What is the limiting reagent in grignard reaction?
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.