[CONTRIBUTION No. 161 FROM THE RESEARCH LABORATORY OF ORGANIC CHEMISTRY, MASSACHUSETTS INSTITUTE OF TECHNOLOGY]
EFFECT OF GROUPS ON REACTION RATE. REACTION OF a,P-DIBROMIDES WITH SODIUM IODIDE TENNEY L. DAVIS
AND
ROBERT HEGGIE
Received August 30, 1937
The reaction of a,p-dibromides with iodides is well suited for a quantitative study of the effect of substituents on reaction rate for the reasons that the dibromides can be prepared readily in a state of purity and the progress of the reaction can be followed easily by titration of the liberated iodine. The reaction was apparently first used by Perkin,l who obtained coumarine from its dibromide by the action of potassium iodide. Since that time it has come to supply one of the standard methods by which the organic chemist is accustomed to prepare ethylene derivatives. Further knowledge concerning the factors which influence its rate will be of immediate practical importance in the laboratory. Slator2studied the rates of the decomposition of ethylene iodide and of ethylene iodobromide under the influence of the iodide ion, Biilmann3 that of the reaction of CY ,/I-dibromopropionic acid with potassium iodide; van Duin4 established the generality of the reaction and measured the rate at 25" in a number of cases, and Dillon and his co-workers5the rates of reaction of ethylene, propylene, and butylene bromides with potassium iodide. Except for the experiments of Dillon, all the rate measurements were made at one temperature. It is now generally recognized, particularly in the light of the discussions of Hinshelwood,6 that the reaction rate constant is the resultant of the several factors which are indicated in the equation, K = PZe-E'RT. The so-called critical increment, E, is calculated from the temperature coefficients of the velocity constant; 2, the collision factor, is calculated from kinetic theory; and P, the probability factor, is the value which must be inserted in the equation, after E and Z have been found, to give the correct value for K . The factor P has been interpreted as measuring the probability of successful collision, and as such is dependent upon the PERKIN, J . Chem. SOC.,24, 437 (1871) SLATOR, ibid., 86, 1697 (1904). 3 BIILMANN, Rec. trav. chim., 36, 319 (1917). 4 VAN DUIN,ibid., 46, 345 (1926). 6 DILLON, YOUNG, AND LUOAS, J . A m . Chem. SOC.,62, 1953 (1930). 6 HINSHELWOOD, J . Chem. SOC.,1936, 1111. 470 1
2
471
REACTION OF a,@-DIBROMIDES WITH IODIDE
shapes of the molecules, their orientation, the nature of the medium in which the reaction occurs, and various other factors. I n the present work we have measured the rates of reaction of a number of a,@-dibromides with sodium iodide in absolutt: alcohol and in dry acetone at several temperatures for the purpose of determining the effect of substituent groups upon the constants, E and E', of the above equation. The duration of the runs and the extent of the reactions in our several experiments are reported in Table I. I n each case the initial concentraTABLE I DURATION AND EXTENT OF THE REACTIONS Experiments in Alcohol Temperature. . . . . . . . . . . . . . . . . .
56.3"
__
)URATION
DIBROMIDE
(HOURS)
Ethylene. ..................... Propylene. .................... Amylene-1 .................... Allyl alcohol. . . . . . . . . . . . . . . . . Acrylic acid . . . . . . . . . . . . . . . . . . . Ethyl acrylate. . . . . . . . . . . . . . . . Crotonic acid.. . . . . . . . . . . . . . . . Maleic acid . . . . . . . . . . . . . . . . . . .
178. 458. 417. 458. 44. 117. -
80.
47 14 11 7 24 81 81
28. 164. 103. 103. 32. 7.5 28. 20.6
41 23 19 8 54 36 17 69
1.2 32.1 32.4 32.1 2.2 1.5 9.7 4.4
1
12 25 31 20 27 38 29 80
~
Experiments in Acetone Temperature. . . . . . . . . . . . . . . . .
0"
__
2 O , .m
DIBROMIDE
_-
cg l o
ga
___
Benzalacetophenone . . . . . . . . . . 0.17 p-Nitrobenzalacetophenone.. , 0.19 m-Chlorobenzalacetophenone. __ Benzalacetone . . . . . . . . . . . . . . . Acrylic acid . . . . . . . . . . . . . . . . . . 20.8
__
!%% D
ff
P 1
si BE -__ 0.025 23 0.017 27 0.017 24
_ _
_ _
tions in the reaction mixture, while known exactly, were approximately 0.015 molal of the dibromide and approximately 0.30 molal of sodium iodide. The longest runs were those with propylene dibromide and allyl alcohol dibromide in alcohol solution at 25.3' in which 14 per cent. and 7 per cent., respectively, of these materials were converted into ethylene derivatives during 458 hours; the shortest runs were those with the dibromides of p-nitro- and mchlorobenzalacetophenone in acetone at 14.3" in which 27 per cent. and 24 per cent. conversion respectively was accom-
472
TENNEY L. DAVIS AND ROBERT HEGGIE
plished in 61 seconds. Acetone was used as a solvent for the chalcone dibromides because of the slight solubility of these substances in alcohol. Acrylic acid dibromide (CY,P-dibromopropionic acid) reacts with sodium iodide much faster in acetone than in alcohol-and acetone has been found in a number of cases in this laboratory to be a much better solvent than alcohol for use in the preparation of ethylene derivatives from CY ,p-dibromo compounds. The experimental data from all of our runs, except those with the dibromides of propylene a'nd amylene-1, gave good second-order reaction constants when calculated in accordance with the formula,
K z = t(2a - b)
In
b(a - 2) a(b - 22)'
where a is the initial concentration of the dibromide in moles per liter, b the initial concentration of sodium iodide, and x the number of moles per liter of the dibromide which have reacted at the end of the time t. The constancy of the reaction constants is illustrated by the data of the run with acrylic acid dibromide, which are reported in detail as a typical example in Table 11, where the constants are calculated for the time, t, in minutes. The reaction was inconveniently slow when 0.03 M sodium iodide was used-and solutions approximately 0.3 M in sodium iodide were accordingly used in all other cases. The data of our experiment with acrylic acid dibromide are especially interesting because they supply an example of the kinetic salt effect which has been discussed by Dillon;7 the constant was increased by about ten per cent. when the initial concentration of sodium iodide was increased to tenfold. In Table I11 the rate constants are calculated for the time, t, in seconds, in order that they may be compared more conveniently with constants which have been reported by others in discussions of the critical increment and probability factors, E and P. The constants reported in the table are average values of the second-order reaction constants, except for the experiments with the dibromides of propylene and amylene-1, in which cases the constants were found to decrease steadily as the reactions progressed-and the reported constants are those which were obtained by extrapolating the pseudo-unimolecular constants to zero time. We do not place great confidence in the constants which we have listed for these two reactions, but believe that they may have some interest as a basis for comparison. Although second-order constants are reported in the table, we have found that our data correspond to good pseudounimolecular constants in all cases except these two. 7
DILLON, J . Am. Chem. SOC., 54,952 (1932).
REACTION OF
Cy
/%DIBROMIDES WITH IODIDE
473
The chalcone dibromides have rates several thousand times as great as those of the aliphatic compounds with which we have worked. The evidence is not sufficient to enable us to judge whether this difference of rate is to be ascribed to the phenyl or to the carbonyl or benzoyl group, or to both, but some of the difference is undoubtedly to be attributed to the use of acetone as a solvent for the chalcone dibromides and of alcohol for the aliphatic compounds, for the reaction rate of acrylic acid dibromide a t TABLE I1 DETAILSOF EXPERIMENT WITH ACRYLICACID DIBROMIDE IN ALCOHOLSOLUTION & (%IN.)
,015 .015 ,015 ,015 ,015 ,015 ,015 .015 ,015 ,015
.030 ,030 ,030 .030 ,300 ,300 ,300 ,300 ,300 .300
.0200 .0200 ,0200 ,0200 .OH6 ,0186 .OB6 ,0186 .OB6 ,0186
0.35 0.92 0.76 2.16 1.42 1.90 1.92 1.89 2.05 3.92
2363 7143 5698 18420 883 1135 1160 1190 1298 2660
z
.OW35 .00092 .00076 .00216 .00132 .00177 .00179 ,00176 ,00191 .00365
a-z
,01465 .01408 .01424 ,01284 .01363 ,01318 .01316 .01319 .01304 ,01130
K , X 10'
3.38 3.05 3.12 3.04 3.51 3.73 3.68 3.54 3.53 3.55 K , X 10'
,01467 ,01467 ,01467 ,01467 ,01467 I
,298 ,298 ,298 .298 ,298
.00183 ,00183 ,0186 .0 186 .0186
6.90 12.45 1.37 3.35 4.27
200 395 455 1360 1915
,00126 .00228 .00255 .00623 ,00793
.01341 ,01239 ,01212 .00844 ,00674
1.52 1.45 1.42 1.40 1.41 Ks X 10'
,01458 ,01458 ,01458 ,01458
,2916 ,2916 .2916 .2916
.OB6 .OH6 .ON6 ,0186
1.98 2.54 3.48 4.10
50
75 110 133
.00188 .00241 .00331 .00390
.01270 .01217 ,01127 .01068
8.08 8.33 8.11 8.13
25.3" in acetone is about 290 times as great as in alcohol. The m-chloroand p-nitrochalcone dibromides react considerably faster than the unsubstituted compound. p-Methoxychalcone dibromide reacted so fast that its rate could not be measured. In the aliphatic series, in accordance with the views of Hughes and Ingold,* electron-attracting substituents like the carboxyl and carbethoxy groups increase the rate, while substituents which 8
HUGHES AND INOOLD, J. Chem. SOC.,1938,244.
474
TENNEY I,. DAVIS AND ROBERT HEGGIE
tend to supply electrons, either by an inductive effect like methyl or by a tautomeric effect like hydroxyl, diminish it. The actual order of the influence of the substituents upon the rates depends upon the temperature which is chosen for the comparison. Thus, at 25.3" the rate is greater for propylene than for amylene-1 dibromide, but at 37.2" and a t 56.3" the order is reversed. Similarly, maleic acid dibromide reacts more slowly than the dibromide of ethyl acrylate at 25.3" and at 37.2", but more rapidly a t 56.3". These cases show that the TABLE I11 REACTIONRATE CONSTANTS ( K = K z X 106 see-1) Experiments in Alcohol Temperature.. . . . . . . . . . . . . . . .
.I
25.3"
56.3"
AV. DEV.
DIBROMIDE
(70)
5
Amylene-1. . . . . . . . . . . . . . . . . . . . 0.355 Allyl alcohol. . . . . . . . . . . . . . . . . . 0.155
12.8 2.12 2.48 0.91 24.0 68.8 6.75 59.8
4
3 5
4
4 2 2 6
AV, DEI'.
K
--
('570)
94.2 20.0 26.7 8.52 136.0 343.0 32.7 372.0
3 4
4
4 1
3 4 3
Experiments in Acetone Temperature. . . . . . . . . . . . . . . . . DIBROMIDE
8.6"
O0 K
AV.
DEV.
1 K
(%)
____
Benxalacetophenone . . . . . . . . . . p-Xtrobenzalacetophenone.. m-Chlorobenzalacetophenone Benzalacetone. . . . . . . . . . . . . . . Acrylic acid.. . . . . . . . . . . . . . . . . .
I
14.3' AV.
DEV.
K
~(%)
1 6 -
990 1660 1520
-
-
-
-
-
_
25.3"
2 1 4
2070 1730
numbers which represent the reaction constants are not alone a sufficient basis for an adequate discussion of the effects of substituent groups. From the reaction constants, K , at the several temperatures (Table 111) the energies of activation listed in Table IV have been calculated. The averages of these values for each reaction (substantially and within the limits of precision the same as the average energies of activation over the temperature range of our experiments), have been substituted in the equation, K = PZe-EIRT,Z being taken as equal to 2 X lo", and have given us the values for P, the probability factor, which are listed in
REACTION OF
(Y
475
,~-DIBROMIDES WITH IODIDE
Table V. The value of 2 has been calculated from kinetic theory for the concentrations and temperatures a t which we have worked, and is in the TABLE IV IZNERGIES O F ACTIVATION: REACTIOXS O F DIBROMIDES WITH SODIUM Reactions in Alcohol
IODIDE
DIBROMIDE
........................................... Propylene. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
20.6 23.6 28.4 25.8 20.3 18.6 18.8
Acrylic acid.. . . . . ....................... Ethyl acrylate.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Crotonic acid.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Maleic acid.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reactions in Acetone
I
DIBROMIDE
Benzalacetophenone . . . . . . . . . . . . . . . . . . . . . . p-Nitrobenzalacetophenone, . . , . . , , . . , , . . , . Acrylic acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
(ked.)
18.0 19.1
-
I
E&?
21.3 28.8 26.2 24.8 19.2 17.8 17.9 19.6
(ked.)
1
(kcal.)
19.5
17.3 19.2
-
TABLE V PROBABILITY FACTORS : REACTIONS OF DIBROMIDES WITH SODIUMIODIDE Reactions in Alcohol DIBROMIDE
I
Ethylene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Propylene. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...........................................
P
0.04 1.4 240. 27. 0.007 0.0015 0.00013 0.022
Crotonic acid.. . . . . . . . . . . . . . . . ................................ Maleic acid.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reactions in Acetone
1
P
Benzalacetophenone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . p-Nit,robenealacetophenone. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acrylic acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .,
31. 1.4
DIBROMIDE
neighborhood of 2 X 1011, which is the value we have adopted for all our calculations. A difference of 25" in temperature does not change the sig-
476
TENNEY b. DAVIS AND ROBERT HEGGIE
nificant figures of this number. 2 is the number of collisions per second. If all the collisions should result in reaction, then K would be equal to 2. The other factors in the equation are the factors which determine the sucis cess of the collisions in producing the reaction. The number, the fraction of all of the molecules which possess sufficient energy to react, and P is the probability of this energy becoming effective through successful collision. P = 1 corresponds to 100 per cent. probability, or to the successful reaction of all of the molecules which have sufficient energy, and 1 is evidently the highest value of P which is theoretically possible. Uncertainties in the estimation of some of the other factors may possibly double the value of P, making it 2, but the values of 27 and 31 calculated from satisfactory data for the reactions respectively of the dibromides of allyl alcohol in alcohol and of p-nitrobenzalacetophenone in acetone make it clear that the notion of P as a probability factor must be interpreted liberally. The probability factors for the reactions of the dibromides of propylene and amylene-1 with sodium iodide have been calculated, and are reported as a matter of interest, but are discounted for the reason that these reactions gave no really satisfactory constants. It is difficult to see why the probability of successful collision should be greater in the cases of the compounds with the longer chains. Our data however appear to warrant the conclusion that ethylene dibromide owes its greater rate of reaction to its smaller energy of activation. The effect of the -CHZOH group in slowing the reaction, somewhat greater than the effect of the methyl and n-propyl groups, is definitely related to an increase in the energy of activation which overbalances the effect of an increase in the probability factor. Of the groups which increase the rate, the carboxyl and carbethoxy groups apparently act by decreasing the energy of activation, for the probability of successful collision is less than for ethylene dibromide. Acrylic ester dibromide reacts more rapidly than acrylic acid dibromide, and has a smaller energy of activation and a smaller probability factor; the greater rate is attributable to the smaller energy of activation. Crotonic acid dibromide reacts more slowly than acrylic acid dibromide, and has a smaller energy of activation and a smaller probability factor; the effect of the smaller probability factor overbalances that of the smaller energy of activation. Acrylic acid dibromide has about the same energy of activation in alcohol as in acetone, but in the latter solvent the probability factor is 200 times as great, and it reacts much faster. The effect of the acetone is to increase the probability factor, a result which may perhaps be related to the fact that acetone forms a molecular compound with sodium iodide and so increases the probability of its reaction.
REACTION OF
(Y
,&DIBROMIDES WITH IODIDE
477
The data give us the following conclusions as to the principal effects of the several groups. The -CH3, -C3H7, and -CH20H groups increase E. The -COOCzHs and -COOH groups decrease E , the former more than the latter. The effect of two -COOH groups in decreasing E is greater than the effect of one. The effect of a -COOH group in decreasing E is more than sufficient to counteract the effect of a -CH3 group tending to increase it. One -COOH group reduces P ; a second -COOH group increases P again, but not however to the value which it would possess if no -COOH group were present. EXPERIMENTS
The temperatures of the reactions were controlled by immersing the vessels a t 0" in a mixture of ice and distilled water in a Dewar flask, a t 8.6" and 14.3" in a water bath cooled by a regulated flow of t a p water, a t 25.3' and 37.2" in electrically regulated thermostats, and at 56.3" in an electrically heated and regulated glycerine bath. For the slower reactions i t was found that satisfactory results could be obtained by mixing the solutions of the dibromide and the sodium iodide in the reaction vessel, and pipetting out from time to time samples for titration of the free iodine with sodium thiosulfate. For the fast reactions i t was found necessary to make up the solutions in separate test-tubes, mix rapidly, and then to stop the reaction by diluting the mixture with water. Preparation of Materials The alcohol which was used for solvent was commercial absolute alcohol. The acetone was purified from the bisulfite compound, dried over anhydrous magnesium perchlorate, and fractionated. It boiled at 56.3" with a boiling-point range of 0.09" (Beckmann thermometer). The International Critical Tables report b.p. 56.1"; H e i l b r ~ n ,56.5". ~ All the temperatures reported in the present paper are corrected temperatures unless otherwise noted. Ethylene dibromide was fractionated twice through a Davis column, and finally distilled twice a t room temperature in a Hickman still; b.p. 131.5-132.0'. Heilbron reports b.p. 131.7". Propylene dibromide was fractionated twice and then distilled five times in the Hickman still. The index of refraction for the sodium D line a t 28" was determined on each fraction, and found t o be: lst, 1.5160; 2nd, 1.5167; 3rd, 1.5164; 4th, 1.5164; 5th (principal fraction), 1.5163 (b.p. 139-140'); and 6th (residue), 1.5158. The value given in the Znternational Critical Tables for n: is 1.5203; b.p. 140". ilnzylene-1 dibromide was prepared by combining bromine a t 0" with pure pentene-1 obtained by fractionating commercial amylene five times through a Davis column. It was distilled once a t atmospheric pressure, (b.p. 183-185"), and twice in the Hickman still. Brochet'o reports b.p. 190-191"; Dykstra, Lewis, and Board,'' 184". HEILBRON, "Dictionary of Organic Compounds," New York, 1934. RROCHET,Bull. soc. chim., [31, 7, 567 (1896). 11 DYKSTRA, LBWIS,AND BOORD, J . Am. Chem. SOL,62,3400 (1930). Io
478
TENNEY 1.,
DAVIS AND ROBERT HEGGIE
Dibromohydrine (allyl alcohol dibromide) .-Kahlbaum's material was distilled twice in the Hickman still. The product boiled a t atmospheric pressure a t 219"with decomposition, in agreement with the reports of Heilbron and of Read and Hurst.12 a, P-Dibromopropionic acid (acrylic acid dibromide).-Kahlbaum's material T? as used directly. I t was found to melt a t 59-64' and to titrate as 100% dibromopropionic acid. As this substance exists in two forms, melting a t 51" and a t 64" respectively, the latter being transformed slowly into the former when the substance is melted, the reverse change occurring on standing, the melting point of our sample is not significant. Ethyl a,P-dibromopropionate (ethyl acrylate dibromide) .-Kahlbaum's material n-as used after three distillations in the Hickman still. The product boiled a t atmospheric pressure a t 214-215" as reported by Heilbron. a,6-Dibromobutyric acid (crotonic acid dibromide) was prepared from crotonic acid. Eastman Kodak Company pure crotonic acid was recrystallized twice from water, combined with bromine in carbon disulfide solution, and the dibromide was recrystallized to constancy of melting point; m.p. 87" as reported by Heilbron. Dibromosuccinic acid (maleic acid dibromide) .-Kahlbaum's material was recrystallized once; it melted in a sealed tube a t 255-256" with decomposition, in agreement with Heilbron. Benzalacetophenone dibromide was prepared by adding the theoretical amount of bromine to benzalacetophenone dissolved in chloroform, and recrystallizing from that solvent; m.p. 156-157" as reported by Allen.13 p-Nitrobenzalacetophenone dibromide was prepared by titrating recrystallzed p-nitrobenzalacetophenone, m.p. 163-164", with bromine in chloroform a t room temperature. The material, recrystallized from alcohol and carbon disulfide, melted a t 152". Sorge14 reports m.p. 148", and Wieland,'s 151". m-Chlorobenzalacetophenonedibromide, prepared by adding the calculated quantity of bromine to m-chlorobenzalacetophenone in chloroform solution, melted a t 187", as reported by Sorge and by Bodforss.'B Benzalacetone dibromide, prepared from benzalacetone by a similar procedure, melted a t 124-125" as reported by Claisen and Claparede" and by Auwers and Brink.18 SUMMARY
The rates of reaction at several temperatures of sodium iodide in alcohol with the dibromides of ethylene, propylene, amylene-1, allyl alcohol, acrylic acid, ethyl acrylate, crotonic acid and maleic acid, and of sodium iodide in acetone with the dibromides of benzalacetophenone, p-nitroand m-chlorobenzalacetophenones,benzalacetone, and acrylic acid have been measured. READAND HURST,J. Chem. SOC.121, 995 (1914). ALLEN,in "Organic Syntheses," Vol. 8, p. 61, New York, 1928. 14 SORGE, Ber., 36, 1068 (1902). 1 5 WIELAND, ibid., 37, 1149 (1904). BODFORSS, ibid., 49, 2801 (1916). 1 7 CLAISENAND CLAPAREDE, ibid., 14, 2462 (1881). 1 8 AUWERS AND BRINK,J. prakt. Chem., [2], 133, 154 (1932). 12
13
REACTION OF CY
,P-DIBROMIDESWITH
IODIDE
479
Good second-order constants have been obtained in all cases except those of the reactions of the dibromides of propylene and amylene-1. For these two neither first-order nor second-order constants appear to be satisfactory. Acrylic acid dibromide reacts with sodium iodide much faster in acetone than in alcohol. The effects of the substituent groups upon the reaction rates have been discussed briefly. Energies of activation, E, have been calculated for the several reactions ; the probability factors, P, have been calculated from the equation K = PZe-E'RT;and conclusions have been drawn as to the effects of the substituent groups upon E and P.
