is a moisture sensitive reagent. If any H2O (water) is present in the system, grignard would react with it and the reagent would be quenched (rendered inactive). Therefore, the flask must be dry in the reaction.
PhMgBr + H2O ---> PhH + MgBr(OH)
Ph = C6H5
(2) The solid formed after the addition of grignard to benzladehyde is a salt (Ph)2CH(OMgBr)
(3) Extra H2SO4 is added to convert solid Mg into water soluble MgSO4.
Mg(s) + H2SO4(aq) --> MgSO4(aq) + H2(g)
(4) Recrystallization : The crude product is dissolved under warm condition in minimal amount of heptane. The hot solution is filterted to remove any solid impurities. The solution is then allowed to cool at room temperature whereby solid crystals of the product comes out of the solution. The crystals formed are filtered and washed with cold heptane.