In organic chemistry, the Knoevenagel condensation (pronounced[ˈknøːvənaːɡl̩]) reaction is a type of chemical reaction named after German chemist Emil Knoevenagel. It is a modification of the aldol condensation.[1][2]
A Knoevenagel condensation is a nucleophilic addition of an active hydrogen compound to a carbonyl group followed by a dehydration reaction in which a molecule of water is eliminated (hence condensation). The product is often an α,β-unsaturated ketone (a conjugated enone).
General Knoevenagel layout
In this reaction the carbonyl group is an aldehyde or a ketone. The catalyst is usually a weakly basic amine. The active hydrogen component has the form[3]
Z−CH2−Z or Z−CHR−Z for instance diethyl malonate, Meldrum's acid, ethyl acetoacetate or malonic acid, or cyanoacetic acid.[4]
Z−CHRR', for instance nitromethane.
where Z is an electron withdrawing group. Z must be powerful enough to facilitate deprotonation to the enolate ion even with a mild base. Using a strong base in this reaction would induce self-condensation of the aldehyde or ketone.
The Hantzsch pyridine synthesis, the Gewald reaction and the Feist–Benary furan synthesis all contain a Knoevenagel reaction step. The reaction also led to the discovery of CS gas.
^Jones, G. Org. React.1967, 15.
^Emil Knoevenagel (1898). "Condensation von Malonsäure mit aromatischen Aldehyden durch Ammoniak und Amine" [Condensation of malonic acid with aromatic aldehydes via ammonia and amines]. Berichte der deutschen chemischen Gesellschaft. 31 (3): 2596–2619. doi:10.1002/cber.18980310308.
^March, Jerry (1985), Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 3rd edition, New York: Wiley, ISBN 9780471854722, OCLC 642506595
^G. Jones (2004). "The Knoevenagel Condensation". Organic Reactions. pp. 204–599. doi:10.1002/0471264180.or015.02. ISBN 0471264180.
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