Allyl Cation
Allyl Cation - Related Videos
01:18
π Molecular Orbitals of the Allyl Cation and Anion
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2025
An allyl group is a three-carbon conjugated system where the sp³-hybridized allylic carbon is bonded to a CH=CH2 group via a single bond. Allyl anions can be obtained by treating propene with a strong base that can deprotonate methyl groups. Allyl cations are formed as intermediates during substitution reactions involving allylic halides. In both cases, the hybridization of the allylic carbon changes from sp3 to sp2, giving rise to a carbon chain with three sp2-hybridized carbons, each with an...
06:46
Facile Preparation of (2Z,4E)-Dienamides by the Olefination of Electron-deficient Alkenes with Allyl Acetate
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Cited by 1 •
2017
The ruthenium-catalyzed olefination of electron-deficient alkenes with allyl acetate is described here. By using aminocarbonyl as a directing group, this external oxidant-free protocol has high efficiency and good stereo- and regioselectivity, opening a novel synthetic route to (Z,E)-butadiene skeletons.
01:27
π Molecular Orbitals of the Allyl Radical
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2025
Allyl radicals are three-carbon conjugated systems. They are readily formed as intermediates in halogenation reactions of alkenes involving the addition of halogen to the allylic carbon instead of the double bond. As seen in allyl cations and anions, each of the three sp2-hybridized carbon atoms in allyl radicals has an unhybridized p orbital. These orbitals combine to give three π molecular orbitals. The allyl systems have identical molecular orbitals but differ in the number of π electrons.
01:31
Radical Substitution: Allylic Chlorination
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2023
Typically, when alkenes react with halogens at low temperatures, an addition reaction occurs. However, upon increasing the temperature or under reaction conditions that form radicals, providing a low but steady concentration of halogen radicals, allylic substitution reaction is favored. This is because allylic hydrogens are very reactive as the formed intermediate is resonance stabilized. For example, when propene is treated with chlorine in the gas phase at 400 °C, it undergoes allylic...
01:27
Radical Substitution: Allylic Bromination
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2025
In organic synthesis, the formation of products can be altered by changing the reaction conditions. For example, a dibromo addition product is formed when propene is treated with bromine at room temperature. In contrast, propene undergoes allylic substitution in non-polar solvents at high temperatures to give 3-bromopropene. In order to avoid the addition reaction, the bromine concentration must be kept as low as possible throughout the reaction. This can be achieved using N-bromosuccinimide...
FAQs
The three carbon atoms can adopt overlapping p orbitals that create a continuous conjugated framework. This arrangement allows π electrons and electron deficiency to spread across the system instead of remaining concentrated on one carbon. The resulting delocalization lowers the effect of a localized positive charge and explains why the allyl cation is a useful model for understanding intermediate stability.
Delocalization changes how the electron deficiency is represented within the three-carbon framework, so reaction behavior cannot be interpreted by considering only one carbon. This feature helps explain why allylic substrates can show distinctive regioselectivity and product distributions. It also provides a mechanistic basis for analyzing substitution, elimination, and rearrangement reactions involving leaving groups.
A localized carbocation places the electron deficiency primarily on one carbon, whereas the allyl cation distributes that deficiency across a conjugated three-carbon system through p-orbital overlap. This difference is chemically important because the charge distribution affects stability and helps account for reaction pathways that would be less readily explained by a single fixed positive-charge location.
A useful analysis begins by identifying the three-carbon allylic framework and considering how p-orbital overlap distributes electron deficiency across it. Chemists can then evaluate which reaction positions are associated with the delocalized intermediate when interpreting a proposed pathway. This approach connects the cationic framework to regioselectivity and helps explain why products may form at different allylic positions.
Allylic halides, allylic alcohols, and other substrates containing suitable leaving groups are central examples. Their reactions can be discussed in terms of substitution, elimination, or rearrangement pathways that involve allylic cation character. Examining these substrate classes shows how the same delocalized framework can influence both intermediate formation and the distribution of products in organic synthesis.
The model helps chemists connect molecular structure with reaction outcomes. By accounting for charge delocalization across the allylic framework, it supports interpretation of reaction intermediates, regioselectivity, and product distributions. This makes the concept useful when analyzing how leaving-group-containing substrates behave and when comparing possible substitution, elimination, or rearrangement pathways during synthetic planning.