Product Ratios from Bromination of Toluene
In Organic Chemistry, we learned that the substitution patterns in electrophilic aromatic substitution depends on the original substituent. The original substituent can be ortho/para directing or meta directing. Since the methyl substituent in toluene is ortho/para directing, the bromination of toluene is expected to give mainly ortho and para bromotoloune but the ratio of ortho and para depends on many factors that are not as easy to predict. There are twice as many ortho positions but they are more sterically hindered than the para position. The literature shows that the percent ortho substitution can be anywhere from about 30 to 70% depending on the catalyst, the solvent, and the concentrations of each species present. The properties of these constitutional isomers are so similar that analysis using chromatagraphic separation (GC, HPLC, etc.) is difficult. In this laboratory, we will use Fourier-Transform Infrared (FTIR) spectroscopy to determine the product composition from the bromination of toluene by using the characteristic frequencies shown above.
Since toluene is a liquid there is no need for a solvent. We'll use a staple as the iron catalyst and add enough bromine to completely react all toluene using the following procedures:
1. Secure an ordinary staple, straighten it and using either a file or sand paper to remove most of the polymer coating, exposing the bare metal.
2. Take a 12 x 75 mm (3") test tube, stopper it with a cork , place it in a small beaker and accurately weigh it, using an analytical balance. Record the weight in your notebook.
3. Remove the tube from the balance and add approximately 182 mg of toluene; about 10 drops using a disposable pipette
4. Stopper the tube, re-weigh it and record the weight.
5. Remove the tube and, in a fume hood, add about 320 mg of bromine, about 15 drops using a disposable pipette.
6. Stopper the tube and re-weigh it. Calculate the mmols of toluene and bromine in your tube and if the moles of bromine are less than the moles of toluene, add an additional drop or two of bromine to the tube.
7. Place the tube in the hood and add the staple to it. An immediate evolution of hydrogen bromide should be apparent.
8. Allow the tube to sit in the hood for about 30 minutes or until the deep reddish-brown color of the bromine has faded to a pale yellow. If the solution becomes colorless, an additional drop of bromine should be added.
9. When the reaction has ceased, resulting in a pale yellow solution, remove the staple and analyze the reaction mixture as detailed below.
Place a drop of the product mixture between two salt plates and collect a spectrum with 64 scans at 2 cm-1 resolution. If insufficient bromine was used, a small residual toluene peak at 729 cm-1 might be observed. If this is the case, add another drop or two of bromine and allow the reaction to go to completion. Measure the baseline-corrected peak areas for the three peaks shown in Table I. Prior calibration measurements show that the peak areas are proportional to concentration at typical reaction product ratios. Percent composition can be calculated by dividing the peak area by the sum of the peak areas for all three isomers and multiplying by 100.
| Isomer | Ref.Peak |
| para-bromotoluene | 802 cm-1 (w/ low freq. shoulder) |
| meta-bromotoluene | 772 cm-1 |
| ortho-bromotoluene | 747 cm-1 |
| toluene | 729 cm-1 |
Your own data should have five different analyses of the same data (using the widest possible integrated areas, the narrowest possible, and at least three more measurements that are closest to what you believe is the correct area with slight differences in the baseline position). Write down the integrated area for each peak and calculate the percent composition by taking the peak area divided by total area for the three peaks.
Being as consistent as possible and using your best guess as to the proper integration areas and baselines, analyze your own and all other students' data files. Only eliminate that data which you believe can be eliminated by use of the Q-test. A sample table like that provided below should be presented in your results section.
| Area | % composition | |||||
|
student initials
|
para |
meta
|
ortho
|
para |
meta
|
ortho
|
|
CZ
|
160
|
16
|
60
|
67.8
|
6.8
|
25.4
|
|
ME
|
165
|
17
|
65
|
66.8
|
6.9
|
26.3
|
|
YU
|
155
|
16
|
55
|
68.6
|
7.1
|
24.3
|
|
ETC
|
161
|
16
|
61
|
67.6
|
6.7
|
25.6
|
|
ETAL
|
159
|
16
|
59
|
67.9
|
6.8
|
25.2
|
|
Average:
|
160
|
16.2
|
60
|
67.7
|
6.9
|
25.4
|
|
Std. Deviation:
|
3.6
|
0.4
|
3.6
|
0.6
|
0.2
|
0.7
|
|
coefficent of variation
|
2.3%
|
2.8%
|
6.0%
|
1.0%
|
2.2%
|
2.9%
|
|
95% CL
|
160 ± 4
|
16.2 ± 0.6
|
60 ± 4
|
67.8 ± 0.8
|
6.9 ± 0.2
|
25.4 ± 0.9
|
|
Relative Rate of Attack
within 95% CL |
|
|
19.7 ± 0.5
|
1.00 ± 0.03
|
3.7 ± 0.1
|
|
Points for Analysis:
When using the Q-test to eliminate errand data is it most appropriate to compare Areas or percent composition?
What are the 95% confidence limits on the percent composition based on the assumption that there is random error associated with each measurement?
The relative rate of attack at each position can be calculated by dividing the percent compositions by the number of possible positions of attack on the benzene ring to get the intrinsic rate of attack at each position. Since these numbers are meaningless, independently of each other, they should be normalized by dividing all the values by the smallest value. For example if the percent compositions were 60:10:30 (for para, meta, and ortho) then the relative rates of attack would be 12:1:3. [60:10:30 --> 60:5:15 after accounting for number of positions --> 12:1:3 after dividing all values by 5 to get the final normalized ratio.] Use the error propagation methods shown in Table a1-5 of the statistics handout (Skoog, Appendix A) to calculate the standard deviation for each relative rate. Use the confidence limits to make sure you express your relative rates to the proper number of significant figures.
