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The relative reactivities of a number of polycyclic aromatic hydrocarbons have been determined by measuring their retarding effects on the benzoyl peroxide initiated addition of carbon tetrachloride to hexadecene-1 and/or of bromotrichloromethane to styrene. The results could be satisfactorily represented by kinetic expressions, which yielded the rate constants of the interaction of trichloromethyl radicals with the aromatics, relative to that for addition to the olefin. The inhibitory effects are attributed to addition of the chain-carrying CC13 radicals to the aromatics.The scale of reactivities thus obtained extended from benzene to 3 : 4-benzopyrene, the latter being over 100, OOO times more reactive than the former. A close parallel appeared to exist between reactivity towards free radicals and the highest free valence numbers as well as the corresponding polarization energies calculated for each molecule by means of the molecular orbital method. The significance of these correlations is discussed. * CBrC13 interferes with the iodometric peroxide determinations. Dehydrogenation of dihydroanthracene by thiyl radicals was found by Bickel and Kooyman to produce some anthracene besides a large amount of 9 : 10 : 9' : 10'tetrahydro-dianthranyl (to be published shortly).

Crouch and Lochfe have described a new synthesis of pyridine derivatives, in which they start from glutarimide or from its alkyl derivativese). By the action of phosphorus pentachloride on glutarimide at a slightly raised temperature they obtained a trichloropyridine, m.p. -7" and they have shown indirectly that this compound is the 2.3,6-isomer. W e have now reacted glutarimide with phosphorus pentabromide and have confirmed the reasoning of Crouch and Lochte, because the tribromopyridine obtained has been identified with 2,3,6-tribromopyridine, prepared by brominating 3-bromopyridine at 500" 3). Moreover, 2,3,5,6-tetra-bromopyridine appeared to be formed, which was identified with a sample obtained by brominating 3,5-dibromopyridine at 500" 3 ) .Glutarimide (1 g) was mixed with phosphorus pentabromide (12 g) and the mixture held at 8 @ -9 0 "for 2% hours. The reaction product was then poured out into water and the cryetals, which separated after a little time, were submitted to a steam distillation. The first fraction in the steam distillation (60 cms) contained a colourless solid, which melted at 82.5-83.5" after crystallisation from ethanol. (Mixed m.p. with a sample of 2.3.6-tribromopyridine: 82-83'.)After an intermediate fraction of 50 cmS had passed over in the steam distillation, a further fraction was obtained (250 cm*), which contained a colourless substance melting at 103-104" after crystallisation from ethanol. (Mixed m.p. with a sample of 2.3,5,6-tetrabromopyridine: 103-104"). November 1947. (Received March 22nd 1948). 1) 70th Communication on derivatives of pyridine a?d quinoline from the t) W. W. Crouch and H. L. Lochfe, J. Am. Chem. SOC. 65, 270 (1943). 8 ) H. I. den Herfog and J. P. Wibauf, Rec. trav. chim. 51, 940 (1932).

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