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Vinylogous reactivity is the behavior of a vinyl
Vinyl

A vinyl compound is any organic compound that contains a vinyl group , −CarbonHydrogenCovalent bondCH2. These are derivatives of ethene, CH2=CH2, with one hydrogen atom replaced with some other group....
 group in conjugation
Conjugation

Conjugation may refer to:*Grammatical conjugation, the modification of a verb from its basic form, including:**Latin conjugation**Spanish conjugation...
 with an electron-withdrawing group analogous to the reactions of the electron-withdrawing group itself; typically, this is a carbonyl
Carbonyl

In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double bond to an oxygen atom : C=O.The term carbonyl can also refer to carbon monoxide as a ligand in an inorganic or organometallic complex ; in this situation, carbon is triple-bonded to oxygen : C=O....
 group. Vinylogous reactions are found when the pi orbitals of the vinyl group and the electron-withdrawing group (EWG) are aligned and conjugated, enabling the EWG to receive electrons through the conjugated system.






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Vinylogous reactivity is the behavior of a vinyl
Vinyl

A vinyl compound is any organic compound that contains a vinyl group , −CarbonHydrogenCovalent bondCH2. These are derivatives of ethene, CH2=CH2, with one hydrogen atom replaced with some other group....
 group in conjugation
Conjugation

Conjugation may refer to:*Grammatical conjugation, the modification of a verb from its basic form, including:**Latin conjugation**Spanish conjugation...
 with an electron-withdrawing group analogous to the reactions of the electron-withdrawing group itself; typically, this is a carbonyl
Carbonyl

In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double bond to an oxygen atom : C=O.The term carbonyl can also refer to carbon monoxide as a ligand in an inorganic or organometallic complex ; in this situation, carbon is triple-bonded to oxygen : C=O....
 group. Vinylogous reactions are found when the pi orbitals of the vinyl group and the electron-withdrawing group (EWG) are aligned and conjugated, enabling the EWG to receive electrons through the conjugated system. This reactivity is seen in vinylogous carboxylic acids, which have an enol
Enol

Enols are alkenes with a hydroxyl group affixed to one of the carbon atoms composing the double bond. Enols and carbonyl compounds are in fact isomers; this is called keto-enol tautomerism:...
 right next to and in conjugation with the carbonyl group, for example in the enol form of acetylacetone
Acetylacetone

Acetylacetone is an organic compound with molecular formula C5H8O2. This diketone is formally named 2,4-pentanedione....
. Vinylogous reactions are for example conjugate addition, where the electrophile reacts at the vinyl, and the vinylogous aldol reaction (a variation of the aldol reaction
Aldol reaction

The aldol reaction is a carbon-carbon bond formation chemical reaction in organic chemistry. In its usual form, it involves the nucleophilic addition of a ketone enolate to an aldehyde to form a Hydroxy ketone, or "aldol" , a structural unit found in many biomolecule and pharmaceuticals....
), where the attacking nucleophile is a vinylogous enolate, attacking with its vinylic ? position rather than its a position like a normal enolate .

Allylic nucleophiles often react by vinylogous nucleophilic addition instead of direct addition; this is called allylic rearrangement
Allylic rearrangement

An allylic rearrangement or allylic shift is an organic reaction in which the double bond in an allyl chemical compound shifts to the next carbon atom....
.

A classical explanation is resonance
Resonance (chemistry)

Resonance in chemistry is a key component of valence bond theory used to graphically represent and mathematically model certain types of molecular structures when no single, conventional Lewis structure can satisfactorily represent the observed structure or explain its properties....
, where the electrons of the double bond move to connect to the carbon geminal to the electron-withdrawing group. A common example of a vinylogous carboxylic acid is ascorbic acid
Ascorbic acid

Ascorbic acid is a sugar acid with antioxidant properties. Its appearance is white to light-yellow crystals or powder. It is water-soluble. The L-enantiomer of ascorbic acid is commonly known as vitamin C....
 (vitamin C). The enol double bond can resonate
Resonance (chemistry)

Resonance in chemistry is a key component of valence bond theory used to graphically represent and mathematically model certain types of molecular structures when no single, conventional Lewis structure can satisfactorily represent the observed structure or explain its properties....
 between the two possible positions. (In exact terms, it is delocalized between the two positions.)
Ascorbic Acidity3
The related term vinylogy has been defined as the transmission of electronic effect
Electronic effect

An electronic effect influences the chemical structure, reactivity, or chemical properties of molecule but is neither a traditional Chemical bond nor a steric effect....
s through a conjugate system . This concept was introduced in 1926 by Ludwig Claisen when he explained acidic properties of formylacetone and related beta-keto aldehydes .