CHEM 322: RATES OF ELECTROPHILIC AROMATIC SUBSTITUTION
The rate-determining step of electrophilic aromatic substitution is the first step in which the aromatic ring
attacks an electrophile. This breaks the aromaticity of the formally aromatic ring. Then, in a subsequent faster
step, a hydrogen is lost to restore aromaticity of the aromatic ring. Therefore, the rate expression for
electrophilic aromatic substitution is:
Rate = k [Ar] [E+]
where [Ar] is the concentration of the aromatic compound and [E+] is the concentration of the electrophile. In
this experiment, bromine is used as the electrophile.
Substituents already on the ring affect the rate of reaction with an electrophile and orientation of attack.
Activators favor attack at positions that are ortho and para to themselves and increase the rate of reaction
compared to that of unsubstituted benzene, while deactivators favor meta attack and decrease the rate of
reaction (with one exception, the halogens). Depending on their electronic characteristics, existing substituents
stabilize or destabilize the (+) charge that develops during electrophilic attack at various ring carbons.
The main purpose of this experiment is to use kinetic data obtained during the bromination of a family of
similar compounds to explore the relationship between structure and reactivity. The related structures chosen
for this experiment are benzoic acid, phenyl acetate, salicylic acid, acetyl salicylate, methyl salicylate, and
methyl benzoate.
The general reaction to be studied is the following:
G
+
Br2
brown
Br + HBr
G
both products colorless
(G = a substituent attached to the ring)
Usually, halogenations of an aromatic ring are performed using an iron catalyst; however, if a catalyst were
used, the reaction would happen too fast to follow using a spectrophotometer. In order to monitor the reaction
using a spectrophotometer, it is assumed that the measured absorbance is directly proportional to the current
concentration of bromine {Beer’s Law: Absorbance = molar absorptivity (L mol-1 cm-1) * cell path length (cm)
* concentration (M)}. All solutions used in this experiment, except the bromine solution, are clear and colorless,
so they should not have much (if any) effect on the total absorbance measured.
EXPERIMENTAL SECTION
CAUTION: Bromine solutions are very corrosive. If any gets on your skin, rub the area
immediately for about a minute with a solution of sodium thiosulfate or
with glycerol, then rinse thoroughly with water. Do not breathe the acetic
acid or bromine vapors. Fill pipetters slowly to prevent spraying corrosive
solutions into the interior of the pipetter. Disassemble both pipetters as
shown after the last use to allow fumes inside to dissipate.
Acetyl salicylic acid hydrolyzes rapidly to salicylic acid and acetic acid in the acetic acid solvent and so must be
prepared fresh. The necessary recipes are on the bottle labels. Be sure to record exactly what you do when
preparing these solutions (e.g., 0.18 g acetyl salicylic acid / 20 mL 90% HOAc; all solutions should be 0.05M).
Strive for accuracy since the data analysis depends on having the same molar concentrations of Br 2 and
aromatic compound.
The instructor will illustrate proper operation of the spectrophotometer. PLEASE do not spill anything down the
sample well of the instrument. If a spill or a drip runs down the outside of the cuvet, the acid will begin to
corrode components inside the instrument. Put a Kimwipe collar around the cuvet well opening. If there is any
possibility of contamination inside, please tell the instructor so it can be cleaned up immediately.
Micropipetter: the 5 mL pipetters must be used properly to get meaningful results.
(a) NEVER tip sideways – always hold nearly vertical.
(b) Secure the tip to the body by pressing as you twist. It must not leak air. Check by slowly aspirating
some liquid, then holding vertical so that you can see the tip. No drop should form.
(c) Always fill s-l-o-w-l-y. Liquid will spurt up inside the tip if you go too fast. This may create
isolated droplets on the wall of the tip or may contaminate the interior of the pipette.
(d) During delivery, touch tip to side of receiving vessel and drag upward as you expel the last bit.
(e) After delivery, check to see that no liquid is left in the tip.
Procedure: Use supplied glass cuvets with tightly fitting covers. We will assume that all cuvets are closely
enough matched to each other that they can be used interchangeably.
CALIBRATION: First, set the instrument wavelength to 500 nm (to approximate the final setting). Fill
one cuvet with plain 90 % acetic acid, cover it and use it to calibrate the instrument to 0 A / 100% T. To another
cuvet, add 1.6 mL of 0.05 M benzoic acid in 90 % acetic acid and, recording the time of day as you do so, also
add 1.6 mL of 0.05 M Br2 in 90 % HOAc. The time of adding Br2 to each substrate is the “zero” time for that
reaction in this experiment. Cover the cuvet , invert it three times to mix the contents, then insert it into the
instrument. Find a wavelength near 500 nm at which the absorbance of the solution is near 1.0 and set the
instrument to that wavelength. Re-insert the blank cuvet and recalibrate the instrument.
MEASUREMENTS: Re-insert the Br2-benzoic acid solution. Record time of day and its absorbance.
Remove the still-covered cuvet to a safe place on the desk top. Prepare similar reaction mixtures in separate
cuvets (1.6 mL substrate + 1.6 mL Br2) for methyl benzoate and phenyl acetate (record time of mixing
separately for each). These three substrates will react very slowly, so at 4-5 convenient intervals between faster
samples (every 10 to 15 minutes will be fine), re-insert these cuvets and take readings. Record all these
absorbances along with the time of day they were taken.
For the remaining compounds, two people must work as a team. The recommended order of testing is
given below. For each, start data acquisition the moment Br2 is added (do not wait until you insert the cuvet!).
You will need to discard early “wild” data points.
 acetyl salicylic acid (freshly prepared but completely dissolved); collect data for at least 200 sec. This
one is pretty slow so can be mixed by inversion. It may not change much during the 200 sec.
 methyl salicylate (mix by inversion; collect data for at least 200 sec), and finally
 salicylic acid, which goes VERY FAST (collect data for 100 sec).
Procedure for salicylic acid: Practice rapid injection into a cuvet using water. Add 1.6 mL of water to
the cuvet. Inject an equal amount as rapidly as possible down one side without causing any splash out of the
cuvet. This technique ensures the best mixing possible with the equipment available. After you are confident in
your technique, rinse the cuvet a couple of times with 0.05 M salicylic acid solution, and shake dry.
For the “real” run, put 1.6 mL of salicylic acid solution into the cuvet and put the cuvet into the
instrument. When ready to begin, one person rapidly injects 1.6 mL of Br2 solution while the other starts the
recording.
LABORATORY REPORT:
Introduction Section (should include answers to the following questions):
 What is the main objective for this experiment?
 What is the overall desired reaction studied? It is best to answer this with a figure.
 What kinetic order do you expect this reaction to be?
 What method is used to collect the data for this experiment (i.e., what quantity is recorded as a function
of time)?
 What assumption must be valid in order for the recorded data to be useful to you?
 How should results be plotted? (Hint: Write the rate law for the reaction in differential form – d[Br2]/dt
= ?. Compare the starting concentrations of Br2 and substrate as well as reaction stoichiometry.
Separate variables, then integrate this rate expression.) Consult a freshman chemistry textbook or your
instructor if you are not sure how to proceed.
 What is the major difference between an activator and a deactivator for EAS? Refer the reader to a
figure which shows the structures of the substituted benzenes you used as substrates; identify
activator and deactivator groups.
Experimental Section: Should be in the normal format as established in CHEM 321 and 322. Avoid BS!
Results Section (should include):
 Two figures (because the various rate constants span such a wide range):
o One figure that shows integrated rate data for the four slow reactions
o Another figure that shows integrated rate data for the two fastest reactions
 A summary table that:
o Contains structures of the substituted benzenes tested, computed rate constants (with units!)
and relative rate constants (rate constant of current compound / rate constant of slowest
compound).
o Is arranged in order of increasing computed and relative rate constant.
Follow the guidelines on how to present figures
and tables in your report (on course web site).
Discussion Section:
 Reflect on the questions answered in the introduction section in terms of what you have learned from
your data analysis.
 Analyze how well your data fit the theoretical expectation.
o How closely do the data for each substrate fit the predicted straight line?
o Are there any outliers that should be removed before computing a rate constant? If any outliers
were found, discuss why you decided to remove them from your plot. Do not randomly remove
data from your plot!
 Discuss whether the data support the relationship between structure and reactivity learned in class.
 Summary Statement