Nitration Of Bromobenzene

NOTE

WEAR GLOVES DURING THE ENTIRE PROCEDURE

Bromobenzene and the two nitrobromobenzenes are irritating to sensitive skin areas.  If you should have these materials on your hands and then accidentally touch your face, can cause a severe burning sensation in the affected area.  If this should  happen, IMMEDIATELY:

             1. Go to the restroom and wash the affected area with lots of soap and water.

                THE SOAP IN THE LAB IS NOT SUITABLE FOR THIS PURPOSE.

             2. Return to the lab and apply mineral oil to the affected area.

             3. The summary to this warning is NOT TO TOUCH ANY PART OF YOUR BODY

                WHILE PERFORMING THIS EXPERIMENT.

If you must leave the lab for any reason:

             1. First dispose of your gloves in the waste container

             2. Immediately go to the restroom and wash your hands

                thoroughly with soap and water.

                AGAIN, LAB SOAP WILL NOT DO A SUFFICIENT CLEANING

                JOB

             3. Upon returning to the lab, obtain another pair of

                gloves from the front of the room, and proceed with

                the experiment.

NITRATION OF BROMOBENZENE

Equation: H2SO4 + C6H5Br + HNO3   -------------->  C6H4BrNO2 + H2O

Table of Physical Constants

Compound   M.W. density  mp  bp
C6H5Br  157.0 1.522 -31 156
o-NO2C6H4Br 202.0  ----- 43 258
p-NO2C6H4Br 202.0 ----- 127  254
HNO3  -----   -----   -----   ----- 
H2SO4   -----   -----   -----   ----- 


Table of Amounts of Reactants and of amounts of Products Theoretically Possible

C6H5Br

HNO3

H2SO4

C6H4BrNO2

H2O

M.W.

157

63

98

202

18

Moles

Grams        

 mL

          2.6   

   130 drops 

5ml

5ml

M

-

15.6

18.0

-

-

------------------------- FOR SAFETY REASONS --------------------------

 1.            Add 700mL of tap water to your 1 L Beaker.

 2.            Discard any acid washings, plus the contents of the filter flask (from step 9 below) into your 1 L Beaker, WITH STIRRING.

 3.            Wash the contents of your 1 L Beaker down the sink.

PROCEDURE

 1. Prepare a mixture of 5 mL conc HNO3 and 5 mL conc H2SO4 in a 25 x 150mm test tube. Cool it to room temperature by means of a water bath, at your hood workstation. Once the tube has reached 300C as read on your glass thermometer, remove the tube from the water bath, wipe off the outside of the tube, and clamp it to your heating mantle ringstand-suspended over an empty 150 mL beaker. (See figure posted in your hood area)

2. To the tube, add 2.6 mL bromobenzene DROPWISE  over a period of about 5-10 min; gently swirlling the tube to mix the contents after each addition.  Keep the reaction mixture between 50 - 55 oC.  DO NOT ALLOW THE REACTION MIXTURE TO EXCEED 60 oC.  The temperature may be controlled by allowing more time between the addition of successive  portions of bromobenzene and by cooling the reaction flask as necessary with an ice water bath.

3. After the addition is completed and the exothermic reaction has subsided, heat the tube for 30 min. in a hot water bath,  maintaining the temperature in the reaction vessel below 60oC during this period.  The settings on your hot plate should be between dial markings 2 & 3.

4. Cool the tube in an ice bath to room temperature.

5. Pour the reaction mixture into 50 mL of distilled water which is in a 150 mL beaker.

6. Isolate the crude product by vacuum filtration.

7. Wash the filter cake thoroughly with cold (0-10oC) distilled water and dry the filter cake by allowing the vacuum apparatus to draw air through it after you have finished washing.

8. Transfer the crystals to a TARED 50 mL Erlenmeyer flask and obtain the weight of your wet product.  Place the washings into the 1L beaker.

9. Calculate the volume of 95%(v/v) ethanol needed to just dissolve the nitrobromobenzenes at 78.2oC (the boiling point of 95%(v/v) ethanol). You will need approx. 5 mL 95% ethanol per gram of crude product. SHOW THIS CALCULATION IN THE PROCEDURE PORTION OF YOUR REPORT.

10. Bring this mixture to boiling to dissolve the crude product.  If the product does not completely dissolve after 2 minutes of boiling, add 5 mL of 95%(v/v) ethanol.  If solid still remains you will have to do a hot filtration.  Once your crude product has dissolved, set the flask onto your lab bench and allow the contents to cool slowly to room temperature.

11. Isolate the nearly pure crystals of 4-nitrobromobenzene by vacuum filtration.  If there is solid material in the filter flask at this point, pour it into a beaker and vacuum filter this solution again through the funnel containing the first crop of nitrobromobenzene.  Save the filtrate.

12. Wash the crystals with a little ICE COLD ethanol, allowing the washes to drain into the filter flask containing the filtrate.

13. Allow air to be drawn through the Buchner funnel for 5 min. then detach the vacuum hose from the filter flask, turn off the vacuum and transfer the solid from the Buchner funnel onto 11cm filter paper which is on a watch glass.  Spread the solid over most of the filter paper, breaking large clumps into small particles and put it in your drawer to dry overnight.  Place another piece of filter paper lightly over the crystals to keep the dust out.

14. Pour the filtrate from step 12 into a beaker, either 50 ml or 100 ml.  The volume of your filtrate should be no more than 2/3 the volume of the beaker.

15. Place the beaker containing the filtrate on a hot plate, adjust the dial to "4", place a small stirring rod in the solution and place the combination in a hood.  If the boiling is too vigorous, change the setting to "3".

16. Reduce the volume by one half, by boiling the solution then remove the beaker from the hot plate onto the bench and let it cool to room temperature.  Keep the beaker in your hood.

17. If crystals are present, remove them by vacuum filtration using a 125 ml filter flask and wash the crystals with 5 ml of ice cold ethanol.

18. Now transfer the contents from the filter flask to a beaker* (choose the correct size) and evaporate the volume to 5-10 ml under a hood using a hot plate.

19. Remove the beaker and cool to room temperature.  Some 4-nitrobromobenzene crystals (white) may be present.  Some yellow crystals (2-nitrobromobenzene)  crystals might be present & a yellow oil might be present.  A yellow solution will be present.  It is composed of 2-nitrobromebenzene in ethanol.  Take another beaker.  Remove the oil, which is probably sitting at the bottom of the beaker, with a plastic disposable pipette, and put it into a tared beaker.  If you find any YELLOW crystals, add them also.   Keep this for T.L.C.

20.Your crystals of 4-nitrobromobenzene should be dry now so weigh them and determine the melting point of the compound.  Record both in the Results section of your lab notebook & in your lab report.  After running the TLC on your 2 and 4-nitrobromobenzene, put the remainders into the appropriate recovery bottles at the front of the lab.

                                                                                                           

                              Theory and Explanation of the Nitration of Bromobenzene

If the temperature of the reaction mixture exceeds 60oC,

is also formed.

       

STEP 3. After the major portion of the reaction has occurred a significant amount can still react, but since the temperature drops to below 50 degrees, the rate of reaction becomes too slow for our purpose so a hotplate is used to keep the temperature between 50-60 degrees for thirty minutes.
STEP 4.  After the thirty minute period, no more of the product will be formed in significant amounts so the flask containing the reaction mixture is cooled in preparation for STEP 6.
STEP 5.

The cold water, whose temperature is between 10-20 degrees performs two tasks:

a.  it absorbs the heat which develops when concentrated acids are mixed with water.  Your reaction mixture contains significant amounts of concentrated HNO 3 and H 2 SO 4 as the reaction did not go to 100% completion.  This heat is called heat of dilution.  Heat of dilution is the reason that you NEVER add water to concentrated acid as the water sits near the surface of the acid and absorbs heat until it boils.  This boiling occurs so quickly that it splatters and the acid solution splashes on everything and everyone near it.

b. The cold water also does not dissolve p-nitrobromobenzene very well but forms colloidal dispersion with some of the o-nitrobromobenzene.  The crude solid does contain some o-nitrobromobenzene but less than before the treatment with water.
STEP 7. You want to wash as many impurities as you can from the p-nitrobromobenzene solid without dissolving it, so cold water is used, preferable 0-10 degrees in temperature.
STEP  9. Mass of your product * 5 ml solvent/grams of solid = ml solvent needed. The addition of 1 ml ensures that the solution will be concentrated at the boiling temperature, so that all the material that should dissolve does.
STEP 10. Assuming all the solid dissolved, setting the flask aside and allowing the contents to cool to room temperature, gives the crystals of p-nitrobromobenzene time to come together and form crystals while the impurities such as o-nitrobromobenzene to remain in solution.  The slow cooling allows the crystals of the main compound to grow by allowing its molecules to attach to the crystal.
STEP 12.

The p-nitrobromobenzene is insoluble in ice cold ethanol but any o-nitrobromobenzene will still be soluble in it.

The reason that p-nitrobromobenzene has a lower solubility in ethanol than does o-nitrobromobenzene is that it is less polar because the dipoles cancel out.

Net Dipole

exists,=>

more polar

                      Net Dipole Zero