Four adjacent ring hydrogen atoms

Four adjacent ring hydrogen atoms, ortho-di-substituted aromatics. As indicated above, orr/zo-di-substituted materials absorb in approximately the same range of frequencies as mono-substituted derivatives, and the appropriate strong band is found in the range 770—73 5 cm The reduction in the number of adjacent free 

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As planar projection diagrams indicate, considerable overcrowding occurs in 1-phenyldibenzothiophene between the hydrogen atoms on the phenyl substituent and the adjacent C-9 proton. To a lesser extent this is also true of the 4-phenyl derivative due to partial overlap of the phenyl substituent with the sulfur atom of the thiophene ring. A detailed study of the UV spectra of the four monophenyl derivatives and the association constants of their 2,4,7-trinitrofluorenone complexes reveals that both 1-phenyl- and 4-phenyldibenzothiophene have the phenyl substituent orthogonal to the plane of the heterocycle, while both 2-phenyl- and 3-phenyldibenzothiophene are essentially planar. 

The signal at 57 in the NMR of / indicates the presence of an aromatic ring (Cg). In addition, / has four other nonequivalent hydrogens that cire not on a carbon atom that has hydrogen atoms on the adjacent Ccirbon atom. J has four NMR signals therefore, the hydrogen atoms cire in four different environments. (Worry about the ratios later.)... 

Apart from the various tautomeric forms, there are four kinds of parent thiadiazine structures, having molecular formula C3H4N2S, in which the sulfur atom is adjacent to at least one ring nitrogen atom (structures I-IV). In order to name a particular tautomer, the extra hydrogen requires designation. Thus, using Chemical Abstracts nomenclature, structure I is named as 6//-1,2,3-thiadiazine, and structures V, VI and VII are named as... 

These correlations hold also for pyridine systems the ring nitrogen atom counts as a substituent so that a 2-substituted pyridine will be expected to show a band due to the vibrations of four adjacent hydrogen atoms. 

This result sounds very like the results already noted for six-membered rings and the explanation is similar. Saturated four-membered rings—even the ketones—are slightly puckered to reduce eclipsing interactions between hydrogen atoms on adjacent carbon atoms, and axial attack by the small nucleophile gives the more stable cis product having both substituents equatorial . 

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