KINETICSO F REACTION
BETWEEN
BROMINE AND ACETYL BROMIDE
469
The Kinetics of the Reaction between Bromine and Acetyl Bromide in Nitrobenzene Solution
by Carl Cicero and Dan Mathews Graduate Institute of Technology, University of Arkansas, Little Rock, Arkansas
+
(Received August 23, 1963)
+
The kinetics of the reaction Br2 CH3COBr = CHzBrCOBr HBr have been studied in nitrobenzene solution between 50 and 70". The second-order rate law and quantities of activation are consistent with addition of bromine to the enol form of acetyl bromide.
Introduction There is a considerable amount of qualitative information available concerning reactions of halogens with compounds containing a carbonyl group.' The mechanistic interpretation of all a-halogenations of carbonyl compounds follows the same line of reasoning as that used by Lapworth* in his explanation of the bromination of acetone. This explanation of cyhalogenation requires that the carbonyl compound be converted to the enol form, which reacts with the halogen t o form the reaction products, presumably by an alkenc addition-type mechanism. This study tests this hypothesis with regard t o the bromination of acetyl bromide and explains some of the uncertainties.
Experimental Reagents. The purity of the Eastman White Label acetyl bromide was determined by the method of Siggia,3 and the purity of the Baker and Adamson reagent grade bromine was determined by titration with a standard solution of sodium thiosulfate. In both cases the impurities were less than 0.1% and these reagents were used without further treatment. The solvent was Eastman reagent grade nitrobenzene and was used without further purification. ,411 titrations were performed with solutions t h a t had been standardized against a primary standard. In all experiments a reaction stock solution was first prepared by adding a quantity of acetyl bromide t o a 100-ml. volumetric flask containing about 80 ml. of nitrobenzene and the weight of the acetyl bromide was determined by difference. Bromine was next added and its weight determined in a similar manner.
The reagents were protected with dry nitrogen a t all times. The reaction mixture was prepared at room temperature and immediately cooled to So, at which temperatiire it was maintained until the kinetic runs were started. Apparatus. During the kiaetic measurements the temperature was maintained within the range of ~ t 0 . 0 2as~ indicated by a Beckmann differential thermometer. A Sargent constant temperature bath was filled with paraffin oil and used to maintain the constant temperature. An N.B.S. calibrated thermometer was used t o determine the temperature of the oil bath and to calibrate the differential thermometer. All apparatus used in the analysis of the reaction mixtures were calibrated to ensure accuracy. Procedure. The minimum temperature a t which the reaction could be followed conveniently was 50'. At this temperature the vapor pressure of bromine and acetyl bromide is sufficiently high that the reaction must be measured in a sealed container. To accomplish this, capsules were prepared by making a constriction in a 10-ml. test tube 2 cm. from the open end. The reaction stock solution was transferred to a 500ml. Pyrex washing bottle, which had been modified by cutting off the capillary tip of the outlet tube. A serum rubber stopper was placed in the outlet tube and used as a means of withdrawing aliquots of the reaction stock solution. The flask and contents were placed in a water bath (1) H. B. Watson, Chem. Rea., 7 , 173 (1930).
(2) A. Lapworth, J . Chem. SOC.,8 5 , 30 (1904). S. Siggia, "Quantitative Organic Analysis Via Functional Groups," John U'iley and Sons, Inc., London, 1954, p. 57. (3)
Volume 68, Number 3
March, 1964
470
CARLCICEROASD DAKMATHEWS
maintained a t 5' by addition of ice. Aliquots for the kinetic runs were 11-ithdrawn with a pre-cooled 2 ml. syringe. The plunger in this syringe was equipped with a Teflon tip. The sample was transferred from the syringe to the dried capsule by inserting the needle through the constriction. The reaction capsules were then sealed. The reaction was started by placing all of the filled and sealed capsules in the constant temperature bath a t the same time. The slight time lag in reaching thermal equilibrium had no measurable effect on the results. The rate was followed by withdrawing capsules at various times and rapidly cooling them with liquid nitrogen. X reaction sample was obtained for analysis by scratching the capsule containing the frGzen reaction mixture with a sharp file and causing it t o crack by touching the scratch with a hot rod. The capsules were transferred to a beaker containing acidified potassium iodide solution, broken a t the crack, and washed with a water-ethanol solution. The liberated iodine was then titrated with sodium thiosulfate solution. Calculations. The initial concentrations of reactants were calculated from the analysis of the reaction stock solution. All other concentrations were calculated from the bromine analysis and the initial concentrations, by utilizing the stoichiometric equation of the reaction. Stoichiometry. The reaction CH3-COBr Br2 = CH2BrCOBr HBr has been reported to occur mithout appreciable side reaction. Samples were analyzed by gas chromatography in this work and no side reactions were found. There is not sufficient thermodynamic data available for an equilibrium calculation, but calculations from the measured bromine concentration indicate a value of 1 X IO2 at 70" for the quantity (bromine reacted) 2/(hromine)(acetyl bromide), where the parentheses represent concentrations. Treatment of Data. The concentration dependence of the reaction was initially determined by plotting the logarithm of the rate vs. the logarithm of acetyl bromide concentration. The result was a line with a slope of minus one, thus indicating a value of 1 for the order with respect to acetyl bromide. A similar procedure was used to determine the order with respect to bromine and a value of 1 was also obtained. Figure 1 shows graphs of the logarithm of (Br2) (AcBr) us. time. The data which have the poorest agreement with a second-order graph (curve h a t 70') were obtained with an excess of acetyl bromide. Even in this case there is good agreement with the second-
+
The Journal of Physical Chemistrv
+
order equation for 50 min., during which time 85% of the bromine had reacted. When equal concentrations of reactants were used a plot of l/(Brz) us. time was linear. The reaction was thus considered to be second order. It is possible to express k ' , the specific velocity constant, as a function of temperature and quantities of activation. Data were collected a t 50.90, 60.60, and 70.30". The temperature variation was f0.02' in each case. The values of IC' determined from the slopes of the second-order graphs for the respective temperatures were 1.21 i 0.14 X 3.00 f 0.32 X lop4, and 6.90 f 0.13 X l./mole sec. The f terms represent the standard deviation from the average of three determinations. Each determination a t a constant temperature was made from data collected from different initial concentrations of reactants. A graph of the logarithm of IC' us. 1 / T resulted in a line with a slope of -4.443 X lo3'. This corresponds to an Arrhenius activation energy of 20.33 kcal./ mole and a frequency factor of 5.84 X lo9 when the standard state is 1mole/l. The data can be conveniently summarized by substituting this expression for k' into the rate expression. Thus, 1, t ( a - b) In b(a - z ) / a ( b - z) = 5.84 X lo9 exp (- 10,23O/T) summarizes the data from 50 to 70" for unequal initial concentrations. The transition state theory postulates that I C ' = k T / h exp(AS*/R) exp(-AE,+/RT) for condensed systems,6 in which case AH* = E , - RT. This leads t o a value of 19,670 cal./mole for AH* a t 60.6'. Substitution of the value of IC' a t 60.6' leads to a value of - 16.0 e.u. for A S * when the standard state is 1 mole/ 1. Using a standard state of 1 mole/ml. leads to a value of -29.7 e.u. for AS*.
Conclusions While it is commonly assumed that the transition from the keto to the enol form of carbonyl compounds is slow, it is necessary to assume that this transition is fast in the reaction under consideration. If the process were slow, the reaction would be first order in the initial stages and would be autocatalyzed by the HBr produced. Various processes were considered that did not include the en01 formation. All of these failed to meet the high negative activation entropy requirement. This entropy requirement is satisfied if it is assumed that a bromine molecule approaches the enol form of acetyl bromide to form the activated complex
(4) 0. Aschan, Ber., 45, 1913 (1912).
( 5 ) W. J. Moore, "Physical Chemistry," Prentice-Hall, Inc., Engiewood Cliffs, N. J., 1962.
KISETICSO F
in an olefin type addition process. A probable steric arrangement is
H
c
E
\
1.0 0.9 0.8 0.7
B m k: 0.6
3c
Br
\
/ c=c / \
H
47 1
BROMIXE A N D ACETYLBROMIDE
RE.4CTrON BETWEES
v *
u? ,? 0.5 3 -
t ---f
0.4
-.:
0-13
Br
*y *
-c 3
Br
0.3
0.2
$2 50
0.1
H
I
Br-
I
I
1.00
/v \ Br+ 0 I
L
/I
H Br
\
\
0
HBr
H
where the formation of the activated complex also causes a charge separation. The charge separation causes an alignment of the polar nitrobenzene molecules, thus creating a large negative activation entropy. The mechanism of the bromination of olefins is similar to the one postulated here.6
Ri
Ri
Br+
Rz
I
1
Br Since the activation energies of homologous reactions vary only slightly and in a saw-tooth manner7 it appears that the groups R1 and Rz affect the potential energy of the activated complex in the same direction and roughly to the same extent as they affect the reacting compound. The activation energy found in this work is very close to that found in the bromination of olefins, thus supporting an olefin addition mechanism. The scheme presented accounts for all of the facts obtained from this investigation. JIoreover, the reaction rate in nonpolar solvents can be estimated by calculating the change in K* as the dielectric constant of the medium variesR assuming the same mechanism in all solvents, In K* = In KO* e,e,/D(r, r,)lcT, where e , and e, are the charges produced in the forma-
+
+
0
2.00 Time, hr.
I
1
I
3.00
I
4.00
I
I 5.00
Figure 1. Second-order rate plots for unequal reactant concentrations a t different temperatures: curves Ainitial (&*) = 1.0 M , initial AcBr = 1.5 M ; curves Binitial (Br?) = 1.5 M , initial AcBr = 1.0 M ( A = 50°, 0 = 60°, 0 = 70”).
+
tion of the activated complex, r , rj is twice the internuclear separation of bromine, and D is the dielectric constant of the solvent. The solvent dielectric constants were calculated from the equation D = Using the values for eie, of -(4.8 X 10-’o)2and 1 X for r , r,, the equation takes the form In K* = C1 50/D a t 60’. The calculated values of the dielectric constant for carbon tetrachloride and nitrobenzene are 2.2 and 30, respectively. The relative rate in carbon tetrachloride is thus calculated to be slower by a factor of one million. Watson’ has stated that the reaction is very slow in carbon tetrachloride solvent and similar reactions in nonpolar solvents have been found to be c ~ m p l e x thus , ~ implying a different mechanism. While the above calculation has little chance of being exact, it is sufficient t o suspect that the reaction must proceed by a different mechanism in nonpolar solvents.
+
Rz
I
+
(6) E. S. Could, “Mechanism and Structure in Organic Chemistry,” Holt, Rinehart and Winston, Kew York, N. Y . , 1959, p. 516. (7) E . A. Moelwyn-Hughes, “The Kinetics of Reaction in Solution,” 2nd E d . , Oxford a t the Clarendon Press, London, 1947, p. 123. (8) E. A. AMoelwyn-Hughes, “Physical Chemistry,” Pergamon Press, Inc., 2nd Ed., New York, N. Y.. 1961, pp. 883-885. (9) J. F. Bunnett, “Technique of Organic Chemistry,” Vol. VIII, S. L. Friess, E. S. Lewis, and A. Weissberger, Ed., Interscience Publishers, Inc., New York, K.Y . , 1961, pp. 272-274.
Volume 68, Number 3
March, 1984
