Alpha hydrogen

Sulfur tetrafluoride [7783-60-0] SF, replaces halogen in haloalkanes, haloalkenes, and aryl chlorides, but is only effective (even at elevated temperatures) in the presence of a Lewis acid catalyst. The reagent is most often used in the replacement of carbonyl oxygen with fluorine (15,16). Aldehydes and ketones react readily, particularly if no alpha-hydrogen atoms are present (eg, benzal fluoride [455-31-2] from benzaldehyde), but acids, esters, acid chlorides, and anhydrides are very sluggish. However, these reactions can be catalyzed by Lewis acids (HP, BF, etc). 

Synthesis and Properties. Polyquinolines are formed by the step-growth polymerization of o-aminophenyl (aryl) ketone monomers and ketone monomers with alpha hydrogens (mosdy acetophenone derivatives). Both AA—BB and AB-type polyquinolines are known as well as a number of copolymers. Polyquinolines have often been prepared by the Friedlander reaction (88), which involves either an acid- or a base-catalyzed condensation of an (9-amino aromatic aldehyde or ketone with a ketomethylene compound, producing quinoline. Surveys of monomers and their syntheses and properties have beenpubhshed (89—91). 

As the temperature approaches the NTC zone, the reversibility of reaction 2 comes into play and the steady-state concentration of alkyl radicals rises. There is a competing irreversible reaction of oxygen with radicals containing an alpha hydrogen which produces a conjugate olefin (eq. 23). 

Sulfuryl chloride in NiiV-dimethylanilme replaces the alpha hydrogen of a polyfluoronitrile [42] (equation 47)... 

The rate of reaction of a series of nucleophiles with a single substrate is related to the basicity when the nucleophilic atom is the same and the nucleophiles are closely related in chemical type. Thus, although the rates parallel the basicities of anilines (Tables VII and VIII) as a class and of pyridine bases (Tables VII and VIII) as a class, the less basic anilines are much more reactive. This difference in reactivity is based on a lower energy of activation as is the reactivity sequence piperidine > ammonia > aniline. Further relationships among the nucleophiles found in this work are morpholine vs. piperidine (Table III) methoxide vs. 4-nitrophenoxide (Table II) and alkoxides vs. piperidine (Tables II, III, and VIII). Hydrogen bonding in the transition state and acid catalysis increase the rates of reaction of anilines. Reaction rates of the pyridine bases are decreased by steric hindrance between their alpha hydrogens and the substituents or... 

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

Acidity of Alpha Hydrogen Atoms Enolate Ion Formation... 

Alpha hydrogen atoms of carbonyl compounds are weakly acidic and can be removed by strong bases, such as lithium diisopropylamide (LDA), to yield nucleophilic enolate ions. The most important reaction of enolate ions is their Sn2 alkylation with alkyl halides. The malonic ester synthesis converts an alkyl halide into a carboxylic acid with the addition of two carbon atoms. Similarly, the acetoacetic ester synthesis converts an alkyl halide into a methyl ketone. In addition, many carbonyl compounds, including ketones, esters, and nitriles, can be directly alkylated by treatment with LDA and an alkyl halide. 

Wada, M., Saknrai, Y., Akiba, K.-y. (1984) Addition of Alkynyl Anions to Aldimines Containing Alpha-Hydrogens A Novel Synthesis of Beta-Aminoacetylenes. Tetrahedron Letters, 25, 1083-1084. 

C. Secondary Alpha Hydrogen-Deuterium Kinetic Isotope Effects. 896... 

TABLE 8. The secondary alpha hydrogen-deuterium kinetic isotope effects for the Menshutkin reaction between 3,5-disubstituted pyridines and methyl iodide in 2-nitropropane at 25 °C... 

TABLE 9. The carbon-12/carbon-14 and secondary alpha hydrogen-tritium kinetic isotope effects for the Sat2 reactions between Y-substituted /VA-dimethylanilines and Z-substituted benzyl X-substituted benzensulfonates in acetone at 35°C ... 

TABLE 13. The secondary alpha hydrogen-deuterium and primary nitrogen kinetic isotope effects for the SN2 reaction between sodium thiophenoxide and benzyldimethylphenylammonium nitrate at different ionic strengths in DMF at 0 C... 

ElNemr, A. Tsuchiya, T., alpha-Hydrogen ehmination in some 3- and 4-triflates of alpha-D-glycopyranosides. Carbohydr. Res. 1997,301, 77-87. 

The photochemistry of ferf-butylazide in a nitrogen matrix at 12 K was followed by IR spectroscopy. Only one product, trimethyhmine, was observed. Thus, the removal of alpha hydrogen from the alkyl azide does not suppress 1,2-migration reactions. 

Deviations from this simple expression have been attributed to mechanistic complexity For example, detailed kinetic studies have evaluated the relative importance of the Langmuir-Hinshelwood mechanism in which the reaction is proposed to occur entirely on the surface with adsorbed species and the Eley-Rideal route in which the reaction proceeds via collision of a dissolved reactant with surface-bound intermediates 5 . Such kinetic descriptions allow for the delineation of the nature of the adsorption sites. For example, trichloroethylene is thought to adsorb at Ti sites by a pi interaction, whereas dichloroacetaldehyde, an intermediate proposed in the photo-catalyzed decomposition of trichloroethylene, has been suggested to be dissociatively chemisorbed by attachment of the alpha-hydrogen to a surface site... 

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