Charge types in nucleophilic substitution

Charge Types In Nucleophilic Substitution a. When the Nucleophile Is Anionic, the transition state involves dispersal of the negative charge, and one factor in its free energy will be lowest when this charge is distributed over the atoms most widely separated in space (33 vs. 35). The benefit of charge dispersal should... 

The state of the art and theory of bimolecular nucleophilic substitution (Sn2) at carbon may be summarized not all processes that go with inversion are SN2, but nearly all SN2 processes go with inversion. Over the past thirty years, Hughes, Ingold et al., took the view that the exclusion principle would always defeat electrostatics and produced a series of examples of various charge types in which SN2 processes went with inversion (Harvey et cd., 1960 Hoffmann and Hughes, 1964). 

Ambident anions are mesomeric, nucleophilic anions which have at least two reactive centers with a substantial fraction of the negative charge distributed over these cen-ters ) ). Such ambident anions are capable of forming two types of products in nucleophilic substitution reactions with electrophilic reactants . Examples of this kind of anion are the enolates of 1,3-dicarbonyl compounds, phenolate, cyanide, thiocyanide, and nitrite ions, the anions of nitro compounds, oximes, amides, the anions of heterocyclic aromatic compounds e.g. pyrrole, hydroxypyridines, hydroxypyrimidines) and others cf. Fig. 5-17. 

Compound (4) reacts with halides, preferably their alkylammonium salts, in acetone or DME with exchange of one CO for a halide, giving anionic complexes of the type Co(CO)2(NO)X (X = Cl, Br, I, CN) (equation 29). Owing to the high charge density at the metal, these anions can act as sources for the hyponitrous ion, NO, which is transferred in nucleophilic substitutions at activated (benzyl) halides, giving aldoximes (e.g. PhCH=NOH from PhCH2Y) (equation 30). ... 

Since charge effects are also important in affecting substitution rates, a recent suggestion has been made that the complex [PtCl(NH3)en]" be used as a reference for n%i with complexes of this charge type. In particidar, before comparison can be made it is emphasized that kinetic data must be collected under the same ionic strength conditions, and care must be exercised when fitting data for biphilic nucleophiles. 

Y is a strongly pi electron donor group. As previously noted in the results section, examples of Y from Table VI include centers of high pi electron charge density at carbon, sulfur, nitrogen, and oxygen. Also included in Table VI are examples of nucleophilic substitution transition states (cf. reactions 21 and 22) of the type... 

It is interesting to note in the latter connection that nucleophilic substitution transition states in which there apparently is not strong delocalization of pi electron density into the substituent tend to fall into the <7 type (cf. reactions 18 and 19 of Table VIII). In set 18, there are two ortho nitro groups which apparently take up much of the pi charge (thus it is unavailable to X), whereas in set 19, the positive piperidinium center may cause (perhaps with assistance from the NH hydrogen bonding permitted by the aprotic solvent) the... 

We can also describe the differences between these reaction types in terms of Pearson s hard-soft description (Pearson, 1966 Pearson and Songstad, 1967). Cationic micellar head groups interact best with soft bases, e.g. relatively large anions of low charge density such as bromide or arenesulfo-nate, or anionic transition states such as those for nucleophilic aromatic substitution. They interact less readily with hard bases, e.g. high charge density anions such as OH ", or anionic transition states for deacylation. 

Chlorinated cyclopenta-l,2,3-dithiazoles such as (105) and (125) (Section 4.11.8.5) are susceptible to nucleophilic substitution (e.g. Equation (13a))) <93JCS(P1)769>, whereas chlorinated cyclohepta-1,2,3-dithiazoles of the type (127)-(129) (Section 4.11.8.5) do not react with nucleophiles. This difference may be explained by the opposite charge separation between fused cycles in the two... 

Many other solvent parameters have been defined in an attempt to model as thoroughly as possible solvent effects on the rate constants for solvolysis. These include (a) Several scales of solvent ionizing power Tx developed for different substrates R—X that are thought to undergo limiting stepwise solvolysis. (b) Several different scales of solvent nucleophilicity developed for substrates of different charge type that undergo concerted bimolecular substitution by solvent. (c) An... 

By the same token that aza substituents retard electrophilic substitution, so they accelerate nucleophilic substitution,19,20 40 41 particularly when positively charged. In an interesting study based on this type of reactivity, the equilibrium 16 = 17 has been investigated,85 and this and the rate of subsequent ring opening leading to substituted anils of glutaconic aldehyde found to correlate with o. ... 

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