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was anticipated, the presence of two additional fused benzene rings led to a more highly strained system in fluoradene than was present in the parent Equilibration temp., Ortho: Para Ratio 2,2a,3,4-tetrahydro-lH-cyclopent [cd]indene.l Of OK. Found Ca1cd.J Fig. 1 particular interest is the unusually strong acidity of -300 3.04iz 0 . 1 5 3.00 A the hydrogen on C,Z. 77.4 0 . 9 7 i.0 . 0 5 0.99 B The structure of fluoradene was established (1) 64 0.66 =k 0 . 0 3 0.63 C by its method of synthesis, (2) by elementary analyses and molecular weight determinations, and as a dilute helium solution, was rechromatographed (3) in particular by its ease of hydrogenation and to check the retention time and purity (Fig. 1, D consumption of perbenzoic acid, reactions which are and E). characteristic for such strained, fused ring comWith a properly activated column, there is no pounds.’.* The hexahydrofluoradene (111) [m.p. detectable “irreversible adsorption.” When 118.4-119.4. Anal. Calcd. for C19H18: C, 92.6; samples of hydrogen were injected slowly between II,7.4. Found: C, 92.5; H, 7.21 resulted from a the end of the chromatographic column and the rapid 300 mole % hydrogen absorption in methanol combustion tube using a syringe driven a t constant a t room temperature in the presence of palladized speeds, the areas of the resultant rectangular peaks carbon. Its ultraviolet spectrum was that of two were the same (=k 3y0)as the sums of the areas of 1,2- or 1,3-dialkylindane~,~ and this lends strong the ortho- and parahydrogen peaks produced when support to the structural assignment. At room samples of identical size were injected a t the front temperature, fluoradene (I) consumed 400 mole % of the column. of perbenzoic acid while under the same conditions T h a t substantial separation of ortho- and para- 9-phenylfluorene consumed 50 mole yo. Both of deuterium occurs is shown by the shoulder on the these reactions reflect the presence of appreciable trailing edge of the deuterium peak (Fig. 1, F). strain in the benzene ring which is doubly-fused to The peak assignment is based upon the ortho: five-membered rings. para ratio of 2.00 for normal deuterium. The The most unusual property of fluoradene is the deuterium isomers are separated completely from acidity of the one remaining non-aromatic hydrothe hydrogen isomers; however, orthohydrogen is gen a t Clz. This was first observed when i t was eluted only slightly more rapidly than HD. discovered that fluoradene could be removed from To aid in the interpretation of these separations, benzene by washing with dilute aqueous alkali and we are in the process of measuring chromatographiwas confirmed by exchanging this hydrogen for cally the heats of adsorption. deuterium in boiling deuterium ethoxide, conditions (3) Ref.2, p. 14. under which 9-phenylfluorene shows no exchange. DEPARTMENT OF CHEMISTRY WILLIAM R. MOORE In alkaline solution fluoradene develops a pink MASSACHUSETTS INSTITUTE OF TECHNOLOGY color reminiscent of phenolphthalein. On acidifiCAMBRIDGE 39, MASSACHUSETTS HAROLD R. WARD cation, fluoradene is recovered, but in the presence RECEIVED APRIL21, 1958 of oxygen the color fades and 12,12’-bifluoradenyl is formed [m.p. 306-307 dec. Anal. Calcd. for C3&T2?:C, 95.4; H , 4.6. Found: C, 95.4; H, FLUORADENE (INDENO [1,2,3-jk]FLUORENE), AN 4.7.1, identical with the product from the reaction UNUSUALLY ACIDIC HYDROCARBON of 12-bromofluoradene with mercury. To obtain Sir: a quantitative measure of this acidity, the distriI n continuation of our study of cyclopentindene bution of fluoradene was examined between hexane compounds‘J we wish to report the synthesis and and 97(r, aqueous methanol as a function of PH. some of the properties of fluoradene (indeno By this procedure, a pK’, of 11*0.5 (extrapolated [1,2,3-jk]fluorene) (I) [m.p. 129.8-130.3’. Anal. to water) was found for fluoradene.6 This is a Calcd. for CloH12: C, 95.0; H, 5.0; mol. wt., 240. truly remarkable acidity and should be compared Found: C, 94.8; H, 4.9; mol. wt. (Rast) 2351, with the most acidic hydrocarbons heretofore prepared in %yoyield by deamination of IIa.S As known, 9-phenylfluorene, phenylacetylene, and indene,6 for which pKi’s of 21 or greater have been reported. Undoubtedly, the high symmetry and complete conjugation in the fluoradene anion are responsible for this extreme acidity. TABLEI CHROMATOGRAPHIC ANALYSES OF HYDROGEN
CHEMISTRY DEPARTMENT OF CALIFORNIA UNIVERSITY BERKELEY, CALIFORNIA
H. RAPOPORT G. SMOLINSKY
RECEIVED APRIL21, 1958 I
I I a , R = NH2 IIb, R = OH
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(1) H . Rapoport and J. 2. Pasky, THISJ O U R N A L , ~ 3788 ~, (1956). H.Rapoport and G. Smolinsky, ibid., 79, 5831 (1957). (3) I t is of interest to note that a recent attempt t o prepare I from 9- (o-aminophenyl)-9-fluorenol by deamination led t o tribenzotropone a$ the only product in addition to the phenol [M. Stiles and A J. Libbey. J r , J . Org. Chem., 22, 1243 (1957)l. (2)
(4) J. Entel, C.H. Ruof and H . C. Howard, A n d . Chcm.. 26, 1303 (1953). ( 5 ) The same method applied to 9-(o-hydroxyphenyl)-fluorene (1Ib) led to a pK:, of 12.2 in 97% aqueou9 methanol. In water, this ;:heno1 has a p K : of 9.7, and the difference, 2.5 p l i units, was used for the extrapolation of fluoradene’s acidity to water. ( 6 ) J. B. Conant a n d G . W. Wheland, THIS JOUKNAL, 64, 1212 (1932); W. K.McEwen, i b i d . , 68, 1124 (1936).
