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Solvolysis

Solvolysis is a special type of nucleophilic substitution or elimination where the nucleophile is a solvent molecule. Many SN reactions are carried out using the solvent as the nucleophilic agent. They are called solvolysis reactions and involve solvents such as water, ethanol, ethanoic acid, and methanoic acid. Two examples are

 


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In these examples, solvolysis is necessarily a first-order reaction, because normally the solvent is in such great excess that its concentration does not change appreciably during reaction, and hence its contribution to the rate does not change. (This point will be much clearer if you work Exercise 8-4e.) However, that the overall rate is first order does not mean the reaction necessarily proceeds by an SN1 mechanism, particularly in solvents such as water, alcohols, or airlines, which are reasonably good nucleophilic agents. The solvent can act as the displacing agent in an SN2 reaction.

To distinguish between SN1 and SN2 mechanisms of solvolysis requires other criteria, notably stereochemistry (Sections 8-5 and 8-6), and the effect of added nucleophiles on the rate and nature of the reaction products. For example, it often is possible to distinguish between SN1 and SN2 solvolysis by adding to the reaction mixture a relatively small concentration of a substance that is expected to be a more powerful nucleophile than the solvent. If the reaction is strictly SN1, the rate at which RX disappears should remain essentially unchanged because it reacts only as fast as R® forms, and the rate of this step is not changed by addition of the nucleophile, even if the nucleophile reacts with R®. However, if the reaction is SN2, the rate of disappearance of RX should increase because RX reacts with the nucleophile in an SN2 reaction and now the rate depends on both the nature and the concentration of the nucleophile.

For certain nucleophiles, there are specific terms for the type of solvolysis reaction for instance, water, the term is hydrolysis; for alcohols, it is alcoholysis; for ammonia it is ammonolysis; for glycols,glycolysis; for amines, amynolysis.

 

SOLVOLYSIS OF TERTIARY ALKYL HALIDES

Substitution: Nucleophilic, Bimolecular (SN2):

 

 

Substitution: Nucleophilic, Unimolecular (SN1):

 

This mechanism involves more than one step, the first of which is the ionization of the alkyl halide substrate, yielding a carbocation and an anion. The planar carbocation then reacts with a nucleophile, forming the product (in the case of a chiral substrate, racemic products generally result). Since the nucleophile is often a solvent molecule, the process is sometimes called solvolysis. A typical SN1 reaction is the solvolysis of t-butyl chloride in water to form t-butyl alcohol. Equation (1) above represents the breaking of a C-X bond, the first step in the solvolysis process. This bond-breaking process possess the highest activation energy of the two steps involved and thus has the slowest rate of reaction. This represents the rate determining step in that the overall two step reaction can only proceed as fast as this step. Since this step only involves the presence of the original substrate molecule, the reaction is considered unimolecular.

Some factors will have an effects on the rate of solvolysis of a 3° alkyl halide substrate molecule such kinetics and rate laws will be discuss later.

 

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