Exp’t 124
The Reaction of 4-Nitrotoluene with Chromic Acid
Adapted by R. Minard and J. Emeigh (Penn State Univ.) from an experiment in Integriertes Organisches Praktikum (Verlag Chemie, 1984) as translated by Scott Denmark at U. of Illinois,
Urbana. Revised 9/22/03
Introduction:
This reaction yields a product whose structure you will deduce by IR or MS analysis.
CH3
Na2Cr2O7, H+
"Chromic Acid"
?
NO2
Prelaboratory Exercises:
Based on what you have learned in lecture about oxidations with chromic acid, predict some possible
structures of the product.
Cautions:
Sodium dichromate (in fact, many compounds containing chromium) are suspected carcinogens. Handle
these substance with great care using gloves when appropriate. Avoid breathing dust or vapors.
Concentrated H2SO4 is very corrosive. Wearing gloves is recommended when handling it.
Synthesis:
Place a 1/2-inch magnetic stirring bar in the 5-mL long-neck flask from your microscale kit and clamp it (use 3
prong microclamp) over the magnetic stirrer. Using the small plastic funnel from this kit, add 390 mg of sodium
dichromate dihydrate (Na2Cr2O7.H2O) and 1 mL water and stir until the sodium dichromate dissolves. In the
solution, suspend 137 mg of p-nitrotoluene (m.p. 52-54°C). To this reaction, add 40 to 44 drops of concentrated
sulfuric acid (H2SO4, density = 1.84 g/mL) drop-wise, from a Pasteur pipet held at a 45° angle, over a period of 15
minutes. A slow enough rate of addition will assure that the reaction doesn’t become so exothermic that it boils
out the top of the flask. Then a sand bath is placed under the flask and heating and stirring continued for 45
minutes at a gentle boil so that the vapors reflux and don’t come out the top of the flask. (What is the approximate
temperature of the refluxing liquid?) Your solution should turn a dark green color.
Isolation and Purification:
While cooling the reaction in a small beaker of ice water, 3 to 6 small chips of ice are dropped into the
reaction mixture, and the reaction is cooled and stirred for 15 to 30 minutes. Filter off the solid product on a
Hirsch funnel with a 1.3-cm filter paper disc placed in it. Wash out any solid adhering to the flask walls by adding
about 1 mL of cold water to it, swirling rapidly and quickly pouring into the filter funnel. Alternatively, one could
wash the product down the sides of the flask using a Pasteur pipet filled with cold water, and withdrawing the solid
suspended in water with the same pipet. If dark brown chromate salts remain on the solid, wash with 10 to 20
drops of 1M sulfuric acid.
Dissolve the solid in 1 mL of 1M sodium hydroxide (NaOH) in a small flask containing a stir bar. Heat if
needed to dissolve all of the solid. Then suction filter this solution through a Hirsch funnel containing a 1/8-inchdeep bed of wetted Celite filter aid [Take a spatula tip full of Celite and place it in a small beaker. Then carefully
dampen it with a few drops of water from a Pasteur pipet and transfer the moist clump onto the Hirsch funnel frit.
Make sure that none of the Celite runs through the filter into the flask.] Rinse the flask and filter with a few drops
of water. Cool the filtrate on ice, while at the same time chilling 1 mL of 1M sulfuric acid in a small beaker or 20
mL vial. When both solutions have been completely chilled, slowly and carefully add the filtrate dropwise with a
Pasteur pipet to the chilled sulfuric acid. After the addition is complete, check that the solution is acidic by testing
with pH paper and, if it is not, add a few drops of 1M sulfuric acid until it is. The precipitate is collected by suction
filtration using the Hirsch funnel, and the crystals are washed dropwise with cold water. Dry the product by
leaving it in an open container until the next period. Weigh the amount of crude product; set aside a small amount
for a melting point. If the crystals are brownish and the m.p. is less than 236°C, recrystallize the product from
ethanol. Determine the yield and m.p. of both the crude and purified products. Analyze the product using the
method given in your Experiment Assignment sheet.
Cleaning Up:
Add saturated sodium bisulfite to the aqueous filtrate from the initial reaction so that all the orange or brownish
dichromate ion is reduced to the greenish chromic ion. This may then be flushed down the drain with lots of water
(it is equivalent to 21 mg of chromium) or placed in the heavy metals waste container. The remaining aqueous or
ethanol/water filtrates can also be flushed down the drain.
Product Determination - Final Report:
There are four conceivable products as shown below:
O H
OH
O
OH
OH
N O2
N O2
N O2
N O2
1
2
3
4
On the basis of your spectral analysis, give the structure of the product in your Results and Discussion section.
On the spectrum, draw the correct structure and point out the spectral features that led you to choose that
structure.
At the end of your lab report answer these questions:
1. What features of an IR spectrum would be different for the three structures that you ruled out as possible
structures for the final product? What evidence is there that the functional groups present are parasubstituted? (See the IR section of Ch. 11 of the Lab Guide)
2. Write equations for the reaction of your product with NaOH and for the subsequent reaction with H2SO4.
Explain why the solubility of the organic species changes with pH.
3. Why are the carbons of the aromatic ring not oxidized by Cr(VI) salts?
4. When Cr(VI) acts as an oxidizing agent, what is the product? What is the visual cue that something has
been oxidized?
Synthetic Experiment PreLab Grading Sheet
Name(s):
TA:
Date:
PreLab For Exp't #:124
The Reaction of 4-nitrotoluene with Chromic Acid
Possible
Points
Date, Name, Desk #, Experiment # & Title(abbreviated after 1st pg), Section &
TA Name
4
Summary
8
Goals
8
Reactions, structures, conditions, diagrams
18
Completeness of Chemical Data Table(s)
14
Chromatographic Behavior Comparison
12
Spectral Features Comparison
12
Work-up - Explanation of the product isolation and purification process
12
PreLab Exercise
12
TOTAL FOR PRELAB
100
Missed
Points
Date Handed in:
General Comments:
Total Points:
Synthetic Experiment Final Report Grading Sheet
Name:
TA:
Date:
Final Report For Exp't #:124
The Reaction of 4-nitrotoluene with Chromic Acid
Possible
Points
Name, Date, Experiment Title (abbreviated after 1st page) and every page
numbered
4
OBSERVATIONS and DATA - Overall organization, readability, completeness
8
Data: Weighing data, molecular weights, moles, density, volumes, Rf’s.
8
Missed
Points
Product analysis conditions i.e. weight of sample and KBr for IR; solvent and
field strength for NMR; ionization mode for MS; solvent and wavelength
range for UV/Vis,
Yield: Show % yield calculations with limiting reagent clearly stated.
12
Purity: Record melting points, color, TLC analyses or other indicators of
purity.
RESULTS AND DISCUSSION - Overall organization, readability, completeness
12
16
Results; Achievement of goals
Product Analysis Data: Quality and Interpretation – Structure(s) drawn on the
product spectrum
24
Interpret major IR or MS peaks or all NMR peaks (including impurities).
Explain how spectra confirm product identity. Discuss all UV/Vis lmax in
terms of conjugation.
Product determination – correctness and rationale.
See Lab Guide Chapter 3, Section 3.4 for guidelines in annotating spectra and
Ch 11 for help with interpretation.
POSTLAB QUESTIONS
16
TOTAL POINTS
100
Date Handed in:
General Comments:
Total Points: