Question

Prelab to accompany bromination of trans-cinnamic acid (4 pts each)

The stoichiometry of the addition is 1:1, meaning that for every one mole of transcinnamic acid, one mole of Br2 is needed to form the addition product. The Br2 presents as a solution in dichloromethane solvent. This often causes problems for students when calculating stiochiometric equivalents. The bromine solution is 10% Br2 by volume. For instance, 100 mL of solution contains 10 mL of liquid Br2. The density of Br2 is 3.12 g/mL.

Calculate the number of mmol of Br2 present in 0.48 mL of bromine solution

Calculate the number of mmol in 100. mg of trans-cinnamic acid (use a MW calculated to 2 decimal places)

Which is the limiting reagent?

What is the theoretical yield of the addition product? (use a MW calculated to 2 decimal places)

Show all calculations.

Answer 1

10% bromine solution of 0.48 ml contain 0.048 ml of Br liquid

mass = volume density = 0.048 3.12 = 0.14976 gm

10% bromine solution of 0.48 ml contain 0.14976 gm of bromine liquid

molar mass of Br₂ = 159.808 gm / mol then 0.14976 gm = 0.14976/159.808 = 0.0009371245 mole of bromine

0.0009371245 = 0.9371245 milimole of Br₂

10% bromine solution of 0.48 ml contain 0.9371245 milimole of Br₂

molar mass of trans-cinnamic acic = 148.1586 gm / mol

100 mg = 0.1 gm

mole of transcinnamic acid = 0.1/148.1586 = 0.0006749524 mole = 0.6749524 milimole

in in 100. mg of trans-cinnamic acid contain 0.6749524 milimole

Br₂ is 0.9371245 milimole and trans-cinnamic acid is 0.6749524 milimole Thus trans-cinnamic acid is limiting reagent.

trans-cinnamic acid is limiting reagent react completely thus, 0.6749524 milimole of trans-cinnamic acid react with  0.6749524 milimole of Br₂ to form  0.6749524 milimole of 2,3-dibromo-pheny propanoic acid.

molar mass of 2,3-dibromo-phenylpropanoic acid = 307.97 gm/mol =

0.6749524 milimole = 0.0006749524 mole

1 mole of 2,3-dibromo-phenylpropanoic acid = 307.97 gm then 0.0006749524 mole of 2,3-dibromo-phenylpropanoic acid = 307.97 0.0006749524 = 0.207865 gm

therotical yield = 0.207865 gm = 207.865 mg