Exp’t 51
Nitration of Nitrobenzene
Adapted by R. Minard and Roderick Black (Penn State Univ.) from A.I. Vogel's Elementary Practical Organic Chemistry, Longman Group Ltd., London (1966). Revised 9/22/03
Introduction:
Nitration is an example of electrophilic aromatic substitution; the electrophile is a nitronium ion, 1, which
displaces a hydrogen ion from the benzene ring. It is generated from a Lewis base, nitric acid, in the
presence of a Lewis acid catalyst, sulfuric acid.
HNO3 + H2S O4
H2O + NO2+
1
HSO4-
NO2
H
+
NO2+
+
H+
When the nitration of a substituted benzene is performed, the activating or deactivating effect as well as
the directing effect of the substituent(s) must be considered. If, for example, a meta-directing,
deactivating substituent Y is present on the aromatic ring, other conditions may be required to drive the
reaction toward a reasonable product yield.
H
+ HNO3 (excess)
NO2
+ H2O
H2S O4
D
Y
Y
The objectives of this experiment are: 1) to nitrate the compound nitrobenzene in the presence of a
sulfuric acid catalyst; 2) to investigate the existence of isomers in the crude product using thin-layer
chromatography; 3) to characterize the final product using melting point and ultraviolet spectroscopy; and
4) to discuss the implications of aromatic ring activation/deactivation by a given substituent on the directing
effect of that substituent.
Data in the following table will be useful in interpreting your experimental results. The data given is taken
from Sadtler Standard Spectra (Sadtler Research Laboratories, 1968).
Table I:
lmax for Selected Nitro-substituted Benzenes
lmax (nm)
Compound
Nitrobenzene
1,2-Dinitrobenzene
1,3-Dinitrobenzene
1,4-Dinitrobenzene
1,3,5-Trinitrobenzene
260 (350 shoulder)
463
233
261
223
Prelaboratory Exercises:
Write the mechanism for the nitration of nitrobenzene. Use electron arrow pushing to show the formation
of the electrophile (NO2+), the intermediate arenium ion, and the final product of the reaction.
Cautions:
Nitrobenzene is readily absorbed into the skin, and the concentrated mineral acids are corrosive. Gloves
should be worn when transferring these reagents.
Exp't 51
Synthesis:
Fill a reaction tube with concentrated sulfuric acid (18.0 M) to just below the 1-mL mark and to this
add nitrobenzene until the 1.5 mL mark. Drop in a 1/2” stir bar, and while stirring, add 40 drops of
conc. Nitric acid slowly over a period of about three minutes. Heat the reaction mixture in a boiling
water bath (NOT a sandbath) for about 25 minutes. Cool the mixture in an ice bath. While stirring,
slowly add about 1.5 mL of ice chips to the reaction tube. A solid should form. Cool the product
mixture in an ice bath. Collect the crude solid product on the Hirsch funnel lined with a 1-cm filter
paper disk using vacuum filtration. Use a few drops of ice cold water to rinse the product. To dry the
product somewhat, leave it in the Hirsch Funnel and draw air through it by leaving the vacuum on for
about 10 minutes. Reserve a small amount of crude product to be used for TLC
comparison described below.
Isolation and Purification:
Transfer the crude yellowish product in the funnel to a tared reaction tube and recrystallize using 95%
ethanol. Add about 0.5 mL of the ethanol to the solid in the reaction tube, insert a boiling stick, and heat
the solution in the water bath. Add additional solvent sparingly to dissolve the remainder of the product,
then quickly wrap the hot reaction tube in several paper towels and set it in a beaker. This insulating
procedure allows slow cooling to room temperature over a period of about 15 min. Large, pale yellow
needles will slowly crystallize from the solution. Now place the reaction tube in an ice bath to further
crystallize product. The solvent is conveniently removed from the crystals using pipet filtration and the
crystals are washed with fresh, ice cold 95% ethanol using the technique explained in your Lab Guide, Ch
4. The crystals should then be dried by connecting the reaction tube directly to the vacuum with heavywalled vacuum tubing for several minutes. Weigh the crystals, determine the melting point, and calculate
the percent yield.
Prepare to run a thin layer chromatography analysis using 1:2 ethyl ether/hexanes as the developing
solvent. Dissolve a few milligrams of both the crude and recrystallized products separately in 1 mL each
of CH2Cl2 and use the solutions to carry out TLC analysis. Chapter 7 of the Lab Guide explains the
procedure for spotting, running, and viewing your TLC plate.
Cleaning-up:
The acidic mother liquor obtained in the filtration of the crude product can be flushed down the drain with
plenty of water.
Ultraviolet Spectral Analysis:
Take the ultraviolet spectrum of your recrystallized product on the Hewlett Packard 5842 Diode Array UV
Visible spectrophotometer. Instructions are in the Lab Guide or near the instrument.
Final Report:
Tape the TLC plates into your lab notebook, properly marked, and calculate Rf's for all spots. By
referring to Table I which lists the lmax for various nitro-substituted benzenes, deduce or confirm the
structure of your final product.
Answer these questions:
1. Draw the resonance structures of the carbocation intermediates formed by ortho, meta, and para
electrophilic attack on nitrobenzene. Use these resonance structures to account for the primary
observation of only one product in this synthesis.
2. What is the role of sulfuric acid in this synthesis (i.e., why will nitration be less effective if only nitric acid
alone is used?)
Synthetic Experiment PreLab Grading Sheet
Name(s):
TA:
Date:
PreLab for Exp't #: 51
Title: Nitration of Nitrobenzene
Possible
Points
Date, Name, Desk #, Experiment # & Title(abbreviated after 1st pg), Section & TA
Name
4
Summary
8
Goals
8
Reactions, structures, conditions
14
Completeness of Chemical Data Table(s)
14
Chromatographic Behavior Comparison
16
Spectral Features Comparison
12
Work-up - Explanation of the product isolation and purification process
12
PreLab Exercise
12
TOTAL FOR PRELAB
Missed
Points
100
Date Handed in:
General Comments:
Total Points:
Synthetic Experiment Final Report Grading Sheet
Name(s):
TA:
Date:
Final Report for Exp’t #: 51
Title: Nitration of Nitrobenzene
Possible
Points
Name, Date, Experiment Title (abbreviated after 1st page) and every page numbered
4
OBSERVATION and DATA - Overall organization, readability, completeness
8
Data: Weighing data, molecular weights, moles, density, volumes, Rf's, TLC
analysis conditions and TLC plates. UV sample preparation
12
Yield: Show % yield calculations with limiting reagent clearly stated.
Purity: Give melting point, color, or other indicators of purity.
12
RESULTS AND DISCUSSION - Overall organization, readability, completeness
Results; Achievement of goals
Missed
Points
8
16
Product Analysis Data: Quality and Interpretation – Structure(s) drawn on UV
Spectrum and lmax explained in terms of conjugation.
Discussion of TLC results
24
See Lab Guide Chapter 3, Section 3.4 for guidelines in annotating spectra and
Ch 11 for help with interpretation.
POSTLAB QUESTIONS
TOTAL POINTS
16
100
Date Handed in:
General Comments:
Total Points: