The dye preparation is carried out in a 250-mL round-bottomed flask equipped wit

ID: 948136 • Letter: T

Question

The dye preparation is carried out in a 250-mL round-bottomed flask equipped with a magnetic

stir bar, reflux condenser, and drying tube. The flask is charged with 0.80 g magnesium turnings,

THF (45 mL), 4-bromo-N,N-dimethylaniline (5.0 g), and a small crystal of iodine. The mixture is

warmed gently to reflux (set the hot plate to 200 °C) and maintained there for 30 min, during

which time the original dark reddish-brown color changes to the typical “dirty dishwater” shade

of the Grignard reagent. Check with your TA to ensure this has indeed happened.

The flask is cooled to room temperature (ice-water bath). Be sure to keep the solution dry. After

15 min. diethyl carbonate (0.49 g) in 5 mL of THF is added in one portion. The mixture is

warmed to reflux for an additional 5 min, and then cooled again to room temperature (ice-water

bath). Aqueous hydrochloric acid (15 mL of a 10% solution) is added slowly while stirring (the

reaction with the remaining unreacted magnesium is vigorous). Keep stirring the mixture for 15

min. The result is a purple solution with some unreacted magnesium in the bottom. Fill one

labeled vial from the side shelf with your solution. This is about 20 mL.

What is the theoretical yield of Crystal Violet?

moles of 4-bromo-N,N-dimethylaniline = 5.0 g/200.076 g/mol = 0.025 mols

moles of crystal violet formed = 0.025/3 = 0.0083 mols

Theoretical yield of crystal violet = 0.0083 mols x 400.979 g/mol = 3.34 g

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