The Guerbet reaction, named after Marcel Guerbet (1861–1938), is an

organic reaction Organic reactions are chemical reactions involving organic compounds. The basic organic chemistry reaction types are addition reactions, elimination reactions, substitution reactions, pericyclic reactions, rearrangement reactions, Mechanistic Organ ...

that converts a primary

alcohol Alcohol most commonly refers to: * Alcohol (chemistry), an organic compound in which a hydroxyl group is bound to a carbon atom * Alcohol (drug), an intoxicant found in alcoholic drinks Alcohol may also refer to: Chemicals * Ethanol, one of sev ...

into its β-alkylated

dimer Dimer may refer to: * Dimer (chemistry), a chemical structure formed from two similar sub-units ** Protein dimer, a protein quaternary structure ** d-dimer * Dimer model, an item in statistical mechanics, based on ''domino tiling'' * Julius Dimer ( ...

with loss of one equivalent of water. The process is of interest because it converts simple inexpensive feedstocks into more valuable products. Its main disadvantage is that the reaction produces mixtures.

Scope and applications

The original 1899 publication concerned the conversion of ''n''-butanol to

2-ethylhexanol 2-Ethylhexanol (abbreviated 2-EH) is an organic compound with formula CHO. It is a branched, eight-carbon chiral alcohol (chemistry), alcohol. It is a colorless liquid that is poorly soluble in water but soluble in most organic solvents. It is prod ...

. 2-ethylhexanol is however more easily prepared by alternative methods (from

butyraldehyde Butyraldehyde, also known as butanal, is an organic compound with the formula CH3(CH2)2CHO. This compound is the aldehyde derivative of butane. It is a colorless flammable liquid with an unpleasant smell. It is miscible with most organic solvents. ...

). Instead, the Guerbet reaction is mainly applied to

fatty alcohol Fatty alcohols (or long-chain alcohols) are usually high-molecular-weight, straight-chain primary alcohols, but can also range from as few as 4–6 carbons to as many as 22–26, derived from natural fats and oils. The precise chain length varies ...

s to afford oily products, which are called Guerbet alcohols. They are of commercial interest to as components of cosmetics, plasticizers, and related applications. The reaction is conducted in the temperature range 180-360 °C, often in a sealed reactor. The reaction requires alkali metal hydroxides or

alkoxide In chemistry, an alkoxide is the conjugate base of an alcohol and therefore consists of an organic group bonded to a negatively charged oxygen atom. They are written as , where R is the organic substituent. Alkoxides are strong bases and, whe ...

Catalyst Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...

s such as Raney Nickel are required to facilitate the hydrogen transfer steps. While the Guerbet reaction is traditionally (and commercially) focused on fatty alcohols, it has been investigated for the dimerization of ethanol to butanol. Organometallic catalysts have been investigated. A small amount of the

diene In organic chemistry a diene ( ) (diolefin ( ) or alkadiene) is a covalent compound that contains two double bonds, usually among carbon atoms. They thus contain two alk''ene'' units, with the standard prefix ''di'' of systematic nomenclature. ...

'' 1,7-octadiene'' is required as a proton acceptor.

Mechanism

The

reaction mechanism In chemistry, a reaction mechanism is the step by step sequence of elementary reactions by which overall chemical change occurs. A chemical mechanism is a theoretical conjecture that tries to describe in detail what takes place at each stage of ...

for this reaction is a four-step sequence. In the first step the

oxidized Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...

to the

aldehyde In organic chemistry, an aldehyde () is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred to as an aldehyde but can also be classified as a formyl grou ...

. These intermediates then react in an

aldol condensation An aldol condensation is a condensation reaction in organic chemistry in which two carbonyl moieties (of aldehydes or ketones) react to form a β-hydroxyaldehyde or β-hydroxyketone (an aldol reaction), and this is then followed by dehydration to ...

to the allyl aldehyde which the hydrogenation catalyst then reduces to the alcohol. The

Cannizzaro reaction The Cannizzaro reaction, named after its discoverer Stanislao Cannizzaro, is a chemical reaction which involves the base-induced disproportionation of two molecules of a non-enolizable aldehyde to give a primary alcohol and a carboxylic acid. : ...

is a competing reaction when two

molecules react by

disproportionation In chemistry, disproportionation, sometimes called dismutation, is a redox reaction in which one compound of intermediate oxidation state converts to two compounds, one of higher and one of lower oxidation states. More generally, the term can b ...

to form the corresponding alcohol and

carboxylic acid In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an R-group. The general formula of a carboxylic acid is or , with R referring to the alkyl, alkenyl, aryl, or other group. Carboxylic ...

. Another side reaction is the

Tishchenko reaction The Tishchenko reaction is an organic chemical reaction that involves disproportionation of an aldehyde in the presence of an alkoxide. The reaction is named after Russian organic chemist Vyacheslav Tishchenko, who discovered that aluminium alko ...

References

{{Reflist

External links

* ''A Review of Guerbet Chemistry'' Anthony J. O’Lenick, Jr. https://web.archive.org/web/20110209074739/http://www.zenitech.com
Link
Condensation reactions Name reactions Fatty alcohols

Scope and applications

Mechanism

See also

References

External links