Planar carbenium ion

To explain the very varied behaviour patterns shown by the various monomers in various solvents, use has been made of a further, hitherto unrealized, implication of the model, namely that the rate of the isomerization-propagation must depend upon the electrochemical environment of the complex. This vague idea has been given precision by concentrating attention on the species which occupies the site at the back-side of the near-planar carbenium ion, the front-side of which is 7t-bonded to the double bond of the monomer. The idea is that the stronger the dipole at the back, the weaker is the Jt-bond, and the lower is the energy of the transition state, and therefore the greater is the rate. 

In the Sj l reaction, a planar carbenium ion is formed first, which then reacts further with the nucleophile. Since the nucleophile is free to attack from either side, this reaction is associated with racemization. 

Fig. 1.3. Energy levels and occupancies (red) of the MOs at the trivalent C atom of planar carbenium ions R3C (left) and pyramidal carbanions RjC (right). The indices of each of the four MOs refer to the AOs from the central C atom.

The overall changes in the chemical shift of the methine protons can be directly correlated with the amounts of ions if ionization is significant (>1%). However, even minute amounts of intermediate cations can be detected in some systems by dynamic NMR. Because ionization leads to the formation of a planar carbenium ion, the chirality at the carbon atom is lost. In the particular case of the isobutyl vinyl ether derivatives, the isobutoxy group has a built-in probe (CH2-CHMe2) separated from the chiral center by oxygen atom. The methylene protons on the ether group are magnetically nonequivalent due to the presence of four different substituents at the electrophilic carbon center. They become equivalent only... 

Furthermore, if it is true that the 1,2 shift leaves a planar carbenium ion IX, a process that erases the configurational identity of the carbon atom bearing the i.sopropyl group (and therefore apparently any further use of the advantages first introduced by the stereoselective step VII - VIII), it is also true that the ensuing 1,2-methyl migration (IX - II) must proceed in a suprafacial fashion... 

Imprinted polyurethanes as coatings for optrodes have been used for the detection of solvent vapours in air (see chapter 20). One per cent of substituted 3,3-diphenylphthalide as indicator has been intercalated within the polymer. The phthalide forms a highly coloured planar carbenium ion by interaction with an acidic component and a subsequent cleavage of the lactone ring - the unreacted phenolic groups in a polyurethane provide enough acidity for this reaction (Fig. 21.4). The incorporation of analytes reduces the acidity and the back-reaction... 

Epoxides have occasionally been used in Friedel-Crafts reactions, and some interesting stereochemical observations have been made in this context. Quite unlike secondary alcohols which give almost fully racemized product, it has been shown that optically pure propylene oxide with AlCh and benzene gives optically pure 2-phenyl-1-propanol with inversion of configuration at the cleaved center. AlBrs leads to much lower levels of optical purity it was demonstrated that both starting material and product are optically stable to the reaction conditions, and therefore partial racemization is intrinsic to the mechanism with AlBrs. It is nonetheless clear from these and other results that even powerful Lewis acids do not assure reaction via simple planar carbenium ions. 

Trivalent ( classical carbenium ions contain an sp -hybridized electron-deficient carbon atom, which tends to be planar in the absence of constraining skeletal rigidity or steric interference. The carbenium carbon contains six valence electrons thus it is highly electron deficient. The structure of trivalent carbocations can always be adequately described by using only two-electron two-center bonds (Lewis valence bond structures). CH3 is the parent for trivalent ions. 

The transition state leading to surface methoxy formation (Fig. 13) is reported to contain a planar CH3 carbenium ion fragment (221, 222). It is now accepted (204) that carbenium ion fragments do not exist freely in zeolites but must be bonded to the lattice oxygen atoms. They have been... 

Trialkyl cations of oxygen, sulfur, and selenenium (and less commonly tellurium) are onium ions by virtue of their having one extra valence, although these three-coordinate ions invite comparison with carbenium ions. These ions (e.g., 28, 29, and 30) are not planar (as opposed to the analogous carbenium ions), but rather are pyramidal. These species have a well-characterized solution acid chemistry concentrated sulfuric acid is required for observation of the trimethyloxonium cation, but the sulfonium and selenonium ions require less strongly acidic solutions. 

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. 

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. 

Figure 2.13 shows an SN1 reaction with optically pure ( )-2-bromooctane carried out as a solvolysis. By solvolysis we mean an SN1 reaction performed in a polar solvent that also functions as the nucleophile. The solvolysis reaction in Figure 2.13 takes place in a water/ethanol mixture. In the rate-determining step, a secondary carbenium ion is produced. This ion must be planar and therefore achiral. However, it is highly reactive. Consequently, it reacts so quickly with the solvent that at this point in time it has still not completely separated from the bromide ion that was released when it was formed. In other words, the reacting carbenium ion is still almost in contact with this bromide ion. It exists as part of a so-called contact ion pair R R2HC LBr . 

A carbocation (also called a carbonium ion or a carbenium ion) is a species that contains a carbon atom bearing a positive charge. The positively charged carbon atom is bonded to three other atoms, and it has no nonbonding electrons, so it has only six electrons in its valence shell. It is sp2 hybridized, with a planar structure and bond angles of about 120°. For example, the methyl cation (+CH3) is planar, with bond angles of exactly 120°. The unhybridized p orbital is vacant and lies perpendicular to the plane of the C—H bonds (Figure 4-13). The structure of +CH3 is similar to the structure of BH3, discussed in Chapter 2. 

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. 

Generally, chiral tricoordinate centers are configurationally stable when they are derived from second-row elements. This is exemplified by sulfonium salts, sulfoxides and phosphines. In higher rows, stability is documented for arsines and stibines. In contrast, tricoordinate derivatives of carbon, oxygen, and nitrogen (first-row atoms) experience fast inversion and are configurationally unstable they must therefore be viewed as conformationally chiral (see Fig. 3, Section 3.b). Oxonium salts show very fast inversion, as do carbanions. Exceptions such as the cyclopropyl anion are known. Carbon radicals and carbenium ions are usually close to planarity and tend to be achiral independently of their substituents [21-23]. 

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