Substituent, structure

Carbenes having aryl substituents, structure and reactivity of, 22, 311... 

More detailed consideration will now be given to the influence of N- and C-14 (=C-11 in benzomorphans) substituent structure upon the pharmacological profile of opoid ligands. 

The effect of substituent structure is more pro-noimced in the nitrogen compounds. The "aromatic oniums" reduce as easily as sulfoniums, but the quaternary ammonium salts are reducible only when the nitrogen has activating substituents. 

Dependence on Substituent Structure. The molecular nature of the side groups has two effects (I) it influences and, presumably, largely determines the mean free energy of defect formation (e) via steric effects on the enthalpy associated with defect creation and possible entropic contri-... 

As has been discussed briefly in Section III above, the ring strain of the cyclopropenes is accommodated, in part, by somewhat unusual bonding to the substituents. Structural and spectroscopic studies suggest that the vinylic C-H bonds have ca. 44% s character and that this is approximately constant in bonding to other substituents. Consequently aspects of the chemistry of the cyclopropenes are expected to reflect this with enhanced acidity of the vinylic proton. Cleavage of an exocyclic bond at C(3) will result in the cyclopropenyl cation, radical or anion with consequent interaction with the n bond and perturbation of the energy of the system. By comparison addition to the n-system to yield a cyclopropane relieves ring strain by ca. llOkJmol" ... 

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