Carbenium ions nucleophilic substitution

S-Substituted thiiranium ions react with water and alcohols to give trans ring opening (Scheme 72). A report that oxygen nucleophiles attack sulfur as well as carbon has been shown to be incorrect (79ACR282). The intermediate thiiranium ion (57) in the presence of lithium perchlorate readily yields the carbenium ion which undergoes a transannular hydride... 

The ionization mechanism for nucleophilic substitution proceeds by rate-determining heterolytic dissociation of the reactant to a tricoordinate carbocation (also sometimes referred to as a carbonium ion or carbenium ion f and the leaving group. This dissociation is followed by rapid combination of the highly electrophilic carbocation with a Lewis base (nucleophile) present in the medium. A two-dimensional potential energy diagram representing this process for a neutral reactant and anionic nucleophile is shown in Fig. 

Carbonyl reactions are extremely important in chemistry and biochemistry, yet they are often given short shrift in textbooks on physical organic chemistry, partly because the subject was historically developed by the study of nucleophilic substitution at saturated carbon, and partly because carbonyl reactions are often more difhcult to study. They are generally reversible under usual conditions and involve complicated multistep mechanisms and general acid/base catalysis. In thinking about carbonyl reactions, 1 find it helpful to consider the carbonyl group as a (very) stabilized carbenium ion, with an O substituent. Then one can immediately draw on everything one has learned about carbenium ion reactivity and see that the reactivity order for carbonyl compounds ... 

If anions R are oxidized in the presence of olefins, additive dimers (24) and substituted monomers (26) are obtained (Scheme 5, Table 8, and Ref. [94]). The products can be rationalized by the following pathway the radical R obtained by a le-oxidation from the anion R adds to the alkene to give the primary adduct (25), which dimerizes to afford the additive dimer (24) with regiospeciflc head-to-head connection of the two olefins, or couples with R to form the additive monomer (26). If the substituent Y in the olefin can stabilize a carbenium ion, (25) is oxidized to the cation (27), which reacts intra- or inter-molecularly with nucleophiles to give (28) or (29). 

The effect of a-silyl substitution on the stability of a carbenium ion was qualitatively unclear for a long time. Early solvolytic studies by the groups of liaborn36 and Cartledge37 suggest a destabilizing effect of a-silyl substitution compared with alkyl. The measurement and interpretation of the kinetic a-silicon effect in solvolysis reactions is, however, often complicated by the fact that steric and ground state effects may play an important role and that, in addition, the rates of ionization often involve a contribution from nucleophilic solvent assistance. 

HO-OH2+.14 This reaction forms two water molecules and a carbenium ion, which is then hydrated to form the protonated alcohol. The degenerate gas-phase reactions between HF and protonated alkyl fluorides RFH+ (R = Me, Et, i-Pr, r-Bu) were investigated by ab initio methods.15 With the exception of MeFFi+, the protonated alkyl fluorides can be viewed as weak complexes of the carbocation R+ and FiF. Both frontside and backside substitutions occur, supporting the proposal (see Introduction)8 that nucleophilic substitution reactions are better understood through such a competition as opposed to the traditional S /S 2 competition. 

The better the solvent stabilizes the ions, the more probable that the reaction will follow an SN1 pathway (e.g., in polar protic solvents such as water/acetone). The more highly substituted is the incipient carbenium ion, the more probable that the reaction will follow an SN1 pathway. The more unreactive the nucleophile, the more probable it becomes that a reaction with secondary and tertiary electrophiles will follow an SN1 pathway. A weaker nucleophile is not as effective in the backside attack, since this location is sterically shielded, especially in the case of tertiary substrates. Carbenium ions are planar and therefore less sterically hindered, and are naturally more reactive as electrophiles than the uncharged parent compound. 

A very interesting paper80 reported studies of the reactions of several substituted benzhydryl carbenium ions, generated by laser flash photolysis, with halide ions in several solvents. This work provided the nucleophilicity N of chloride and bromide ions in several solvents. These data, along with the ionization rate constants and the solvolysis rate constants for the reactions of substituted benzyhdryl halides, was used to construct quantitative energy surfaces for the. S N 1 reactions of substituted benzhydryl halides in several solvents. 

Substitution reactions according to the SN1 mechanism take place in two steps (Figure 2.11). In the first and slower step, heterolysis of the bond between the C atom and the leaving group takes place. A carbenium ion is produced as a high-energy, and consequently short-lived, intermediate. In a considerably faster second step, it reacts with the nucleophile to form the substitution product Nu—R. 

When in carbenium ions of this type R1 R2 R3, these carbenium ions react with achiral nucleophiles to form chiral substitution products R1 R2R3C—Nu. These must be produced as a 1 1 mixture of both enantiomers (i.e., as a racemic mixture). Achiral reaction partners alone can never form an optically active product. But in apparent contradiction to what has just been explained, not all SN1 reactions that start from enantiomerically pure alkylating agents and take place via achiral carbenium ions produce a racemic substitution product. Let us consider, for example, Figure 2.13. 

It is the carbenium ions that interest us in this chapter because they are the intermediates in some nucleophilic substitutions. The simplest possible carbenium ion would be CH3, the methyl cation, and it would be planar with an empty p orbital. 

Cationic intermediates are considered the active species in many organic reactions, as well as in cationic polymerizations. For example, cationic intermediates are postulated in both electrophilic addition and elimination reactions. They are also involved in electrophilic aromatic substitutions and in some nucleophilic aliphatic substitutions. The latter reactions may involve either onium or carbenium ions. The current understanding of... 

There is some spectroscopic evidence that aromatic compounds complex carbenium ions [42]. For example, the complexation equilibrium constant between trityl ions and hexamethylbenzene is K = 68 mol-1 L at 0° C [43]. Complexation should be stronger with more electrophilic carbenium ions such as those derived from styrene and a-methylstyrene. On the other hand, the monoalkyl-substituted phenyl rings attached to the polymer chain are weaker nucleophiles than hexamethylbenzene. A complexation constant K = 4 mol 1 L was reported for trityl cation and styrene [43]. Similar complexes have been proposed to explain the red color observed in inifer systems based on l,4-bis(I-chIoro-l-methyl-ethyl)benzene and BCI3 in CH2C12 at low temperature [44],... 

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