Steric Effects Causing Decreased Electronic Interactions

Steric Effects Causing Decreased Electronic Interactions... 

Analyzing the frequency factor v, Bursey43 was able to show that the details of the reaction mechanisms of the electron impact induced ketene elimination from ortho substituted phenyl acetates and from acetanilides, 36, are dramatically influenced by the nature of the substituent R = F, Cl, Br, J. The results demonstrate that the decrease of v going from the voluminous iodine to the small fluorine is not caused by steric effects (which should operate in an opposite direction) but is the result of an electronic interaction of both substituents, leading to a tighter transition state in the case of the more electronegative fluorine. Additional factors which are not yet completely understood play a decisive part in the fragmentation of the anilides. 

Electronic interactions may be increased or decreased by steric effects and in certain cases totally new interactions may result. Extended conjugation of 77 orbitals requires coplanarity of the atoms involved in the 77-cloud delocalization for maximal resonance interaction. If large bulky groups are in positions that cause perturbation of the coplanarity of the 77 system, is usually shifted to shorter wavelengths and max also decreases. For example, diphenyl (An,ax = 246 nm, e ax = 20,000) has coplanar rings and shows higher molar absorptivity than its derivative, o,o -dialkyl-diphenyl which has nonplanar rings (A ax = 250 nm, e ax = 2000). 

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