Nitration of Bromobenzene Worksheet

The Nitration of BromobenzeneFor being so highly unsaturated (degree of unsaturation = 4), aromatic hydrocarbons are
remarkably unreactive. Unlike other unsaturated compounds such as alkenes, dienes, and
alkynes, aromatic hydrocarbons do not undergo addition reactions. The most common reactions
that these compounds undergo are called electrophilic aromatic substitution reactions, reactions
that are much slower than the electrophilic addition reactions of other unsaturated hydrocarbons.
There are five basic electrophilic aromatic substitution reactions: halogenation, nitration,
sulfonation, alkylation, and acylation.
H
Y
+
+ Y
+ H+
In this experiment, we will carry out the nitration of bromobenzene. Bromine, substituted on
an aromatic ring, is an ortho-para directing group. It will direct an incoming electrophile to the
ortho and para positions. It is also deactivating, meaning that its reactions will be slower than
the identical reactions of benzene. For nitration, the excellent electrophile is the nitronium ion.
It is produced from the reaction of nitric acid with sulfuric acid.
HNO3 + H2SO4 → NO2+1 + HSO4-1 + H2O
The nitronium ion will react with bromobenzene primarily at the ortho and para positions to form
two products, 1-bromo-2-nitrobenzene and 1-bromo-4-nitrobenzene.
Br
Br
Br
+
NO2+
NO2
+
NO2
The para product, 1-bromo-4-nitrobenzene, has a melting point of 127ºC. The ortho product has
a melting point of 43ºC. In this experiment, both products will form and crystallize together
during the purification process. Recrystallization of this mixture from ethanol will result in the
formation of crystals of the less soluble para product. The more soluble ortho product remains in
the ethanol. Evaporation of the solvent from the recrystallization filtrate will produce an oil, the
impure low melting ortho product. In this experiment, we will isolate only the para product, the
1-bromo-4-nitrobenzene.
 Pre-lab Preparation
Before coming to lab, you should have done the following things:
1.
Read the background and procedure sections of this experiment thoroughly.
2.
Review the procedure of recrystallization.
3.
Answer the following questions about the procedure in this experiment:
a. What is the purpose of keeping the reaction temperature low during the reaction?
b. Consider the solubility properties of the products. What are the crystals that
form when the reaction mixture is poured into water? Why do they form at this
point?
c. Why is it so important to wash these crystals so thoroughly with water after the
first filtration?
Experimental Procedure:
Safety Considerations
• Concentrated sulfuric acid and concentrated nitric acid are both strong acids and will cause
serious burns if you spill them on the skin. If either of these acids comes in contact with
your skin or clothing, wash the area affected area thoroughly with water. Then report to
your instructor. When dispensing either of these acids, if some of it spills, clean the spill
thoroughly with water. If acid spills down the side of the bottle, wash it thoroughly so that
the next person who touches the bottle won’t be burned on the fingers.
• Nitric acid reacts violently with acetone when mixed in high concentrations. During this
lab, no acetone should be used.
• Bromobenzene is a skin irritant. Wearing gloves is recommended. If you get any of this
material on the skin, wash it thoroughly with soap and water. Avoid touching sensitive skin
on your face while working with these compounds.
• The products of this reaction are toxic if ingested. Wash your hands at the conclusion of the
lab to prevent accidental ingestion.
1.
In the hood, prepare a warm water bath (50o – 60oC) using a large beaker on a hot plate.
This will be needed in step 4. Heat the bath gently to keep it below 60 degrees Celsius.
2.
Use gloves! Into a 50-mL Erlenmeyer flask, place 4.0 mL of concentrated nitric acid
(15.8M, d =1.42 g/mL) and 4.0 mL of concentrated sulfuric acid (17.8M, d = 1.84 g/mL).
Swirl the mixture to mix the acids and cool it to around room temperature with an ice
water bath in a beaker in the hood.
3.
At this point you will begin very slowly adding bromobenzene to the nitrating acid
mixture. Because the reaction is exothermic, it is important to keep the temperature under
60 degrees Celsius to minimize formation of the dinitro byproduct. Measure 3.0 mL of
bromobenzene into a graduated cylinder and, in the hood, and over a 5 minute period, add
about 0.5 mL portions of bromobenzene to the reaction flask. This addition is done
slowly over time in order to keep the temperature from rising too high as excess heat will
cause dinitration or degradation of the starting materials. If the flask begins to feel very
warm through your gloves, or exceeds about 55 degrees Celsius, cool it using the ice bath.
During the addition of bromobenzene, swirl the flask regularly to mix the reagents. You
should begin to see solid product forming.
4.
After you have added all of the bromobenzene, swirl the flask for another 15 minutes in
your warm water bath. If your water bath has become too warm, add cool water to adjust
the temperature. Continue to monitor your reaction visually for overheating. If you see a
large amount of dark yellow-brown gas forming, remove the flask immediately from the
heat, and cool it briefly. You may resume heating once the reaction has cooled and the
gas has dissipated. (If you significantly overheat your flask, you will continue to see
excessive yellow-brown color in the contents of the flask. This also may result in failure
to form significant solid product. If this happens, check with your instructor for further
guidance.)
5.
After the reaction period is complete, cool on ice. You should see significant amounts of
solid product. Carefully pour the acidic mixture into a beaker containing 50 mL of water.
(This step is done to dilute the acid before suction filtration.) Use a spatula to scrape any
crystals that have stuck to the reaction flask to transfer them to the beaker. You may also
use water to help transfer the solid.
6.
Filter the crystals with suction and wash them thoroughly with cold water. The crystals
are insoluble in water, so don’t be afraid to use too much water. Keep suction on the
funnel and the crystals to dry them as much as possible. Press them with a spatula tip to
force out as much water as possible. Dispose of the filtrate in the waste container in the
hood.
7.
Place the solid product into an Erlenmeyer flask and recrystallize with 95% ethanol. First
bring some ethanol to a boil in a separate flask and then transfer about 5 mL of ethanol to
the flask containing the solid product. Bring the mixture back to a boil with swirling.
Don’t forget to use a boiling stone in the ethanol flask. Keep adding hot ethanol until
all of the crystals have just dissolved at the solvent’s boiling point.
8.
Allow the solution to cool slowly, then put it in an ice bath. Once all of the crystals
appear to have formed, filter them with suction and wash them with a small amount of ice
cold ethanol. Dispose of the recrystallization filtrate in the waste container in the hood.
9.
Remove the crystals from the funnel and store them in an open container to dry. After at
least 24 hours, weigh your crystals and take their melting point.
Post-Lab
1.
Write an equation for the reaction of bromobenzene with nitric acid in the presence of
sulfuric acid.
2.
Write a detailed mechanism for the formation of 1-bromo-4-nitrobenzene from
bromobenzene. Your mechanism must show all relevant resonance, including all
resonance contributing structures, as well as the formation of the excellent electrophile
from nitric and sulfuric acids.
3.
Calculate the moles of bromobenzene and the moles of nitric acid you used in this
synthesis. From this data, calculate the theoretical yield of bromonitrobenzene product.
Note: In this reaction two organic products are formed. One mole of bromobenzene
will react with one mole of nitric acid to produce one mole of mononitration product.
That one mole will be made up of both the ortho and para products. You cannot predict
accurately how much ortho or how much para product is formed. You can only calculate
the theoretical yield of the combined products.
4.
Using the theoretical yield of the combined products calculated in question 3, calculate
the percentage yield of 1-bromo-4-nitrobenzene you obtained based on your starting
quantity of bromobenzene.
5.
Give a detailed description of sources of yield loss. Give both chemical reasons (side
reactions of the limiting reagent or further reactions of the product) and physical reasons
(procedural loss of limiting reagent or product) why you got a lower than 100% yield.