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Allylic Hydrogen Atom

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Allylic Hydrogen Atoms: A Comprehensive Q&A



Introduction:

Q: What is an allylic hydrogen atom, and why is it important?

A: An allylic hydrogen atom is a hydrogen atom bonded to a carbon atom that is directly adjacent (allylic position) to a carbon-carbon double bond (alkene). These seemingly unremarkable hydrogens possess unique chemical reactivity that makes them crucial in organic chemistry, particularly in synthesis and reaction mechanisms. Their increased reactivity stems from the resonance stabilization of the resulting allylic radical or carbocation formed upon abstraction or removal of the hydrogen atom. Understanding allylic hydrogen reactivity is critical for predicting reaction outcomes and designing efficient synthetic routes in various chemical processes, including petroleum refining, polymer chemistry, and pharmaceutical synthesis.

Section 1: Reactivity of Allylic Hydrogens

Q: Why are allylic hydrogens more reactive than typical sp<sup>3</sup> hybridized hydrogens?

A: The enhanced reactivity of allylic hydrogens arises from the resonance stabilization of the intermediate formed after hydrogen abstraction. When an allylic hydrogen is removed (e.g., by a radical or electrophilic species), it generates an allylic radical or carbocation. This intermediate is stabilized through delocalization of the unpaired electron (in the radical) or positive charge (in the carbocation) over two carbon atoms via resonance. This delocalization lowers the energy of the intermediate, making the hydrogen abstraction process more favorable compared to the abstraction of a hydrogen atom from a saturated carbon, which lacks this resonance stabilization.

Q: Can you illustrate resonance stabilization with an example?

A: Consider the abstraction of an allylic hydrogen from propene:


```
CH2=CH-CH3 -----> •CH2-CH=CH2 + H• (Allylic radical formation)
<---> CH2=CH-CH2•
```

The dot represents the unpaired electron. Notice how the unpaired electron can be delocalized across both terminal carbon atoms, leading to two resonance structures. This resonance stabilization significantly lowers the energy of the allylic radical compared to a primary alkyl radical. A similar resonance stabilization is observed for allylic carbocations.

Section 2: Reactions involving Allylic Hydrogens

Q: What are some common reactions that specifically target allylic hydrogens?

A: Several important reactions selectively involve allylic hydrogens due to their increased reactivity. These include:

Allylic halogenation: Reactions using reagents like N-bromosuccinimide (NBS) or N-chlorosuccinimide (NCS) in the presence of light or peroxides selectively brominate or chlorinate allylic positions. This reaction proceeds via a free radical mechanism.

Allylic oxidation: Reagents like selenium dioxide (SeO2) or chromium trioxide (CrO3) can selectively oxidize allylic carbons to form allylic alcohols or ketones. These reactions are often used in the synthesis of complex molecules.

Allylic substitution: Reactions where a nucleophile replaces an allylic leaving group (like a halogen) are also common. These can proceed through S<sub>N</sub>1 or S<sub>N</sub>2 mechanisms, depending on the substrate and reaction conditions.

Section 3: Real-world Applications

Q: Where are allylic hydrogens and their reactivity important in the real world?

A: The unique reactivity of allylic hydrogens plays a significant role in various applications:

Petroleum refining: Allylic oxidation is utilized in the production of certain lubricating oils and other petroleum products.

Polymer chemistry: Allylic chemistry is crucial in the synthesis of polymers with specific properties, such as controlled branching and reactivity.

Pharmaceutical synthesis: Many pharmaceuticals contain allylic functionalities, and their selective modification is often critical for drug discovery and development. For example, allylic oxidation can be a key step in synthesizing complex natural products with medicinal properties.

Conclusion:

Allylic hydrogen atoms, though seemingly simple, exhibit unique chemical behavior due to resonance stabilization of the intermediate formed upon their removal. Their enhanced reactivity makes them crucial targets in various organic reactions, with important applications across diverse fields. Understanding their reactivity is fundamental for organic chemists to design efficient synthetic routes and predict the outcomes of various chemical transformations.


FAQs:

1. Q: Can allylic hydrogens participate in other reactions besides those mentioned? A: Yes, allylic hydrogens can also participate in reactions such as hydroboration-oxidation and epoxidation, albeit with potentially lower selectivity compared to the reactions discussed.

2. Q: How does the stereochemistry of the starting material affect the outcome of allylic reactions? A: The stereochemistry of the starting alkene can significantly impact the stereochemistry of the product in allylic reactions. For example, in allylic halogenation, the stereochemistry can be influenced by the radical mechanism.

3. Q: Are there any limitations to using allylic reactions in synthesis? A: Yes, limitations include the potential for multiple allylic positions in a molecule leading to mixtures of products, and the possibility of competing reactions at other sites. Careful choice of reagents and reaction conditions is crucial.

4. Q: How can one predict the regioselectivity of allylic reactions? A: Regioselectivity is often influenced by steric factors and the stability of the intermediate (radical or carbocation). More substituted allylic positions are generally more reactive.

5. Q: What spectroscopic techniques can be used to confirm the presence of an allylic hydrogen and the products of reactions involving them? A: Nuclear Magnetic Resonance (NMR) spectroscopy, particularly <sup>1</sup>H NMR, is a powerful tool to identify allylic hydrogens (by their chemical shifts) and analyze the products of reactions involving them. Infrared (IR) spectroscopy can also provide evidence of functional group changes during allylic reactions.

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Search Results:

Chapter 10 Conjugation in Alkadienes and Allylic Systems Two resonance forms are not equivalent; gives mixture of isomeric allylic bromides.

Photoinduced palladium-catalyzed asymmetric allylic C–H amination In 2016, Gevorgyan and co-workers demonstrated that aliphatic C(sp3)–H bond can be cleavaged with a photoin-duced generated hybrid aryl Pd(I) radical through hydrogen atom transfer (HAT) process.8Inspired by this work, very recently they dexterously con-ceived that this kind of homolytic mode can be applied to Pd catalyzed allylic C–H amination,...

Chapter 11 Conjugation in Alkadienes and Allylic Systems abstraction of an allylic hydrogen to given a resonance stabilized allylic radical. The allylic radical then reacts with a halogen, X 2, to form the allylic halogen and generate a new radical. The fact that the second propagation step generates a new radical means that this is …

Kinetic Barriers of H-Atom Transfer Reactions in Alkyl, Allylic, and ... Hydrogen-atom transfers involving allylic radical formation demonstrated barrier heights that were 15-20 kJ mol -1 lower than those in corresponding alkyl radicals, whereas those involving oxoallylic species (R-site radicals of aldehydes and ketones) were 20-40 kJ mol -1 lower.

Direct allylic acylation via cross-coupling involving ... - Nature In this work, we disclose a strategy for coordinating triple catalysis to direct allylic acylation of alkenes with carboxylic acids (Fig. 1d). Speci cally, a carboxylic acid would be activated...

Microsoft Word - CHE_P12_M16_e-Text.doc - INFLIBNET Centre There are two types of hydrogens in alkenes. Hydrogens directly attached to the double bond are called vinyl hydrogens, whereas those located on the carbon atom attached to the double bond are called allylic hydrogens. Each type of hydrogen resonates in a particular chemical shift region.

10.2: Allylic Carbocations - carbocation with a vinyl group 10.6: Allylic Halogenation - Allylic halogenation of an alkene takes place through a free radical mechanism. (2 x 126 = 252) the double bonds of conjugated dienes are more stable than isolated double bonds. When the carbons of a conjugate diene all lie in the same plane, the π-molecular orbitals overlap. There are three conformations of butadiene.

Aerobic C C Bond Cleavage of Allylic Alcohols via Co-Catalyzed Hydrogen ... General Information: All Co-catalyzed hydrogen atom transfer reactions were carried out under an atmosphere of air. Glassware was dried for 12 hours at 100℃ prior to use.

Allylic C(sp3)–H alkylation via synergistic organo- and photoredox ... A new catalytic method for the direct alkylation of allylic C(sp3)–H bonds from unactivated alkenes via synergistic organo- and photoredox catalysis is described. The transformation achieves an efficient,

The direct arylation of allylic sp3 C-H bonds via organic and ... Figure 1 | The direct arylation of allylic C–H bonds via the synergistic merger of photoredox and organic catalysis. a, Arylation of allylic bonds is generally accomplished via transition-metal-catalysed couplings with pre-functionalized substrates. b, Installation of …

Rapid allylic 1,6 H-atom transfer in an unsaturated Criegee … A novel allylic 1,6 hydrogen atom transfer mechanism is established through infrared activation of the 2-butenal oxide Criegee intermediate, resulting in very rapid unimolecular decay to hydroxyl (OH) radical products.

Insights into the Mechanism of an Allylic Arylation Reaction via ... Transient absorption wavelength traces were collected at varying wavelengths from 390 nm to 1000 nm at a 415 nm excitation wavelength with 1.0mJ/cm2 pulse at sample. Time lengths of 6.4 ns to 10 ms were captured for each wavelength examined.

Allylic hydrogen abstraction II.w H-abstraction from 1,4 type ... Three types of the H-atom abstractions were distinguished: direct H-abstraction with CH3, indirect abstraction with a higher barrier height with iso-C3H7, OOH and a non-direct quasi-barrierless...

Alkyl Halides - Rutgers University An allylic hydrogen has been substituted for a bromine. The bromine atom abstracts an allylic hydrogen because the allylic radical is resonance stabilized. The radical then reacts with a bromine molecule to continue the chain.

Chemical Kinetics of Hydrogen Atom Abstraction from Allylic Sites … Chemical Kinetics of Hydrogen Atom Abstraction from Allylic Sites by 3O 2; Implications for Combustion Modelling and Simulation Supporting Information Chong-Wen Zhou Combustion Chemistry Centre & School of Chemistry National University of Ireland, Galway H91 TK33, Ireland John M. Simmie, Kieran P. Somers, C. Franklin Goldsmith, Henry J. Curran

Lewis Acid Catalyzed Carbonyl-Ene Reaction - University of … bearing an allylic hydrogen atom (the carba-ene) and an activated alene or alkyne (the carba-enophile) H C O + H O C R R R R (2) The hetero-ene-reaction: describes a reaction between an ene or enophile, either of which contains at least one heteroatom (a) Type I: reaction between all-carba-ene components with hetero-enophiles

Lewis Acid-Catalyzed Carbonyl-Ene Reaction: Interplay between ... 24 Jul 2023 · This transformation belongs to the family of group transfer pericyclic reactions and forms a C−C bond with concomitant 1,5-hydrogen shift by the reaction of an alkene bearing an allylic hydrogen atom (ene) with a multiple bond (enophile).2 Owing to its compatibility with a number of functional groups attached either to the ene or enophile moieti...

Quantum dots enable direct alkylation and arylation of allylic … Here, we report a general and mild strategy using semiconductor quantum dots (QDs) as photocata-lysts for coupling a broad range of available allylic C(sp3)–H bonds with a-amino C–H bonds or heteroarenes, respectively, under sun-light irradiation (> 85 examples).

Insights into the Mechanism of an Allylic Arylation Reaction via ... as potent hydrogen atom abstractors. In general, HAT catalysts achieve selectivity bytargeting weakened C-H and X-H bonds (X = S, N, O), in-cluding those in olefins, tertiary amines, dihydro-furan, piperidines, and benzylic ethers.1 Despite the importance of tandem photore-dox/HAT reactions, the nature of these radical re-

Chemical kinetics of hydrogen atom abstraction from allylic sites … Hydrogn atom abstraction from allylic C-H bonds by molecular oxygen plays a very important role in determining the reactivity of fuel molecules having allylic hy- drogen atom.