Bromination of Acetone
Adapted from an Exercise developed by the
Physical Chemists at the University of Kansas by J. M. McCormick
Last Update: November 3, 2013
Under acidic conditions ketones react with halogens to
give substitution at the α-carbon, as shown in
Fig. 1 for the reaction of Br2 for acetone. Shown in Fig. 2 is a
proposed mechanism for this reaction.1
In this exercise you will test whether this proposed mechanism is consistent
with the experimental rate law by first experimentally determining the rate law
using the isolation method (click here
to review kinetics) and deriving the rate law predicted by this mechanism.
In addition, because this mechanism involves the breaking of a C-H bond it is
expected that it should display a kinetic isotope effect when D is substituted
for H. Therefore, the rate of bromination of deuterated acetone will be
measured to determine if there is a kinetic isotope effect, and thus provide
further evidence for the proposed mechanism.
Figure 1. Reaction of Br2 with acetone
to give α-bromoacetone, Br-
Figure 2. Proposed mechanism for the reaction of Br2
CAUTION! Concentrated HCl is corrosive. Acetone is
flammable and volatile. Br2 is a corrosive strong oxidant and very
toxic. Avoid breathing Br2 fumes. Work with the concentrated
bromine water solution only in the hood. The products of these reactions are
lachrymators and moderately toxic. Handle all solutions with care and dispose of
all waste properly.
Note that Br2 absorbs in the UV.
Therefore, you will only be able to use either an Ocean Optics spectrometer
whose range is set for the UV (the Ocean Optics spectrometers in MG 1026 are
not set for the UV) or the
Before preparing the solutions, you will need to warm up the spectrometer.
Refer to the
schemes for the procedure
for determining the order with respect to each reactant. Note that you will
prepare the solutions on the left and right hand side of each scheme and then
mix these together, as indicated, to give the solutions that you will make the
kinetics measurements on. These solutions are in the center of each scheme in a
red box. It is suggested that you first determine the order with respect to Br2
Use distilled water to prepare all solutions.
Prepare a 1 M aqueous HCl stock solution from concentrated HCl, and a 4 M
aqueous acetone stock solution. The concentration of neither solution needs to
be precise, but you should have at least two or three significant figures for
each. Note that the concentration of the acetone stock solution will change
appreciably if it is left uncapped.
As the Br2 solution is colored (extinction
coefficient at 400 nm, ε400, is 160.
M-1∙cm-1), you will follow the reaction by the
disappearance of the Br2 absorbance at 400 nm (A400). A
saturated solution of Br2 in distilled water (bromine water) will be
provided. The [Br2] in the saturated solution is approximately 0.16
M, but it will change with temperature. Therefore, the [Br2] must be
determined at the start of each laboratory period. To find the [Br2],
first take 2.00 ml of the stock solution and dilute to 100.0 ml with water in a
hood. Measure the absorption spectrum of this diluted solution and from A400
determine the [Br2] using Beer’s Law.2
The dilutions in Schemes 1-3 assume a [Br2] of 0.16 M, and as such
you will need to work out the actual bromine concentrations based on your
To perform a kinetics run,
the spectrometer (for Ocean Optics spectrometers,
click here; for the Cary 50,
here) to record data at the maximum Br2
absorbance and make any necessary background/baseline corrections. You will need
to set the spectrometer’s software to acquire the data at some set interval and
to delay the start of data acquisition long enough for you to mix the reagents
and place the cuvette in the sample holder. This may require some trial and
error. It is important that you take as much data as possible right after the
reagents are mixed. The absorbance change will deviate from ideal behavior as
the reaction proceeds because of further halogenation reactions of the
bromoacetone. Only data within the first one to two minutes after mixing may be
useable and you may observe significant deviations after this time.
To start the kinetics run, place the two reagents to be
mixed, as per the particular scheme you are following, in separate beakers. Pour
one into the other and start the software countdown when they are half mixed.
Swirl the solution for a few seconds to mix the reagents well and quickly pour
the solution from the beaker into a cuvette. Place the cuvette in the cell
compartment of the spectrophotometer before the delay time is up. Once you have
finished a run you should immediately make a graph of your data to check whether
you need to change the delay time, or the sampling frequency. Take at least one
measurement at a sufficiently long time to represent the residual absorbance at
t = ∞. Based on this result you will know whether you need to make
absorbance readings at t = ∞ for all of your kinetics runs. Make at least
three runs for each different starting condition.
After you have determined the order with respect to each
reactant, determine the rate of reaction for d6-acetone (where all
1H are replaced with 2H = D). Determine whether
there is any kinetic isotope effect (kH/kD,
where the kH and kD are the rate constants
for the reaction with and without deuterium, respectively).4
Deuterated acetone is fairly expensive, so carefully plan which set of reaction
conditions given in the schemes will give you the best results. You
are to prepare only enough of the 4 M d6-acetone to perform the
Graph the raw kinetics data in Excel as described in the
kinetics review document. Extract the orders
with respect to each reactant, rates and rate constants from a best fit of the
data. Include a representative example of each graph in the results section of
Report your results (order with respect to each reactant,
the overall rate constant and kH/kD)
including uncertainties. Propagate the error in each slope and rate of reaction
through to the order of reaction and rate constant. A detailed propagation error
for the dilutions is not necessary because of the large number of dilutions
involved. However, make sensible estimates of these errors where appropriate.
Note that the order of reaction is usually has integer or half-integer values.
So, a value of 0.91±0.03, for example, may be taken as being equal to one.
Derive the rate law for the mechanism given in Fig. 2,
assuming that the first equilibrium is rapidly established (K = k1/k-1),
the second step (k2) is slow and the final step (k3)
In your Discussion be sure to address the
following: 1) whether this mechanism is consistent with your experimentally
determined rate law consistent; 2) if the rate law is consistent with the
mechanism, does it necessarily prove the mechanism and the structures of the
intermediates shown? 3) what rate constants in the mechanism does the measured
rate constant correspond to? 4) what is the role of H+ in this
reaction? 5) is your measured kinetic isotope effect consistent with the
- 1. Solomons, T. W. G. Organic Chemistry, 4th
Ed.; Wiley: New York, 1988, p. 788.
- 2. Beer’s law states that the absorbance, A, is
directly proportional to the extinction coefficient, ε, the
C, and the pathlength, b, or A = ε∙b∙C.3
- 3. Skoog, D. A.; West, D. M. and Holler, F. J.
Fundamentals of Analytical Chemistry, 5th Ed.; Saunders
College Publishing: New York, 1988, pp. 464-469.
- 4. Sykes, P. A Guidebook to Mechanism in Organic
Chemistry, 5th Ed.; Longman: New York, 1981, p. 46-48.
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