Experiment 42
Reactivities of Aromatic Compounds
Ashly Dowdy
3-17-09
Abstract
This experiment prepared, separated, and purified p-Bromoacetanilide with a percent yield of
33.5%. The use of NMR, GC/MS, and melting point was used to identify the product. The 13C
NMR showed the expected number of carbon environments for the molecule. Proton NMR was
utilized to determine that one Br atom was present due to the ratio of area for hydrogen peaks.
GC/MS data showed a close 1:1 ratio between the M peak and the M+2 peak for Bromine,
which indicates one bromine atom was added to the molecule. This was confirmed by the 97
quality match with N-(4-bromophenyl)-Acetamide. Also, the melting point range 165.2-168 °C
matched closely to the melting point from the literature value of 168 °C for p-Bromoacetanilide.
Introduction
Based on this experiment, the reactivity’s of three different aromatic compounds can be
distinguished. The starting materials anisole, aniline or acetanilide was added to a hydrobromic
acid/bromine mixture that undergoes an electrophilic aromatic substitution reaction that adds
bromine atoms to the aromatic ring. When the addition occurs, the positioning of the new
atom on the ring depends on the group already present on the ring. These groups can be
activators, which place new atoms at the ortho/para positions or deactivators which place new
atoms at the meta position on the ring. Not only can the positioning of the new bromine be
determined by the original group but also the amount of bromine atoms that are able to be
added, called the reactivity. Carbon and Proton NMR and GC/MS data help illlustrate the
number of bromine atoms added to the aromatic region. Based on the addition of bromine
atoms the reactivity of each molecule can be ranked in relation to one another.
Experimental Procedure
The experimental procedure was followed on pages 347-352 in Introduction to Organic
Laboratory Techniques.
A drying tube was not applied to the apparatus. The amount of acetanilide that was
added was 0.453 g instead of 0.45 g. The 5.0mL of bromine/hydroromic acid mixture was
directly added to the claisen head. The addition of 2.5mL of unsaturated sodium bisulfate
solution instead of 2.5mL of saturated was added first, then a second addition of 2.5mL of
saturated sodium bisulfate solution corrected the error. Carbon and Proton NMR and GC/MS
data was also collected.
O
H
N
H
+
Br2
N
O
Br
Acetanilide
Starting:
0.453 g
0.00335 mol
Limiting
Bromine
p-Bromoacetanilide
15.514 g
0.194 mol
Theoretical: 0.00335 mol
Theoretical: 0.717 g
Isolated: 0.24 g, yield of 33.5%
Data
Proton NMR Table
Position, ppm
9.608
Splitting Pattern
Singlet
7.616
7.420
Solvent
Doublet
Observed Area
10.81
No. of H Atoms
1
Interpretation
N-H
A
22.93
7.260
Doublet
22.96
3.065
2.014
Impurity- possible water
Singlet
35.02
2
2
C-H on aromatic
ring- closest to
N-H bond
B
C-H on aromatic
ring- closest to
Br atom
C
3
CH3 group
connected to
C=O
D
1.852
Impurity
Carbon NMR Table ( in 3:1 mixture of DMSO/CDCl3)
Position, ppm
168.949
138.487
131.446
121.285
115.414
24.284
Interpretation
C=O
C connected to aromatic ring and C-N-H bond
C on aromatic ring at the ortho position relative to the activating group
C on aromatic ring at the meta position relative to the activating group
C on aromatic ring C-Br at the para position relative to the activating group
CH3 group directly connected to C=O
GC/MS Table – Peak at 11.104 minutes
Abudance
43
O
H
H H
92
H
N
H
+
171
H
N
H
Br
173
* Using Br79
H
N
Br
213/215
H
* Using Br81
H
N
Br
O
* Br could be isotope 79 or 81
A
F
E
D
C
B
Data gathered from Reference GC/MS:
Starting Material: Aniline 2,4,6-tribromobenzenamine
Starting Material: Anisole 2,4-dibromo-1-methoxybenzene
Results/Discussion
Mechanism for Bromination:
+
+
H
Br
Br-Br
+
O
H
O
H
N
+
Br
+
N
+
H
H
O
H
O
H
N
+
Br
N
-
+
H
Br
As multiple additions of bromine occurred, the reaction would continue using the same mechanism.
The reaction would get slower due to hindrance formation as each Br was added.
Proton NMR was useful when analyzing the position and number of bromine atoms present
after the reaction. Based on where the Br atom is placed on the aromatic ring, the hydrogen
environments will change. For example, if the –CH3 group has an area of 30 and the Br was arranged in
the para position, then the total area of hydrogens on the aromatic ring would be 40. On the other
hand, GC/MS better defines the number of bromine’s added to the molecule. Observing the M peak,
M+2 peak, and M+4 peaks for a mass spectrum will help identify the number of bromine’s present. If
there only one bromine present, the M peak and M+2 peak will be a 1:1 ratio. More than one bromine
atom present will create a distinct M+4 peak in addition to the M peak and M+2. For example, the
reference spectrum of 2,4-dibromo-1-methoxybenzene shows a 1:3:1 ratio between the M, M+2, and
M+4 peaks. It appears that on the reference spectrum 2,4,6-tribromobenzenamine, the difference
between the three peaks is less due to the addition of a third bromine atom.
Reactivity: Least Most
Acetanilide – Anisole - Aniline
Questions:
1. The two resonance structures are shown for the amino group below. The para substitution
allows for the positive charge to be delocalized onto the substituent.
H
H
N
H
H
H
+
H
N
H
N
H
E
H
E
N
H
2. The least activating substituent was acetanilide. Bromination only took place once at the para
position because that was the least hindered position.
Appendix
Attached!
References:
C222-2009-03-10-lmthomas-1.D\data.ms, Lauren Thomas obtained by Ashly Dowdy
C222-2009-03-10-kablaney-1.D\data.ms, Katie Blaney obtained by Ashly Dowdy
Calculations: Attached!