Analysis of 2-Nitroaniline

Author: Lisa Stojowski

The goal of this lab is to compare the theoretical 13C NMR of 2-Nitroaniline with its experimental 13C NMR.  The theoretical spectra for the compound was obtained using Gaussian 98.  Several different models were explored in Gaussian.  I personally used the RHF/3-21G//RHF/3-21G model and the extremely time consuming B3LYP/6-311+G(d)//B3LYP/6-311+G(d).  The experimental spectra was obtained using our Brucker FT-NMR.  In this study of 2-Nitroaniline, we want to assign peaks of the experimental 13C NMR spectrum to the carbons that they belong to and explore the reason why carbon 4 has more electron density than carbon 6.
 

Introduction:

2-Nitroaniline is a highly toxic orange solid with applications in agriculture, dyes/pigments, engineering polymers, rubber chemicals, veterinary pharmaceuticals, and water treatment chemicals (http://www.solutia.com/products/nitroaniline.html).

2-Nitroaniline has a NO2 group ortho to its NH2 group. The NO2 group is a deactivating meta director whereas the NH2 group is a strongly activating ortho/para director.  Because the NO2 group is a deactivating group, it withdraws electron density from the aromatic ring and it decreases its reactivity.  The NO2 withdraws electrons from the ortho and para positions of the aromatic ring leaving the meta positions with higher electron density and as the site for substitution.


Molecular orbital representation of nitrobenzene showing the decreased electron density at the ortho/para positions
(http://www.uis.edu/~trammell/organic/aromatics/reactivityAndOrientation.htm)






Because the NH2 group is an activating group, it increases the electron density from the aromatic ring and it increases its reactivity.  The NH2 group increases electrons at the ortho/para positions of the aromatic ring increasing the electron density and becoming the sites for substitution.


Representation of aniline showing the increased electron density at the ortho/para positions.
(http://www2.ccc.uni-erlangen.de/tagung/10_cic/boegel/)






The above explanation of the effects of the NO2 and NH2 groups would leave one to believe that carbon 6 should have a greater electron density than carbon 4.  However, observations show that the electron density of carbon 4 is greater than that of carbon 6.  There must then be some interaction between the NO2 group and the NH2 group that allows the NO2 group to withdraw electrons more easily from carbon 6 than from carbon 4.  The goal of this lab is to come to an understanding of this interaction.
 
 

Literature 13C NMR Spectra of 2-Nitroaniline in CDCL3

Assignment
ppm
1
145.05
2
135.76
3
132.20
4
126.01
5
118.99
6
116.55

 



Theoretical 13C NMR of 2-Nitroaniline using RHF/3-21G//RHF/3-21G
 

Assignment
ppm
Shift wrt to Benzene**
1
80.9394
12.9217
2
111.2331
-17.372
3
90.0630
3.7981
4
86.3746
7.4865
5
74.5894
19.2717
6
108.7926
-14.9315
**The RHF/3-21G//RHF/3-21G was ran on benzene and we obtained
an average integration of 93.8611ppm.  To determine the shift with respect
to benzene, the integrations of the 2-Nitroaniline are subtracted
from this average of benzene.
 
 


Theoretical 13C NMR of 2-Nitroaniline using B3LYP/6-311+G(d)//B3LYP/6-311+G(d)
 

Assignment
ppm
Shift wrt to Benzene**
1
64.1670
-13.762
2
43.0283
7.3767
3
63.2221
-12.8171
4
32.2320
18.173
5
44.5248
5.8802
6
50.4875
-.0825
**The B3LYP/6-311+G(d)//B3LYP/6-311+G(d) was ran on benzene
and we obtained an average integration of 50.405ppm.  To determine
the shift with respect to benzene, the integrations of the 2-Nitroaniline are
subtracted from this average of benzene.
 

2-Nitroaniline Orbitals

Highest Occupied Molecular Orbital (HOMO)
Lowest Unoccupied Molecular Orbital (LUMO)

 


Experimental 13C NMR Spectra of 2-Nitroaniline in CDCL3

Assignment
ppm
Shift wrt to Benzene**
1
144.667
16.07
2
135.670
7.31
3
132.255
3.895
4
126.198
-2.162
5
118.757
-9.603
6
116.963
-11.397
**The experimental shift relative to benzene is 128.36 (number obtained from
instructor).  To determine the shift with respect to benzene, the average of
benzene is subtracted from the integrations observed.
 


Comparison of Methods*
Shifts Relative to Benzene

Assignment Literature RHF/3-21G B3LYP/6-311+G(d) Experimental 
1 +16.69 +19.2717 +18.173 +16.307
2 +3.74 +7.4865 +5.8802 +3.895
3 -2.35 +3.7981** -.0825 -2.162
4 -11.51 -17.372 -13.762 -11.397
5 +7.4 +12.9217 +7.3767 +7.31
6 -9.37 -14.9315 -12.8171 -9.603
Mean Absolute Deviation*** 4.61 1.93
*Once the several different conventions of numbering of 2-Nitroaniline were straightened out and made universal to the above numbering of the structure (the conventional numbering system) the comparisons could be made.
**Sign opposite to what it should be.  This seemed to happen to all students in class except for the B3LYP/6-311+G(d) method.
***Obtained by taking the absolute value of the sum of the (theory integrations - experimental integrations) and dividing by 6.
 
 

Mean Absolute Deviation of All Methods used In Class

NMR//Geometry
Mean Absolute Deviation
B3LYP/6-31G(d)//RHF/3-21G
1.24, 1.33 (two students deviations)
B3LYP/6-31G(d)//B3LYP/6-31G(d)
1.4
B3LYP/6-311+G(2d,p)//RHF/3-21G
1.85
B3LYP/6-311+G(d)//B3LYP/6-311+G(d)
1.93
RHF/3-21G//RHF/3-21G
4.61, 4.90 (two student deviations)

 

Conclusions:

The goal of this lab was to compare the theoretical 13C NMR of 2-Nitroaniline with its experimental 13C NMR.  My theoretical NMR spectra was obtained using two different models on Gaussian 98 - the RHF/3-21G//RHF/3-21G model and the extremely time consuming B3LYP/6-311+G(d)//B3LYP/6-311+G(d).  After analyzing the data, I found that the more time consuming B3LYP/6-311+G(d)//B3LYP/6-311+G(d) method was better than the RHF method.  However, when compared to other methods used by other students, the B3LYP/6-311+G(d)//B3LYP/6-311+G(d) method wasn't the best.  However, the B3LYP/6-311+G(d)//B3LYP/6-311+G(d) was the only method that returned all the correct signs for the shifts with respect to benzene (woohoo)!  The experimental spectrum of 2-Nitroaniline was obtained using our fabulous new FT-NMR - and it certainly proved to be fabulous.  Our experimental results were very close to literature values and our experimental results were much better than those produced by Gaussian.

All results indicate that the electron density of carbon 4 is greater than that of carbon 6.  These results are not what is expected.  Because of the nature of the substituents on the benzene ring, it is expected that the electron density of carbon 6 should be greater than that of carbon 4. There must therefore be some interaction between the NH2 and the NO2 that allows the NO2 to withdraw electrons more easily from carbon 6 than carbon 4.  Although I would love to understand this interaction, I don't.  It most likely has something to do with the molecular orbital interaction between NO2 and those two carbons but further analysis is needed to understand this.

All in all, I think that this lab was pretty successful.  I gained valuable experience with using Gaussian and with using the new NMR.  I also have a greater understanding of the properties of NMR spectra and how to analyze the data.  With more time, I believe that I could further analyze 2-Nitroaniline and come to an understanding of the results.  However, I am out of time.
 
 


last modified 07-May-01