TAUTOMERISM beween methyl-acenes (I, II, III, IV, V) ygtnd methylene-dihydroacenes (la, Ila, Ilia, IVa^Va, respectively) has often been postulated tcxexplain side -chain reactivity of aromatic compound^ but hitherto no methylene-dihydroacene has beprt isolated. This weakness in the hypothesis has rfow been removed by the synthesis of pure 6-methylene-6: 13-dihydropentacene (Va). The 'anellation principle'1 requires a gradual transition in properties in the acene series, and it may be inferred, therefore, that (IV), (III), (II) and (I) will also contain the corresponding methylene forms, but in decreasing proportions. Absorption spectrum of 9-methylanthracene in alcohol. Band maxima at: 3850, 3640, 3460, 3290. 3140, 3000; 2540, 2460 A.Absorption spectrum of 5-methyltetracene in alcohol. Band maxima at: 4805, 4490, 4220, 3980; 3750, 3560; 2950, 2755,2650 A. Calculated absorption spectrum of 6-methylpentacene in 1-methyl-naphthalene. Band maxima at: 5850, 5410, 5010; 4300, 4120; 3390, 3150 A.(Lower curve.) Observed absorption of methylpentacene at 210° in 1-methylnaphthalene: 5790, 5360, 4960 A. (intensity qualitative); on rapid cooling the fading band maxima shift to: 5850, 5410, 5010 A. Absorption spectrum of 6-methylene-6:13-dihydropentacene (Va)in alcohol. Band maxima: 3770, 3585, 3420; 3175, 3020, 2900; 2690, 2570 A.5000 40007000 6000This view is supported by study of the absorption spectra of the hydrocarbons (see accompanying graph). The spectrum of 9-methylanthracene (III) resembles very closely that of anthracene, but does not exclude the presence of a small proportion of the methylene form (Ilia), which would have only feeble absorption. The orange 5-methyltetracene (IV) shows an absorption similar to that of tetracene, but the intensity of the bands is considerably lower than in the case of tetracene, whereas with 9-methylanthracene and anthracene the reverse relationship holds. This suggests the presence in (IV) of an appreciable proportion of a weakly absorbing methylene form (IVa). When we come to the pentacene series, we find that the spectrum of the pale yellow methylene-dihydropentacene (Va) shows no resemblance to that to be expected for methylpentacene (V), which may be calculated by application of the anellation principle1. If the pale yellow solution of (Va) in 1-methylnaphthalene is heated with exclusion of air to 200°, the solution becomes violet-red and shows the first three bands calculated for methylpentacene (V). The colour fades slowly on cooling.The fact that methylpentacene exists at room temperature almost entirely in the methylene form (Va) is important from the point of view of resonance in aromatic compounds. As each member of the pairs of structures formulated above contains the same number of double bonds as the other member, the energy-difference between the methyl form and the methylene form must consist of the difference in resonance energy. Wave mechanical treatment predicts a decreasing difference in resonance energy of these pairs of hydrocarbons with increasing numbers of rings, until the difference becomes zero with an infinite number of rings2. Hence the ratio between methylene and methyl forms should never exceed 1:1. This prediction has thus been shown to be entirely erroneous, and the evidence now adduced may perhaps provide a new basis for the calculation of the decreasing energy per ring in the higher acenes. 9-Methylanthracene (III) was prepared as described by Sieglitz and Marx3. 5-Methyl-tetracene (IV) was obtained by dehydration of the crude carbinol formed by interaction of methylmagnesium iodide with tetracenone, prepared by reduction of 5: 12-tetracene-quinone4 with alkaline sodium hydrosulphite solution. It formed orange-red prisms (from benzene), m.p. 160°, and gave a bright green solution in sulphuric acid (found: C, 94-2; H, 5-7. C19H14 re-quires C, 94-2; H, 5-8 per cent).6-Methylene-6: 13-dihydropentacene (Va) was similarly prepared from pentacenone6, and formed pale yellow needles (from xylene), which dissolved in sulphuric acid to give a blue solution which afterwards turned to brown (found: C, 94-4; H, 5-65. C23H16 requires C, 94-5; H, 5-5 per cent). The melting point was between 244° and 254°, and in an evacuated capillary the melt was deep red. Oxidation with selenium dioxide in boiling nitrobenzene gave pentacene-5: 13-quinone. A sparingly soluble by-product from the Grignard condensation formed colourless crystals, m.p. 420° (decomp.), which gave a greenish-yellow solution in sulphuric acid (found: C, 94-2; H, 5-8. C46H32 requires C, 94-5; H, 5-5 per cent). We thank the Department of Scientific and Industrial Research for a grant which enabled one of us (J. W. W.) to participate in the experimental section of this investig