Carbocations charge distribution

Modeling to view the carbocation repre sented by resonance struc tures A and B How is the positive charge distributed among its carbons ... 

Use Learning By Modeling to view the charge distribution in the allylic carbocation shown in the equation. 

Despite the cyclic character of these TSs, both the bond distances and charge distribution are characteristic of a high degree of charge separation, with the butenyl fragment assuming the character of an allylic carbocation. 

Effects of nitrogen substitution can be predicted by quantum chemical calculations (42,43), or more qualitatively by examining the charge distribution in the relevant carbocations. When a carbocation is generated at C-l, the positive charge can be delocalized as shown below. 

Ab initio MO calculations have been carried out for two carbocation-generating reactions the 6 nI reaction of protonated 1-phenylethanol (H2O leaving group) and the acid-catalysed hydration of styrene. Optimizations were done at the MP2/6-31G level. The 6 nI transition state lies half way between the reactant and the product with respect to the bond lengths, charge distribution, and secondary deuterium isotope effects. 

Earlier progress in these studies has been summarized in reviews published in the last decade, emphasizing carbocations and oxidation dications, RCs, as well as reactive intermediates from the nitro- and nitroso-derivatives. The review article published in 1996 emphasized groundwork studies on protonation as well as oxidation (both RCs and stable dications) of polycyclic arenes and explored possible relationships between charge distribution and carcinogenicity, in concert with its... 

Formation of both 1,2- and 1,4-addition products occurs not only with halogens, but also with other electrophiles such as the hydrogen halides. The mechanistic course of the reaction of 1,3-butadiene with hydrogen chloride is shown in Equation 13-1. The first step, as with alkenes (Section 10-3A), is formation of a carbocation. However, with 1,3-butadiene, if the proton is added to C1 (but not C2), the resulting cation has a substantial delocalization energy, with the charge distributed over two carbons (review Sections 6-5 and... 

The intermediate carbocation formed by SN1 reactions of either 3-chloro-3-methyl-1-butene or 4-chloro-2-methyl-2-butene reacts with water to give a mixture of 2-methyl-3-buten-2-ol and 3-methyl-2-buten-1-ol. Which alcohol would you expect to predominate under conditions of equilibrium control On the basis of steric hindrance, charge distribution in the cation, and so on, which alcohol should be favored under conditions of kinetic control (Review Sections 6-5C, 10-4A, and 11-3.) Give your reasoning. 

This modified charge distribution in the transition state leads to a mismatch between substituent effects on the rate of reaction and on the equilibrium constant. With respect to the fluorine substituents in Scheme 31, these decrease both the stability of the carbocation and the stability of the transition state. However, while there must be less carbocation character in the transition state than in the carbocation itself the positive charge is located to a greater degree on the benzylic carbon atom and therefore will be more sensitive to stabilization by substituents. If substituent effects at the a-carbon atom in the carbocation and in the transition state are then of comparable magnitude, there will be no net effect on the rate of reaction, as is observed. 

In the case of cyclopropyl methyl ketone, disconnection of either the 1,2- or 1,3-carbon-carbon bond of the cyclopropane ring results in the preferred charge distribution shown in (4), namely, the carbanion site is adjacent to the meso-merically stabilising carbonyl group, and the carbocation site may be viewed as a halide-carrying carbon. The reagent equivalent may therefore be 5-chloro-pentan-2-one. 

Table 1 Charge Distribution on /3-H Atoms and sp2-Hybridized C + Atoms Calculated by RHF/6-31G for Carbocations Derived from Various Alkenes...

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