91
endo derivative decreased by more than 120 kcal. The latter result was to be expected from simple steric consideration. An analysis of the bond indices reveals at this point that the observed differences are produced both by a steric inhibition f o r the endo derivative and a uparticipationf o r the exo derivative (Figure 1). However, even the exo derivative appears t o be lfss stable than his analog with a 2-6 CC distance of 2.4 A indicating that the closing of the cyclopropane ring (u assistance) occurs later in the process. These results are tentatively interpreted as in the Figure 1 where the exo derivative appears to react faster and its transition state involves more u participation than the endo position whose reactivity is inhibited by steric hindrance. Sim-
ilarly, the formation of a new bond, C--Y, to form the product is more favorable in the exo position. In a very nucleophilic medium the quenching of the carbonium occurs very fast and before any extensive rearrangement of the I4C occurs whereas in a less nucleophilic medium the longer life of the protonated nortricyclene permits the tautomeric rearrangement of the protons. In this case, a complete scrambling of the hydrogen and the carbon atoms may occur. Acknowledgment. I wish to thank Professor G. A. Olah for his interest and stimulating suggestions. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research.
The Oxibase Scale and Displacement Reactions. XVII. The Reaction of Nucleophiles with Ethyl Tosylate and the Extension of the Oxibase Scale' R. E. Davis, R. Nehring,2a W. J. Blume,2band C. R. ChuangZc
Contribution from the Department of Chemistry, Purdue University, Lafayette, Indiana 47906. Received January 23, 1968 Abstract: The rate constants of ten nucleophiles reacting with ethyl tosylate have been measured in aqueous solution. From these rate constants the oxibase scale parameters for ethyl tosylate have been obtained; a = 1.48 and /3 = 0.03 at 25" in water. The nucleophilic constants of several nucleophiles have been obtained for the temperature range of 0-100" in water. The uses of the oxibase scale are extended to include the energetics of isotopic exchange reactions as well as the relationship between the oxidation potential and the basicity.
T
he tosylate group, -OSO2CBH4CH3-p,is an often used leaving group to study structure-reactivity effects in physical-organic chemistry. In the present study ethyl tosylate has served as the substrate for ten nucleophiles in aqueous solution (see section I below). The data of McCleary and Hammett3 on the s N 2 reaction of water, hydroxide, and halide ions with ethyl p-toluenesulfonate (or ethyl tosylate) were determined in 60% aqueous dioxane (by volume) at 50". Edwards4 computed a to be 1.68 and /3 to be 0.014. However, the plot5 of E-l log (klk,) us. HIE has (1) Paper XVI: R. E. Davis in "Inorganic Sulphur Chemistry," G. Nickless, Ed., Elsevier Publishing Co., The Netherlands, in press. (2) (a) Postdoctoral Fellow, 1962-1963. (b) Undergraduate Research Thesis Student, 1963-1964. (c) Taken in part from the Ph.D. Thesis, August 1967. (3) H. R. McCleary and L. P. Hammett, J . Amer. Chem. Soc., 63, 2254 (1941). (4) J. 0. Edwards, ibid., 76, 1540 (1954). (5) The equation now called the oxibase scales is
+
log (klko) = CYE OH with k + AY + AX f Yka HzO + AY +AOH + Y - + H+ E = + 2.60 v 2X- e X, + 2e-
X-
€0
€0
X - + H + e H X pK, H = pK. 1.74
+
(0
a large amount of scatter. The scatter is due to several factors; an important factor is the requirement of accurate pK, values of the halide ions. A second problem is that kinetic data in dioxane-water at 50" should not necessarily correlate well the thermodynamic values in water' at 25 " or with empirical kinetic constants* obtained in water at 25 ". In parts I1 and 111, the oxibase scale is extended as the result of theoretical work to cover the temperature range of 0-100" in water, Further theoretical work on the oxibase scale relates the parameters to more fundamental properties of the reacting species. Results I. The Present Investigation. The use of the oxibase scale on ethyl tosylate in pure water at 25" has been made so that the substrate can be used as a convenient substrate for determining the kinetic E values of the mercaptoethylamine and other nucleophiles (see the following paper). A plot can be prepared from (i) by dividing by E.
E-' log (klko) =
CY
+ @(H/E)
(ii)
(6) R. E. Davis in "Survey of Progress in Chemistry," A. Scott, Ed., Academic Press, New York, N. Y., 1964, pp 189-238. (7) The same problem of using nucleophilic data obtained in one solvent at one temperature with data in other solvents at different temperatures is wide spread. Lack of data forces such correlations to be tried, nevertheless. (8) C. G. Swain and C. B. Scott, J . Amer. Chem. Soc., 75, 141 (1953).
Davis, Nehring, Blume, Chuang J Reaction of Nucleophiles with Ethyl Tosylate
92
lo9k/k. E
-
-1
I
I
Ethyl Tosylote
Table I. Reaction of Nucleophiles with Ethyl Tosylate in Aqueous Solution at 25.00 f 0.01 '
-
new data
Nucleophile
k?,
H D
-
01-5
0
I
I
5
IO
NaOH
15
HIE
Na2S203
Figure 1. Oxibase scale plot of the data on ethyl tosylate at 25". Divisions on the E-' log ( k / k o ) axis are 0.10 unit each. The points for NOa-, Ns-, and CIO- have been placed on the line and the E value computed. Note that the value of k o ( k a , o ) is included in each ratio of k/ko. Therefore, all ten data points are used.
Using the general rules,6 an a priori estimate of the two substrate constants ( a and p) can be made. The methylene carbon is only slightly positive due to the electronegativity of the adjacent oxygen. Therefore, p will be small and positive. The E of the tosylate group is An estimate from the data of Hammett3 on the rates of displacement of tosylate ion on the ethyl halides formed allows an estimate of E as about 0.6 f 0.2 V. This E value is quite uncertain but at least it allows a prediction concerning the a of ethyl tosylate. Using the solubility product of silver sulfate and of silver tosylate in water at 25", a value of E = 0.50 f 0.05 V can be estimated for the tosylate anion.ga The leganion orderg would predict an a of 1.5 (see eq 18). Experimentally the value has been determined to be 1.48 in pure water at 25". The experimental data are presented in Table I. The oxibase scale plot of E-' log ( k / k o ) = a p( H I E ) is presented in Figure 1. The correlation is satisfactory. The values of E reported for the nucleophiles in Table I1 for nitrite, hydrosulfide, azide, and hypochlorite must be considered to be accurate for ethyl tosylate as a substrate but subject to verification using other substrates. Using the E and H values6 of hydroxide, thiosulfate, sulfite, iodide, thiocyanate, and water the CY of ethyl tosylate is 1.48 and the p is +0.03. 11. The E and H Values at Other Temperatures.lo The E value was defined5 as the oxidative dimerization relative to that of water. I n i t i a l l ~ , the ~ potential values were those listed by Latimer at 25". The standard oxidation potential at a temperature, eo(t), can be related to the eo at 25" using
+
e"l)
=
€0
+f()"(
bT
Na2S03
NaN3 NaClO
NaSHl
KI
KSCN NaN02
Journal o j the American Chemical Society
91:I
55.5 55.5 55.5 55.5 55.5 55.5 55.5 0,0600 0.0918 0.168 0.180 0.221 0.598 0.0854 0.169 0.199 0.0729 0.0844 0.107 0.123 0,0993 0.149 0.192 0.0250 0.0250 0.0125 0.0475 0,0400 0.0641 0.0985 0.0984 0,0676 0.100 0.153 0.187 0.0992 0.0998 0.1521 0.155 0.155 0.157
4.0 5.2 7.0 6.8s 7.01 7.00 7.0h
8.54 8.56 8.56 8.57 6.75 9.14 6.75 9.14 8.74 8.74 8.90 10.7 10.7 10.0 11.2 11.4 10.74 10.55 11.4 8.56 8.55 8.60 8.55 6.75 8.50 8.50 6.75 8.37 9.80
+
k , , sec-'
M-l sec-l
6.38 x 10-6 I . I S x 10-7 6.40 X 1.16 X 6.39 X loWG1.15 X 6.48 X 6.44 X 6.39 X 6.28 X 1 . 1 6 x 10-5 1.23 x 10-4 1.78 x 10-5 1 . 2 5 x 10-4 2.83 X 1.24 X lo--' 2.89 x 10-5 1.22 x 10-4 3.43 x 10-5 1.20 x 10-4 3.37 x 10-5 8.60 x 10--41 4.31 x 10-5 7.45 x 10-5 8.60 x 10-5 3.09 X 5.41 x 10-5 4.88 X 1.13 X 10-3i 7.45 x 10-5 1.11 x 10-3 j 1.39 x 10-5 1.15 x 10-4~ 1.76 x 10-5 1.50 x 10-4 2.06 x 10-5 1.48x 10-4 2.89 X 10V 0 . 9 0 X 3.26 X 10V 1 . 0 4 X 1 . 8 4 X 1O-j 0.96 X 4.95 x 10-5 0.90 x 10-3 3.40 x 10-5 1.60 x 1 0 - k 5 . 8 7 x 10-5 1 . 6 3 x 10-3 9.00 x 10-5 1.70 x 10-3 9.02 x 10-5 1.70 x 10-3 8.06 X 6.54 X 1.29 X 10-5 6.50 X 1.63 X 6.50 X 1.84 X 6.48 X 9.77 X 6.84 X 9.78 X 6.82 X 1,64 X 6.50 X i , 8 4 x 10-6 i , 5 5 x 10-4k 1.83 x 10-6 1 . 5 5 x 10-4 1 , 80 x 1 0 ~1.50 x 10-4
+
Table 11. E, Values Computed from k,, Ethvl Tosvlate
CY,and
p
of
Nucleophile
E
H
NO$SHN3-
1.97 2.60 1.95 2.43
5.09 8.70 6.46 9.04
c10-
which is a Taylor expansion equation. Accurate thermodynamic data are available on seven nucleo(leaving group) anion to predict the LY value of the substrate. Methyl iodide is quite sensitive to the oxibase scale values of the attacking nucleophile; the 01 is almost 3.0. Since iodide ion, the legate anion, is so easily oxidized, this means that the substrate methyl iodide is, thercfore, very sensitive to the E value of the nucleophile. (10) Parts I1 and 111 of this paper are discussed in greater detail in ref 2c.
uHh
a Molarity. * pH rt 0.01 using dilute buffer solutions of phosObserved first-order rate constant: phates. Ionic strength, 0.1. Second-order rate constant com-d[EtOTs]/dr = kl[EtOTs]. puted from kl after correction for the water reactions; rate = ka[EtOTs][N] ko[EtOTs][H20] k[EtOTsJ[OH-1. The value of kz is about that at infinite dilution except for the data on hydroxide ion. e Pure water, no buffer present, Initial pH. J Sodium perchlorate added to very dilute pH 7.0 phosphate buffer. Ionic strength p = 0.03. 0 S e e f , p = 0.07. h p = 0.14. s e e f Slope of the line using the four points. j Five second-order rate constants computed at each pH. k Second-order rate constant computed at each concentration of the nucleophile. E Added as NazS.9H20.
- 25) +
( 9 ) (a) R. E. Davis, J . A m e r . Chem. SOC.,87, 3010 (1965); paper VI. (b) Some misunderstanding has arisen using the E value of the legate
Ma
philes; these data are listed in Table 111. From these data the eo(t) values have been computed. The E value is defined as EO 2.60 V at 2 5 " ; the value of 2.60 V is the negative of the value arbitrarily assigned5 for the process
January I, 1969
+
2Hz0
+
H40zZ+ 2e-
(2)
93 Table III. Standard Oxidation Potentials of Various Nucleophiles eo (25"),
X
Vu
Reaction
+ + + + + + + +
HZe 2H+
2e2S2- e Szz- 2e2SzOa2-$ S40622e21- e Iz 2e2Br- e Br2 2e2C1- c Clz 2e2S042- s SzOs2- 2e2F- e Fz 2e-
H? S 2SzOsz-
IBrCISO,2F-
0 0.480 -0.080 -0.5355 -1.087 -1.3595 -2.01 -2.87
deO/dT, de0/dT2, eo (@),a mV/dega (pV/deg)za V
0 0.93 1.11 0.148 0.478 1.260 1.26 1.83
0 5.965 5.454 5.34
0 0.457 -0.108 -0.537 -1.099 -1.389 -2.04 -2.91
eo (50"),
€0 (75"),
V
V
0 0.503 -0.052 -0,530 -1.075 -1.326 -1.98 -2.82
0 0,526 -0.024 -0.521 -1.063 -1,290 -1.94 -2.77
€0
(loo'), V @ 0.550
0.003 -0,508 -1.051 -1.250 -1.92 -2.72
A. J. de Bethune and N. A. S . Loud, "Standard Aqueous Electrode Potentials and Temperature Coefficients at 25"C," C. A. Hampel, Publishing Co., Skokie, Ill., 1964. All values computed using eq 1. Standard. a
The EO of (2) would also vary with temperature; however, since no data are available it is convenient to eliminate the value of 2.60 from the oxibase scale. Thus at 25 " log (ki/ko) and
=
aEi
+ pH1
+ 2.60) + pHi
= ~ ( E I
CNS 2S042-
F-
11.66 15.41 3.43 4.76
25 O 10.97 14.04 3.74 4.90
50 10.4 13.86 4.06 5.13
9.92 13.1 4.41 5.39
+ AC,, -1nT+ R
9.52 12.3 4.8 5.68
AS" - AC,
R
1
AGQ
+ L-(g)
0.5Hdg)
7
AGz = AG,\
+ 0.5Lz(g)
AGs
H(g)
+ L(g)
Thus
=
=
AG3 - AGg
+
AH3 - AH8 T(AS3 - AS,)
+ AGq - AGg
+ AG, - AGs
(7)
(8)
or 1.364pKa - 23.066'
The K, values of the nucleophile would vary as -AH" RT
H W
loo0
75O
Data have been obtained from J. Bjerrum, G. Schwarzenbach, and L. G. Sillen, "Stability Constants," Part 11, The Chemical Society, London, 1958.
= ___
AGE
AGj - AGIO
a
In K,
AGi =
AGj - AGIO
Table 1V. H Values of Selected Nucleophiles as a Function of Temperaturea 0"
AG4
(3)
In eq 5 another nucleophile (with a rate constant kz on the substrate) would serve as the standard. Then eq 5 can serve as a more general oxibase scale from 0 to 100" using the E and H values at the proper temperature (See Table IV). In eq 5 it will be assumed that a and p are independent of temperature until accurate kinetic data are available to test this hypothesis.
Nucleophile
t
(6)
In Table V, the pK, values are listed. Since H is defined as pK, log [HzO], the log [HzO]term will vary as the density of water varied from 0 to 100". The variation of the density is about 47& thus log [H20] varies from 1.744 to 1.728 and 0 to 100". T o a good approximation log [HzO]is constant and a value of 1.74 will be used. 111. The Relationship of the Oxibase Scale to the Properties of the Nucleophiles. For an acid, HL, the following cycle may be written1'
+
(11) Part 111 of this paper is a further development of the work of McDaniel. See D. H. McDaniel and A. Yingst, J. Amer. Chem. SOC., 86, 1334 (1964). In that paper the entropy terms (AS3 - A&) and the solvation terms (AG4 - AGo) were neglected. In part 111 of this present study, these terms are computed.
+
+
D H L- 0 . 5 [ D ~ , DL,] T(AS3 - As,) - AGq AGg (9)
=
+
since AGIO = -nn5e0 = -23.06~0, AH3 = -DHL, AH, = -0.5[DH, DL,I at 25". The terms on the left of the equation are the pK, value (a base term) and the oxidation term. They are related to the bond dissociation enthalpies, the entropies (AS3 and A&) and the solvation terms (AG4 and AGg) of the neutral species. If one restricts one self to L = F, C1, Br, I, OH, SH, SeH, and TeH, enough data are available so that D H L and DL* are known. Likewise enough structural data are known so that the entropies (AS3 and Ass) can be computed in the gas phase using the standard methods of statistical mechanics. For example, A S 3 - AS, for a monoatomic L case is given by
+
AS3 - AS8
= 1.5R In mHL - 0.75R In mH, 0.75R In mLz R In ( I H L / ~-) 0.5R In (IH,/2) - 0.5R In (ZLJ2)
+
(10)
when R is the gas constant, m is the mass, and I is the moment of inertia.
Davis, Nehring, Blume, Chuang
1 Reaction of Nucleophiles with Ethyl Tosylate
94 Thermodynamic Functions of Some Selected Nucleophiles and Their pK, Values at Different Temperatures (0-100") H + X-n
Table V.
+
for the Reaction Scheme HX-n+' Nucleophile, X-"
AH", kcall mol
cc
bb
a5
12.81 -8.91 42.60 2.7995 X l o 3 4.4841 -12.63 84.68 -499.25 -2.7595 X lo3 -42.619 12.34 -58.46 306.92 2.6962 X l o 3 29.420 15.55 -62.44 331.03 3.3988 X lo3 31.424 -20.74 10.07 -84.86 -4.5324 X IOa -5.066
CNS2S042-
FCIa
AC,, call AS,', (mol cal/(mol deg) deg)
AHoo/2.303R.
-AC,/R.
'(-ASoo
-
0"
-11.2562 127.608 -79.848 -85.986 20.744
9.92 13.68 1.69 3.02 -8.2
PKa 50"
25"
75"
100"
7.78 8.18 11.36 10.60 3.04 2.67 3.65 3.94 -5.16 -4.44
8.66 9.23 12.90 12.12 2.32 2.00 3.16 3.39 -6.0 -7.0
ACp)/2.303R.
Parameters for the Diatomic Molecule, HL, and the Value of A S 3 - ASSa
Table VI.
~
ML X 1024 g
Molecule H-L
MH-L X loz4g
rH X 108cm
rL X lOscm
rH
~~
AS3 - ASS: cal/(mol deg)
X 108 cm
&-L
+ rL
Obsd
H-H H-F
1.674 31.54
3.348 33.22
0.37 0.28
0.37 0.709
0.740 0.989
0.74 0.918
H-C1
58.87
60.54
0.28
0,991
1.274
H-Br
132.68
134.36
0.28
0.140
H-I
210.71
212.39
0.28
1.333
Reaction
1.27
+ o.SFz(g) * HF(g) 0.5Hz(g) + 0.5CIz(g) S HCI(g)
2.668
1.420
1.42
0.5Hz(g)
+ 0.5Brz(g) e HBr(g)
3.228
1.613
1.61
0.5Hz(g)
+ 0.5Ikd S H I ( d
3.642
0.5Hz(g)
1.763
Bond distances taken from L. Pauling, "The Nature of the Chemical Bond," 3rd ed, Cornell University Press, Ithaca, N. Y . , 1960, p 225. At 298°K and 1 atm. All values in this column are calculated. Parameters on Diatomic Nucleophiles and the Value of AS,
Table VII.
HzX
