Dipole rate-determining

I > Br > Cl > F. In nucleophilic aromatic substitution, the formation of the addition intermediate is usually the rate-determining step so the ease of C—X bond breaking does not affeet the rate. When this is the ease, the order of reactivity is often F > Cl > Br > I. This order is the result of the polar effeet of the halogen. The stronger bond dipoles assoeiated with the more eleetronegative halogens favor the addition step and thus inerease the overall rate of reaetion. 

The evidence for the involvement of 1,3-dipoles as discrete intermediates includes the observation that the reaction rates are independent of dipolarophile concentration. This fact indicates that the ring opening is the rate-determining step in the reaction. Ring opening is most facile for aziridines that have an electron-attracting substituent to stabilize the carbanion center in the dipole. 

The solvent reorganization term reflects the changes in solvent polarization during electron transfer. The polarization of the solvent molecule can be divided into two components (1) the electron redistribution of the solvent molecules and (2) the solvent nuclear reorientation. The latter corresponds to a slow and rate-determining step involving the dipole moments of the solvent molecules that... 

This microwave effect is consistent with the fact that the rate-determining step may be certainly the generation of aldimine A from aldehydes and 2-amino 3-pico-line. It is in agreement with the transition state develops a dipole and, consequently more polar than the ground state. To ascertain this hypothesis, it was effectively next shown that the reaction involving reaction with preformed aldimine A, no revealed any microwave effect [119]. 

Because r is larger than r then a linear plot ofln k vs. 1 /eR with a positive slope will be obtained irrespective of the charge on the ion. The observation3 that a positive slope of log/c vs. l/sR was obtained for the oxidation of triphenylmethane and diphenylmethane by [Fe(CN)6]3 in various mixtures of aqueous acetic acid was taken as proof that the rate determining step was of the ion dipole type, i.e. involved [Fe(CN)6]3- and the arylalkane molecule. 

Criteria for determining whether an intermediate is formed in simple reactions previously thought to occur in one step have been suggested.125 A mechanism that requires the formation of an intermediate ion-dipole complex in a rapid pre-equilibrium step before the rate-determining substitution is proposed for the XN2 reaction between 4-nitrophenoxide ion and methyl iodide. 

In the end, the stereochemistry of the reacting dipole is determined by the relative rates of the reversible ring closure of the iminium carboxylate betaine into 2,4-trans-5-oxazolidines (kj), the decarboxylative ylide generation (kj), the isomerization of anti-ylides into syn-ylides (kj), and cycloaddition trappings 1(4 and kj), as shown in Eq. (23). 

Several explanations have been offered for the preference of amination in the 2-position, If aromatization of the intermediate is the rate-determining step, this behavior may be due to the fact that u-adduct 108 loses a hydride ion more readily than 109 (Scheme 43). This argument is weakened by the lack of a kinetic isotope effect in the Chichibabin amination (65CJC725). Alternatively, the amide ion may be attracted to the 2-position by an ion-dipole interaction with the 3-substituent in the addition step. For a substrate like 3-dimethylami-nopyridine (110), this is a very significant interaction (113), leading exclusively to the 2-amino product (Scheme 44) (73CHE1119). 

To explain this result, it may be assumed that the transrmination by reaction of the starting imine, 1, with 2-amino-3-picoline, 2, might be the rate-determining step. In such a situation, the transition state is expected to be more polar and, therefore, sensitive to MW dipole-dipole stabilization. The qualitative polarities between ground and transition states have been determined by PM3 computations, and clearly showed the enhancement of the polarity during the process (Eq. 19) ... 

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