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Concept
Vinylogy
Summary
In organic chemistry, vinylogy is the transmission of electronic effects through a conjugated organic bonding system. The concept was introduced in 1926 by Ludwig Claisen to explain the acidic properties of formylacetone and related ketoaldehydes. Formylacetone, technically , only exists in the ionized form or . Its adjectival form, vinylogous, is used to describe functional groups in which the standard moieties of the group are separated by a carbon–carbon double bond. For example, a compound with a carbon-carbon double bond (, a "vinyl" moiety; actually a vinylene group) between a carbonyl group () and a hydroxyl group () is referred to as a vinylogous carboxylic acid. The figure to the right shows two resonance forms of the anion.
Due to the transmission of electronic information through conjugation, vinylogous functional groups often possess "analogous" reactivity or chemical properties compared to the parent functional group. Hence, vinylogy is a useful heuristic for the prediction of the behavior of systems that are structurally similar but contain intervening C=C bonds that are conjugated to the attached functional groups. For example, a key property of carboxylic acids is their Brønsted acidity. The simplest carboxylic acid, formic acid (), is a moderately strong organic acid with a pKa of 3.7. We would expect vinylogous carboxylic acids to have similar acidity. Indeed, the vinylog of formic acid, 2-formyl-1-ethen-1-ol, has a substantial Brønsted acidity, with an estimated pKa ~ 5–6. In particular, vinylogous carboxylic acids are substantially stronger acids than typical enols (pKa ~ 12). Vitamin C (ascorbic acid, see below) is a biologically important example of a vinylogous carboxylic acid.
The insertion of a o- or p-phenylene (i.e., a benzene ring in the 1,2- or 1,4-orientation) also results in some similarities in reactivity (called "phenylogy"), although the effect is generally weaker, as conjugation through the aryl ring requires consideration of resonance forms or intermediates in which aromaticity is disrupted.
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