The limiting reagent in a Grignard reaction is usually the substance to which you add the Grignard reagent, but you have to confirm this by calculation

The limiting reagent in a Grignard reaction is usually the substance to which you add the Grignard reagent, but you have to confirm this by calculation.

EXAMPLE

Assume that, in a preparation of triphenylmethanol, you prepared phenylmagnesium bromide by reacting 2.1 mL of bromobenzene (density 1.50 g/mL) with 0.50 g of magnesium in anhydrous ether. To this, you then slowly added a solution of 2.4 g benzophenone in anhydrous ether. What was the limiting reactant?

Solution

The equations (##"Ph" = "C"_6"H"_5##) are:

##"PhBr" + "Mg" → "PhMgBr"## ##"Ph"_2"C=O" + "PhMgBr" → "Ph"_3"COMgBr"## ##"Ph"_3"COMgBr" + "H"^+ → "Ph"_3"COH" + "Mg"^(2+) + "Br"^"-"##

Calculate the moles of each reactant

##2.1 color(red)(cancel(color(black)("mL PhBr"))) × (1.50 color(red)(cancel(color(black)("g PhBr"))))/(1 color(red)(cancel(color(black)("mL PhBr")))) × "1 mol PhBr"/(157.0 color(red)(cancel(color(black)("g PhBr")))) = "0.020 mol PhBr"##

##0.50 color(red)(cancel(color(black)("g Mg"))) × "1 mol Mg"/(24.30 color(red)(cancel(color(black)("g Mg")))) = "0.021 mol Mg"##

##2.4 color(red)(cancel(color(black)("g Ph"_2"CO"))) × ("1 mol Ph"_2"CO")/(182.2 color(red)(cancel(color(black)("g Ph"_2"CO")))) = "0.013 mol Ph"_2"CO"##

Identify the limiting reactant

All theoretical molar ratios are 1:1.

Since benzophenone has the fewest moles, it is the limiting reactant.