Solvolysis

Solvolysis is a type of nucleophilic substitution (S_N1) or elimination where the nucleophile is a solvent molecule.^[1]

Mechanism

Solvolysis of a chiral reactant results in the racemate (as expected in S_N1) but is often accompanied by Walden inversion. This is explained by postulating an intimate ion pair whereby the leaving anion is still in proximity to the carbocation and effectively shielding it from an attack by the nucleophile.

Forms

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

Hydrolysis

While solvolysis often refers to an organic chemistry context, hydrolysis is very common in inorganic chemistry, where aqua complexes of metal ions react with solvent molecules due to the Lewis acidity of the metal center. For example, aqueous solutions of aluminium chloride are acidic due to the aqua-aluminium complex losing protons to water molecules, giving hydronium ions which lowers the pH.

In organic chemistry, hydrolysis reactions often give two fragments from an initial substrate. For example, the hydrolysis of amides give carboxylic acids and amines; the hydrolysis of esters give alcohols and carboxylic acids.

Alcoholysis

An example of a solvolysis reaction is the reaction of a triglyceride with a simple alcohol such as methanol or ethanol to give the methyl or ethyl esters of the fatty acid, as well as glycerol. This reaction is more commonly known as a transesterification reaction due to the exchange of the alcohol fragments.^[2]

Ammonolysis

Ammonolysis refers to solvolysis by ammonia but can also describe nucleophilic attack by ammonia more generally. Ammonia boils at −33°C and so it rarely used as a solvent in its pure form, it is however readily miscible with water and is commonly used in the form of a saturated aqueous solution. For this reason ammonolysis may be considered a special case of solvolysis, as the ammonia is itself dissolved in a solvent. Despite this the reactions are usually highly selective, due to the greater nucleophilicity of ammonia compared to water.

In Recycling

Researchers are experimenting with ways to use solvolysis in recycling. As an example, solvolysis is used to recover carbon fibers from end-of-life carbon fiber parts.^[3]

See also

• Electrolysis
• Pyrolysis

References