Sigma-substitution mechanism

Once formed, the electrophile behaves like any other electrophile, so the mechanism of the attack is the same as that for the previous situation where a nucleophile attacked the electrophile (described in the earlier section Basics of Electrophilic Substitution Reactions ). The attack leads to the formation of the resonance-stabilized sigma complex, followed by the loss of a hydrogen ion to a base. 

The magnitude of the solvent isotope effect and the absence of a carbon isotope effect confirm that proton transfer is rate-determining in the reactions referring to s. As far as the reactions referring to are concerned, the experimental values of these rate coefficients for the decarboxylation of 2- and 4-aminobenzoic acids, as well as the Arrhenius parameters, are comparable to those of the substituted salicylic acids if expected substituent effects are taken into account (Table 21) there is a correlation between log A and Ea. Therefore, it is reasonable to expect that the mechanism is the same. The observed general catalysis supplies additional evidence for rate-determining proton transfer from H30+ to S (sigma complex formation) in the decarboxylation of 4-aminobenzoic acid. 

If the radical is formed by the loss of an electron from a given a bond, such as a bond to a highly substituted carbon, the cleavage at this location will be favored sigma electron ionization mechanism). 

The negative sigma coefficient in eq 26 indicates that electron withdrawing groups are unfavorable. But it isn t clear from this equation that this is due to an unfavorable effect on distribution and that the a component relating to mechanism is positive (i.e. a +a corresponds to a -pKg term in eq 25). ortho-Derivatives required an E term. Since ortho substitution also reduces acidity, the true influence of steric factors on uncoupling, separated from influences on acidity-distribution, can only be determined by using E with log D. 

Media pH errors and media pH span errors are common. Since electrophilic aromatic substitution is almost exclusively an acidic media process, do not make any strong bases during the mechanism. The proton on the aromatic ring becomes very acidic after the electrophile attaches and forms the carbocationic sigma-complex. However, before the electrophile attacks, the aromatic H is not acidic at all, p Ta = 43, so do not get your steps out of order and try to pull the H off first. 

Now let s back off from the problem far enough to see the entire process as a whole (Fig. 10.9). The mechanism for this transformation or mechanistic sentence can be made from our mechanistic phrases or path combinations A proton transfer from the acid catalyst improves the leaving group, then an SnI substitution followed by a Dg rearomatization step that sets the alcohol up for a second SnI substitution, ending with a second Dg rearomatization step. The only alternative route is to use solvent to shuttle the proton from the sigma-complex to the alcohol. 

While the addition of water to the sigma complex can be shown in a reasonable mechanism, the product is not aromatic. Thus, it has lost the 152 kJ/mol (36 kcal/mole) of resonance stabilization energy. The addition reaction is not favorable energetically, and substitution prevails. 

Characteristically, electrophilic substitution, which was once thought to proceed by a rather simple mechanism, turns out to be quite a complex reaction. A key development in understanding mechanistic pathways has been the studies involving kinetic isotope effects. Much of this work indicated a two-step reaction process with the implied formation of an intermediate but the use of isotopes has also added additional complexities to the interpretation of electrophilic substitution. In addition to the kinetic isotope effect, studies on the formation, stability, structure, and reactions of sigma and pi complexes have also shed light on the role of intermediates in the substitution process. 

Plietker B, Dieskau A, Moews K, Jatsch A (2008) Ligand-dependent mechanistic dichotomy in iron-catalyzed allylic substitutions sigma-allyl versus pi-allyl mechanism. Angew Cbem Int Ed 47 198-201... 

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