Sigma complex stable

Strictly speaking there is only indirect evidence for the occurrence of sigma complexes as Side Note 5.1. short-lived intermediates of Ar-SE reactions. Formed in the rate-determining step, sigma com- Stable Cyclohexadienyl plexes completely explain both the reactivity and regioselectivity of most Ar-SE reactions (cf. Cations Section 5.1.3). In addition, the viability of short-lived sigma complexes in Ar-SE chemistry is supported by the fact that stable sigma complexes could be isolated in certain instances. 

Fig. 5.2. NHR studies and geometry of a stable sigma complex. The C,C bond lengths in the six-membered ring correlate with the bond order resulting from the superimposition of the three resonance forms. The 13C-NMR shifts of the sp2-hybridized ring atoms by means of a low-field shift, i.e., an increased Svalue, indicate the centers that—according to the resonance forms—bear positive partial charge.

Which sigma complexes are the most stable This is determined to a small extent by steric effects and to a considerably greater extent by electronic effects. As a carbocation, a substi-... 

Because of completely analogous considerations, every acceptor-substituted sigma complex E—C6H5—EWG is less stable than the reference compound E— C6H6+ (Figure 5.11). From this analysis, one derives the following expectations for Ar-SE reactions of acceptor-substituted benzenes ... 

In this example, the phenyl substituent with its pi electron donating (+M) effect determines the structure of the most stable sigma complex and thus the regioselectivity. The competing inductive (+1) effect of the alkyl substituent cannot compete. This is understandable because it is not as effective at stabilizing an adjacent positive change as the phenyl group. 

Like an alkene, benzene has clouds of pi electrons above and below its sigma bond framework. Although benzene s pi electrons are in a stable aromatic system, they are available to attack a strong electrophile to give a carbocation. This resonance-stabilized carbocation is called a sigma complex because the electrophile is joined to the benzene ring by a new sigma bond. 

Because the sigma complexes for ortho and para attack have resonance forms with tertiary carbocations, they are more stable than the sigma complex for nitration of benzene. Therefore, the ortho and para positions of toluene react faster than benzene. 

Draw all the resonance forms of the sigma complex for nitration of bromobenzene at the ortho, meta, and para positions. Point out why the intermediate for meta substitution is less stable than the other two. 

The stability of the sigma complex is the guiding principle when determining the site of electrophilic attack on any aromatic ring. The more approximately equal energy resonance structures the carbocation has, generally the more stable it is. Resonance forms that place the positive charge next to an ewg are poor and should not be counted. 

The more stable the intermediate, the lowpr th S greater stabilization to this sigma complex... 

Because electron-withdrawing groups would destabilize an adjacent carboca-tion, addition of the electrophile to the meta position gives a more stable sigma complex intermediate. Since meta is the major product obtained, electron-withdrawing groups are known as meta directors. ... 

Ortho attack and para attack are preferred because each of these pathways involves a sigma complex with four resonance structures (shown below). Attack at the meta position involves formation of a sigma complex with only three resonance structures, which is not as stable as a sigma complex with four resonance structures. The reaction wiU proceed more rapidly via the lower energy sigma complex, so attack takes place at the ortiio and para positions in preference to the meta position. 

When compound 2 undergoes an electrophilic aromatic substitution reaction, the intermediate sigma complex exhibits a positive charge that is spread over three carbon atoms and three oxygen atoms. This intermediate is more stable than the intermediate formed when compound 1 undergoes an electrophilic aromatic substitution reactioa It is therefore expected that compound 2 wiU lose tritium at a faster rate than compound 1. 

The other elements of group 10 and gold do not form stable binary carbonyls. The stabilization of carbonyl complexes of these elements requires the introduction of other ligands, preferably sigma-bonded, that is the compounds [Pd(CO)Cl2] , [Pt(CO)2Cl2] and [Au(CO)Cl] are relatively stable. 

Loosely bound aggregates (chemical effects) are formed with the hydrocarbons acting as electron donors (Lewis base) and the solvents acting as electron acceptors (Lewis acid). The hydrocarbon that forms the most stable complex with the solvent experiences a decrease in volatility. Electron donors are rated by ionization potential, and electron acceptors are rated by their electron affinities. The selectivity will be higher, the larger the difference in ionization potential between the hydrocarbons and the larger the electron affinity of the solvent (9). While data on ionization potentials of hydrocarbons can be found (15, 16), electron affinities data are rare because of difficulties in their experimental determination. Prausnitz and Anderson (8) recommend that the sigma scale, proposed by Hammett (17), be used to determine approximately the solvents relative ability to form complexes with the two hydrocarbons. Attempts by this author, however, to use this scale were not conclusive. Prausnitz and Anderson (8) should be consulted to understand better the physical and chemical effects. 

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