Hyperconjugation, substituent effect

Hammett s equation, and substituent effects, 137-43 heteromolecules, 130 Holleman s product rule, 3 hyperconjugation, in nitration of alkyl-benzenes, 165-7 in nitration of positive poles, 169... 

How do you interpret the values of p and r in these equations Which system is more sensitive to the aryl substituent How would you explain this difference in sensitivity Sketch the resonance, field and hyperconjugative interactions which you believe would contribute to these substituent effects. What, if any, geometric constraints would these interactions place on the ions ... 

The net effect is, therefore, a balance of these two components. Some of the results of the separation may be surprising, such as the relatively small contribution that resonance makes to the overall substituent effect of p-NOj. Another interesting result is the predominant resonance contribution in the alkyl series this (if real) must be attributed to hyperconjugation. 

SUBSTITUENT EFFECTS IN THE BENZENE SERIES through the contribution of the hyperconjugative form ... 

Some other mechanisms will be discussed in later sections The Grant-Cheney formulation (88) of steric compression effects will appear in the context of y-gauche substituent effects (Section III-C), and hyperconjugative orbital interactions will be cited in interpretations of y-anti substituent (Section III-D) and intramolecular interaction effects (Section IV). 

Substituent effects Carbocations are formed in the S l reactions. The more stable the carbocation, the faster it is formed. Thus, the rate depends on carbocation stability, since alkyl groups are known to stabilize carbocations through inductive effects and hyperconjugation (see Section 5.2.1). The reactivities of SnI reachons decrease in the order of 3° carbocation > 2° carbocation > 1° carbocation > methyl cation. Primary carbocation and methyl cation are so unstable that primary alkyl halide and methyl halide do not undergo SnI reachons. This is the opposite of Sn2 reactivity. 

Substituent effects do not appear to be reliable probes for hyperconjugation, however. Even in the 2-norbornyl cation, where considerable C-C bond delocalization is generally considered to be present16S), substituents fail to indicate any electron deficiency at the 6-position of the developing 2-norbornyl cation 162,166>16 ). Methyl substituent effects may also not be expected to provide a reliable test for C-C hyperconjugation in the 1-adamantyl system. 

Substituent effects and the role of negative hyperconjugation in [l,2]-silyl migration and decarbonylation of methoxy(substituted-siloxy)carbenes have been investigated using quantum chemical calculations and natural bond orbital analysis (Scheme 38).62... 

Adcock, W. Abeywickrema, A. N. Substituent effects in the bicyclo[2.2.2]octane ring system. A carbon-13 and fluorine-19 nuclear magnetic resonance study of 4-substituted bicyclo[2.2.2]oct-l-yl fluorides, / Org. Chem 1982,47,2957-2966. Laube, T Ha, T. K. Detection of hyperconjugative effects in expaimentaUy dete-mined structures of neutral molecules, J. Am. Chem. Soc. 1988,110, 5511-5517. Rozeboom, M. D. Houk, K. N. Stereospecific alkyl group effects on amine lone-pair ionization potentials Photoelectron spectra of alkylpiperidines, / Am Chem Soc. 1982,104,1189-1191. 

---