Carbenium ions positive charge substituents

In vinyl cations (Figure 3.13), as in trisubstituted carbenium ions, the positive charge is stabilized by electron donating substituents or by aryl or vinyl groups via jt-conjugation. Further stabilization may be achieved by c participation— that is, by hyperconjugation of a substituents, by complexation to a metal, or by the (3-silyl effect. 

A carbon radical has seven valence electrons, one shy of the octet of a valence-saturated carbon atom. Typical carbon-centered radicals have three substituents (see below). In terms of electron count, they occupy an intermediate position between the carbenium ions, which have one electron less (a sextet and a positive charge), and the carbanions, which have one electron more (an octet and a negative charge). Since both C radicals and carbenium ions are electron deficient, they are more closely related to each other than to carbanions. Because of this, carbon radicals and carbenium ions are also stabilized or destabilized by the same substituents. 

What can be said about the stereochemistry of SN1 reactions In the carbenium ion intermediates R1 R2R3C , the positively charged C atom has a trigonal planar geometry (cf. Figure 1.3). These intermediates are therefore achiral, if the substituents R1 themselves do not contain stereogenic centers. 

In the first step of the actual Ar-SE reaction, a substituted cyclohexadienyl cation is formed from the electrophile and the aromatic compound. This cation and its derivatives are generally referred to as a sigma or Wheland complex. Sigma complexes are described by at least three carbenium ion resonance forms (Figure 5.1). There is an additional resonance form for each substituent, which can stabilize the positive charge of the Wheland complex by a pi electron-donating (+M) effect (see Section 5.1.3). This resonance form is an all-octet formula. 

Alkenes polymerize cationically by electrophilic addition of the monomer to a growing carbenium ion. Therefore, the monomer must be nucleophilic and capable of stabilizing the resulting positive charge. In addition, the double bond must be the most nucleophilic functionality in the monomer. Some vinyl monomers which can polymerize cationically are listed in Eq. (34) in their order of reactivity, which corresponds to the electron-donating ability of their substituents. 

Substituents with a-heteroatoms such as a-alkoxy, thio, halo, and amino groups stabilize through resonance to form, for example, oxonium (R—0+=CH2) or halonium ( + Br=CR2) ions. Aryl groups, especially those with electron-donating substituents, also delocalize the positive charge through resonance, with the stability of carbenium ions increasing... 

Carbenium ions are sp2 hybridized with the empty orbital perpendicular to the plane containing the three substituents. Calculations confirm both the flat structure of carbenium ions and the shorter linkage with aryl substituents due to the partial double-bond character. As discussed previously, the positive charge is not localized just on the sp2-hybridized carbon of the carbenium ion, but is dispersed onto neighboring substituents. Table 1 presents the charge distribution in model carbenium ions formed by protonation of propene, isobutene, styrene, a-methylstyrene, methyl vinyl ether, and methyl propenyl ether. 

The precise reactivities of various types of carbenium ions (free, paired, and aggregates) are not known, but it seems that the differences in reactivities are not very large. This may be due to solvation effects, which are similar for all types of carbocations, and also due to the large size of the counterions which interact weakly with the cations. A positive charge in carbocations is located partially on the sp2-hybridized C atom and is widely delocalized over the /3-protons and substituents, especially in the case of aromatic and alkoxy a-substituents. 

Silylenium ions are more thermodynamically stable than carbenium ions, but are also much more reactive. Larger size of silicon atoms and longer bonds to substituents make them more accessible to nucleophilic attack than carbon. Thus, reactions at silicon are generally less sensitive to steric hindrance. Silicon is also more electropositive than carbon, and the positive charge in sSL is highly localized on silicon, while in =C it is largely dispersed over the substituents. This results in stronger electrostatic attraction forces between silylenium cation and electron-rich species [14]. 

Poor Hammett a correlations are often obtained for reactions in which a positively charged centre is formed that can resonate with an electron donating substituent, because of the additional resonance transmission for these substituents This phenomenon is illustrated by the solvolysis of substituted 2-phenyl-2-chloropropane (Figure 5). Scheme 6 shows how a transition structure may obtain extra stabilisation by resonance interaction between carbenium ion and 4-methoxy and 4-dimethylamino groups. In these cases there is no effect on the energy of the reactant molecules. 

Maximum resonance interaction is likely to occur in the transition structure for the solvolysis reaction of Scheme 6 when the transition structure fully resembles a carbenium ion. In reactions where the transition structure does not completely resemble the reference state the substituent will not exert its full potential in resonance transmission with the reaction centre. The r parameter of Yukawa and Tsuno (Equation 12) provides a measure of the extent of the resonance interaction for a reaction centre which builds up positive charge. 

The preference of this conformation (the barrier hindering the rotation of the p-XCgH — at X = Cl, CHj, CFj amounts to 8.5 kcal/mol., cf. also ° 0 is assumed to be due, at least partially, to the donor-acceptor interaction between the K-system of the aryl residue and the carbenium centre at Cjg in the electronic absorption spectra of the 9-p-X-phenyl-9,10-dimethylphenanthrenium ions the charge transfer bands have been revealed whose position correlates well with the ionization potentials of respective X-substituted benzenes. Judging by the NMR- C spectra, however, the extent of this interaction in the main state of ions is insignificant — the chemical shift of the Cjo atom nearly remains unchanged as the X substituent... 

A characteristic property of carbenium ions is their aptitude to rearrangements due to the shift of the substituent R from a-sp -hybridized carbon atom to the positively charged sp -hybridized atom (1,2-shift of the R substituent) ... 

Reactions where the reaction centre becomes positively charged and may attract electrons from a substituent via a resonance pathway will give poor correlations with o. The solvolysis of 2-phenyl-2-propylchlorides (Eqn. 53) gives a transition state where the almost fully formed carbenium ion can stabilise by resonance with a para... 

Table 11.7 shows a series of Hammett sensitivity parameters for various SnI reactions using o substituent constants. The p value is indicative of the amount of positive charge present in the transition state. The larger the magnitude of p, the more positive the charge. Some trends are noteworthy. The smallest negative number (entry 1) is for triphenylmethyl cation, the most stable carbenium ion. This is as expected, because the reaction would be the least endothermic, and by the Hammond postulate the transition state would be most reactant-like, thereby having the least cationic character. 

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