615
Organic and Biological Chemistry
The Nature of the ortho Effect. 11. Composition of the Taft Steric Parameters Marvin Charton Contribution f r o m the Department of Chemistry, Pratt Institute, Brooklyn, New York 11205. Received June 13, 1968 Abstract: The Taft Es values are shown to be a linear function of the van der Waals radii. They are independent of electrical effects. The Taft Eos values intended for use with ortho substituents are completely independent of the van der Waals radius; they are solely a function of electrical effects. The ortho effect for most substituents is elec-
trical rather than steric in nature.
T
he most widely used quantitative measures of steric effects among organic chemists are the ES and Eos values proposed by Taft.’ Numerous papers have appeared reporting correlations with these “steric effect” constants.* Taft, in his review,l reported that the Es values parallel the van der Waals radii. It has been suggested that these parameters may include electrical as well as steric effect^.^ Koppe14 has reported that for alkyl groups the equations ES = a -I-bu*
+ can
=
a
+ bu* + cAn
(2) are obeyed. Other substituents generally did not follow eq 1 and 2. In view of the uncertainty regarding the nature of the Es and IPS values it seemed of interest to investigate their composition. To this end we have carried out correlations of Es and eo^ values with the equations
Esx EoSX
=
aqx
= agI,X
+ h , x + *rv,x + h + + f pUR,X
gYV,N
Table I. Substituent Constantsa
(1)
and
E”s
The up constants are from McDaniel and Brown’ or from Exner and J o n k 7 Constants from other sources are given in Table I. The atomic van der Waals
X
UI
Ref
UP
Ref
CH2SMe CBrsH CBr3 CChH CFzH
0.07 0.28 0.38
a
0 0.34 0.57 0.36 0.41
b
C C
d
f e e
a Calculated from the equation U I , X C H * = 0 . 3 6 9 ~ 1, ~ 0.02. *Calculated from the equation U ~ . X C H ~= 0 . 5 2 2 ~ 1 . ~ 0.13. Calculated from the equation UI,X*CH = 0 . 3 2 4 ~ ~1,~ 0.01. d Estimated from the equation U ~ . X * C H = 0 . 5 2 2 ~ ~ 1 -. ~0.13. Calculated from the equation U I , C X ~= 0 . 3 0 8 ~ ~-10.03. . ~ 1 Estimated from the equation uP.x3c= 0 . 5 2 2 ~ ~-1 .0.13. ~
radii are from the excellent study of B 0 n d i ~ 8(Table ~ 11). F o r symmetric top substituents of the type CX3 (e.g.,
(3)
Table 11. van der Waals Radii Used in Correlationsa
(4)
X rv
F 1.47
c1
1.75
Br 1.85
I 1.98
0 1.52
S 1.80
H 1.20
where uI and uR are the localized (field and/or inductive) See ref 8 and 9. and delocalized (resonance) effect substituent constants, and rv is the van der Waals radius. The uI constants required are taken from our ~ o m p i l a t i o n ; ~ Me, CF3, t-Bu, CCI3,. , . ) radii for the entire group were calculated. F o r such a substituent there are four the uR constants were obtained from the equation6 quantities of interest. They are shown in Figures 1 b R = up - U I (5) and 2. These quantities are: (1) rv,min, minimum perpendicular to CX3, minimal van der Waals radius (1) R. W. Taft, Jr., “Steric Effects in Organic Chemistry,” M. S . perpendicular to the group axis; (2) Y ~ , maximum ~ ~ ~ , Newman, Ed., JohnWiley & Sons, Inc., New York, N. Y., 1965, p 565. Q
(2) See references cited in the review by C. D. Ritchie and W. F. Sager, Progr. Phys. Org. Chem., 2,342 (1964); R. T. M. Fraser, Nature. 205, 1207 (1965); K. Bowden, N. B. Chapman, and J. Shorter, J . Chem, SOC.,5239 (1963); 3370 (1964). (This is only a sampling of references using Es values in correlations.) (3) J. E. Leffler and E. Grunwald, “Rates and Equilibria of Organic Reactions,” John Wiley & Sons, Inc., New York, N. Y.,1963, p 228; C. K. Hancock, E. A. Myers, and B. J. Yager, J . Am. Chem. Soc., 83, 4211 11961). (4) I. Kdppel, Reakts. Sposobnost. Org. Soedin., 2 (2), 24 (1965). \ - - - -
(5) M. Charton, J . Org. Chem., 29, 1222 (1964). (6) R. W. Taft, Jr., and I. C. Lewis, J . A m . Chem. SOC.,80,2436 (1958).
(7) D. H. McDaniel and H. C.Brown, J . Org. Chem., 23,420 (1958); 0. Exner and J. Jon& Collection Czech. Chem. Commun., 27, 2296 (1 962). (8) A. Bondi, J . Phys. Chem., 68, 441 (1964). (9) Complete tables of the correlations obtained have been deposited as Document No. NAPS-00156 with the ASIS National Auxiliary Publication Service, % CCMInformation Sciences, Inc., 22 West 34th St., New York, N. Y. 10001. A copy may be secured by citing the document number and by remitting $1.00 for microfiche or $3.00 for photo-
copies. Advance payment is required. Make checks or money orders payable to: ASIS-NAPS.
Charton / Taft Steric Parameters
616
V I , V K , r Y,mln
L
3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 195 200
Figure 1. Bottom view of CX3 group.
$cz
rvII
rgmqg
- - - - - -1
1
-
-i
group
QX;Z
I I
(TI, U R ,
rTtmax
rv,x rv.min rv.max rv.x rlr,min 01, UR, rv,max rv.x rv,min rv,max UI, UR, rv.x UI,O R , rv,min UI, UR, rvfmar rv,x
CHXz
(TI, UR,
UI, U R ,
CX4
rv.min
rv.mnx UI, UR, rv.x
X X
UI, UR
Sets 19 and 20 represent correlations of Eos constants defined for ortho substituents in benzene derivatives. All other sets represent correlations of Ea values defined for aliphatic systems. Q
Figure 2. Side view of CX3 group.
perpendicular to CX3, maximal van der Waals radius perpendicular t o the group axis; (3) rv,,,, van der Waals radius parallel to the group axis; (4) I, equivalent to a covalent radius for the entire group, CX3. By means of simple geometry and trigonometry these quantities can be calculated. Values for some common groups are given in Table 111. The values for the t-Bu and NMe3+ groups were obtained using for the van der Waals radius of X the value of rv,mincalculated for the Me group.
Table 111. van der Waals Parameters for BABSymmetric T o p Substituents X Me CF3 Me&
so3-
t-Bu NMe3+
IX,G
rv,lI
1.912 1.933
1.575 1.866 2.12 2,077 2.283 2.214 2.357 2.470 2,684 1.996 2.510
2.332 2.100 1.973 2.144 2.157 2.251 2.013 2.247
rV.mnx
rv.min
2.23 2.743 3.987 2.852 3.150 3.114 3.408 3.670 3.996 2.864 3.473
1.715 2.107 2.60 2.186 2.435 2.417 2.579 2.760 2.988 2.192 2.637
Results ES,CH?X Values. The results of the best correlations with eq 3 and 4 are given in Table VI. The correlations of the E X , C values ~ * ~ with ry,x, rv,minand rv,mar (Figure 1, sets 1, 2, and 3, respectively) all gave significant, although poor, correlation with eq 3. The t test shows that II. is much more significant than is a! or 0. Elimination of the Es value for X = Me gave slight improvement in the case of correlation with rv,x (set 1A) and slightly worse results with rv,minand rY,,,, (sets 2A and 3A). Again t tests showed to be the most significant coefficient. This suggests a far greater dependence of the Es values on rv than upon uI or uR. T o determine whether this is indeed the case correlations were carried out with the equation
+
ES
=
$rv
+h
(6)
The results of these correlations are presented in Table VII. A good correlation of E s , c H ~ X with rV,x was CBr3 obtained (set 4); exclusion of the value for X = Me CI, gave excellent results (set 4A). Very good correlations cos with eq 6 were obtained using rV,minand rv,max(sets CS? 5 and 6 ) . Exclusion of the value for X = Me gave excellent correlations (sets 5A and 6A). Correlations with rv,x, rv,min,and E S , C H ~Values. X rV,max(sets 7, 8, and 9) gave no significant, poor but For purposes of correlation with eq 3 Es values were significant, and fair results, respectively. The value for divided into three categories: CH2X, CHX2, and hydrogen (H in place of CHX2)was included in sets 8 and symmetric top (CX3 and H) substituents. Correlations were then made with each set using rv, rV,max,and rV,min 9. The van der Waals parameter used for the hydrogen substituent was its rv value. Exclusion of the value for as steric parameters. The sets studied are listed in Table IV. The Es values are taken from Taft;' the hydrogen from set 8 gave a nonsignificant correlation values used are given in Table V. (set SA) whereas exclusion of the value for X = Me gave fair results (set 8B). Exclusion of the value for hyIn correlating the Es values for MeOCHz and MeSCHz drogen from set 9 also gave a nonsignificant corin set 1, it was assumed that the OMe and SMe groups would be oriented in such a way as to minimize their relation (set 9A) whereas exclusion of the value for X = Me gave good results (set 9B). Again, in all corsize. Thus rv values for 0 and S , respectively, were relations, was the most significant coefficient as shown used for these groups. For the Me group, the van by the t tests. Correlations were therefore carried der Waals radius was taken to be the value of rV,min. cc13
+
Journal of the American Chemical Society
91:3 1 January 29, 1969
617 Table V. ES Values Used in Correlations'
X Es
Me 0
Et -0.07
Sets 1-6 CHzCl -0.24
CHzF -0.24
CHzBr -0.27
CHzI -0.37
CHzOMe -0.19
CH2SMe -0.34
X Es
Me 0
MeEH -0.47
Sets 7-12 FzCH -0.67
ClzCH -1.54
BrzCH -1.86
H 1.24
x
Me 0
CF3 -1.16
Sets 13-18 CCll -2.06
CBr3 -2.43
CMea -1.54
H 1.24
Es
X
Me0 0.99
Eos
Sets 19 and 20 c1 Br 0.18 0
F
Et0 0.90
0.49
I -0.20
Me 0
NOz -0.75
Ph -0.90
Table VI. Results of Correlations with Eq 3 and 4 1A 2A 3 7 8B 9B 13 14 15 19 20A
-0.476 -0,648 -0,410 3.12 2.87 -0.641 -0.374 1.32 -0.300 -3.11 -2.45
-0.139 -0.126 -0.552 -3.71 -1.49 0.289 -0.662 -0,604 -0.401 -0.687 -0.433 Set 1A 2A 3 7 8B 9B 13 14 15 19 20A
Set 1A 2A 3 7 88 9B 13 14 15 19 20A
-0.361 -0.216 -0,119 -1.69 -1.81 -1.28 -3.12 -2.20 -1.44 0,484
tcYQ
0.238 0.201 1.O?O 0.277 1.707 0.654 24.52 1,782 0,930 2,987 2.503
0.583 0.628 0.536 13.4 0.873 0.442 0.270 0.339 0.431 0.230 0.173
CLh
