Question

An Alkene addition is being performed. The reactants are .1504 g of trans-cinnamic acid and .3843 g of pyridinium tribromide, these components heated and are reacted within 2 mL of glacial acetic acid to produce 2,3-dibromo-3-phenylpropanoic acid. How much of this final product would theoretically be produced? Would the product be majority threo or erythro?

Answer 1

During the reaction, pyridinium tribromide produces molecular bromine, which reacts with the alkene part to form a cyclic bromonium ion. The bromonium ion eventually gets attacked from backside by the bromide anion, to yield the dibromide. As evident from the mechanism, the product contains -Br substituents on same sides in the Fischer projection, i.e. erythropoietin isomer. So, erythro isomer is the major product.

Now, as per the reaction, 1 mole trans-cinnamic acid produces 1 mole of 2,3-dibromo-3-phenylpropionic acid

So (0.1504 / 148.1586) gm mole trans-cinnamic acid produces (0.1504 / 148.1586) gm mole of 2,3-dibromo-3-phenylpropionic acid = {(0.1504 / 148.1586) x (307.97)} gm = 0.3126 gm 2,3-dibromo-3-phenylpropionic acid.

So, theoretically, 0.3126 gm 2,3-dibromo-3-phenylpropionic acid will be produced.