[CONTRIBUTION FROM THE DEPARTMENT OF CHEMISTRY OF THE UNIVERSITYOF MARYLAND AND THE GEORGE HERBERT JONESLABORATORY OF THPJ UNIVERSITY OF CHICAGO]
THE DECOMPOSITION O F UNSYMMETRICAL ORGANOMERCGRIC COMPOUNDS: A METHOD OF ESTABLISHING THE RELATIVE ELECTRONEGATIVITIES OF ORGANIC RADICA.LS. V* M. 5. KHARASCH, R. R. LEGAULT,
AND
WILLARD R. SPROWLS
Received August $6, 1938 INTRODUCTION
The more electronegative of two organic radicals was defined by Kharasch and Flenner’ as the one which is cloven from the mercury atom by hydrogen chloride in the following reaction: RHgR’
+ H C 1 4 RHgCl + R’H
Reference was also made to the fact that the replacement of a hydrogen atom by a chlorine atom in the phenyl radical decreases the electronegativity of that radical and that for the three positions in the ring the decrease is greatest for the meta and least for the para position. A tentative hypothesis was advanced that groups capable of causing direct replacement of a hydrogen atom in a benzenoid compound decrease the electronegativity of the initial radical. A subsequent paper2 showed that this rule holds for the mfluorophenyl radical and the three brqmophenyl radicals, but that the pfluorophenyl radical is slightly more electronegative than the phenyl radical. The present paper extends this hypothesis to the 2,P and 2,5-dichlorophenyl radicals. DISCUSSION
The results of the present work are summarized in Table I, which lists the radicals newly placed. Our results show that the 2 ,Pdichlorophenyl radical is less electronegative than mchlorophenyl, and more electronegative than benzyl, and that 2 ,5-dichlorophenyl is less electronegative than mchlorophenyl and more electronegative than methyl. Another experiment which shows that m-chlorophenyl is more electronegative than * This paper is a condensation of the Master’s theses of R. R. Legeult, the University of Maryland, 1928, and of Willard R. Sprowla, the University of Chicago, 1936. 1 KHARASCII AND FLENNER, J . A m . Chem. SOC.,64,674 (1932). * KHARASCH,PINES,AND LEVINE,J. ORG.CHEM.,3, 347 (1938). 409
410
M. S. KHARASCH, R. R. LEGAULT, AND W. R. SPROWLS
benzyl merely confirms the position of these two radicals. Other cleavage reactions carried out in the present work show that the 2,4 and 2,5dichlorophenyl radicals are less electronegative than the phenyl radical, and that the former is also less electronegative than o-chlorophenyl, but these serve only to confirm previous work which compared the phenyl and chlorophenyl radicals. Previous work also showed that methyl is more electronegative than benzyl. The present work furnishes further evidence for the rule of Kharasch and Flenner that direct substitution decreases the electronegativity of the phenyl radical. The introduction of a second chlorine atom in the aromatic nucleus decreases still more the electronegativity of the chlorophenyl TABLE I RELATIV ELECTRONEGATIVITIES ~ OF SOME D I C H L O R O P ~ NRADICALS YL p-Chlorophenyl o-Chlorophenyl
2,4-Dichlorophenyl
II
2,8Dichlorophenyl
II
,Methyl 4.
4.
. I d Benzyl
radicals. It is also of interest that all the substituted aromatic radicals thus far tested are more electronegative than any of the aliphatic radicals. EXPERIMENTAL
Analysis for mercury.-Three methods were used for the determination of mercury in its organic compounds. They have been described by Kharasch and Flenner,' Kharasch, Pines, and Levine2 (electrolytic method), and Whitmore and Sobatski.' Analysis for chlorine was made by the Carius method. Preparation of organomercuric chlorides.-(Summarized in Table 11.) Chlorophenylmercuric acetates were prepared from the chloroanilines through the sulfinic acids according to the procedure described by Hanke,' with the following modifications. 2,4,6-Trichloroaniline was diazotized according to the method of Chattaway, Deighton, and Adair6 instead of according to that of Hanke. The large excess
5
WHITMORE AND SOBATZKI, J. Am. Chem. SOC.,66, 1128 (1933). HANKE,ibid., 46, 1321 (1923). CHATTAWAY, DEIQHTON, AND ADAIR,J. Chem. SOC., 1931, 1925.
411
DECOMPOBITION OF ORQANOMERCURIC COMPOUNDS
(about 15 moles) of sulfur dioxide required for all these preparations was weighed out in a trap cooled with solid carbon dioxide, and then passed into the cooled (ice and salt) solution of the diazonium salt under a pressure of two atmospheres with agitation. We have found that only one-fifth the quantity of catalytic copper recommended by Hanke is necessary for the satisfactory decomposition of the resulting solution. The separation of the benzenesulfinic acid from the catalytic copper by solution in aqueous sodium carbonate was effected in the case of the 2,Cdichloro derivative, but with the 2,5-dichloro and 2,4,6-trichloro derivatives the separation was more easily carried out with alcohol because of the low solubility of the sodium salts. The arylmercuric acetates in acetic acid solution were converted to arylmercuric chlorides by addition of 10% aqueous sodium chloride. The chlorides precipitated and were crystallized from alcohol. TABLE I1 PREPARATION AND PROP~RTIES OF ORGANOYERCURIC CHLORIDES YIELD (% THEORETICAL)
R IN RHoCl
m.p., '0.
50
35
85
196 205 184 104 250-1 147 208 170
I
YEBCUBY CONTENT (%)a
Calc'd
Found
52.51 52.51 48.15 61.33 64.06 57.71 57.71 79.89
52.72 48.6
By electrolytic method. One and two-tenths grams of this compound was treated with 0.5 g. of bromine in 10% aq. KBr solution. The product was l-bromo-2,4-dichlorobenaene;m.p. 27" (recorded m.p. 25"). e Treatment as above gave l-bromo-2,5-dichlorobenzene;m.p., 35-6" (recorded m.p. 36"). d This and succeeding derivatives are listed only to facilitate identification of cleavage products in Table 111. 0
b
Benzylmercuric chloride was prepared from benzylmagnesium chloride and mercuric chloride by the method of Hilpert and Griittner.6 Preparation, properties, and cleavage of unsymmetrical mercury compounds.(Summarized in Table 111.) The unsymmetrical mercury compounds were usually prepared by the addition of the organomercuric chloride to 2.5 equivalents of Grignard reagent in ether as described by Kharasch and Flenner.1 Adding the Grignard reagent to an ether solution of organomercuric chloride gave better results in the preparation of phenyl-2,4-dichlorophenylmercury. In the preparation of benzyl2,4-dichlorophenylmercury,benzylmagnesium chloride was added to a suspension (instead of a solution) of 2,4-dichlorophenylmercuricchloride in ether; the resulting unsyrn.metrica1compound had a high mercury content, indicating the presence of 6
H I L P ~ RATN D G R ~ T T N EBer., R , 48, 913 (1915).
412
M. 8. KHARASCH, R. R. LEGAULT, AND W. R. SPROWLB
Y
Y
cd
d
2;
0
DECOMPOSITION OF ORGANOMERCURIC COMPOUNDS
413
unchanged organomercuric chloride. Several preparations of some of the unsymmetrical compounds were made, but data for only the best preparations are listed. Cleavages of the unsymmetrical mercury compounds with mercuric chloride and hydrogen chloride were carried out according t o the methods of Kharasch and Flenner. On treatment with mercuric chloride phenyl-2,4- and phenyl-2,ij-dichlorophenylmercury and methyl-2,5-dichlorophenylmercurygave mixtures from which the expected organomercuric chlorides were isolated. Mixtures obtained from some of the other unsymmetrical compounds could not be resolved. After cleavage with hydrogen chloride, the products obtained by evaporation of the solvents were not recrystallized but were usually washed with ligroin. SUMMARY
1. Some new organomercuric compounds of the type RHgCl and RHgR’ have been prepared. 2. The 2,4-dichlorophenyl radical is less electronegative than mchlorophenyl radical, and more electronegative than the benzyl radical. 3. The 2,5-dichlorophenyl radical is less electronegative than the mchlorophenyl radical, and more electronegative than the methyl radical.
