Alpha -hydrogen abstraction

The aldol condensation involves the reaction of two molecules of an aldehyde or ketone that has alpha hydrogens. Abstraction of an alpha hydrogen by base produces a carbanion which attacks the carbonyl carbon of the other molecule by base-initiated nucleophilic addition an alcohol group is formed. Often the alcohol dehydrates to form the final product, an unsaturated aldehyde or ketone. In a crossed aldol condensation, a carbonyl compound with alpha hydrogens reacts with one without alpha hydrogens. 

The Claisen condensation is a carbanion reaction in which the carbanion produced by alpha hydrogen abstraction on an ester displaces the alkoxy group of another ester molecule the reaction produces keto esters. 

As a class of compounds, the two main toxicity concerns for nitriles are acute lethality and osteolathyrsm. A comprehensive review of the toxicity of nitriles, including detailed discussion of biochemical mechanisms of toxicity and stmcture-activity relationships, is available (12). Nitriles vary broadly in their abiUty to cause acute lethaUty and subde differences in stmcture can greatly affect toxic potency. The biochemical basis of their acute toxicity is related to their metaboHsm in the body. Following exposure and absorption, nitriles are metabolized by cytochrome p450 enzymes in the Hver. The metaboHsm involves initial hydrogen abstraction resulting in the formation of a carbon radical, followed by hydroxylation of the carbon radical. MetaboHsm at the carbon atom adjacent (alpha) to the cyano group would yield a cyanohydrin metaboHte, which decomposes readily in the body to produce cyanide. Hydroxylation at other carbon positions in the nitrile does not result in cyanide release. 

Cross-linked PVP can also be obtained by cross-linking the preformed polymer chemically (with persulfates, hydrazine, or peroxides) or with actinic radiation (63). This approach requires a source of free radicals capable of hydrogen abstraction from one or another of the labile hydrogens attached alpha to the pyrrohdone carbonyl or lactam nitrogen. The subsequently formed PVP radical can combine with another such radical to form a cross-link or undergo side reactions such as scission or cyclization (64,65), thus ... 

O Base abstracts an acidic alpha hydrogen from one acetaldehyde molecule, yielding a resonance-stabilized enolate ion. 

Tertiary peroxy radicals have no alpha hydrogen that can be abstracted by 02 in the second step. However, a unimolecular decomposition of the alkoxy radical results in the formation of a peroxy radical, which can be measured. 

Oxidation under basic conditions evidently involves the alkoxide ion rather than the neutral alcohol, The oxidizing agent, Mn04e, abstracts the alpha hydrogen from the alkoxide ion either as an atom (one-electron transfer) or as hydride, H e (two-electron transfer). The steps for the two-electron sequence... 

The products from the pyrolyses of four higher alpha-olefins have been accounted for by an empirical model involving three competing decomposition pathways a molecular decomposition involving a six-membered ring transition state (which yields both propylene and an alpha-olefin with three less carbon atoms than the reactant) and two free radical chain pathways. One free radical channel involves hydrogen abstraction from, the second radical addition to the reactant olefin. The relative contributions of these three paths have been estimated from experimental data from the laboratory pyrolysis of higher alpha-olefins. The pyrolyses were carried out at low conversions in a quartz flow reactor at 475°-550°C. 

With respect to previous work by B. Ignasiak (14), we are in agreement that alpha proton abstraction from tetrahydrofuran is occurring under Sternberg conditions. The nature of our hydrogen evolution in the model system is different, however, for it comes from a reverse quench into D2O, not from alkylation conditions. Our results implicate hydride formation during the naphthalene-sodium-tetrahydrofuran reaction. 

Ethoxide base abstracts an acidic alpha hydrogen atom from an ester molecule, yielding an ester enolate ion. 

Alpha hydrogens are hydrogens on carbons directly attached to a carbonyl group. They are weakly acidic and can be abstracted by base to form a carbanion. The carbanion is called an enolate ion and is resonance stabilized. Neutralization of the enolate ion results in an enol, a compound in which an alcohol group is directly bonded to a carbon involved in a carbon-carbon double bond. The enol is in equilibrium with the original aldehyde or ketone in an equilibrium referred to as keto-enol tautomerism. The equilibrium usually favors the keto form. 

Unlike the three fundamental mechanisms of olefin formation already outlined, the a -ji (or ylid) mechanism is only applicable to elimination from onium salts . The base abstracts an alpha hydrogen from the leaving group to form an ylid which subsequently acts as an internal base and abstracts the beta hydrogen atom, viz. 

For a recent application of this reaction, see (a) Boto, A., Hernandez R., and Suarez, E., Oxidative decarboxylation of alpha-amino acids a mild and efficient method for the generation of N-acyliminium ions, Tetrahedron Lett., 40,5945,1999 (b) Francisco, C. G., Herrera, A. J., and Sudrez, E., Intramolecular hydrogen abstraction reaction promoted by alkoxy radicals in carbohydrates, synthesis of chiral 2,7-dioxabicyclo[2.2.1]heptane and 6,8-dioxabicyclo[3.2.1]octane ring systems, /. Org. Chem., 67, 7439, 2002. 

According to Wall and co-workers [2] the pyrolysis of polyethylene proceeds by a radical chain mechanism. The products formed result from the process of random-chain cleavage, followed by intermolecular or intramolecular hydrogen abstraction. Hydrogen abstraction occurs preferentially at tertiary carbon atoms, and product formation results from homolysis of the carbon-carbon bond at the beta position relative to the radical site. The major products formed are the -alkanes and alpha-omega-diolefins. The peaks between the triplets result from chain branching. 

Bulk Polymerization. The bulk polymerization of acryUc monomers is characterized by a rapid acceleration in the rate and the formation of a cross-linked insoluble network polymer at low conversion (90,91). Such network polymers are thought to form by a chain-transfer mechanism involving abstraction of the hydrogen alpha to the ester carbonyl in a polymer chain followed by growth of a branch radical. Ultimately, two of these branch radicals combine (91). Commercially, the bulk polymerization of acryUc monomers is of limited importance. 

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