SOLVENT AND PEROXIDE EFFECT IN THE ADDITION OF

[CONTRIBUTION FROM

THE

CHEMICAL LABORATORY OF HARVARD UNIVERSITY]

SOLVENT AND PEROXIDE EFFECT IN THE ADDITION OF HYDROGEN BROMIDE TO UNSATURATED COMPOUNDS.

IV. ISOPROPYLETHYLENE (1)’ ARTHUR MICHAEL

AND

NATHAN WEINER

Received February 13, 1940

Wishnegradski (2) first examined the action of concentrated aqueous hydrogen bromide on isopropylethylene and, in agreement with the socalled Markownikoff rule (3), obtained pure secondary isoamyl bromide. Later, Ipatieff (4)confirmed that the secondary derivative appeared in an almost pure state in the reaction, but found that the primary bromide, mainly, was formed when the addition was carried out in acetic acid solution. On the other hand, hydrogen iodide gave practically the secondary derivative under both conditions. Michael and Leupold ( 5 ) analyzed 1 In a preliminary notice in the May issue of the J . Org. Chem., 4, 132, footnote 13 (1939), and 3, 379, footnote (1938), it was announced that we had proved the course of addition of hydrogen bromide to isopentene-2 to be susceptible t o solvent and also t o peroxide influence, although Kharasch [ J . Org. Chem., 2,288 (1937)l had concluded that only alkenes with terminal unsaturation show the latter property. The report of the completed research was sent to this Journal on May 24, 1939, and appeared in the November issue, p. 532. On August 3, 1939, Kharasch and coworkers submitted a paper t o the J . A m . Chem. Soc., 61,2694 (1939)in which peroxide effect on isopentene-2 is now admitted, but no reference is made t o our announcement in September 1938 or in May 1939. We are now engaged in an investigation on tertiary butylethylene, in a manner similar t o that here presented on isopropylethylene. Ipatieff and Dechanoff (Chem. Zentr., 1904,II, 901)found that trimethylethylene in acetic acid yielded 10-15%, not 10-25% as stated by Kharasch, of the secondary bromide and 8690% of the tertiary bromide. Michael and Zeidler, Ann., 386, 245 (1911), showed t h a t the appearance of the secondary derivative in the solvent-free reaction was due to a slight impurity in the hydrocarbon and that with pure hydrocarbon, tertiary bromide alone is formed. Kharasch and co-workers (Zoc. cit.) consider these results “similar.” These chemists state the secondary bromide easily undergoes isomerization in the addition, since “methyl isopropyl carbinol or isopropylethylene with hydrogen bromide” yield more or less of the isomeric tertiary bromide. This carbinol is dehydrated by strong acids into trimethylethylene mishnegradski (2)], which yields solely the tertiary bromide, and which is therefore formed by addition, not by rearrangement. Its formation from isopropylethylene is explained in this paper and is not connected with Kharasch’s assumption. There is no support for the statement that the secondary bromides are rearranged t o the tertiary isomers under the conditions used in the hydrogen bromide additions. 389

390

ARTHUR MICHAEL AND NATHAN WEINER

the addition-products semi-quantitatively and found that with concentrated aqueous hydrobromic acid, the addition-product consisted of a mixture of 520/, of the secondary and 48y0of the tertiary isoamyl bromide. The investigation was continued by Michael and Zeidler (6), who confirmed the latter result and found, contrary to Wishnegradski (2), that the addition of hydriodic acid, in concentrated solution, gave a mixture of secondary and tertiary iodides in about the same proportion as the bromides in the above experiment. Nor did the results of hydrogen bromide addition in acetic acid coincide fully with Ipatieff’s (4); instead of mainly the primary bromide, a mixture of the isomeric isoamyl bromides was formed, containing the primary, secondary, and tertiary bromides in about the proportion of 65.5 :28: 5.6. The experiments with isopropylethylene were carried out before the influence of oxidants on the addition of hydrogen bromide to alkenes was known. The appearance of the primary bromide could possibly be ascribed to the presence of peroxide in the hydrocarbon or solvent. However, the appearance of the tertiary amyl halides in the reaction with the hydrogen halides involves a novel course of organic addition, since it can occur only with simultaneous intramolecular migration of the tertiary hydrogen atom; the reaction may, therefore, be called an intramolecular rearrangement-addition : cH3)C(HvCH=CH~ CH3

+ HBr = CH3>~CH&H3 CH3

The formation of the tertiary halide is obviously opposed to the so-called Markownikoff rule (3), which has long since been shown to be untenable for other groups of unsaturated compounds (7)2 and is not even applicable to all hydrocarbons of the alkene-1 type. There can be no doubt that alkenes of the type (Alk)ZCHCH=CHz will yield with hydrogen bromide considerable tertiary halide and that the relative amount will increase with the combined relative positivity of the alkyl groups. The affinity and energy relations of those atoms in the system, which mainly activate the rearrangement-addition are: (a) the chemical hindrance, Le., the energy required to separate the tertiary hydrogen from the attached carbon atom is relatively slight, owing to the direct positive influence of the two

* The primitive Markownikoff rule is repeatedly mentioned in “Organic Chemistry” (edited by Gilman); only in one place (p. 549), do Allen and Blatt note its limitations and inadequacies. They then state that “no wholly satisfactory explanation” why “addition follows this rule has been advanced.” This remark illustrates how the theoretical foundation of organic addition phenomena upon the law of degradation of energy, and the principle of partition, with the experimental confirmations (7), have been entirely overlooked in the above treatise. (A. M.)

ADDITION TO ISOPROPYLETHYLENE

391

alkyl groups upon the attached carbon (8); (b) the heat of formation of the tertiary bromide is considerably greater than that of the secondary, which correspondingly increases the energy degradation of the reaction in that direction; (c) isopropylethylene itself adds hydrogen bromide with relative difficulty, doubtlessly due to the greater reduction of the affinity of the unsaturated methylene carbon for the additive hydrogen, by the influence of the six methyl hydrogens in the spatially near 5-positions, over that of the carbon of the unsaturated methinyl group, which is in the spatially, relatively distant 4-position. The different spatial relations bring the relative polarities of the unsaturated carbons nearer together and thus reduce the differences in their relative additivity,s as indicated in the structural formulas and tends, therefore, to the formation of a mixture of isomeric bromides. The above affinity-energy factors mainly determine the energy degradation in the addition to the isoalkene and so favor the maximum energy degradation through a rearrangement-addition, that it approximates that occurring by direct union of hydrogen bromide at the unsaturated carbons. From the above viewpoint, the changes in the proportion of tertiary and secondary halide produced by addition of hydrogen bromide to (Alk)&HCH=CH2, with change in alkyl, may be predicted. The more positively the alkyls act upon the attached methinyl group, the greater will be the proportion of tertiary to secondary halide. Less definite is the effect of replacing a hydrogen of the unsaturated methylene group by alkyl since, while the relative value of (a) wouldbeincreased, that of (b) would be lessened. Probably, however, the relative proportion of isomeric halides obtained would not be greatly changed from that appearing with isopropylethylene. The influence of peroxides and solvents upon the course of the addition of hydrogen bromide to isopropylethylene is of considerable theoretical interest. In contrast to trimethylethylene (IC), in the absence of solvent and peroxide, the addition of hydrogen bromide to isopropylethylene takes place quite slowly at -78" and even at 0";for this reason the additions were carried out in sealed tubes at room temperature. Under these conditions, it was found that isopropylethylene and dry hydrogen bromide (see Table I) yielded ea. 61% of the tertiary and 39% of the secondary derivative; the primary bromide could not be detected. A comparison of this result with that obtained with a saturated aqueous solution of hydrobromic acid under comparable conditions, which gave 48% and 52% of the respective bromides, shows that even water may exert a noticeable effect as a solvent; the proportion of the tertiary bromide decreased about 12%. Hence, in the future, the solvent effect of water in additions upon an 8 See Michael and Brunel, Am. Chem. J . , 41, 128 (1909),for an explanation of the relative additivities of other alkenes from this viewpoint.

392


addendum should be taken into consideration. The active agent is undoubtedly the hydrated acid, HBr.H20.4 There is no indication in our results that halogen hydrides function, either in normal or abnormal additions, in the ionic state.* We next examined the effect of ascaridole upon the action of fuming hydrobromic acid. Addition of 0.012 mole of ascaridole scarcely changed the percentage of tertiary bromide (49.3%), but 37% of the primary bromide now appeared, formed at the expense of the secondary bromide; this relationship underwent only a slight change when the amount of ascaridole was increased four-fold. In the presence of water, therefore, ascaridole exerted a very slight, if any, action upon the tertiary hydrogen of the hydrocarbon, but functioned characteristically TABLE I REACTIONS BETWEEN ISOPROPYLETHYLENE AND HYDROGEN BROMIDE, WITHOUT SOLVINT,AT ROOMTEMPERATURE RATIO I I Ern.

1 2 3 4 5 6 7 8 9 10 11 12 13

PmB c

mm

MOLES ASCABIDOL= YOLES (CHI)*C H C H S H z

PBIMABT

0.0006 o.Ooo9 0.002 0.003 0.003

36.5 36.4 45.9 44.4 43.4 66.3 80.0 75.8

0.009 0.02 0.02 None None 0.2 g. hydroquinone 0.2 g. hydroquinone 0.1 g. ferric chloride

PER CENT SECONDABT

PEB CENT TEBTIARY

11.0 10.4 20.6 20.0 23.3 14.8 13.6 18.6 39.3 38.8 41.5 40.7 18.5

52.5 53.2 33.5 35.6 33.3 18.9 6.4 5.6 60.7 61.2 58.5 59.3 81.5

upon the addition relationship between the primary and secondary derivatives (Table II), causing the appearance of the former. Under the same experimental conditions, addition of antioxidant hydroquinone t o a mixture of dry hydrogen bromide and pure isopropylethylene did not materially affect the proportion of tertiary to secondary bromide. On the other hand, ferric chloride showed a marked influence, increasing the percentage of tertiary bromide from 61% to 81.5% and decreasing ‘Michael and Brunel, Am. Chem. J., 48, 267 (1912)) showed that the system, yielded directly an addition-product containing an equal isopentene-2-HBr .8Hn0, number of molecules of the saturated bromide and carbinol and that any system containing the acid in greater or lesser concentration leads t o the formation of the halide in a greater or lesser proportion. Other strong acids showed a similar relationship.

393


the percentage of secondary bromide correspondingly.6 On the other hand, the presence of ascaridole diminished the proportion not only of tertiary bromide, in opposition to its influence in the hydrobromic acid system, but that of the secondary as well, with the appearance of a corresponding amount of the abnormal primary bromide. The abnormal reaction is remarkably sensitive to the influence of the peroxide. The addition of 0.0006 mole of ascaridole caused the appearance of 36.5% of the primary bromide, by reduction in the yields of the tertiary and secondary bromides by ca. 8% and 28.5%, respectively (Table I). With an ascaridole molar concentration of 0.003, the percentage of the abnormal primary bromide increased t o 44y0,this time mainly at the expense of the tertiary bromide, since the percentage of the secondary bromide was only slightly greater than the initial low amount formed at the lowest concentration investigated. The effect of further increase in ascaridole concentration proceeded TABLE I1 REACTIONS BETWEEN FUMING HYDROBROMIC ACID,SATURATED AT 0", AND ISOPROPYLETHYLENE The reactions were carried out at 25". BATIO E m .

MOLE8 ABCABIDOLE MOL188

14 15 16 17

PEE CENT

(CH:): CHCH%H,

None None 0.012 0.05

37.0 36.0

PmB CENT BECONDABP

PEB ClNT TEBTUBY

49.7 48.1 13.7 15.4

50.3 51.9 49.3 48.6

in the same direction, Le., with increased formation of the primary derivative and corresponding decrease in the tertiary bromide. Thus, over the range of ascaridole concentrations investigated (0.0006-0.08 molar), the net change in the secondary bromide was only 13% (10.4-23.3%), whereas the primary bromide increased by 43.5y0 and the tertiary bromide decreased by 47.3%. The experimental data are collected in Table I. We next turned t o the influence of organic solvents upon the course of the addition, using those we had found effective in producing addition reversals with trimethylethylene (IC). In opposition to the result with that hydrocarbon in methanol solution, we found that no reaction took place at -78" and 0" (Table 111),even after twenty-four hours, a period which sufficed, in the absence of solvent, to cause addition with excellent SKharasch, J. Org. Chem., 2, 288 (1937), and previous papers, contrariwise, found that the chloride, with the compounds he examined, caused a decided increase in the normal addition.

394


yields. Even in the presence of ascaridole, under conditions where addition took place in methanol solution at -78", no reaction took place at 0". However, at -78", addition occurred when a certain minimum ratio of ascaridole to methanol (0.2 g. to 10 g.) was used. Contrary to the influence of water, a large increase in the proportion of primary bromide (7843%) took place at the expense of the secondary bromide, which was reduced to 14-19% and of the tertiary bromide, which was formed only to the extent of 3-6%. Beyond the minimum concentration of ascaridole, a change in concentration exerted practically no influence on the proportions of the isomeric bromides formed. Ether is the most effective solvent in reversing the normal addition of hydrogen bromide to trimethylethylene (IC). The use of this solvent with isopropylethylene involved diEculties. A t - 78", conforming with the TABLE I11 REACTIONS BETWEEN ISOPROPYLETHYLENE AND HYDROQEN BROMIDE IN METHANOL SOLUTION There was no reaction when the reagents were allowed to stand in methanol solution at 0" under otherwise duplicated experiments of the tabulated successful experiments at -78". E m .

(CH& CHCH=CHa, 0.

AKABI-

CHIOH,

DOLI, 0.

Q.

TZ3M.P.

--

18 19 20 21 22 23 24

5 5 6.5 4.5 7.0 7.0 6.0

0.1 0.2 0.2 0.4 0.4

20 10 10 20 10 20 10

0" -78" -78" -78" -78" -78" -78"

No reaction in two weeks No reaction in 24 hours No reaction in 24 hours No reaction in 24 hours 82.9 80.6 77.9

relatively slight additivity of isopropylethylene, no reaction had taken place after five days, and at room temperature the cleavage of ether by hydrogen bromide, t o ethyl bromide and alcohol, was so extensive that the results could not be considered wholly as due to an ether effect. However, by allowing the reaction to proceed for only eighteen hours at O", reproducible results and fair yields (50-60%) of bromides were realized. In the presence of antioxidants the yields were about half the above, but were also reproducible. Corresponding to the results with trimethylethylene, ether caused considerable abnormal addition (5344% primary bromide) ;even in the presence of the antioxidants appreciable amounts of the abnormal primary bromide were formed. The usually more effective inhibitor of abnormal addition, hydroquinone, was less effective than diphenylamine (about 28% and 14%, respectively). This abnormal be-

395


havior is significant; the solvent effect of ether may be even more effective than the above figures indicate, for Kharasch (9) has recently shown that ethyl bromide, one of the products of ether cleavage, is an effective inhibitor of the abnormal addition. The results are tabulated in Table IV. Hydrogen bromide added to trimethylethylene in acetic acid solution to give, besides the normal tertiary bromide, about 16% of the abnormal secondary derivative; the proportion was not changed when ascaridole was added to the mixture. The solvent in this addition neutralized the oxidant influence, as it did also, to a very large extent, the antioxidant influence of diphenylamine. When this solvent was used with isopropylethylene the formation of 4448% of the abnormal, primary, 30-34% of the secondary, and 18--22% of the tertiary bromide was observed. The result was not due to “undetectable traces of peroxide,’’ since about the same yields were obtained by Kharasch’s vacuum technique and, anomalously, in the presTABLE IV ADDITIONOF HYDROGEN BROMIDE TO ISOPROPYLETHYLENE IN ETHER AT O”, 3.5 o. OF HYDROCARBON IN 5 cc. OF ETHER Em.

ANTI-OXIDANT

PEB CENT PRZMART

PEB CENT BECONDARY

PER CENT TEBTIABY

25 26 27 28 29 30

None None 0 . 2 g. hydroquinone 0 . 2 g. hydroquinone 0 . 2 g. diphenylamine 0 . 2 g. diphenylamine

54.0 53.4 30.2 26.0 14.7 13.0

29.6 32.3 43.6 49.7 62.9 57.0

16.4 14.3 26.2 24.3 22.4 20.0

ence of hydroquinone and diphenylamine, in a concentration of 0.2 g. to 10 cc. of glacial acetic acid. In contrast to comparable experiments with trimethylethylene, the addition of 0.0025 mole of ascaridole increased the formation of the primary bromide by about 30%) but a ten-fold increase in its concentration (0.025 mole) made very little, if any, difference in the proportions of the products formed; Le., 71-79010 primary, 15-19% secondary, and 6-10% tertiary bromide. In acetic acid solution ascaridole increased the proportion of primary bromide, with an equal reduction in the percentages of the tertiary and secondary bromides. In view of this solvent effect of acetic acid, it was of interest to attempt to determine the underlying factor in its influence. For this purpose the effect of dichloro- and trichloroacetic acids (K = 5 X 10+ and 3 X lO-l, respectively) was examined. With the first acid, a reduction in the percentage of the abnormal primary bromide occurred, to 18-22%, along with 20-25% secondary, and 57-58% tertiary bromides. Therefore, this

396


acid, with a much higher K value than acetic acid (1.8 X was far less effectivein inducing abnormal addition; the lessened influence appeared also in the ascaridole catalyzed reaction. Using ascaridole in a ten-fold increase in concentration (0.005-0.05 molar) a nearly constant proportion of addition-products was obtained, vix., 5 5 4 8 % primary, 15-22% secondary, 22-26% tertiary bromide. In agreement with the decreased abnormal TABLE V ADDITION O F HYDROGEN BROMIDETO ISOPROPYLETHYLENE I N ACETIC, DI- AND TRI-CHLOROACETIC ACIDS 6.5 g. of hydrocarbon in 10 cc. of the liquid acids; 6.5 g. of hydrocarbon with 10 g. of ClsCCOOH. The reactions were all carried out a t room temperature. CATALYET, WHIIQHT, Q.

PER CENT PRIMARY

PER CENT BECONDARY

PER CHINT TERTIARY

46.8 43.9 47.6 45.9 78.8 78.0 77.6 73.8 71.5 72.7 71.3

29.9 34.7 34.4 32.1 15.3 15.7 14.7 17.5 19.3 18.4 18.9

23.3 21.4 18.0 22.0 5.9 6.3 7.7 8.7 9.2 8.9 9.8

24.7 20.4 15.3 22.5

57.2 57.9 26.1 22.1

Acetic acid 31 32 33 34 35 36 37 38 39 40 41

Vacuum technique None 0.2 diphenylamine 0.2 hydroquinone 0.04 ascaridole 0.04 ascaridole 0.04 ascaridole 0.1 ascaridole 0.2 ascaridole 0.2 ascaridole 0.4 ascaridole

Dichloroacetic acid 42 43 44 45

18.2 21.7 58.6 55.4

None 0.2 g. hydroquinone 0.08 g. ascaridole 0.8 g. ascaridole

Trichloroacetic acid None 0.2 g. hydroquinone

~

None None

~

25.7 25.7

I

74.3 74.3

addition induced by dichloroacetic acid, it was found that no irregular addition could be detected in a saturated solution of trichloroacetic acid in isopropylethylene ; over 74% of the tertiary bromide was produced. The results with these acids are tabulated in Table V. According to Kharasch and Potts (lo), acetic acid functioned as an antioxidant in alkene-1 additions; on the other hand, with isopentene-2 it acted mildly as an oxidant (IC). This acid functioned likewise with iso-


397

propylethylene, causing the appearance of the abnormal primary bromide. From the viewpoint (IC),that the primary phase leading to abnormal additions is the formation of a polymolecule of hydrogen bromide with the added reagent, followed by its union, in accordance with the principle of partition (7), at the relatively positive unsaturated carbon, it is evident that the stability of the double molecule of fatty acid and hydrogen bromide should decrease with increase in its negativity, as represented by that of the organic acid. In agreement, the abnormal effect of dichloroacetic acid was much less than that of acetic acid and it entirely disappeared with the use of the far more acidic trichloroacetic acid. Probably a similar relationship would appear in corresponding experiments with other alkenes, EXPERIMENTAL

Materials. Isopropylethylene was prepared by the dehydration of isoamyl alcohol over activated alumina (4-8 mesh) a t 410°, according to the directions of Norris and Joubert (11). The product was separated from the water formed, distilled, and the fraction of b.p. 20-24" was collected separately. This was shaken with sulfuric acid(6) (2 vols. conc'd H2S0, and one vol. water) a t 5", until a fresh portion of acid no longer decreased the volume of hydrocarbon. The hydrocarbon was then shaken with ice-water and once with 20% caustic soda. It was dried over calcium chloride and distilled in a %foot partial reflux column, packed with glass helices, and the fraction of b.p. 20.1-20.5"/757 mm. collected. This was dried over sodium wire and redistilled, b.p. 20.3"/757 mm. The hydrocarbon was stored in a tightly stoppered flask, over sodium wire. The solvents were purified as previously described (1 c). Dichloroacetic and trichloroacetic acids were freshly distilled in vacuo and used immediately. The former boiled a t 88"/14 mm., and the latter a t 88"/3 mm. Hydrogen bromide was prepared by the method of Ruhoff and Reid (12), and dried by passage over phosphorus pentoxide. A p p a r a t u s and technique. The reaction-vessels consisted of two chambers of approximately 5 cc. and 25 cc. capacities, connected by an inverted U-tube of 8 mm. tubing. The larger chamber was calibrated to show 5, 10, 15, and 20 cc. The openings t o both chambers were 8 mm. tubing ending in standard taper 10/30 female joints, which permitted a direct connection with the hydrogen bromide generator, or the condenser through which the hydrocarbon was distilled. The experiments were carried out as follows: either 5 or 10 cc. of isopropylethylene (sp. gr. 0.66) was distilled into the larger chamber, cooled to -20", directly from the storage flask through an 18-inch column. The other opening was connected with a calcium chloride tube. When catalysts were used, the specified amount was already present. The solvent was added with a pipet, and then both chambers were cooled to -78" in a transparent Dewar cylinder. Between 2 and 3 cc. of hydrogen bromide was condensed in the smaller bulb and then the entire apparatus was sealed off. The bulb containing the hydrogen bromide was brought to room temperature and the hydrogen bromide allowed to distill into t h e larger chamber, still a t -78". When all the acid had volatilized, the larger chamber was placed in a bath a t the specified temperature. At the end of the reaction-period the apparatus was again cooled to -78" and opened. The products were worked up in the same manner as those from trimethylethylene, except that the products from the water-miscible

398


solvents were first distilled i n uucuo at room temperature before final drying over phosphorus pentoxide. The reagents, in the reaction with hydrobromic acid (Tab1e 11) were mixed a t 0' in glass-stoppered flasks and shaken mechanically a t room temperature for two hours. The organic layer was separated, washed once with ice-water, dried over potassium carbonate, distilled i n vacuo a t room temperature and finally dried over phosphorus pentoxide. Analytical method. The products were analyzed for tertiary bromide by the method previously described (1 c). Secondary bromide was determined by the 0.1 N silver nitrate method of Michael and Leupold ( 5 ) . The products were shown to consist of the isomeric amyl bromides by giving 97-100% of silver bromide after two hours heating with 0.1 N alcoholic silver nitrate a t the boiling point. SUMMARY

1. The appearance of a rearrangement-product in the addition of concentrated aqueous hydrobromic acid to isopropylethylene (4,5,6) has been confirmed. In the presence of air, the yield of the secondary bromide amounted to 49% and that of the tertiary bromide to about 51% of the theory. 2. Ascaridole induced the formation of the abnormal, primary isoamyl bromide, formed a t the expense of the secondary bromide. 3. In the absence of a solvent, dry hydrogen bromide adds to isopropylethylene to yield more of the tertiary and less of the secondary bromide; water, therefore, showed a solvent effect, and the long-accepted conclusion that dry hydrogen bromide and aqueous hydrobromic acid yield identical addition-products in the same proportion can no longer be upheld. It is probable that hydrobromic acid functions in addition-reactions as the hydrated form. The change in the course of the addition may be explained by an approach in the relative positivities of the unsaturated carbons in isopropylethylene, due to the polymolecular union of the hydrated acid to a greater extent at the relatively positive methinyl carbon. 4. In the dry hydrogen bromide system, ascaridole induced the formation of primary isoamyl bromide. Up to 0.009 molar concentration, it was formed mainly at the expense of the secondary bromide, but a further increase involved the tertiary bromide, which a t 0.02 molar concentration almost disappeared. 5. The. unusual fall in reaction velocity with rise in temperature, previously observed with trimethylethylene in methanol solution, was likewise encountered with isopropylethylene. Its slight reactivity manifested itself in no addition a t -78" and at 0' in methanol alone, or at the higher temperature in the presence of ascaridole. However, at -78', a certain critical concentration of ascaridole induced the addition and then over 80% of the abnormal primary product appeared; further increase in concentration was ineffective in altering the relative proportion of the products.

ADDITION TO IBOPROPYLETHYLENE

399

6. Ether exerted a marked solvent effect; it led to the formation of ca. 53% of the primary bromide, at the expense of the tertiary and secondary isomers. With isopropylethylene, contrary to the general results with other alkenes, the antioxidant diphenylamine was more effective in reducing the extent of abnormal addition than hydroquinone. The influence of these antioxidants is far less with isopropylethylene than with normal alkenes-1. 7. Acetic acid also showed a remarkable solvent effect, inducing in vacuum, or in the presence of antioxidants, the appearance of 4447% of the abnormal primary bromide. Small amounts of ascaridole decidedly augmented the proportion of the primary bromide, which decreased slightly in amount with increasing concentration of ascaridole. 8. With dichloroacetic acid a much smaller percentage of the abnormal primary bromide was obtained. Compared with the result of solvent-free hydrogen bromide addition to the hydrocarbon, the amount of the tertiary bromide was only slightly lower, while that of the secondary bromide fell off considerably. In comparison with the influence of acetic acid, drastic changes occurred; the percentage of the secondary bromide decreased slightly but that of the primary bromide decreased by more than half, while the relative amount of the tertiary product was more than double. This result was independent of the presence of hydroquinone. Ascaridole (0.05-0.005 molar) reduced the yield of tertiary bromide and increased that of the primary derivative, but comparatively far less than in acetic acid. 9. In the presence of trichloroacetic acid, addition became normal, in the sense that only the secondary and tertiary bromides were formed. The presence of this strong acid increased the formation of the tertiary bromide at the expense of the secondary product. Exactly the same result was obtained in the presence of hydroquinone. 10. The formation of tertiary amyl bromide by the action of hydrogen bromide on isopropylethylene should not be considered an abnormal addition. It is a normal consequence of the affinity and energy relationships existing in the chemical system. The chemical behavior of this system manifests itself, alone and in the presence of solvents, oxidants and antioxidants, by changes peculiar to the hydrocarbon. CAMBRIDGE, MASS.

REFERENCES (1) (a) MICHAEL, J . Org. Chem., 4, 128 (1939). (b) J . Org. Chem., 4, 519 (1939). ( e ) MICHAEL AND WEINER,J . Org. Chem., 4, 531 (1939). (2) WISRNEGRADSKI, Ann., 190, 339 (1878). Ber., 14, 2071 (1881). (3) MARKOWNIKOFF,

400


(4) IPATIEFF AND DECHANOFF, Chem. Zenlr., 1904, 11, 691. AND LEUPOLD, Ann., 379, 297 (1910). (5) MICHAEL (6) MICHAEL AND ZEIDLER,Ann., 386, 244 (1911). (7) MICHAEL, J . prakt. Chem., 60, 335, 446 (1899);Ber., 38,2138-2163 (1906). Am. Chem. J . , 10, 1 (1877); 14, 481 (1878);43, 338 (1910);J . Org. Chem., 4, 519, footnote 2 (1939). (8) MICHAEL,J . prakt. Chem., 60,341 (1899);Ber., 39, 2139 (1906);J . Am. Chem. SOC.,32, 990, 996-998 (1910). (9) KHARASCH AND WALLING,J . Am. Chem. SOC.,61, 2694 (1939). (IO) KHARASCH AND POTTS, J . Am. Chem. SOC.,68, 157 (1936). (11) NORRIS AND JOUBERT, J . Am. Chem. Soc., 49, 2624 (1927). (12) RUHOFFAND REID,Organic Syntheses, 16, 24 (1935).

SOLVENT AND PEROXIDE EFFECT IN THE ADDITION OF

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