FACTORS AFFECTING THE ADDITION OF BROMINE TO

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[CONTBIBUTION FROM THE GEORGE HERBERTJONES LABORATORY OF THE UNIVEE6ITY OF CHICAGO]

FACTORS AFFECTING THE ADDITION OF BROMINE TO PHENANTHRENE M. S. KHARASCH, P. C. WHITE,

AND

F. R. MAYO

Received April 11, iQS8

A preliminary study of the side-chain and nuclear bromination of toluene in this laboratory has been reported.' The effects of light and of peroxides were found to be similar in that both accelerated side-chain bromination. It was further shown that the use of bromine-toluene ratios greater than 1:10 materially counteracts the effects of light and/or oxygen on side-chain bromination. The working hypothesis proposed is that: (a) the side-chain bromination of toluene is a chain reaction in which bromine atoms play the major rale; (a) light and oxygen are both effective in the production of bromine atoms; ( c ) an excess of molecular bromine has a chain-breaking effect. Study of the bromination of other hydrocarbons, as yet unreported, has yielded data consistent with this hypothesis. It seerped desirable, therefore, to re-investigate the addition of bromine to phenanthrene with special attention to the effects of light and oxygen. The addition of bromine to phenanthrene has been studied previously by Price: and the (presumably) essentially similar addition of bromine to several phenanthrene derivatives by Fieser and Price.s Price concluded that the addition of bromine to phenanthrene takes place through a chain mechanism involving bromine atoms, but his observations as to the effects of light and oxygen on the reaction were not in all details those that past experience has led the present authors to regard as characteristic of such reactions. According to Price the addition of bromine to phenanthrene is somewhat accelerated by light, but proceeds a t a measurable rate in the dark. In our own experiments, when oxygen and other catalytic impurities were rigidly excluded, no detectable bromine addition took place in the dark in 24 hours. We believe that the discrepancy is attributable to Price's colorimetric method of determination, which necessitated repeated brief periods of intense illumination. Our own experiments (1 and 2, recorded 1

KHARASCH, MARQOLIS, WHITE,AND MAYO,J . Am. Chem. Soc., 69, 1405 (1937).

8

F I ~ SAND ~ RPBXCB, ibid., 58,1838 (1936).

* PBICE,ibid., 68, 1834, 2101 (1930).

674

575

ADDITION OF BROMINE TO PHENANTHRENE

in Table I) indicate that intermittent illumination, particularly in the presence of air, is almost as effective in promoting the reaction as is steady illumination. The difference between the rates of the air-exposed and the vacuum reactions recorded in Table I is also notable. Price's statement that the E F F E OF ~ AIR ON

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THE

TABLE I PHOTOBROMINATION OF PHENANTHRENE

CONDITIONS OF ILLUMINATION EXPT. NO.

EXTENT OF REACTION

(%)'

TIME (MINUTES)

Air-Exposed

Vaouum

15

66

15

59

12 24 12

62 54 59

53 20 41 39 24

Lamp Wattage ~~~~~~

1 2 3 4 5b

400 400 100

24 54

30

100 100

40

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73

The light was turned on 30 seconds every 5 minutes for a total period of 50 minutes (aggregate time of illumination, 5 minutes). * One mole per cent. of diphenylamine was present in both reaction systems. The experiments were c All analyses were by titration of unreacted bromine. carried out in carbon tetrachloride solution, the concentrations of both bromine and phenanthrene being 0.05 mole per liter. 0

EFFECT

TABLE I1 BROMINATION O F PHENANTHRENE TETRACHLORID~ SOLUTION IN THE DARK

OF PEROXIDES ON THE

IN

CARBON

~

BROYINE CONCENTRATION~ (MOLEI/LZTER)

PEROXIDE

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0.5 0.5 0.5 0.05 0.05 0.05 0.05 0.05 a

None Ascaridole Ascaridole None Ascaridole Triacetone perox. Benzoyl perox. Perbenzoic acid

PEROXIDE CONCENTRATION (MOLE %)

3 10 -

3 3 3 3

TIME (HOURS)

1 1 1 24 1 24 24 24

EXTENTOFRBACTION (%)

0 17 92 0 40, 40, 45 18 45 29

Equimolecular proportions of bromine and phenanthrene were used in all cases.

removal of oxygen from the solvent by boiling at diminished pressure had no demonstrable effect on the rate of the reaction is interpretable as indicative that mere traces of oxygen, especially in conjunction with intermittent illumination, may be highly effective. In harmony with this is the observation that in the presence of diphenylamine the bromination of phenanthrene is much slower when oxygen is excluded (experiment 5).

576

M. S. KHARASCH, P. C. WHITE AND F. R. MAY0

We believe, therefore, that in Price’s experiments the reaction was accelerated by intermittent illumination, and catalyzed to some extent by atmospheric oxygen. These factors would account for his observation that the presence of benzoyl peroxide did not materially affect the rate. In our own experiments (Table 11) benzoyl peroxide did facilitate bromination in the dark, although not to the same extent as did ascaridole-a less stable peroxide. The experiments recorded in Table I1 also show that ascaridole, at least, is relatively more effective as a catalyst in the presence of low concentrations of bromine than in the presence of higher concentrations. On the basis of these observations we conclude that the measurements of Price (and, by inference, those of Fieser and Price) apply to a photochemical, oxygen-catalyzed reaction, rather than to a dark, uncatalyzed reaction, as they have assumed.