448
JOSEPH
MILLER
Vol. 76
completely volatile under the conditions of distillation. As the data prove that the high molecular weight products are not formed exclusively through simple polysubstitution of biaryls, the above calculation of relative reactivities based entirely upon the yields of binuclear products seems the most logical method available. The percentages of isomers in the biaryl fractions are listed in Table 11. The accuracy of the analytical technique is indicated by the deviations shown.
reactivity of the nuclei toward free radical substitution. The isolation of phenylpyridine fractions of constant composition despite the variation of competitive phenyl halide solvent indicated that association of the phenyl halide and pyridine did not occur. However, since the completion of the major portion of this work, Hey7has reported that the reaction of benzoyl peroxide with chlorobenzene in the absence of a competitive solvent produces 62% 0-,24% mand 14% p-chlorobiphenyl. This deviation from the values found in the present work indicates that TABLE I1 association of the solvents in each competition reOrtho Meta Para action might have occurred but that the effect on Chlorobiphen yl 54 rt 3.0 31 f 2 . 0 15 i 1.0 the isomers produced from pyridine must be conBromobiphenyl 48.5f 3 . 0 33 =t2.0 18.5 f 1 . 0 stant. Because of this possible association, the apIodobiphenyl 55 f 3.0 28 f 2.0 16.5f1.0 plication of the relative reactivities determined in Phenylpyridine 68 f 3.5 28 f 3.0 14 rt 1.5 this paper to other systems of solvents may not be Nethyl phenylbenzoaccurate. ates 49 f 2 . 0 20 i 3.0 31 5 1.0 The limited data available to date indicate that substitution by free radicals is properly described Since the reaction of benzoyl peroxide with isomer mixtures of the above percentages has been found as invariably ortho, but that the ratio of meta to to leave biaryl fractions of unchanged composition, para substitution may be statistical with groups the percentages of isomers shown are significant which are ortho para directing for the common despite the accompanying formation of large quan- ionic substitution reactions. Acknowledgment,-We are indebted to the Ketities of tar. I n other words, subsequent reactions of biaryls initially formed would not alter the per- search Corporation for financial aid to one of the centage of isomers. Conversely it has been shown authors, Ilk. Robert E. Phelps, in the pursuance of that the orientation of a single substituent in a sub- this work. stituted biphenyl does not appreciably affect the CLEVELAND, OHIO
[CONTRIBUTION FROM T H E
DEPARTMENT OF CHEMISTRY,
U N I V E R S I T Y O F \VESTERN A U S T R A L I A ]
The SN Mechanism in Aromatic Compounds. VI. Substituents
Carbonyl and Nitrile
BY JOSEPII MILLER KECEIVEDMARCH 2G, 1953 The activating power in aromatic SV reactions of substituent groups containing C=O and C=N bonds has been measured and discussed in terms of electronic effects. The Arrhenius parameters for the compounds containing the aldehyde and nitrile groups are abnormal and require either a special mechanism or the occurrence of side reactions which are reversible and d o not affect the products.
I n discussing a series of substituents of the general type -C/
0 X ‘
Ingoldl pointed out that
as the electron donating power of X increases so the external effect of the whole group decreases. In aromatic SN reactions i t has already been pointed out by the author2 that these substituents are activating and that, following Ingold, one should then have the sequence of activating power
-c-cy 0
>-C
“H*
It should be noted that only the permanent electron repulsion effects of X should operate. These predictions have been tested by measuring the rate constants and Arrhenius parameters for the replacement of C1 by Ohle- in dry methanol, in the series NO2
\ c
where Y =
,-c//o\o-
AS is common, the C=N group is t o be included with this series. (1) C. K.Ingold, Chem. Revs., 1 5 , 223 (1Y34). ( 2 ) J. Miller, Revs. Pure A p p l . Chem. ( A u s f . ) ,1, 171 (1951).
1
0
-
- k N
Y
or H.
The com-
pounds actually investigated were : (i) o-chloronitrobenzene, (ii) sodium 4-chloro-3-nitrobenzoate, (iii) 4-chloro-3-nitrobenzamide, (iv) methyl 4chloro-3-nitrobenzoate, (v) 4-chloro-3-nitroacetophenone, (vi) 4-chloro-3-nitrobenzophenone,(vii) 4-chloro-3-nitrobenzaldehvdeand (viii) 4-chloro-3nitrobenzonitrile. The relevant results are given as Table I, which \
I
SNMECHANISM OF CARBONYL AND NITRILESUBSTITUENTS
Jan. 20, 1954
449
TABLE I c
Compd. i ii
Sub-
stituent H
co9-
. ..
Rate constant: lOskr (I. moles-1 sec-1) Calcd. from the Exptl. a t temperatures Arhennius parameters shown in parentheses 00 50' 1000
4.85 (59.9)
.. .
iii
CONH:
iv
COnCHr
v
COCHr
vi
COCsHi
...
vii
CHO
...
viii
CN
...
55.75 (30.2) 48.2 (26.1)
3.62 (75.3) 20.45 (75 3) 12.95 (35.2) 99.9 (34.8) 76.2 (30.2) 170 (34.8) 20.95 (25.0) 52.2 (30.2)
00
6.82 37.4 0.03297 0.2515 34.95 1 (81.6) (100.8) 13.2 36.4 165 0.02293 1.79 157.5 (81.6) (100.6) 40.3 178 0.152 65.8 5605 512 (45.3) (60.2) 258 937 2.15 392.5 17700 7240 (45.3) (59.9) 117.3 325 2.40 500 24900 8080 (35.0) (45.3) 12760 442.5 1600 3.79 668 29500 (45.35) (60.0) 1435 85.0 316 0.426 564 109000 (35.2) (45.3) 1590 102 391.5 0.473 706 148500 (35.0) (45.3)
include the values of the substituent rate factors (S.R.F.'s3). These results show clearly: (i) T h a t CHO and CN groups appear t o act by a different mechanism from the rest of the groups, which may be designated the main sequence. (ii) The S.R.F.'s of the main sequence groups are in the predicted order, with the additional information that -C/ precedence over -C
