[CONTIRIBUTION FROM THE GEORQE HERBERT JONES LABORATORY OF THE UNIVERSITY OF CHICAGO] 'THE HYDROLYTIC INSTABILITY OF THE CARBON- TO-CARBON BOND M. S. KHARASCK AND JULIUS PORSCRE Received July 8, 1936 INTRODUCTION Kharasch and co-workers have that the substitution of a strongly electronegative* group for a hydrogen atom in methane decreases the electronegativity of the methane carbon atom. It is a logical conse- quence of this concept that, beyond a certain liminal value of decreased electronegativity of the methane carbon atom, the bonds linking the methane carbon atom to strongly electronegative substituents should be distinctly polart in character. An investigation was undertaken to de- termine whether or not such a threshold may be reached among carbon- to-carbon bonds. The literature contains but little evidence for the existence of markedly polar carbon-to-carbon bonds.3 The necessary search for a molecule or group containing a powerfully electronegative carbon atom was simplified by a clue obtained from the work of Kharasch and H ~ w a r d . ~ Their study of the stability of the carbon-to-nitrogen bond demonstrated that the 1 carbon atom of 2-naphthol is very strongly electronegative-a con- clusion supported by the chemical behavior of 2-naphthol. The molecular 1 KHARASCK AND MARKER, J. Am. Chem. SOC., 48, 3130 (1926). KH-4RASCH AND REINMUTH, J. Chem. Educ., 6, 409 (1928). * Electronegative is here used in the sense defined by Kharasch and co-workers, loc. cit. t Electrovalence is not here implied, although that would be the extreme type of the phenomenon under consideration. This paper, however, deals with what the authors believe to be unsymmetrical covalences. 3 (a) KOSTANECKI AND ZIBELL, Ber., 24, 1695 (1895). (b) MIETZKI AND GUITERMAN, ibid., 20, 1274 (1887). (c) MOEHLAU AND STROBACH, ibid., 33, 804 (1900). (e) BRASS AND FIEDLER, Ber., 66B, 1654 (1932). The foregoing references cite some of the more important instances of marked 4 KHARASCH AND HOWARD, J. Am. Chem. Soc., 66, 1370 (1934). (d) I30EHM, Ann., 318, 259 (1901). polarity of carbon-to-carbon bonds. 265
Transcript
[CONTIRIBUTION FROM THE GEORQE HERBERT JONES LABORATORY O F THE UNIVERSITY
O F CHICAGO]
'THE HYDROLYTIC INSTABILITY OF THE CARBON- TO-CARBON BOND
M. S. KHARASCK AND JULIUS PORSCRE
Received July 8, 1936
INTRODUCTION
Kharasch and co-workers have that the substitution of a strongly electronegative* group for a hydrogen atom in methane decreases the electronegativity of the methane carbon atom. It is a logical conse- quence of this concept that, beyond a certain liminal value of decreased electronegativity of the methane carbon atom, the bonds linking the methane carbon atom to strongly electronegative substituents should be distinctly polart in character. An investigation was undertaken to de- termine whether or not such a threshold may be reached among carbon- to-carbon bonds.
The literature contains but little evidence for the existence of markedly polar carbon-to-carbon bonds.3 The necessary search for a molecule or group containing a powerfully electronegative carbon atom was simplified by a clue obtained from the work of Kharasch and H ~ w a r d . ~ Their study of the stability of the carbon-to-nitrogen bond demonstrated that the 1 carbon atom of 2-naphthol is very strongly electronegative-a con- clusion supported by the chemical behavior of 2-naphthol. The molecular
1 KHARASCK AND MARKER, J. Am. Chem. SOC., 48, 3130 (1926). KH-4RASCH AND REINMUTH, J . Chem. Educ., 6 , 409 (1928).
* Electronegative is here used in the sense defined by Kharasch and co-workers, loc. cit.
t Electrovalence is not here implied, although that would be the extreme type of the phenomenon under consideration. This paper, however, deals with what the authors believe to be unsymmetrical covalences.
3 (a) KOSTANECKI AND ZIBELL, Ber., 24, 1695 (1895). (b) MIETZKI AND GUITERMAN, ibid., 20, 1274 (1887). (c) MOEHLAU AND STROBACH, ibid., 33, 804 (1900).
(e) BRASS AND FIEDLER, Ber., 66B, 1654 (1932). The foregoing references cite some of the more important instances of marked
4 KHARASCH AND HOWARD, J. Am. Chem. Soc., 66, 1370 (1934).
(d ) I30EHM, Ann., 318, 259 (1901).
polarity of carbon-to-carbon bonds.
265
266 M. S. KHARASCH AND JULIUS PORSCHE
types r /
and $&-phenol suggest themselves because of the possibility of wide varia- tion in the electronegativity of R and of the phenol.
One of the criteria of bond polarity adopted was susceptibility to acid hydrolysis under comparatively mild experimental conditions. Hydro- chloric acid was chosen as the hydrolytic agent because of its strongly acid and exceedingly feeble reducing properties. Acetic acid has been used as a solvent for convenience in manipulation and in the isolation of the compounds studied and their decomposition products.
RESULTS
The series of compounds listed in the accompanying tables has been synthesized and the various members have been subjected to treatment with two per cent. hydrochloric acid in boiling glacial acetic acid.
Two distinct reactions indicated by the following equations, occur al- most instantaneously.
R H
C \ /
R H
C \ /
R H
C \ /
The chief reaction is that involving scission of the carbon-to-carbon bond. The results obtained are summarized in Table I. l-Benzyl-2- naphthol, submitted to this treatment, was recovered unchanged.
HYDROLYTIC INSTABILITY O F CARBON-TO-CARBON BOND 267
With a view to preventing dibenzoxanthene formation, the correspond- ing methoxynaphthyl compounds have been prepared and subjected to the ijame treatment. Rupture of the carbon-to-carbon bond was again observed with consequent formation of an aldehyde and 2-methoxynaph- thalene. The extent of formation of 2-methoxynaphthalene, correspond- ing to variations in the group R is recorded in Table I.
TABLE I EXTENT OF HYDROLYSIS OF THE DI-1-(2-HYDROXYNAPHTHYL)METHANES AND THEIR
The dibenzoxanthenes were not affected by two per cent. hydrochloric acid.
Nuclear triphenylmethyl derivatives of phenol, resorcinol, resorcinol dimethyl ether and 2-naphthol have been subjected to the same treatment without evidence of decomposition.
268 M. S. KHARASCH AND JULIUS PORSCHE
Further evidence for the marked polarity of the dinaphthol methane derivatives has been obtained by treating them in acetic acid solution at room temperature, with the antidiazotate of 2,5-dichloroaniline. They are found to react with the resultant diazo reagent at varying rates, de- pending on the nature of the group R, in accordance with the following equation.
R H
C \ / / \
c1 \
_____) N2-OH
($1 H o g J + 2 ( ) - \
c1 c1 OH
d l
1-Triphenylmethyl-2-naphthol does not undergo decomposition under these conditions, but couples, as a unit, with the diazo compound. The results of the experiments performed are presented in Table 11.
DISCUSSION
The results obtained demonstrate strikingly the necessity of passing a certain threshold* of decreased electronegativity of the methane carbon atom before it is possible to obtain perceptible evidence of marked polarity. The stability of the tetraarylmethanes seems to indicate that three phenyl groups and one 4-hydroxyphenyl or one 2,4-dihydroxyphenyl group are insufficient to induce the liminal degree of electronic displacement. Simi- larly one phenyl and one naphthol group are insufficient, as evidenced by the stability of 1-benzyl-2-naphthol toward the reagents employed. One might conclude further, from the stability of l-triphenylmethyl-haph- tho1 to two per cent. hydrochloric acid, that three phenyl groups and one naphthol group are also insufficient mere it not for the observation that
* This concept is discussed in greater detail by KHARASCH, REINMUTH AND MAYO, J . Chem. Educ., 11, 82 (1934).
HYDROLYTIC INSTABILITY OF CARBON-TO-CARBON BOND 269
the molecule couples, as a whole, with the diazo reagent. This reaction suggests the possibility that the 1 position of the naphthol group is free and that the structure is not 1-triphenylmethyl-2-naphthol as proposed by Hardy.5 Two naphthol groups and two hydrogen atoms, however, as in di-l-(2-hydroxynaphthyl)methane1 are just sufficient to exceed the liminal degree of electron displacement. Two per cent. hydrochloric acid effects a scission of the carbon-to-carbon bond that is just perceptible, as indi- cated in Table I, and a more striking indication is obtained by removing 2-naphthol as 2 , 5-dichlorobenzeneazo-2-naphthol and displacing the equilibrium as shown in Table 11. Further decrease in the electronegativ- i ty of the methane carbon atom through the substitution of a strongly electronegative group for one of the remaining hydrogen atoms in di-l- (2-hydroxynaphthy1)methane results in augmented hydrolytic instability of the bonds between the naphthol groups and the methane carbon atoms. The very weakly electronegative o-nitrophenyl and trichloromethyl groups were the only substituents employed which did not enhance the polarity of the molecule.
Examination of the data reveals a correlation, although not an exact parallelism between the relative electronegativities of the R groups and the rates of carbon-to-carbon scission. However, consideration of the side reactions which might occur simultaneously with the scission of the carbon-to-carbon bond makes it evident that the electronegativity of the radicals is the dominant although not the sole factor determining the rate of scission. Some of the complications introduced by side reactions are avoided by methylation of the reactive naphtholic hydroxyl groups, or by the removal of 2-naphthol as an insoluble azo compound, and it is clear from the results tabulated that when this is done a much closer cor- relation may be obtained between the relative electronegativity of R and the relative rate of carbon-to-carbon bond scission.
In order to demonstrate that the carbon-to-carbon bond scission is hydrolytic rather than thermal in nature, two grams of l,l’-benzalbis-2- naphthol was subjected to the same treatment that led to fifty per cent. decomposition, except that the hydrochloric acid addition was omitted, and was recovered without change. Similarly, l,l’-p-anisalbis-2-naphthol was unchanged after heating in benzene solution at 150” for five hours.
EXPERIMENTAL
Preparations
1 , 1 ’-(p-Dimethylaminobenzal) bis-$-naphthol.-The method of Hewitt, Turner and Braadley6 for the preparation of the hydrochloride was employed with but slight
modification.' The hydrochloride was made into a paste with acetone and treated with an excess of sodium bicarbonate solution. The free base was extracted with an ether-acetone mixture. Evaporation of the dry (sodium sulfate) solution yielded a brown solid from which a snow-white crystalline solid was obtained by repeated crystallizations from alcohol. Yield, 50-60%; m.p., 175-6" (dec.)
1 , l '-(p-Anisa1)bis-I-naphthol.-Anisaldehyde (13. 6g.) and 28.8 g. of 2-naphthol were dissolved in 120 cc. of glacial acetic acid. The solution was cooled t o about 5" and treated with 4 cc. of concentrated hydrochloric acid. The solution was kept at 5" in a tightly stoppered flask. After 48 hours a precipitate separated, and was collected on a filter and washed thoroughly with 70% acetic acid. The product was crystallized twice by the addition of water to its boiling alcoholic solution until crystallization commenced. The substance was completely soluble in 2% NaOH. Yield, 60%; m.p., 190-2" (dec.).
1 , l '-BenzaZbis-d-naphthol:-The following modifications of Hewitt and Turner's* procedure facilitated the isolation of a pure product. The condensation was al- lowed t o take place a t 5" and 0.1% sodium acetate was added to the glacial acetic acid used for crystallization.
The melting point observed was 203-4" (dec.). 1 , l '-[o- (and m-) NitrobenzaZ]bis-I-naphthols.-Great difficulty was experienced
in the isolation of products free from the corresponding nitrophenyldibenzoxan- thenes. This difficulty may be obviated by mixing the reagents and permitting the condensation to proceed at a temperature of 5-10'. Furthermore, the rate of addi- tion of the condensing agent must be slow in order to avoid local heating effects. 1 ,l'-(0-Nitrobenzal) bis-2-naphthol is very sensitive to light and must be kept in the dark. Observation of these precautions made i t possible t o duplicate the results of Dischendorfer.9 1,1 '-(o-Nitrobenzal)bis-2-naphthol, m.p. 205-7" (dec.) ; 1,l '-(m- nitrobenzal)bis-2-naphthol, m.p. 182-3" (dec.).
1 ,l'-(~-Phenylpropional)bis-S-naphthol,-All efforts to synthesize this molecule by direct condensation of hydrocinnamaldehyde with 2-naphthol led almost ex- clusively to products insoluble in alkali. The following procedure, however, gires a satisfactory yield of the pure compound.
y-Phenyl-a-l-(2-hydroxynaphthyl)propyl-~-phenylpropylidenamine (26.2 g.) and 40 g. of 2-naphthol were dissolved in a minimum quantity of boiling benzene. The temperature of the mixture was kept at 90"-100" for ten hours. Completion of the reaction was recognized by triturating a sample of the gum with cold alcohol. The Schiff base is very insoluble in cold alcohol and separates in the form of white needles if present in appreciable quantities.
When the reaction was complete, the brown gum was extracted four times with one-liter portions of boiling water. The residue was dissolved in about 250 CC. of cold acetone, and treated with 750 cc. of two per cent. sodium hydroxide solution. A small amount of insoluble material was removed by filtration. Acidification of
PORSCHE, Doctorate Dissertation, University of Chicago (1933). * Considerable difficulty was experienced in burning many of the di-2-naphthol
8 HEWITT AND TURNER, Ber., 34, 202 (1901). g DISCHENDORFER, Moncxtsh., 48, 543 (1927); 49, 137 (1928).
methanes.
HYDROLYTIC INSTABILITY OF CARBON-TO-CARBON BOND 27 1
the filtrate precipitated an oil which solidified on standing for a short time. The latter was subjected to three crystallizations from alcohol by the addition of suffi- cient water to produce a permanent turbidity a t the boiling point. This treatment yielded a tan-colored crystalline substance. Two crystallizations from toluene were sufficient to remove all the color from the compound. The product was completely soluble in dilute NaOH. Yield, 6045%; m.p., 172-3".
Anal.
T h e Di-l-(%'-methoxynaphthyl)methanes (2, i '-formalbis-8-methoxymphthalenes) .- With the exception of l,l'-trichloroacetalbis-2-methoxynaphthalene, all the di- methyl ethers were prepared in the following manner.
A ten per cent. solution of di-l-(2-hydroxynaphthyl)methane in acetone was stirred vigorously. Two per cent. sodium hydroxide and methyl sulfate (four moles of each) were added from dropping funnels in such a way that the solution remained faintly alkaline. The resulting mixtures were stirred for one hour to permit com- pletion of the reaction. The products were precipitated by addition of water and redissolved in acetone. Four more moles of sodium hydroxide and methyl sulfate were added, and the products were precipitated completely with water. These substances were invariably very pure, although not always obviously crystalline.
1 , I'-(p-Anisal) bis-2-methoxynaphthalene.-The product was crystallized from gla- cial rtcetic acid; m.p. 194-5".
l,i '-Benzalbis-2-methoxynaphtha2ene.-The product was crystallized from glacial acetic acid; m.p. 184'.
A n d .
1, /'-(m-Nitrobenzal) his-2-methoxynaphtha1ene.-Dilute acetic acid was used to crystallize this compound. The observed melting point was 172-3", whereas Disch- endoi-fer9 reported 216".
tion 'was again dilute acetic acid. dorferg reported 191".
from alcohol; m.p., 146-7". Calc'd for CsIH2s0~: C, 86.09; H, 6.53. Found: C, 86.00, 86.05; H, 6.60.
Hydrolyses
The following method has been adopted for a comparative study of the extent of carbon-to-carbon scission in the presence of two per cent. hydrochloric acid.
Two grams of each compound was dissolved in 50 cc. of boiling glacial acetic acid. The source of heat was removed, and when ebullition ceased, 2.4 cc. of hydrochloric acid ('sp. gr. 1.18) was added with agitation of the solution. The resulting mixture
10 KOHN AND OSTERSETZER, ibid., 38, 402 (1918).
272 M. S. KHARASCH AND JULIUS PORSCHE
was dropped into 150 cc. of boiling water through which a rapid current of steam was passing. The rate of addition was such that three to five minute were required for its completion. Distillation was continued for ten to fifteen minutes.
1 , 1 '-Benzalbis-2-naphthol.-The steam distillate was collected in a flask con- taining one cc. of phenylhydrazine dissolved in a little glacial acetic acid. After two hours, 0.49 g. of benzaldehyde phenylhydrazone (m.p., 154-5') was obtained by filtration of the distillate.
The suspension remaining in the distillation flask was removed by filtration while the mixture was hot, and dissolved in acetone. The addition of an excess of two per cent. sodium hydroxide to the acetone solution precipitated 0.40 g. of a crystal- line substance which was identified as 14-phenyldibenzo(aj)xanthene. Acidification of the alkaline solution yielded 0.52 g. of 1 ,lt-benzalbis-2-naphthol. Neu- tralization of the dilute acetic acid from the distillation flask and subsequent satura- tion with sodium chloride precipitated 0.77 g. of nearly colorless crystals (m.p. 117-20') which were identified as 2-naphthol.
1 , 1'-(p-Dimethylaminobenzal) his-$-naphthol.-None of the starting material or the corresponding dibenzoxanthene could be isolated from the brown gum which re- mained in the distillation flask.
Neutralization of the supernatant dilute acetic acid solution and saturation with salt yielded a small quantity of a green gum from which 0.20 g. of 2-naphthol was obtained by extraction with boiling water containing Norite.
1 , l '-(p-Anisal) bis-8-naphthol.-None of the starting material could be isolated from the red gum that remained in the distillation flask. It was possible, however, to obtain 0.18 g. of colorless needles which were identified as 14-p-anisaldibenzo(aj)- xanthene.
The dilute acetic acid solution yielded 0.60 g. of a red-brown crystalline sub- stance; m.p., 114-19". Extraction of the solid with ten per cent. sodium hydroxide left 0.05 g. of a light-brown solid of unknown constitution; m.p., 240-3'. Acidifica- tion of the alkaline extract precipitated 0.45 g. of 2-naphthol; m.p., 118-20'.
l,I'-(m-Nitrohenzal)bis-2-naphthol.-The alkali-insoluble fraction of the gum re- maining in the distillation flask weighed 0.23 g. and was identified as 14-m-nitro- phenyldibenzo(aj)xanthene. The alkali-soluble portion yielded 0.90 g. of the start- ing material and a small quantity of gum that resisted all efforts at purification.
2-Naphthol (0.42 g.) was recovered in the manner previously described. 1 , l '-(0-Nitrohenzal) bis-8-naphthol.-14-o-Nitrophenyldibenzo (uj)xanthene (0.07 g . )
was obtained by the usual technique from the red-brown gum in the distillation flask. The alkali-soluble fraction contained 1.70 g. of the starting material con- taminated with a dark, tarry impurity which could be removed only by repeated crystallization from acetic acid.
A small amount of red gum was obtained from the hot solution in the distilling flask. Two crystallizations from water (Norite) resulted in the isolation of 0.01 g. of 2-naphthol; m.p., 118-20".
1,1'-(-pPhenyEpropional)bisJ-naphthol.-Repeated crystallizations of the alkali- insoluble fraction of the residual yellow gum from benzene-alcohol yielded 0.07 g. of a white crystalline substance (m.p., 173") which was assumed to be 14-phenethyl- dibenzo(uj)xanthene.
Anal. Calc'd for C Z ~ H Z ~ O : C, 90.11; H, 5.74. Found: C, 89.51, 89.86; H, 6.07, 6.13.
It was impossible to separate the alkali-soluble fraction into its constituents. The usual treatment of the dilute acetic acid solution yielded 0.48 g. of 2-naphthol. Di-l-(2-hydroxynaphthyl)methane (I , 1 '-formalbis-2-naphthol).-Alkali-insoluble
HYDROLYTIC INSTABILITY OF CARBON-TO-CARBON BOND 273
material (0.29 g.; m.p., 183-90") was obtained from the residue in the distillation flask. Of the starting material, 1.48 g. was recovered. Semicrystalline material (0.15 g.) was obtained upon neutralization of the dilute acetic acid. Extraction with hot water yielded .055 g. of 2-naphthol, and recrystallization of the residue from alcohol-water left .OS g. of the original material.
f -Benzyl-2-naphthol.-l-Benzyl-2-naphthol (1.97 g . ; m.p. 112-13") was recovered. Treatment a-ith an aqueous solution of the anti-diazotate of 2,5-dichloroaniline proved the absence of 2-naphthol in the distillate and in the distilling flask.
The Di-f -(2-methoxynaphthy1)methanes (1 , f '-jormaEbis-2-methox ynaphthalenes) .- The same hydrolytic procedure was followed as in the case of the di-l-(2-hydroxy- naphthy1)methanes except that steam distillation was continued until no more methoxynaphthalene appeared in the distillate. Two grams of material was used in each experiment.
1, I'-Benzalbis-2-methoxynuphthalene.-Extraction of the residual ta r with ether yielded 0.26 g . of crystalline material; m . p , 17M". This was identified as the starting material. All attempts to separate the remaining resinous substance into its c-onstituents failed.
It Fas identified as dibenzo(aj)xanthene.
2-Methoxynaphtlialene (0.70 g.) was obtained from the distillate. 1 ,l'-(p-Anisal) bis-8-methoxynaphtha1ene.-None of the starting material could be
The distillate yielded 0.65 g. of 2-methoxynaphthalene. 1, 1 '-(m-Nitrobenzal) bis-8-methosynaphtha1ene.-Fractional crystallization of the
residual material from acetone-water mixtures resulted in the recovery of 1.35 g. of the starting material. The remainder of the residue was a brown oil containing a small amount of m-nitrobenzaldehyde.
recovered from the residual gum.
The distillate yielded 0.13 g. of 2-methoxynaphthalene. 1,1'-(o-Nitrobenzal)bisd-methoxynaphthalene.-A residue (1.95 g.; m.p., 194-6')
was obtained. Only a faint odor of 2-methoxynaphthalene could be detected. 1 , l '-(y-Phenylpropional) bis-8-methoxynaphtha1ene.-A yellow resinous substance,
which was not purified, remained in the distillation flask. 2-Methoxynaphthalene (0.52 g.) was isolated from the distillate. Di-1(8-methoxynaphthyl)methane (1,1'-jormalbis-8-methoxynaphthalene).-The
cryetalline residue weighed 1.86 g.; m.p., 136-42'. After several crystallizations from alcohol, the mixture was shown to consist chiefly of the starting material.
The distillate contained 0.10 g. of 2-methoxynaphthalene. 1,1'-Triehloroaeetalbis-2-methoxynaphthalene.-A half-gram of the substance was
subjected to the treatment previously described; 0.47 g. was recovered. No 2-me- thoxynaphthalene could be detected.
T'he dibenzoranthenes.-Half-gram portions of 14-phenyl- (and l.l-p-anisyl-) dibenzo(aj)xanthenes were recovered quantitatively after treatment with two per cent, hydrochloric acid.
The tetraarUlmethanes.-No triphenylcarbinol could be isolated, following the previously described treatment with two per cent. hydrochloric acid of any of the four substances examined.
Reactions with Diazo Reagent.-A 0.5 g. portion of each substance investigated mas dissolved in 50 cc. of glacial acid, and the solutions were cooled to room tempera- ture. Twenty cc. of water, containing four moles of the anti-diazotate of 2,5- dichloroaniline for each mole of compound, was added dropwise to each of the solutions, with agitation. After five to seven minutes, the red precipitates were col-
274 &I. S. KHARASCH AND JULIZTS PORSCHE
lected on filters washed with 50% acetic acid and water. The n-eights and melting points of the products have been recorded in Table 11.
One gram of I-triphenylmethyl-2-naphthol yielded 1.50 g. of a red substance, (m.p., 265-73’). After crystallization from pyridine-methyl alcohol, the m.p. 275-7” was obtained.
Anal. Calc’d for C35H2aClZN20: N, 5.01. Found: N, 5.00.
SUMMARY
1. The validity of the general theory of the stability of the carbon-to- carbon bond‘ j 2 has received further experimental confirmation in the case of markedly polar carbon-to-carbon bonds.
2. Carbon-to-carbon bonds which possess the remarkable property of undergoing hydrolytic scission in the presence of dilute mineral acids have been investigated.
3. The following new compounds have been prepared: (a) l,l’-(p-anisal)bis-2-naphthol,
