Hydrogen atom reactivity

Bursten, B. E. and Gatter, M. G. (1984). Molecular orbital studies of organometallic hydride complexes 2. The correlation of hydrogen atom reactivity with valence orbital energetics. Organometallics 3, 895. 

Tetraoxo derivatives of 5-5 ring type (e.g., (210)-(212)) have very acidic hydrogen atoms. Reactivity of these tetrones as nucleophiles is well documented and it is summarized in Scheme 32 <66AP(299)441, 78TL543, 86AP(319)646>. 

Method C. The third system we have used to measure hydrogen atom reactivities involves die photolysis of fcrf-butyl peroxyformate (BUP), H-C02-0-C(CH8)8, to produce hydrogen atoms ( ). The kinetic system involves a competition in which the H atoms either abstract deuterium from thiol-d (added as a standard reactant) or from QH. The reactions are shown below. 

Organic isocyanates tend to react, in many cases quite readily, with substances containing active hydrogen atoms, that is, with hydrogen atoms reactive enough to be replaced in their compounds by alkali metals or by the Zere-witinoff reagent, methyl magnesium iodide. " ... 

Here the phenylacetic anhydride, possessing more reactive a-hydrogen atoms, condenses with benzaldehyde to give a-phenylcinnamic acid. The preparation of the latter is an example of the Oglialoro modiflcation of the Perkin reaction. 

Each chlorine atom formed m the initiation step has seven valence electrons and IS very reactive Once formed a chlorine atom abstracts a hydrogen atom from methane as shown m step 2 m Figure 4 21 Hydrogen chloride one of the isolated products from... 

The relative rates of reaction of ethane toluene and ethylbenzene with bromine atoms have been measured The most reactive hydrocarbon undergoes hydrogen atom abstraction a million times faster than does the least reactive one Arrange these hydrocarbons in order of decreasing reactivity... 

Other elements have atoms that can have different ratios of protons to neutrons. Indeed, hydrogen actually consists of three types of atoms. All hydrogen atoms have the same number of protons (one for hydrogen), giving each a mass of 1 Dalton, but some atoms of hydrogen also contain one neutron in the nucleus as well as the proton (mass of 2 Da), while yet others have two neutrons with each proton (mass of 3 Da). Thus hydrogen has three naturally occurring isotopes of mass 1, 2, and 3 Da. Chemically, there are only small differences between the reactivities of the different isotopes for any one element. Thus isotopes of palladium aU react in the same way but react differently from all isotopes of platinum. 

Rea.ctlons, Butyrolactone undergoes the reactions typical of y-lactones. Particularly characteristic are ring openings and reactions in which ring oxygen is replaced by another heteroatom. There is also marked reactivity of the hydrogen atoms alpha to the carbonyl group. 

Oxidation begins with the breakdown of hydroperoxides and the formation of free radicals. These reactive peroxy radicals initiate a chain reaction that propagates the breakdown of hydroperoxides into aldehydes (qv), ketones (qv), alcohols, and hydrocarbons (qv). These breakdown products make an oxidized product organoleptically unacceptable. Antioxidants work by donating a hydrogen atom to the reactive peroxide radical, ending the chain reaction (17). 

Hydrogen atoms can be produced in significant quantities in the gas phase by the action of radiation on or by extreme heating of H2 (3000 K). Although hydrogen atoms are very reactive, these atoms can persist in the pure state for significant periods of time because of the inabiUty to recombine without a third body to absorb the energy of bond formation. 

Miscellaneous Reactions. The A/-hydrogen atom of diphenylamine is reactive and readily replaced by deuterium by treating with C2H OD. The addition of acid cataly2es the exchange of the hydrogen atoms on the ring system (11). 

The weight in grams to provide one reactive hydrogen atom for every epoxide group in lOOg of liquid epoxide resin of epoxide equivalent 190. 

Radicals also rapidly abstract hydrogen atoms from many types of solvents, and most radicals are highly reactive toward oxygen. 

The selectivity observed in most intramolecular functionalizations depends on the preference for a six-membered transition state in the hydrogen-atom abstraction step. Appropriate molecules can be constmcted in which steric or conformational effects dictate a preference for selective abstraction of a hydrogen that is more remote from the reactive radical. 

For the acetoxy radical, the for decarboxylation is about 6.5 kcal/mol and the rate is about 10 s at 60°C and 10 s at —80°C. Thus, only very rapid reactions can compete with decarboxylation. As would be expected because of the lower stability of aryl radicals, the rates of decarboxylation of aroyloxy radicals are slower. The rate for p-methoxybenzoyloxy radical has been determined to be 3 x 10 s near room temperature. Hydrogen donation by very reactive hydrogen-atom donors such as triethylsilane can compete with decarboxylation at moderate temperatures. 

In some cases, diene polymers (for instance polychloroprene rubbers) can add to the growing polymer chain by 1,2 addition (also called vinyl addition). This creates labile hydrogen or reactive halogen on tertiary carbon atoms. A few percent of this type of structure in the rubber will assist cross-linking reactions. 

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