A reaction starting with 8.0 g of benzoic acid, excess methanol, and excess phenyl magnesium bromide.What would be the expected theoretical yield of triphylmethanol?? If 5.36 g of triphenylmethanol is end product, what is % yield?Show all calculation.

1 Answer
Sep 13, 2015

This all belongs to the same lab report you are probably doing right now. See this for the mechanism after you form methyl benzoate:

http://socratic.org/questions/write-the-overall-reaction-for-reaction-of-methyl-benzoate-with-excess-phenyl-ma

The only difference here is that now you are forming methyl benzoate using benzoic acid and excess methanol. Excess because this reaction is slow.

You know that benzoic acid must be the limiting reagent in the entire reaction from start to finish because everything else is in excess. You're really just driving the equilibrium forward.

You really only need one equivalent of methanol to react for this to work, and you only get one equivalent of methyl benzoate if this works.

When you react with excess phenyl magnesium bromide with this, you need excess of that due to some steric hindrance of having three benzene rings on one product. You started with one ring and you get two, so you get two equivalents of phenyl magnesium bromide that need to react.

Dehydration of the carboxylic acid into an ester:
#C_6H_5COOH + CH_3OH rightleftharpoons C_6H_5COOCH_3 + H_2O#

Dry over #MgSO_4# to prep for the Grignard...

Grignard reaction to form a tertiary aromatic alcohol:
#C_6H_5COOCH_3 + 2C_6H_5MgBr rightleftharpoons (C_6H_5)_3 CO^(-) + "MgBr side products"#

#(C_6H_5)_3 CO^(-) + "MgBr side products" + H_3O^(+) rightleftharpoons (C_6H_5)_3 COH + "hydrolyzed Grignard side products"#

Our focus is the main product, triphenylmethanol. You only need one equivalent of benzoic acid combined with two equivalents of phenyl magnesium bromide, so we're good to go now.

#"8.0" cancel"g benzoic acid" xx ("1" cancel"mol benzoic acid")/("122.12" cancel "g benzoic acid")#
# xx ("1" cancel"mol triphenylmethanol")/("1" cancel"mol benzoic acid") xx "260.33 g triphenylmethanol"/("1" cancel"mol triphenylmethanol")#

#= "17.05 g triphenylmethanol"#

Therefore:

#% "Yield" = 5.36/17.05 ~~ color(blue)(31.4%)#

To be honest, that's a pretty normal yield to get. (On the final lab you may do this year, don't be surprised if you only get #25%# yield.)

If you are still unsure, feel free to ask your professor to confirm.