Question

The following work-up steps are part of our second synthesis (“Hydrogenation of a Chalcone”).

a) Add 15 mL of brine and 10 mL of ethyl acetate to the mixture. Shake vigorously, then separate the ethyl acetate layer from the aqueous phase.

b) Extract the aqueous phase again with an additional 5 mL of ethyl acetate.

c) Combine the two ethyl acetate solutions and wash this ethyl acetate layer with 5

mL of brine. Drain the aqueous phase and pour the ethyl acetate solution into a

clean 25-mL Erlenmeyer flask.

d) Dry the organic phase with anhydrous magnesium sulfate for 5 min.”

Which (if any) changes to the outcome of the work-up would you expect for each of the following changes to the procedure?

i) In step a), replace ethyl acetate with hexane.

ii) Omit step b).

iii) In step c), use water instead of brine.

iv) Omit step d).

Answer 1

i) If we are replacing Ethyl acetate with hexane in step (a) - Ethyl acetate is a polar solvent while hexane is a non-polar solvent .Chalcones being polar dissolves only in polar solvents. Since Chalcones are solids ,ethyl acetate is usually used to recrystallise chalcone in the pure form.

ii) Aqueous phase is againe extracted with ethyl acetate so as to dissolve any chalcone left behind in aqueous phase.

iii) Brine is usually used in extraction technique to push or extract water from organic layer to the aqueous layer since brine solution (NaCl solution ) have a stronger affinity to water than organic solvents.if water is used it will not be possible to remove water present in the organic layer or complete extraction of chalcone from organic layer becomes difficult.

iv ) if not dried using anhydrous magnesium sulphate- chalcone formation is an aldol condendsation reaction between an aromatic aldehyde and aromatic ketone and if water is present there is a possibility of retro-Aldol reaction i.e hydrolysis of chalcone may take place.