Procedure
Figure 6-2. Nitration of bromobenzene setup.
NITRATION OF BROMOBENZENE
1. Set up a stirring hot plate at your workstation, and make sure that the heat stays off.
Prepare an ice bath in a 400 mL beaker, and place atop the hot plate (see Figure 62 above).
2. In the ice bath, securely clamp a 125 mL Erlenmeyer flask with a 1-inch stir bar
(obtain from TA). Measure 10 mL of concentrated nitric acid, and carefully transfer
into the cold 125 mL flask. Turn the stirring on slowly to avoid acid splashing out
of the flask. Add some sodium bicarbonate to the sink, and carefully rinse out the
graduated cylinder used to measure the acid with a copious amount of water.
3. Measure 10 mL of concentrated sulfuric acid and carefully add this to the cold nitric
acid in the 125 mL flask. Stir for 5 minutes to ensure this mixture is cold before
proceeding. Again rinse the graduated cylinder used to measure the acid as in step 2.
Caution: Sulfuric acid and nitric acid are very strong acids. Handle with
EXTREME care. Do not take the reagent bottles out of the fume hood. Dispense the
acids only when you are actually ready to use them. Rinse the graduated cylinder with
copious amount of water immediately after use.
4. Measure 6.5 mL of bromobenzene. Over 5 minutes, slowly pipet this bromobenzene
into the icy-cold nitration mixture (HNO3/H2SO4) in the 125 mL Erlenmeyer reaction
flask with stirring.
5. Once the addition of bromobenzene is completed, continue to stir the reaction mixture
in the ice bath for 30 minutes (DO NOT heat).
Note: As the reaction progresses, you should observe formation of a solid. If you do
not observe any solid, slowly increase the speed of stirring (remember to do this
carefully to avoid acid splashing out).
6. In a new 250 mL Erlenmeyer flask, place ice and water for a total volume of 75 mL.
Carefully pour the reaction mixture onto the ice water mixture. A yellow pasty solid
should immediately form.
7. Collect the solid by vacuum filtration. While still in the Büchner funnel, rinse the
product with 100 mL of cold tap water. Allow the product to vacuum for 5 minutes
after rinsing. Make sure your solid is sufficiently dry before proceeding to
purification. Return the cleaned stir bar to your TA.
PURIFICATION OF PARA-BROMONITROBENZENE BY RECRYSTALLIZATION
1. Transfer the solid from step 7 into a clean and dry 125 mL Erlenmeyer flask. Add 10
mL of 95% ethanol to this flask.
2. Place the flask on the hot plate on a low setting only enough to bring the mixture to a
gentle boil. DO NOT OVERHEAT!
3. If needed to fully dissolve the crude product, add up to 5 mL more of hot ethanol in
small portions, allowing the mixture to heat up between additions.
4. Once dissolved or no change is seen with addition of ethanol, move the 250 mL flask
to the benchtop, and allow to cool to room temperature undisturbed. Crystals should
form as the solution cools. If none form within 15 minutes, return the flask to the hot
plate and evaporate off some of the ethanol.
5. Once cooled to room temperature with sufficient crystal formation, place the flask in
an iced bath for 5 minutes to complete crystallization.
6. Collect the purified product by vacuum filtration rinsing with 5 mL ice-cold ethanol 3
times. Allow the vacuum to run for 5 minutes before transferring the product to a
tared watch glass. Store the product in your lab drawer, and allow to air dry until next
lab period.
7. Next lab, after the crystals have air dried, determine the yield. Show your TA your pbromonitrobenzene. Then, prepare a capillary for m. p. and a sample for 1H NMR (see
Part II step 2 in Experiment 5 for NMR sample preparation). Take the m. p. while
waiting on your turn to use the NMR. The m. p. of pure p-bromonitrobenzene is 124–
126°C.