Charge types, in nucleophilic

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). 

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. 

There is evidence, both experimental and theoretical, that there are intermediates in at least some Sn2 reactions in the gas phase, in charge type I reactions, where a negative ion nucleophile attacks a neutral substrate. Two energy minima, one before and one after the transition state, appear in the reaction coordinate (Fig. 10.1). The energy surface for the Sn2 Menshutkin reaction (p. 499) has been examined and it was shown that charge separation was promoted by the solvent.An ab initio study of the Sn2 reaction at primary and secondary carbon centers has looked at the energy barrier (at the transition state) to the reaction. These minima correspond to unsymmetrical ion-dipole complexes. Theoretical calculations also show such minima in certain solvents, (e.g., DMF), but not in water. "... 

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. 

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... 

No matter what the charge of the rr-adduct formed, we feel it is more essential to look at the charge type of the attachment reaction by which the adduct is formed. Thus ff-adduct formation using nucleophilic reagents usually belongs in the following charge types (Eqs. 1-4). 

An alternative explanation is based on the charge type of the reaction. The zwitterions which are initially formed on reaction with amines show two characteristic features (a) The oonformers (112) and (114) with eclipsed nucleophile and electron pair have extra stabilization resulting from the interaction of opposite charges, (b) An ammonium proton is available in the vicinity of the negative charge. 

Besides spectroscopic methods, quenching processes have been utilized to differentiate between various types of radical ion pairs, too. These chemical methods make use of the different reactivities of CIP and SSIP which are caused by the unequal solvation and distance of the charged species in the ion pairs. Depending on the ambivalent character of radical ions, these intermediates may be scavenged either by electron transfer quenchers (Q) or by nucleophilic and electrophilic scavengers (Scheme 7 and Eqs. (5—7)). 

---