Homologation reaction

A homologation reaction, also known as homologization, is any chemical reaction that effects an overall increase of the carbon skeleton of a saturated reactant molecule. The reactants undergo a homologation converting them into the next member of the homologous series. For example the reaction of aldehydes and ketones with diazomethane or methoxymethylenetriphenylphosphine effectively inserts a methylene (-CH2-) unit in the hydrocarbon chain and the reaction product is the next homologue.

Examples of homologation reactions include:


 * Seyferth-Gilbert homologation
 * Displacement of a halide by a cyanide group, which can be reduced to an amine
 * Kiliani-Fischer synthesis, where an aldose molecule is elongated through a three-step process consisting of:
 * Nucleophillic addition of cyanide to the carbonyl to form a cyanohydrin
 * Hydrolysis to form a lactone
 * Reduction to form the homologous aldose
 * Wittig reaction of an aldehyde with methoxymethylenetriphenylphosphine, which produces a homologous aldehyde.
 * Arndt-Eistert synthesis is a series of chemical reactions designed to convert a carboxylic acid to a higher carboxylic acid homologue (ie. contains one additional carbon atom)
 * Kowalski ester homologation, an alternative to the Arndt-Eistert synthesis. Has been used to convert β-amino esters from α-amino esters through an ynolate intermediate.

Some reactions increase the chain length by more than one unit. For example, the following are considered two-carbon homologation reactions:


 * Nucleophilic addition to ethylene oxide, resulting in a ring-opening and producing a primary alcohol with two extra carbons.
 * Malonic ester synthesis, which produces a carboxylic acid with two extra carbons.