ON THE INFLUENCE OF SOLVENTS ON THE STEREOCHEMICAL COURSE OF ADDITION OF HYDROGEN BROMIDE TO MONOBASIC ACETYLENIC ACIDS AND THE RELATION OF SOLVENT EFFECT TO CHEMICAL STRUCTURE ARTHUR MICHAEL (EXPERIMENTAL PARTWITH G. H. SHADINGER) Received January 6, 1 1 8
About fifty years ago, Michael and co-workersl showed that the two bromocinnamic acids, obtained by the action of alkali upon cinnamic acid dibromide2, were not the isomeric, a- and j3-bromocinnamic acids, as was then accepted, but stereomeric a-bromocinnamic acids and that phenylpropiolic and hydrobromic acids gave two stereomeric 8-bromocinnamic acids. Corresponding results were obtained from crotonic acid dibromide and tetrolic acids. The formation of 8-bromoderivatives in the additions agreed with, and were explained by, the positive-negative addition rule4, but observations, which. did not concord, were later made by Sudborough and Thompson6,in a study of the addition of hydrogen bromide to phenylpropiolic acid in non-aqueous solvents. Light, temperature and concentration of aqueous hydrogen bromide were practically without effect upon the course of addition. However, in acetic acid solution the relative proportion of j3-bromocinnamic acid (m.p. 134') increased decidedly and, with benzene, chloroform, and carbon disulfide as solvents, the main product was the trans a-bromo acid (m.p. 120'). The formation of the a-bromo acid was directly opposed to the positivenegative addition rule4, which hitherto had been applicable to the addition reactions of all classes of unsaturated, organic compounds and addenda, except hypochlorous and hypobromous acidss. This rule was based upon 1 MICHAEL et al., Ber., 18, 1378 (1886);19,887 (1887);20,550 (1888);Am. Chem. J . , 9,221,281 (1887). a GLASER, Ann., 143, 330 (1867). * MICHAEL AND CO-WORKERS, Am. Chem. J . , 2 , 12 (1879);J . prakt. Chem., 36, 257 (1887);38, 1 (1888);40, 62, 96 (1889);46,266 (1892). 4 MICHAEL, J . prakt. Chem., 37, 525 (1888);40, 171 (1889);60, 332 (1899);J . Am. Chem. SOC.,32, 1005 (1910). SUDBOROUOH AND THOMPSON, J . Chem. SOC.,85, 1152 (1903). 6 MICHAEL, J . prakt. Chem., 60,454 (1899);64,1028 (1901);J . Am. Chem. Soc., 32, AND LEIOHTON, Ber., 39,2167 (1906). These acids add in dilute 990 (1910);MICHABL aqueous solution and, being strong oxidants, exert a peroxide effect upon the couree
128
SOLVENT EFFECT ON COURSE OF ADDITION
129
the affinity-energy relations of the respective, unsaturated atoms for the components of the addenda; the heats of formation of the possible isomers are regarded as the major factors contributing to the energy degradation in the reactions and, therefore, largely determining the course of the additions7. When the thermal difference is large, the addition should yield the isomer or stereomer with the larger heat of formation almost exclusively. But, when the thermal values are nearer together, both derivatives should be formed and in relative amounts proportionate to the approximation deduced from the principle of partition7. Under these conditions, physical energy factors, associated in determining the maximum energy degradation, should play a more important r61e in directing the mode of addition. Thus, the solvent may alter, more or less, the course of addition when the heats of solution of the formed isomers or stereomers differ materially and when their solubility relations differ decidedly. When a mixture results in an addition, the physical factors, by favoring the formation of one or the other of the possible intermolecular structures of the preliminarily-formed polymolecules8, will function the more decisively the nearer together are the values of the involved chemical energy factors. No theoretical explanation of the mechanism of the addition process is probable unless it can cobrdinate the course of addition with the chemical structures of the unsaturated compounds and successfully predict the outcome of unexamined reactionsg. of addition. For an explanation of partially abnormal additions to several unsaturated hydrocarbons, that proceed with intramolecular migration of tertiary hydrogen, see A m . , 386, 244-247 (1911). MICHAEL, J. prakt. Chem., 80, 348 (1899); 68, 199 (1903); Bey., 39, 2138 (1906). 8 MICHAEL, Ber., 34, 4029 (1901); A m . Chem. J., 39, 2 (1908). 9 The frequently quoted Markownikoff addition rule is wholly empirical and is not even entirely valid for the two groups to which it was applied, Le., alkenes-1 and alkines-1. For other classes of unsaturated, organic compounds it usually leads t o wrong conclusions, e.g., with the a,,9-unsaturated acids and, as a general rule for addition to double and triple linkages, i t is only of historic interest. Lauer and Stodola [ J . Am. Chem. SOC.,66, 1216 (1934)l found that pentene-2 adds hydrogen bromide t o form a mixture of 2- and 3-bromopentane in nearly equal amounts and therefore concluded that the Wagner-Sayzeff rule [Ann., 179,313 (1873)l is not valid. These chemists, and also Lucas and Morse [ J .Am. Chem. Soc., 47,1460 (1925)1, overlooked that this subject had been theoretically and experimentally developed previously [Michael, J . prakt. Chem., 80, 348 (1899); Ber., 39, 2141, 2143, 2149 (1906)l and that the same conclusions had been reached. I t ia of interest that Lauer and Stodola concluded that the results conflict with the current views on electronic displacement and Kharasch’s partial polarity speculations. Their criticism is supported by the experimental results obtained by the writer, which are consistently explained by the principle of partition.
130
ARTHUR MICHAEL
With a view to explain the abnormal results obtained in the addition of hydrogen bromide to phenylpropiolic acid in non-aqueous solutions, the reaction was re-examined. We confirmed the above observations in benzene solution and found that the trans a-acid was also largely formed in toluene and bromobenzene. On the other hand, with nitromethane and nitrobenzene as solvents, the trans 8-bromo acid was formed exclusively, and the same derivative was obtained in ethyl bromide, ether, and acetone. Since the mineral acid can transform the cis 8-derivative (159') catalytically into the trans form (134'), the latter product may have resulted from a secondary change, coincident with or subsequent to, the addition. We proved that a stereomerization of the cis 8-bromo acid (159') did occur. In the latter solvents, hydrogen bromide readily converted the cis 8bromo acid into the trans 8-modification, but, after only a short reaction period in nitromethane, the reaction yielded the cis &bromo acid (type A, see formulae below) as the sole product of the addition. The proof of the direct formation of the trans a-acid was more difficult, because the cis a-bromo acid (type B), in all the solvents leading to the formation of the a-derivative, was isomerized rapidly into the trans stereomer, although the ease of the conversion varied somewhat with the solvent. When hydrogen bromide was passed into a saturated solution of phenylpropiolic acid in benzene, the trans a-bromo acid (type D) was precipitated almost immediately; but under the same conditions, only 30 per cent. of the cis a-bromo acid (type B) was stereoisomerized during onehalf hour. These facts make it probable that the trans a-bromo acid is a direct product of addition in benzene; accordingly this addition would be a trans process. Sudborough and Thompson5 concluded that associating and ionizing solvents favored formation of the a- and the 8-bromocinnamic acids, respectively. In our experiments with phenylpropiolic acid, however, more solvents were used, and the results show that no connection exists between the course of addition and the ionizing or associating properties of the solvents (see Table V). The above results place the abnormal formation of the a-bromo acid from hydrogen bromide and phenylpropiolic acid in certain solvents in a different theoretical aspect. Unfortunately for the development of organic theory, a systematic study of thermal data for organic compounds and reactions has been not only largely neglected but undervalued for years. Fortunately, however, for the subject of this paper, the K values of the stereomeric a- and P-chlorocinnamic acids are known. According to Stohmann's rule, the heats of combustion of position-isomericlo and stereomeric acids rise and falls 10 Stohmann, in his first two papers on this subject [ J . prakt. Chem., 40,357 (1890); 46,341(1892)],showed the validity of this rule for isomeric acids, formed by replacing hydrogen, in different positions with respect to the carboxyl, by the same group of
SOLVENT EFFECT ON COURSE O F ADDITION
131
with their K values. Cis-addition of halogen hydride to phenylpropiolic acid may yield 8-haloisocinnamic (type A) and a-haloisocinnamic acid (type B); the trans process, 8-halo- (type C) and a-halocinnamic acid
(4
R-C-Hal.
II
HOOCCH
R-C-H
(B)
R-C-Hal. (C>
II
H-CCOOH
II
HOOCC-Hal. R-C-H
(D)
II
Hal.-CCOOH
(type D). The K values of the 8-chloro acids (A and C) differ but slightly, and 27.2X 10-5J1, and those of the a-chloro acids are 107 X 10-5 and 97 X The K values for the corresponding bromo acids have not been determined, but there can be no doubt that the numerical relationship is approximately the same as that for the corresponding chloro acids, which is about as 1:3.5. The K values for the a- and 8-chloro- and bromohydrocinnamic acids are undetermined, but they may be deduced approximately from those for the corresponding a- and p-halogen-sub28 X
atoms. Such acids were called “Stellungsisomere” (position-isomers). In the following paper, Stohmann [ibid., 46, 630 (1892)l gave a resume of his conclusions with an extension of the experimental work. In rule (l), “Isomere” instead of “Stellungsisomere” was used, although this alteration was not supported by new experimental work. It was apparently an erratum, as in the following, final paper on this subject [ibid., 49, 118 (1894)l “Stellungsisomere” was again used to designate the groups of acids in question. Unfortunately rule (1) has been discredited through misrepresentation of Stohmann’s viewpoint. Verkade [Rec. trav. chim., 44, 1006 (1925)l found that the respective experimental data associated with the “stereomeric” meso- and d-tartaric acids do not conform with the rule. However, by no stretch of imagination can these compounds be considered as “Stellungsisomere”, or as stereomerio acids. Roth and Ostling [Ber., 46,309 (1913)] showed that the relation between the isomeric, stereomeric, and cyclic acids of the formula CaH6COOH and CnH&OOH conform to Stohmann’s rule connecting the heats of combustion and K values of the respective acids, but believed that the values for the isomeric tanacetone carboxylic and pinonic acids probably do not conform. However, with these acids the comparison is between cyclopropane and cyclobutane derivatives, which contain highly energetic groups in positions exerting a slight influence upon the K values, and that the relationship between the thermal and electrolytic constants should prevail for such isomers is evidently improbable. The same misconstruction of rule (1) generally occurs in the literature, e.g., Hiickel, “Theoretische Grundlagen der orgianischen Chemie, 11, p. 298 (1931), but no experimental results are known discrediting Stohmann’s rule in its original form. Between the a- and p-carboxylic derivatives of acids considerable differences in the heats of combustion and, also, in the K values occur and the first relation has been accepted in this paper for the corresponding chloro and bromo derivatives. 11 MULLIKEN, Dissertation, 1890. Beilstein, IX, 595 (1926).
132
ARTHUR MICHAEL
stituted, aliphatic acids, ie., about as 1 :16-17. Accordingly, solvents may affect the mode of addition of hydrogen bromide to phenylpropiolic acid, but should not with cinnamic acid. Actually, with the first acid, in aqueous and nitromethane solution, fl-bromoisocinnamic and in benzene solution the corresponding a-acid was formed, while with cinnamic acid no solvent was found that changed the mode of addition, i.e., ,9-bromohydro-’ cinnamic acid was formed under all tested conditions. The stereomeric course of addition of hydrogen bromide to tetrolic acid differs from that to phenylpropiolic acid. In aqueous solution, the latter acid yielded fl-bromoisocinnamic acid (type A), while tetrolic acid gave fl-bromocrotonic acid (type C), and the same acid appeared in nitromethane. On the other hand, in benzene and propyl bromide solutions, the a-bromocrotonic acid (m.p. 104’; type D) was formed. The K values for fl-chloroisocrotonic (type A), and for the trans a-acid (type D), 14.4 X lP5and 72 X respectively, are in the ratio of 1:4.9, and, on the basis that these relations, hold approximately for the corresponding bromo derivatives, it is evident that the course of addition of hydrogen bromide to tetrolic acid may be subject to solvent effect. I n accordance, it was found that the course of addition of hydrogen bromide to tetrolic acid varied with the solvent used, but the addition to crotonic acid, concordant with the comparatively large difference in the K values (ratio of 1 :16-17) for the respective, possible addition isomers, showed no solvent susceptibility; i.e., the reaction gave only fl-bromobutyric acid12. Experiments on the behavior of cis a- and trans fl-bromocrotonic acids towards hydrogen bromide in nitromethane solution confirmed the conclusion, which had been drawn previously from other results, viz., that the cis bromocrotonic derivative (type A) is much more stable towards the mineral acid than the corresponding cis bromocinnamic derivative. Acand only a partial concordingly, no change was noticed with the cis @-, version with the a-bromocrotonic acid. The above results make the conclusion probable that in additions of halogen hydride to structurally, closely related groups of unsaturated compounds, the smaller the difference in the heats of combustion of the addition products, and with acids the smaller the divergence in the K values, the greater may be the solvent effectla. MICHAEL,J . prakt. Chem., 62, 289 (1895). See Table 111. The mechanism of “oxygen and peroxide effect” upon hydrogen bromide addition is analogous to that of solvent and that it haa the corresponding relation to the structures of the unsaturated, organic compoundsis evident from the existing experimental data. Contrary to the conclusion of Kharasch [J. ORG. CHEM.,2, 289 (1937)l “peroxide effect” in alkenes does not depend upon the existence of terminal unsaturation. With N. Weiner, the previous observation [Michael and Zeidler, 11
I*
133
SOLVENT EFFECT ON COURSE OF ADDITION EXPERIMENTAL
BY ARTHURMICHAELAND G . H. SHADINGER General procedure.-Hydrogen bromide, prepared from C.P. bromine, red phosphorus and water, was passed through a long U-tube filled with red phosphorus, and dried by passing successively through tubes filled with calcium bromide and phosphoric anhydride. The dried gas waa absorbed in solutions, or suspensions, of the unsaturated organic acids listed in the tables. Solid products were collected by filtration and purified by recrystallization, or the bromo acids were isolated as barium salts. These salts are not completely insoluble, and the aqueous filtrates, after acidification, yielded acidic products which were purified as indicated in the footnotes t o the tables summarizing the results. In the early experiments, the bromo acids were dissolved in 40 parts of water and neutralized with a 10% barium hydroxide solution, but the salts separated incompletely a t this dilution14 and, in
TABLE I RATESOF HYDROQEN BROMIDE ELIMINATION
I
1
a-Bromoallom.p. 120’
1
CINNAMIC ACIDS
a-Bromom.p. 130’
0
yoAvailable Br Eliminated
11.0 37.0 79.0
1
&Bromoabm.p. 159’
12 60 93 100
1
&Bromom.p. 134’
100
TIME,
Ens.
2 20
46 118
the later work, the bromo acids were dissolved in only 20 parts of water. For identification, the bromo acids, obtained from the precipitated barium salts and from the aqueous filtrates, were treated in the cold with alkali of known concentration, and the Ann., 386,271 (1911)l of a solvent effect with isoamylene-2 has been confirmed and it has been shown that the hydrocarbon is sensitive t o “peroxide effect.” The extent of the latter influence depends upon the solvent and with methanol increases with fall of temperature. It seems probable, therefore, that “oxygen and peroxide effects” function through the formation of a double molecule (“polymolecule”) in which contact between oxygen and unsaturated carbon linkage occurs more largely at the more positive of the atoms. When the difference between the positivities of the unsaturated carbons is comparatively slight, the previously relatively positive of the unsaturated carbons may become the relatively negative and thus lead to abnormal additivity. Solvent effect may be attributed to an analogous mechanism. Corresponding influences undoubtedly occur in other chemical processes. An interesting illustration was noticed by Roth and Stoermer [Ber., 46, 276 (1913)l who examined the velocity of stereomerisation of cinnamic and cumarinic acids and derivatives in ultraviolet light. They found that the smaller the difference in the K values of the stereomeric pair of acids, the larger was the amount of acid stereoisomerised. In other words, the percentage change decreased with the increase in work required to effect the isomerisation. 14 MICHAEL AND BROWNE, Ber., 20, 554 (1887).
134
ARTHUR MICHAEL
TABLE I1 ADDITION OF HYDROQBN B R O M I DTO ~ PHBNYLPROPIOLIC ACID
8
I
s:
E m .
No*
ad 16
/
4 1 SOLVENT,
0.
3 4
~-
8
PBODUCT
E-
Crude
8 8 "8.0' ---
Acid
g.
%s;@
&
_ . -
5.0 Benzene, 30 50 R.T. IIb 0.1 Toluene, 1 R.T. IIIC 1 .o Bromobenzene, 30 20 0
152.5 128-129 a-Bromo- 2.5 128-129 a-Bromo128-129 a-Bromo- 0.73128.5129.5 IVd 0. l! Acetone, -20 @-Bromo134-135 ve 0.1! Ethylbromide, R.T. @-Bromo132-133 5 45 -20 2 1.44113-116 @-Bromo134-136 VI' 1.0 Ether, R.T. 12 0.37 128-140 @-Bromo133 VIIU 0.5 Nitromethane, 15 @-Bromo- 0.9 133-135 0 24 VIIIh 1 .o Nitrobenzene, 30 15
Solvent waa evaporated from the benzene filtrate, and the residue, purified through the barium salt; yield, 1.9 g. of a-bromo acid, m.p. 1'27-128'. The aqueous filtrate, from which the barium salt had separated, waa concentrated, and yielded 0.4 g. of acid, which, after recrystallization from benzene, melted a t 123-133". Elimination of hydrogen bromide from the bromo acid waa complete after treatment with alkali for 2 hours; the bromo acid, therefore, was @-bromocinnamicacid (m.p. 134") and i t constituted 10% of the addition product. b Hydrogen bromide was absorbed until.al1 the organic acid dissolved. No solid separated from the bromobenzene solutions during 24 hours a t 0'. Solvent was distilled off with steam, and the residue was converted to the barium salt; acidification of this salt liberated a-bromocinnamic acid. A solution of the bromo acid in 2% alkali was acidified after 1.5 hours; the precipitate, after conversion t o the barium salt, and recrystallization of the free acid, melted a t 129.5-130.5'. In a similar experiment, the solvent was distilled off with steam, and the dried residue (0.8 g.; m.p. 119-124') was converted into the barium salt, which, on acidification, yielded 0.6 g. of a-bromo acid, m.p. 131-132". The filtrate from the barium salt was acidified, and yielded 0.2 g. of a-bromo acid, m.p. 119-124'. The aqueous filtrate from the original crude addition product yielded 0.52g. of acid, which, after purification through the barium salt, gave 0.1 g. of a-bromo acid, m.p. 131-132". * The hydrogen bromide was absorbed a t the temperature of an ice-salt mixture. Solvent wm distilled off in uucuo, and the residue was purified through the barium salt. The filtrate from the salt was acidified, and gave a small amount of acid, m.p. 0
0
117-122'. Solvent was distilled off i n vacuo;the residue, m.p. 128-128', was purified through the barium salt, and yielded the p-bromo acid. /Absorption was made a t the temperature of a salt-ice mixture. The crude product WM purified through the barium salt; the aqueous filtrate was acidified, and yielded 0.7 g. of acid, m.p. 123-130'. Absorption of hydrogen bromide was stopped when solid began to separate from the reaction mixture; the crude product wm purified through the barium salt. In a second experiment, 1 g. of phenylpropiolic acid wm used; the crude addition product, after purification through the barium salt, yielded 0.8 g. of acid, m.p. 117-131', which,
SOLVENT EFFECT ON COURSE OF ADDITION
135
TABLE 11-Continued repurified through the barium salt, gave pure @-bromocinnamicacid, map. 134135". The aqueous filtrate waa acidified, and yielded 0.1 g. of acid, m.p. 11&150", which, after recrystallization from alcohol, gave pure 8-bromoallocinnamic acid, m.p. 158.5159.5". In a third experiment, absorption of hydrogen bromide was stopped after 10 minutes; the solid which had separated from the solution during 1 hour was filtered off, washed with water and ligroin, and then gave 0.15 g. of pure @-bromoallocinnamic acid, m.p. 156-157". h No solid separated from the nitrobenzene solution during 24 hours a t 0'. Solvent was distilled off with steam, the organic residue, since i t gave no insoluble barium salt, was recrystallized from alcohol and then yielded @-bromocinnamicacid, which readily gave an insoluble barium salt, and from which 0.9 g. of the 8-bromo acid waa recovered. Halogen waa completely removed from the bromo acid by alkali during 2 hours. amount of liberated hydrogen bromide was determined by titration. This method is applicable because the rate of elimination of hydrogen bromide from the fumaroid is much greater than from the maleinoid product16. The relative rates of elimination were determined by dissolving 1 g. each of the bromo acids in 31.75 cc. of 1.25% NaOH (3 moles) solution, prepared from metallic sodium; a t intervals aliquot portions (6 cc.) of the solutions were titrated by the Volhard method. Typical results are given in Table I. Addition of hydrogen bromide to phenylpropiolic acid.-The acid was dissolved in carbon tetrachloride, and the hot solution filtered to remove insoluble cinnamic acid. Solvents were dried over phosphoric anhydride. The results of the additions are summarized in Table 11. Addition of hydrogen chloride to phenylpropiolic acid.-A solution of phenylpropiolic acid in chloroform, saturated with hydrogen chloride, yielded, after 6 weeks, only a very small amount of addition product (calc'd for CpHrOd31: C1, 19.45;found: C1, 1.89) and, during 3 days a t 60°, only a trace of the halogen acid was formed. Similar results were obtained in toluene and ether: a t room temperature only very little of the addition product was formed and, in toluene, after heating for 3 days a t 60", the product contained only 2.5% C1. Addition of hydrogen bromide tQ cinnamic, tetrolic, and crotonic acids.-The solutions, or suspensions, of the unsaturated acids were saturated with hydrogen bromide a t 0", and the reaction mixtures were heated in sealed tubes; when addition was very slow a t room temperature. Solid addition products were seperated by filtration, solvents were distilled off i n uacuo from the filtrates, and the residual products were purified as indicated in the tables and footnotes. The results are summarized in Table 111. No relation exists between the course of addition of hydrogen bromide to phenylpropiolic acid and the dielectric and dissociation constants of the solvents (Table V). SUMMARY
1. Aqueous hydrogen bromide unites with phenylpropiolic acid by cisaddition to yield &bromoisocinnamic acid. 2. The course of addition of hydrogen bromide to phenylpropiolic acid 15
MICHAEL AND WHITEHORNE, Ber., 34, 3647 (1901).
ee:
ee:
c; Pi
e
a I
2
e
pc
Y
? 0
0
-e
0'
0"
.d
Id .3
Y
Y
e 2
2 D
i U
F-
7 136
137
SOLVENT EFFECT ON COURSE O F ADDITION
in non-aqueous solvents may vary with the solvent. In benzene, bromobenzene, and toluene, a-bromocinnamic acid was formed, whilst in nitromethane, nitrobenzene, ether, and acetone, the trans &derivative appears exclusively. However, in nitromethane, the cis 8-acid is the primary TABLE IV ELIMINATION OF HYDROGEN BROMIDEFROM
U- AND
~-BROMOBUTYBIC ACIDS
BUTYRIC ACID
% Available Br Eliminated
{I
u-Bromo-
1
5.84 11.70 16.80
TIYE, EBB.
&Bromo-
4 27 72
98.16 100.00
TABLE V SOMEPHYSICAL CONSTANTS OF PRODUCTS OF ADDITION OF HYDROGEN BROMIDIC TO PHENYLPROPIOLIC ACID PHENYLPBOPIOLlC ACID QAVE
Bromo acid
Solvent
a-Bromocinnamic a-Bromocinnamic a-Bromocinnamic @-Bromocinnamic 8-Bromocinnamic @-Bromocinnamic @-Bromocinnamic 8-Bromocinnamic 8-Bromocinnamic
Benzene Toluene Bromobenzene Acetone Ethylbromide Ether Nitromethane Nitroethane Nitrobenzene
DIELECl'BIC CONSTANT AT", O c .
DIBBOCIATIOR TAcTOBb
L88OCIATION QA~TOB~ __I
4
2.26 2.31 5.2 20.2 9.5 4.37 38.2 29.5 36.45
1.18 1.08
20
Weak ( 1 ) Weak (8) Weak (3) 74 (4, Strong (6) Weak (6) 92 (7)
18 18
88 (3)
1.82
19 19 18 17 20 18
1.53 1.28 1 .OO
The dielectric constants are taken from the International Critical Tables.
* The dissociation factors were reported by:
( 1 ) BECKMANN AND LOCKEMANN, Z . physikal. Chem., Bo, 398 (1907).
(8) KAHLENBERG AND LINCOLN, J . Phys. Chem., 8, 19 (1899). (3) WALDEN,Z.physikal. Chem., 64, 129 (1905). These values were obtained with triethylammonium iodide as electrolyte a t a concentration of 1/1OOO. ( 4 ) TIMMERMANN, Bull. SOC.chim. Belg., 20, 305 (1906); Chem. Zentr., 78, I, 1006. (6) KABLUKOFF, Z. phyeikal. Chem., 4, 430 (1889). c The values for the association factor are those reported by TRAUBE, Ber., SO,
273 (1897).
product, but it is quickly converted by the bromide into the trans 8derivative. This stereomeric rearrangement occurs in all solvents of the latter group. 3. Similar solvent relations have been found in the addition of hydrogen bromide to tetrolic acid in water and in non-aqueous solvents. However,
138
ARTHUR MICHAEL
the catalytic transformation of the bromoisocrotonic acids proceeds much less readily than that of the corresponding bromocinnamic acids and the products isolated under the experimental conditions are mainly the primarily-formed bromo acids. 4. The difference between the degradation of energy in the formation of a- and @-halogen derivatives of saturated acids is very considerable. Accordingly, the addition of hydrogen bromide to cinnamic and crotonic acids yielded, respectively, /3-bromohydrocinnamic and @-bromobutyric acids, irrespective of the nature of the solvent. 5. From these results, it is concluded that solvents exert an influence upon the course of addition of hydrogen bromide to a,@-unsaturatedacids only when the involved physical energy factors are strong enough to depress the degradation of chemical energy sufficiently to favor formation of abnormal addition products, Le., those opposed to the course indicated by the positive-negative addition rule. The partial or complete reversal in the mode of addition, caused by solvent effect, occurs only when the difference between the degradation of energy in the two possible directions of addition is relatively small. The above relation of solvent effect to structure is believed to prevail in all energetically analogous reactions. 5. No direct relationship existed between the examined solvent effect and the associating, dissociating, or dielectric constants of the solvents. 6. Markownikoff’s wholly empirical addition rule is valid solely in the alkene-1 and alkine-1 series and then only when the addition does not proceed with migration of hydrogen or methyl. Applied to other classes of organic compounds with unsaturated carbon linkages, it generally leads to conclusions opposed to those obtained by experiment. As a general rule, it is now a hindrance to the development and recognition of a rational addition theory.
