CONTINUATION OF SUBSTITUTION; ELIMINATION

Solvent Effects upon SN1 and SN2 Reactions.

The transition state of the rate determining step for the SN1 reaction has developed ionic character and is well on the way (see Hammond Postulate)to forming a carbocation and a leaving group anion. Thus, the TS has carbocation and anionic character. Since ions benefit greatly from solvation by polar solvents, it is clear that an increase in the solvent polarity will yield an increased stabilization of the TS (compared to the energy of the reactant halide, which is moderately polar, but not ionic) , thus greatly increasing the rate. Thus, SN1 reactions occur especially rapidly in polar solvents like water, methanol, ethanol, or acetic acid.


Solvent Effects upon SN1 Reactions.

The situation with SN2 reactions is more complex. In the familiar type of SN2 reaction in which the nucleophile (a reactant) is negatively charged, solvation of the nucleophile by a polar solvent would tend to greatly slow the reaction rate (stabilization of the reactants increases the activation energy). However, the TS is also anionic, and is similarly stabilized by a polar solvent. So the net effect is not so large as in the SN1 case, but there is still a significant effect in the direction of slowing the reaction in a more polar solvent. This must mean that the reactant anion is more stabilized by the solvent than is the TS. This is fundamentally because the TS is a much larger anion, with the negative charge smeared out over a greater volume. The larger an ion, in general, the less efficiently it is solvated. Nevertheless, it is important to note that in such SN2 reactions an at least relatively polar solvent is required in order to dissolve the anionic nucleophile (as a salt).



 


SOLVENT EFFECTS UPON NUCLEOPHILICITY

 

 

 

 


NUCLEOPHILIC POWER (NUCLEOPHILICITY).

We have seen that the potency of a given anionic nucleophile can be sharply increased by assuring that the anion is not stabilized by hydrogen bonding (using a dipolar, aprotic solvent). For a series of different nucleophiles, obviously, the potency as a nucleophile, which would be measured by the relative rates of reaction with a standard alkyl halide like methyl iodide, can also vary quite widely. The nucleophilicity depends on a number of different factors, including:

 

 

Polarizability of the Nucleophile at the Reactive Atom of the Nucleophile.

 

 


ELIMINATIONS

 

 

 

 

THE E2 REACTION


THE TS MODEL FOR THE E2 REACTION:


REGIOCHEMISTRY OF E2 ELIMINATIONS.

 

 

 

 

 

COMPETING REGIOCHEMISTRIES IN E2 REACTIONS


METHOD OF COMPETING TRANSITION STATES




STEREOCHEMISTRY OF E2 ELIMINATIONS.

 

 


COMPETITION BETWEEN THE E2 AND SN2 REACTIONS