NOTES

RESONANCE PARAMETERS FROM REACTIVITY CONSTANTS

Recently, McDaniel and Brown (I) have compiled an extensive set of Hammett u values based upon the ionizatioil of benzoic acids. A number of electrophilic substituent constants, u+, have also been calculated based on the thermodynamic dissociation con- stants of beilzoic acids by Brown and Okamoto (2). The u+ constants differ from the u constants by an extra resonance factor resulting from the greater opportunity for resonance interaction by the substituents in electrophilic reactions. Such strong resonance interactions in electrophilic reactions occur between the electron-donating substituents and the electron-deficient centers of the reacting systems. Accordingly the a,+ values differ greatly from the u, values for the ortho-para-directing- groups. But the u and 6 constants for the meta-directing substituents are very similar and the deviations fall within the limit of the estimated uilcertainties of the constants (3). This is understand- able, since the meta-directing groups exhibit no significant resonance interaction with the incipient carbonium ion. As one would expect, the a,, and urn+ values for groups are also similar, since the major contribution of a meta-substituent is by inductive inter- action.

Although the resonarice parameters of the substituents should vary with the electronic demands of the reaction, it has been possible to propose a single set of u+ values which would correlate the available data on electrophilic aromatic substitution and electrophilic side-chain reactions with good precision (2, 4). This suggests that the u+ constants can also be separated to independent inductive and resonance contributions similar to the quantitative separation of the Hammett u constants by Taft (j),

where (TI and un+ are the inductive and resonance parameters respectively. The inductive

TABLE I IIESONAXCE PARAMETERS FROM u A N D U+ CONSTANTS

I U I ~ = up-ul ' ~ r n - u ~ UR+ = uP+--u1 u ~ ~ + - u I

NH, $0. 10 -0.76 -0.26 -1.40 -0.26 OH $0.25 -0.62 -0.13 -1.17 -

OCHr +O .25 -0.52 -0.14 -1.03 -0.20 F $0. 52 -0.46 -0.18 -0.59 -0.17 SCH, 4-0.25 -0.25 -0.10 -0.85 -0.09 C1 +0.47 -0.24 -0.10 -0.36 -0.07 NHCOCH3 $0.28 -0.28 -0.07 -0.88 -

B r $0.45 -0.22 -0.06 -0.30 -0.05 (CH3)3C -0.07 -0.13 -0.03 -0.19 $0.01 I $0.39 -0.21 -0.04 -0.26 -0.03 CH3 -0.05 -0.12 -0.02 -0.26 -0.02 Gel-I s $0. 10 -0.11 -0.04 -0.28 -0.01

. , . . . . . . . H 0.00 0.00 0.00 0.00 0.00

. . . . . . . . & ( C H ~ ) ~ $0.86 -0.04 $0. 02 -0.45 -0.50 CFr $0.41 $0.13 $0.02 $0.20 $0.11 (CH,),Si -0.12 4 0 . 0 5 +O .08 +O .14 +O. 13 C N $0.59 $0.07 -0.03 + O . 07 -0.03 C?H,OrC + O . 32 +0.13 $0. 05 $0.16 $0. 05 NO? + 0 .63 $0.15 +O. 08 $0. 16 $0.04

*Prese,ztly at the Deparl?nent of Chemistry, University of Californi:l, Berkeley, Calif.

Can. J. Cl~ern. Vol. 36 (195.8)

1596

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IVE

RSI

TA

T D

E G

IRO

NA

on

11/1

1/14

For

pers

onal

use

onl

y.

NOTES 1597

parameters in this equation are the same as those proposed by T a f t (5, 6). This is reasonable, since the inductive effect should esseiltially be iildepeildeilt of the reaction.

We have now calculated the a, - a I , a,,- a I, a,+- a I and a,,+- a I for several sub- stituents based on the recent compilatioils of the a and 6 values (1, 2). The results are summarized in Table I. In Fig. 1 are plotted the a,-a, versus the a,+-a, for the

i- electron-donating groups. With the exceptioil of N(Me)3, NCOi\iIe, SRIIe, and F groups, all the other groups fall on a straight line. By least-square treatment the line can be represented by the equation :

aRf = 1 . 9 1 ~ ~ - 0 . 0 3 .

This linearity further coilfirms the suggestion that rR+ of a group is essentially a constant to a first approximatioil and that aRf is not a very sensitive fuilctioil of p (4).

FIG. 1. Plot of ua+ vs. UR for electron-donating groups.

The authors' thanks are due to Professor H. C. Brown for his kind interest.

1. MCD.~SII?I., D. H. and BROWN, M. C. J. Org. Chem. 23, 420 (1958). 2. BROWS, M. C. and OKAMOTO, Y. I n press. 3. OKAMOTO, Y., INUKAI, T., and BROWX, H. C. 111 press. 4. OKAMOTO, Y. and BROWN, H. C. J. Org. Chern. 22, 485 (1957). 5. TAFT, R. W. I n Steric effects in organic chemistry. Edited by M. S. Newmarl, John Wiley & Sons,

Inc., New Yorlc. 1956. Chap. 13. 6. TAFT, R. W. Technical Report No. 18. Office of Naval Research Contract No. 656(05), Project

NK055-328. Pennsylvania State University. 1958.

RECEIVED JULY 28, 1958. RICHARD B. WETHERILL LABORATORY OF CIIBMISTRY, DEI'ARTMONT OF CHE~IISTRY, PURDUE UNIVERSITY, LAFAYETTE, INDIANA, U.S.A.

REARRANGEMENT STUDIES WITH CL4 VI. THE FRIEDEL-CRAFTS ALKYLATION OF BENZENE WITH ETHYL-8-C" IODIDE*

In 1955, Roberts, Ropp, and Neville ( I) reported that the Friedel-Crafts alkylation of benzene with ethyl-p-C14 chloride gave an ethylbenzene with no isotope position rearrangement in the ethyl side chain. When the ethyl-p-CI4 chloride was allowed to stand over aluminum chloride a t room temperature for I hour before being recovered and used to alkylate benzene, the ethylbenzene obtained showed rearrangements which

*Presented at the 41st Annzral Coltference of the Che?uical Ins t i t z~te of Canada, Toronto, iMny 26-28, 1968.

Can. J. Chem. Vol. 30 (1058)

Can

. J. C

hem

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

ess.

com

by

UN

IVE

RSI

TA

T D

E G

IRO

NA

on

11/1

1/14

For

pers

onal

use

onl

y.