In the Laboratory

Discovering Electronic Effects of Substituents W

in Nitrations of Benzene Derivatives Using GC–MS Analysis
Malgorzata M. Clennan* and Edward L. Clennan
Department of Chemistry, University of Wyoming, Laramie, WY 82071; *mclennan@uwyo.edu

The nitration of substituted benzenes is a classical ex- tures of those six substrates are then analyzed during the labo-
ample of the electrophilic aromatic substitution reaction and ratory period with GC–MS.2 At the beginning of the next
it is still, despite its age, commonly taught in the second se- laboratory period students are given their chromatographs
mester of undergraduate second-year organic courses (1). This and the mass spectrum of each of the observed peaks. Stu-
reaction is often carried out in the laboratory as an illustra- dents then identify the unreacted starting material and prod-
tion of lecture material, but the majority of the existing pro- ucts and assign their structures based upon retention times
cedures in laboratory textbooks focus on isolation of a major and analysis of fragmentation patterns. The results of each
product and validation of its identity (2). Detection of other section are then pooled and tabulated. Pooling results enables
isomers formed in these nitrations has been virtually neglected students to draw conclusions about the directing effects of
thus giving students the misleading impression that other iso- substituents (ranging from strongly activating to strongly
mers are not formed. In fact, only a few experiments for the deactivating). In addition, writing down the resonance struc-
undergraduate laboratories described in the literature deal tures of possible intermediates allows them to draw conclu-
with the determination of isomer distribution in nitration sions about the mechanism of the nitration reaction. It is up
reactions but their scope is limited to nitration of alkyl sub- to the instructor to decide how much additional informa-
stituted benzenes (3a), aniline (3b), and benzoic acid (3c). tion should be given to the students.
We describe a discovery-based (4, 5) experiment in which The reactions are conducted on a millimolar scale to re-
students, by applying GC–MS analyses to the reaction mix- duce the quantity of waste while providing sufficient mate-
tures, discover the identity of all isomers formed in the ni- rial for GC–MS analysis. In addition, the MSDS categorized
tration of a series of substituted benzenes. The instructional toxic nitration products are never isolated minimizing stu-
value of introducing GC–MS analyses in the organic second- dent contact.
year undergraduate laboratory is well established (3a, 6) and
in this experiment students exercise their knowledge of prin- Experiment
ciples of chromatographic separation (7a) as well as fragmen-
tation patterns (7, 8) to identify the substrates and isomer Synthesis
products. One of the substrates (4 mmol) (1 through 6 in Table
The effectiveness of a discovery-based approached to 1) was added to 2 mL of 98% H2SO4 and the solution was
learning has been discussed previously (4) but only Mohan chilled in an ice bath. A chilled nitrating mixture (0.5 mL of
et al. (5) have described a discovery-oriented nitration ex- 70% HNO3 and 98% H2SO4, 1.25兾0.25, v兾v) was then
periment. However, even in this experiment the isolation and added. The reaction mixture was kept at room temperature
purification protocol used allows NMR identification of the for 15 min followed by dilution with 20 mL water and ex-
major product but does not allow detection of other isomers. traction with two 5 mL portions of ethyl acetate. The com-
bined extracts were dried with sodium sulfate and gravity
Overview of the Experiment filtered. This solution was then used for GC–MS analysis.

During a three-hour laboratory period each student per- GC–MS Conditions

forms the nitration of one of six1 substrates using identical An Agilent 6890 GC兾5973 MSD system with an auto-
reaction conditions (procedures). Representative product mix- matic liquid sampler (split injection 100兾1) and a HP-5, 30 m

Table 1. Relative Peak Areas and Retention Times [min] of Nitration Products
Unreacted Isomers
Products Substrates Ortho Meta Para o,p-Disub
N-Methylaniline, 1 trace [2.50] 09.8 [4.72] 27.3 [5.23] 62.9 [6.14] none
Anisole, 2 01.1 [1.99] 04.0 [3.97] none 06.3 [4.26] 84.1a [6.43]
Fluorobenzene, 3 36.1 [1.96] 02.2 [4.52] none 52.3 [4.24] 09.4b [6.79]
Ethyl Benzoate, 4 42.2 [2.97] 10.8 [4.86] 45.5 [5.17] 01.5 [5.05] none
c
Acetophenone, 5 62.1 [2.52] 07.0 [4.25] 30.9 [4.57] not detected none
α,α,α-Trifluorotoluene, 6 72.7 [2.05] Trace [4.02] 25.3 [4.09] 01.9b [4.48] none
a b c
A second di-nitro isomer at 5.11 min was not identified (rel area 4.5). Original samples were not analyzed. The mass spectra of the original
samples of 3’- and 4’-nitroacetophenone show only minute differences. The meta isomer may be contaminated with para, but attempts to separate
them on our GC–MS failed.

www.JCE.DivCHED.org • Vol. 84 No. 10 October 2007 • Journal of Chemical Education 1679

 In the Laboratory

A B

Figure 1. (A) Chromatogram (TIC) of a crude reaction mixture obtained from nitration of ethyl benzoate. (B) Mass spectrum of the product
with retention time 5.17 min identified as ethyl 3-nitrobenzoate.

× 0.25 mm column was used. Temperatures were inlet- Conclusions

250 ⬚C, detector-280 ⬚C, and oven-110 ⬚C (or 60 ⬚C for fluo-
robenzene and α,α,α-trifluorotoluene) ramped to 250 ⬚C, In this experiment students discover that the reactivity
20 ⬚C兾min. and the substitution pattern in the nitration of aromatic rings
depend strongly on the electronic properties of the substitu-
Hazards ent. Writing and examining the resonance structures for pos-
sible intermediates allow them to elucidate the directing
Ethyl acetate and substrates 2–6 are irritants. N- mechanism. Especially appreciated by the students is the con-
methylaniline and all products are toxic. Fluorobenzene, nection between the material presented in lecture and in this
α,α,α-trifluorotoluene, and ethyl acetate are flammable. Sul- laboratory experiment that provide them with insight on how
furic and nitric acids are corrosive. Gloves should be worn the data presented in the textbook are obtained.
throughout the experiment. The reactions and the work up This simple discovery-based experiment keeps students
of the reaction mixtures should be carried out in hoods. involved and their enthusiasm intact throughout the labora-
tory session. Surprisingly, students involvement in data analy-
Results sis increases when they are told that the distinction between
isomers cannot be achieved based solely on fragmentation
An example of a total ion chromatogram from the ni- patterns and that NIST98 library program will not help.
tration of ethyl benzoate and the mass spectrum of the ma-
jor isomer formed are presented in Figure 1. The uncorrected Acknowledgment
(7a) peak areas for isomers expressed as a percent of total peak
area in the chromatograms and the corresponding retention The authors would like to thank the National Science
times are compiled in Table 1. Foundation for the funds necessary for the purchase of the
All substrates exhibit molecular ions and their identifi- GC–MS (DUE-0125911).
cations by fragmentation patterns are accomplished by the
students without major problems.3 However, the identifica- W
Supplemental Material
tion of the products create a real-life dilemma. On one hand,
GC–MS analysis, using the “nitrogen rule” and the mass of Instructions for the students, notes for the instructor,
the molecular ions as well as the diagnostic peaks (8) deriv- chromatograms of reaction mixtures, and mass spectra of the
ing from the loss of NO2 and NO from the molecular ions, major isomers are available in this issue of JCE Online.
allow students to identify those peaks that are from mono-
and those that are from di-substituted products. On the other Notes
hand, the distinction between the ortho, meta, and para
mono-substituted isomers based on fragmentation patterns 1. Numerous substrates were used in the preliminary testing
cannot be achieved. However, armed with retention times of (among them acetanilide, bromobenzene, and toluene) but the ones
authentic samples and a basic tenet of GC separation theory selected gave the most reproducible and illustrative results and were
(7a) 4 the identifications can be accomplished. The unex- most amenable to chromatographic separations.
pected formation of meta isomer in nitration of N- 2. The rest of the samples are analyzed during the ensuing week.
methylaniline can be explained by existence of the protonated 3. Provided some GC–MS experiment(s) are introduced in
substrate in the acidic medium that directs nitration to the the first semester of the course as the gradualism approach to teach-
meta position (9). ing dictates, see ref 6a.

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 In the Laboratory

4. Elution of compounds is in the order of their increasing Chem. Educ. 2003, 80, 1319–1321. (b) Burns, D. S.; Berka,
boiling points (vapor pressure) with the exception of substrate 4. L. H.; Kildahi, N. J. Chem. Educ. 1993, 70, A100–A102.
5. McElveen, S. R.; Gavardinas, K.; Stamberger, J. A.; Mohan,
Literature Cited R. S. J. Chem. Educ. 1999, 76, 535–536.
6. For example see: (a) Clennan, M. M.; Clennan, E. L. J. Chem.
1. (a) Vollhardt, K. P. C.; Schore, N. E. Organic Chemistry: Struc- Educ. 2005, 82, 1676–1678. (b) Fleisher, J. M. J. Chem. Educ.
ture and Function, 5th ed.; W. H. Freeman & Co: New York, 2002, 79, 1247–1248. (c) Zahedkargaran, H.; Smith, L. R.
2007; pp 726–735. (b) Yurkanis Bruice, P. Organic Chemistry, J. Chem. Educ. 2001, 78, 1379–1380. (c) Pelter, M. W.;
4th ed.; Prentice Hall: Upper Saddle River, NJ, 2004; pp 609– Macudzinski, R. M. J. Chem. Educ. 1999, 76, 826–828. (d)
610. (c) Solomon, G.; Fryhle, C. Organic Chemistry, 7th ed.; Kjonaas, R. A.; Soller, J. L.; McCoy, L. A. J. Chem. Educ. 1997,
John Wiley & Sons, Inc.: New York, 2000; pp 667–680. 74, 1104–1105.
2. (a) Wilcox, C. F. W.; Wilcox, M. F. Experimental Organic 7. (a) Pavia, D. L.; Lampman, G. M.; Kriz, G. S.; Engel, R. G.
Chemistry: A Small Scale Approach, 2nd ed.; Prentice-Hall: Microscale and Macroscale Techniques in the Organic Laboratory;
Englewood Cliffs, NJ, 1995; pp 392–396. (b) Campbell, B. Harcourt College Publishers: Philadelphia, 2002; pp 333–354,
N.; McCarthy Ali, M. Organic Chemistry Experiments. Micro- 470–491. (b) Palleros, D. R. Experimental Organic Chemistry;
scale and Semi-Microscale; Brooks/Cole Publishing Company: John Wiley & Sons, Inc.: New York, 2000; pp 783–800.
Belmont, CA, 1994; pp 292–305. 8. Silverstein, R. M.; Webster, F. X. Spectrometric Identification
3. (a) Asleson, G. L.; Doig, M. T.; Heldrich, F. J. J. Chem. Educ. of Organic Compounds, 6th ed.; John Wiley & Sons: New York,
1993, 70, A290–A294. (b) Hurlbut, J. A. J. Chem. Educ. 1971, 1998; pp 31–32.
48, 411–412. (c) Feldman, M.; Wheeler, J. W. J. Chem. Educ. 9. March, J. Odvanced Organic Chemistry. Reactions, Mechanisms
1967, 44, 464–465. and Structure, 3rd ed.; John Wiley & Sons: New York, 1985,
4. For example see: (a) Moroz, J. S.; Pelino, J. L.; Field, K. W. J. pp 453–457.

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