Part 7: LABORATORY

Part A:

Experimental yields.... First you need to balance the reaction equation and then work out the moles of each reagent used to determine the limiting reagent:

 C6H5CO2CH3 + 2 CH3MgI --> C6H5COH(CH3)2 MW (g/mol) 136.2 166.2 136.2 amount (g) 2.00 3.32 1.00 mmoles 14.7 20 7.3 mmoles/ coefficient 14.7 /1 = 14.7 20.0 / 2 = 10.0 7.3 / 1 = 7.3

Therefore, in the last row of the table where the stoichiometric coefficient from the balanced equation is divided into the number of moles, we can see that the limiting reagent is the Grignard reagent, methyl magnesium iodide. This means that the maximum amount of product that can be formed is 10.0 mmol (based on the reaction stoichiometry). Hence the % yield, based on obtained / max. possible = 7.3 / 10.0 = 73%.

Common general errors: (1) not balancing the reaction equation, (2) not determining the limiting reagent correctly and (3) not knowing how the calculate an experimental yield. This section was done poorly considering the number of times yields are calculated in the laboratory during the semester and similar questions were on the finals for the last two years. Note that it doesn't matter whether you work with grams or moles of product the answer should be the same.

Part B:

• dissolve mixture in ether and water (if needed) and transfer to a separatory funnel.
• separate the layers
• wash ether layer three times with water (removes inorganic salts)
• extract the ether layer three times with 10% sodium hydroxide (converts acid to carboxylate which dlssolves in the water and separates from bromobenzne and biphenyl)
• neutralise the basic layer with conc. HCl to form a white precipitate (the benzoic acid). Collect by vacuum filtration and allow to dry.