Carbonium ions electronic effect

Through its inductive effect halogen tends to withdraw electrons and thus to destabilize the intermediate carbonium ion. This effect is felt for attack at all positions, but particularly for attack at the positions ortho and para to the halogen. 

Above, the phenyl and related groups at the C atom were assumed to display an accelerating effect relative to the methyl one in the case of favourable orientation and electron deficiency on this atom (cf. Ref. 228,241,242) However, the opposite view is also possible. For example, according to the data of the a-CHj group stabilizes some types of carbonium ions more effectively than the phenyl one does. One can expect this order to be retained in the transition state of the 1,2-methyl shift as well (cf. Ref. p. 285). 

A tertiary carbonium ion is more stable than a secondary carbonium ion, which is in turn more stable than a primary carbonium ion. Therefore, the alkylation of ben2ene with isobutylene is much easier than is alkylation with ethylene. The reactivity of substituted aromatics for electrophilic substitution is affected by the inductive and resonance effects of a substituent. An electron-donating group, such as the hydroxyl and methyl groups, activates the alkylation and an electron-withdrawing group, such as chloride, deactivates it. 

Measurements by electron impact methods have led to experimental stabilization energies of a number of alkylcarbonium ions. Muller and Mulliken (1958) compared these with calculated values obtained by a procedure based on the LCAO-MO approximation, the large stabilization energies found for carbonium ions being attributed to the combined effects of hyperconjugation and charge redistribution (Table 1). 

The combined effects of an electron donating alkyl substituent and the adjacent alkene bond in 23 leads to reaction solely by the carbonium ion intermediate and in acetic acid the more stable ally acetate is formed [119],... 

The action of nitrous acid on a primary, aliphatic amine is still a complex process, inasmuch as it is necessary to postulate certain steps that are imperfectly understood, but it can nevertheless be considered that the process leads ultimately to a carbonium ion.26,27 The carbonium ion thus formed is termed hot, that is to say, it is non-solvated and chemically activated the nature of the products resulting from its decay is essentially determined by the conformation of the initial state the electronic factors in effect at the time of the transition state exert little influence.27... 

The reactions of carbonium ions occur via transition states having precise stereochemistry in which the electron pair of the attacking nucleophile must be col inear with the empty p-orbital of the electron-poor carbon atom. Thus, powerful stereoelectronic effects control these reactions. 

It was correctly assumed and later determined unequivocally (64) that in each of these two reactions the BH vertex that was removed was always one of the two equivalent vertices found as the nearest neighbors of the two equivalent CH vertices. The fact that the carbon atoms in polyhedral surfaces of C2Bb 2H carboranes are electronic counterparts of boron atoms in the corresponding isoelectronic B H 2 polyhedral ions (62) requires these carbon atoms to resemble C+, a species present in carbonium ions. Consequently, the BH vertices which are nearest neighbors of two such carbon atoms will be activated for nucleophilic attack by base through the advent of a strong inductive effect. 

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