Vinylogy

In organic chemistry, vinylogy is the transmission of electronic effects through a conjugated organic bonding system.^[1] The concept was introduced in 1926 by Ludwig Claisen to explain the acidic properties of formylacetone and related ketoaldehydes. Formylacetone, technically CH₃(C=O)CH₂CH=O, only exists in the ionized form CH₃(C−O⁻)=CH−CH=O or CH₃(C=O)−CH=CH−O⁻.^[2] 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 carboxylic acid is defined as a carbonyl group (C=O) directly attached to a hydroxyl group (OH): O=C–OH. A vinylogous carboxylic acid has a vinyl unit (−HC=CH−, vinylene) between the two groups that define the acid: O=C–C=C–OH. The usual resonance of a carboxylate can propagate through the alkene of a vinylogous carboxylate. Likewise, 3-dimethylaminoacrolein is the vinylogous-amide analog of dimethylformamide.

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 (HC(=O)−OH), is a moderately strong organic acid with a pK_a 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, HC(=O)−CH=CH−OH has a substantial Brønsted acidity, with an estimated pK_a ~ 5–6. In particular, vinylogous carboxylic acids are substantially stronger acids than typical enols (pK_a ~ 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.^[3][4]

Vinylogous reactions are believed to occur when orbitals of the double bonds of the vinyl group and of an attached electron-withdrawing group (EWG; the π orbitals) are aligned and so can overlap and mix (i.e., are conjugated). Electron delocalization enables the EWG to receive electron density through participation of the conjugated system.