Allylic Hydrogen
Allylic Hydrogen - Related Videos
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.
06:06
Hydrogenation
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2023
Source: Vy M. Dong and Zhiwei Chen, Department of Chemistry, University of California, Irvine, CA This experiment will demonstrate the hydrogenation of chalcone as an example of an alkene hydrogenation reaction (Figure 1). In this experiment, palladium on carbon (Pd/C) will be used as a heterogeneous catalyst for the process. A balloon will be used to supply the hydrogen atmosphere. Figure 1: Diagram showing the hydrogenation of chalcone to 3-phenylpropiophenone.
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.
08:56
Catalytic Reactor: Hydrogenation of Ethylene
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2023
Source: Kerry M. Dooley and Michael G. Benton, Department of Chemical Engineering, Louisiana State University, Baton Rouge, LA The hydrogenation of ethylene (C2H4) to ethane (C2H6) has often been studied as a model reduction reaction in characterizing new metal catalysts.1-2 While supported nickel is not the most active metal catalyst for this reaction, it is active enough that reaction can take place at < 200°C. The reaction typically involves adsorbed, dissociated hydrogen (H2) reacting...
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...
FAQs
The key energetic advantage comes from resonance stabilization. After hydrogen abstraction, the unpaired electron can delocalize across the neighboring carbon-carbon π system rather than remaining confined to one carbon. This delocalization lowers the energy of the radical intermediate and helps explain why reactions that remove allylic hydrogen can proceed more readily than less stabilized radical-forming pathways.
Resonance distributes the radical character across the allylic framework, giving the intermediate more than one contributing electronic arrangement. That stabilization influences which reaction pathway is energetically accessible after C–H bond cleavage. As a result, the π system does more than provide unsaturation: it directly shapes the reactivity and subsequent functionalization of the molecule.
An allylic position can be favored during hydrogen abstraction because radical stabilization lowers the energy required for C–H bond cleavage. This preference helps direct reactions toward allylic sites and contributes to selective outcomes in allylic oxidation, radical bromination, and related functionalization processes. The resulting site preference is important when predicting which products may dominate.
Begin by locating the carbon-carbon double bond, then identify adjacent C–H bonds that could undergo abstraction. Next, consider whether cleavage would produce a radical stabilized by delocalization through the neighboring π system. Finally, relate that intermediate to the proposed oxidation, bromination, or other functionalization pathway to anticipate reaction direction and product distribution.
Its distinctive reactivity provides a way to modify unsaturated molecules through allylic functionalization. In synthesis, recognizing this behavior helps chemists anticipate reaction pathways and interpret the products formed during allylic oxidation or radical bromination. The concept therefore connects molecular structure with practical choices about how an unsaturated framework may be selectively transformed.
In materials chemistry, allylic C–H reactivity helps explain how unsaturated molecules may undergo chemical modification. Resonance-stabilized radical formation can influence the pathways available for functionalizing structures that contain carbon-carbon double bonds. Understanding these effects supports interpretation of reaction behavior in molecular materials and helps relate local bonding features to broader chemical transformation outcomes.