REACTIVITY OF ~-BROMOALKANES WITH THIOCYANATE
2083
Reactivity of 1-Bromoalkanes with Thiocyanate
by T. P. Wallace and C. H. Stauffer Department of Chemistry, State University of New York at Potsdam, Potsdam, New York (Received November 8, 1968)
This investigation was concerned with the evaluation of the average rate constants, E., and AS* for the S N reaction ~ of 1-bromoalkanes with the nucleophilic thiocyanate ion in a 95% ethanol medium. The 1-bromoalkanes studied contained the two-, four-, six-, and eight-membered carbon chains. The values of E, for the reactions involving the four compounds showed no deviation within the experimental error. The values of A S * showed no deviation within the experimental error for the four-, six-, and eightmembered carbon chains. However, the value of AS* for 1-bromoethane was substantially larger. The results indicate that the two-carbon chain is the most reactive of the four compounds studied owing to a difference in the entropy of activation.
Introduction Numerous reports in the literature indicate that the reactivity of st'raight-chain alkyl halides undergoing displacement reactions decreases from ethyl through butyl, but then increases as the length of the chain increases. In 1953, Crowel12 used the nucleophilic thiocyanate ion to determine the rate constants for the S N re~ action of various alkyl bromides with alcoholic sodium thiocyanate at 25". The homogeneous reaction may be expressed by RBr
+ NaSCN +RSCN + NaBr (95% ethanol solvent)
(1)
The rate of reaction can be followed by determining the amount of thiocyanate ion present at different time intervals. Crowel12 used Lang's iodine-cyanide method3 for the quantitative determination of the thiocyanate ion, but it was found that the Andrews method* gave more reproducible results. CrowellW investigation was concerned only with the reaction of alkyl bromides with sodium thiocyanate a t 25" in a solvent of 95% ethanol. Therefore, it was not possible to calculate the activation energies or the entropies of activation from the data. It was the purpose of this investigation to determine the activation energy as well as the entropy and enthalpy of activation for certain alkyl bromides as shown in eq 1. In order to do this, similar data were obtained a t 0,
40, 60, and 70" for 1-bromoethane, 1-bromobutane, 1-bromohexane, and 1-bromooctane, thereby making available five rate constants over a 70" temperature differential. From these data, the activation energy and the entropy of activation for each alkyl bromide reacting with thiocyanate were calculated.
Experimental Section A . Materials. Commercially available l-bromoalkanes were shaken with cold concentrated sulfuric acid, washed with cold water, and then shaken with solid sodium bicarbonate. After being washed again with cold water, the alkyl bromides were dried for 24 hr over anhydrous potassium carbonate before being fractionated through a 60-cm vacuum-jacketed column packed with beryl saddles. The 1-bromohexane and 1-bromooctane were distilled a t reduced pressures because of their higher boiling points. Fisher Certified Reagent sodium thiocyanate was dried a t 130" for 24 hr and then used to prepare an approximate 0.40 M solution, using 95 vol % ethanol as the solvent. A standard solution of approximately 0.3 N potas(1) (a) J. B. Conant and R. E. Hussey, J . Am. Chem. Soc., 47, 488
(1925); (b) J. Semb and 8. M. McElvain, ibid., 53, 690 (1931); (c) P. D.Bartlett and L. J. Rosen, ibid., 64, 543 (1942). (2) T. I. Crowell, ibid., 75, 6046 (1953). (3) R. Lang, Z . Amrg. Allgem. Chem., 122, 332 (1922); 142, 239, 280 (1925); 144, 75 (1925). (4) L. W.Andrews, J . A m . Chem. SOC.,25, 756 (1903).
Volume 71, Number 7 June 1067
T. P. WALLACE AND C. H. STAUFFER
2084
Table I : Reaction of SCN- and l-Bromoethane a t 40”“ Reaction time,
ao
- x,
bC -
Z,
8BC
x,* M
M
M
0 7,740 16,440 25,340 42,660 79,380 90,660 104,160
0 0.01580 0.03060 0.04363 0.06046 0.08517 0.08958 0.09568
0.19663 0.18083 0.16603 0.15300 0.13617 0.11460 0.10705 0.10095
0.13403 0.11823 0.10343 0.09040 0.07357 0.04886 0.04445 0.03835
log b ( a a(b
-
x)/
- x)
0.01809 0.03909 0.06107 0.09743 0.19172 0.21427 0.2538
k X 106, 1. mole-1 8BC -1
8.60 8.75 8.53 8.40 8.88 8.73 8.97 Av 8.68 3 ~ 0 . 1 8
a Significant figures are considered in last column only (i.e., values of k). * x represents the number of moles per liter which reacts in time t. a and b represent the initial molar concentrations of SCN- and RBr.
sium iodate was prepared after drying Fisher Certified Reagent potassium iodate a t 120” for 2 hr. For the quantitative analysis of thiocyanate, a concentrated stock solution of iodine monochloride was prepared5 and stored in the dark. This stock solution was prepared by adding 38.64 g of potassium iodate, 60 g of potassium iodide, and 450 ml of HzO to 450 ml of concentrated hydrochloric acid. After the stock solution was prepared, 20 ml of carbon tetrachloride was added and the solution titrated with potassium iodate until the carbon tetrachloride layer was slightly pink. This provided a solution of iodine monochloride containing only a slight amount of iodine. This excess iodine was later removed by the addition of a drop of two of potassium iodate in the preparation of the solution for use in the quantitative determination of sodium thiocyanate. B. Procedure. A quantity of alkyl bromide necessary to give a concentration of about 0.1 M was added to a 100-ml volumetric flask containing about 25 ml of 95% ethanol (solvent). All solutions and the flask were a t the reaction temperature in a constant-temperature bath. After adding 50 ml of standard 0.4 M sodium thiocyanate, the solution was diluted to the 100-ml mark with solvent at the reaction temperature. At different time intervals, 5-ml samples were withdrawn from the reaction flask and added to an iodine monochloride solution prepared6by adding 25 ml of the IC1 stock solution to 50 ml of water, 5 ml of carbon tetrachloride (indicator), and 75 ml of concentrated hydrochloric acid. A separatory funnel was used to contain the above iodine monochloride mixture, as the two-phase system needed frequent shaking. Potassium iodate solution was added dropwise until the carbon tetrachloride layer was colorless. The sample from the reaction flask was then added to the separaThe Journal of Phyeical Chemistry
tory funnel. The displacement reaction was quenched by rapid reaction of the thiocyanate with iodine monochloride. After the initial shaking, the sample was titrated with standard potassium iodate with frequent additional shaking until the carbon tetrachloride layer again just became colorless. The amount of thiocyanate present was calculated from the volume of standard potassium iodate added.
Calculations Average rate constants, activation energies, and the precision of each were obtained in the usual manner’ by a linear least-squares analysis. The entropies of activation were then calculated using the appropriate quantities listed above. The computations were carried out by an IBM 1620 computer and appropriate Fortran programs.
Discussion Table I indicates the variation in the rate constants with time. The standard deviation of the rate constants was between 2 and 3%. The average rate constants in Table I1 follow the trend that has previously been reported for straightchain alkyl halides.’ The differences between the values for l-bromohexane and l-bromooctane are admittedly very close to the experimental error. However, repeated trials indicated that the l-bromooctane was definitely the more reactive of the two compounds. Table I11 shows the calculated values of the entropies of activation at various temperatures studied. (5) I. M. Kolthoff and R. Belcher, “Volumetric Analysis, Titration Methods: Oxidation-Reduction Reactions,” Vol. 111, Interscience Publishers, Inc., New York, N. Y., 1957, p 456.
(6) See ref 5, pp 456, 457.
(7) S. Benson, “The Foundations of Chemical Kinetics,” McGrawHill Book Co., Inc., New York, N. Y., 1960.
REACTIVITY OF ~-BROMOALKANES WITH THIOCYANATE
2085
Table II: Average Rate Constants at Various Temperatures (Eq 1) (1. mole-' sec-1) Temperature, OC 40
Alkyl bromide
0
25'
1-Bromoethane 1-Bromobutane 1-Bromohexane 1-Bromooctane
6.74 X lo-' 5.60 X lo+ 6.20 x 10-7 6.41 x 10-7
1.67 x lo-& 1.14 X 1.22 x 10-6 1.27 x 10"
8.68 x 10-6 5.99 x 10-6 6.46 x 6.88 x 10"
70
80
5.10 x 3.93 x 4.38 x 4.54 x
10-4 10-4 10-4 10-4
12.8 9.50 9.98 10.2
x 10-4 x 10-4 x 10-4 x 10-4
Table III: Entropy of Activation at Various Temperatures (cal deg-1 mole-') (Reaction 1) 273.2
298.2
Temperature, OK 313.2
333.2
343.2
-15.56 -16.63 -16.66 -16.65
-15.31 -16.71 -16.79 -16.76
-15.57 -16.60 -16.65 -16.58
-15.57 -16.66 -16.64 -16.62
-15.50 -16.65 -16.74 -16.74
Compound
1-Bromoethane 1-Bromobutane 1-Bromohexane 1-Bromooctane
It is observed that the deviations with temperature are extremely small. Table IV contains the thermodynamic quantities and the appropriate precision of each value. It is observed that the values of activation energy are the same within the experimental error as are the entropy of activation values, except for the entropy of activation of 1-bromoethane. 1-Bromoethane has the highest activation energy, yet it is the most reactive of the four compounds. The greater reactivity of 1-bromoethane compared to the longer chain homologs is due to a difference in the entropy of activation. ~~
~~~
Table IV: Thermodynamic Quantities (eq 1) Alkyl bromide
koa1 mole-1
1-Bromoethane 1-Bromobutane 1-Bromohexane 1-Bromooctane
20.02 i 0.22 19.83 f 0.07 19.76 i0.10 19.75 d= 0.12
Ea,
AS *, cal mole-1 dag-1
-15.4 -16.6 -16.7 -16.7
f 0.1 f 0.1 =!= 0.1 f 0.1
Similar reactions have been observed where a d e crease in reactivity caused by a decrease in entropy of activation has been accompanied by a decrease in activation energy. Crowell and Hammetta investigated the reaction of ethyl and n-propyl bromides with thiosulfate ion. In this reaction, they observed a rate decrease in progressing from 1-bromoethane to the higher homologs, despite the fact that the acti-
vation energy became less. They attributed this situation to a decrease in the entropy of activation, as has been lound for the thiocyanate reaction reported in this paper. They also pointed out that this effect was observed in ester hydrolysisDand in the formation of semicarbazones.'O The decrease in the relative reactivity of the 1-bromoalkanes on progressing from the ethane member to higher homologs may possibly be attributed to an increase in the interference with the rotation about the C,-C, bond in the transition state due to the size of the /3 substituent. It has been suggested" that the differences in the entropy of activation among l-bromo-2-chloroethane, l-bromo-2-fluoroethane, and l,2-dibromoethane reacting with thiophenolate may be due to differences in the size of the /3 substituents. It is believed that the same situation exists in comparing the reactivity of 1bromoethane with the higher homologs. Therefore, the reactivity of 1-bromoethane relative to the other three homologs reacting with thiocyanate in a 95% ethanol medium results from a difference in the entropy of activation rather than any difference in the activation energy. It is believed that this difference in the entropy of activation is due to differences in the interference with the rotation about the C,-C, bond. (8) T. Crowell and L. Hammett, J . Am. Chem. Soc., 70,3444 (1948). (9) H. Smith and J. Steel, ibid., 63, 3466 (1941). (10) F.Price and L. Hammett, ibid., 63,2387 (1941). (11) J. Hine, "Physical Organic Chemistry," McGraw-Hill Book Co., Inc., New York, N. Y.,1962,pp 172-175.
Volume 71, Number 7 June 1967
