Disubstituted benzene rings steric effects

The low yields of 6,6 -disubstituted-2,2 -bipyridincs recorded in Table I are probably the result of steric retardation of the adsorption of 2-substituted pyridines. This view is supported by the observation that 2-methylpyridine is a much weaker poison for catalytic hydrogenations than pyridine. On the other hand, the quinolines so far examined (Table II) are more reactive but with these compounds the steric effect of the fused benzene ring could be partly compensated by the additional stabilization of the adsorbed species, since the loss of resonance energy accompanying the localization of one 71-electron would be smaller in a quinoline than in a pyridine derivative. 

These effects can occur when the active site at which a measurable phenomenon occurs is in close proximity to the substituent. Among the many systems exhibiting direct steric effects are ortho-substituted benzenes, 1, cis-substituted ethylenes, 2, and the ortho- (1,2-, 2,1- and 2,3-) and peri- (1,8-) substituted naphthalenes, 3, 4, 5 and 6, respectively. Other examples are d.v-1,2-disubstiUited cyclopropanes, c/ s-2,3-disubstituted norbornanes and ci.s-2,3-disubstituted [2.2.2]-bicyclooctanes, 7, 8 and 9, respectively. Some systems generally do not show steric effects. Vicinally substituted systems such as disubstituted methanes, 10, and 1,1-disubstituted ethenes, 11, are examples, 2,3-Disubstituted heteroarenes with five-membered rings such as thiophenes and selenophenes... 

Copolymerization of substituted and unsubstituted monomers was also investigated [74]. The density of side-chains along the PPP backbone was slowly decreased by varying the ratio of substituted to non-substituted monomers in the reaction mixture. Copolymers exhibit statistically distributed repeating units no evidence for alternation or for block-sequences was found. Their was a marked effect on the UV spectra, indicative of a modification in the extent of conjugation between adjacent benzene rings induced by side-chains. This effect is linked to steric hindrance and it shows that a disubstituted benzene unit acts as an electronic insulator between unsubstituted blocks of PPP. DPs of 50 are reported for these copolymers. 

IDENTIFYING STERIC EFFECTS FOR DISUBSTITUTED AND POLYSUBSTITUTED AROMATIC BENZENE RINGS... 

Identifying Steric Effects for Disubstituted and Polysubstituted Aromatic Benzene Rings... 

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