6048longer C,-Cl bond distance in I, 1.809 (6) A, is similar to the C, 3-cl bond distance found in tert-butyl chloride, 1.828 (5) A,!' 1.807 (4) A.I2 Furthermore, the observed R-C,-Cl bond + 4 J p -~ = 8.6Hz; 6(OMe) 5.07 ppm, 4Jpt-~ = 6 HZ; &Me) 2.12 ppm, COU-(14) R. A. Bell and M. H. Chisholm, results to be submitted for publication.
NOR 88C)Sztwmary Consideration of tautomeric equilibria allows the estimation of the aromatic stabilization energy of 2-pyridone as 25 kcal mol-l: the method is applied to related compounds.THE concept of aromatic character remains a cornerstone in the teaching and practice of organic chemistry, despite difficulties in its precise definition and quantitative measurement. The significant interest is in additional stabilization energy that aromatic compounds possess over and above that for hypothetical similar but non-conjugated systems.estimated by a comparison of the corresponding tautonieric equilibria [(l) t (2)] and [(3) + (4)J. This is a
N.m.r. spectra indicate that 1.2-dihydronaphthalene and its 4-phenyl derivative are in mobile equilibrium. Substituents in the 1 -position are shown to exist preferentially ' pseudo-axial ' by a detailed study of coupling constants of dihydronaphthalenes and related model compounds.University of East Anglia, Norwich BUTCHER 2 analysed the microwave spectrum of cyclohexa-1'3-diene and estimated the angle of torsion between the 1,2-and 3'4-bonds as 17.5 & 2". Earlier, Beckett and Mulley suggested that 9 10-dihydrophenanthrene is nonplanar, and Mislow et al. calculated the angle of torsion between C-4a and C-5a as 15.3°.4 In these compounds and in related 1,2-dihydronaphthalenes the allylic and benzylic protons may assume ' pseudo-axial ' and ' pseudo-equatorial ' conformations (I). The mobile equilibrium between two conformers (Ia) and (Ib) should be influenced by substituent groups. computer analysis and decoupling techniques. Illustrative spectra are shown in Figures 1-3. n < 5 0 c.lscc. , FIGURE 1 Observed and calculated spectra of 1,2-dihydronaphthalene-1,2-dicarboxylic acid a t 100 Mc./sec., showing the ABC part of the ABCX system n a R (11; R = H)
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