Atom abstractions

Atom abstraction occurs when a dissociation reaction occurs on a surface in which one of the dissociation products sticks to the surface, while another is emitted. If the chemisorption reaction is particularly exothennic, the excess energy generated by chemical bond fomiation can be chaimelled into the kinetic energy of the desorbed dissociation fragment. An example of atom abstraction involves the reaction of molecular halogens with Si surfaces [27, 28]. In this case, one halogen atom chemisorbs while the other atom is ejected from the surface. 

Li Y L ef a/1995 Experimentai verification of a new mechanism for dissociative chemisorption atom abstraction Phys. Rev. Lett. 74 2603... 

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... 

Step 2 Hydrogen atom abstraction from methane by a chlorine atom... 

This behavior stems from the greater stability of secondary compared with primary free radicals The transition state for the step m which a chlorine atom abstracts a hydro gen from carbon has free radical character at carbon... 

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... 

Two other important commercial uses of initiators are in polymer cross-linking and polymer degradation. In a cross-linking reaction, atom abstraction, usually a hydrogen abstraction, occurs, followed by termination by coupling of two polymer radicals to form a covalent cross-link ... 

Most solvents for hydroperoxides are not completely inert to radical attack and, consequendy, react with radicals from the hydroperoxide to form solvent-derived radicals, either by addition to unsaturated sites or by hydrogen- or chlorine-atom abstraction. In equation 15, S—H represents solvent and S is a solvent radical. 

Selective chlorination of the 3-position of thietane 1,1-dioxide may be a consequence of hydrogen atom abstraction by a chlorine atom. Such reactions of chlorine atoms are believed to be influenced by polar effects, preferential hydrogen abstraction occurring remotely from an electron withdrawing group. The free radical chain reaction may be propagated by attack of the 3-thietanyl 1,1-dioxide radical on molecular chlorine. 

Important differences are seen when the reactions of the other halogens are compared to bromination. In the case of chlorination, although the same chain mechanism is operative as for bromination, there is a key difference in the greatly diminished selectivity of the chlorination. For example, the pri sec selectivity in 2,3-dimethylbutane for chlorination is 1 3.6 in typical solvents. Because of the greater reactivity of the chlorine atom, abstractions of primary, secondary, and tertiary hydrogens are all exothermic. As a result of this exothermicity, the stability of the product radical has less influence on the activation energy. In terms of Hammond s postulate (Section 4.4.2), the transition state would be expected to be more reactant-like. As an example of the low selectivity, ethylbenzene is chlorinated at both the methyl and the methylene positions, despite the much greater stability of the benzyl radical ... 

The efficiency of the halomethane addition process depends on the relative rate of halogen-atom abstraction versus that of addition to the alkene ... 

This result shows than the initially added trichloromethyl group has little influence on the stereochemistry of the subsequent bromine atom-abstraction. The intermediate 2-(trichlor-omethyl)cyclohexyl radical presumably relaxes to the equatorial conformation faster than bromine-atom abstraction occurs. In contrast with addition to A -octahydronaphthalene, the addition is exclusively /ran -diaxial ... 

Cyclizations involving iodine-atom transfers have been developed. Among the most effective examples are reactions involving the cyclization of 6-iodohexene derivatives. The 6-hexenyl radical generated by iodine-atom abstraction rapidly cyclizes to a cyclo-pentylmethyl radical. The chain is propagated by iodine-atom transfer. 

No product derived from the transannular hydrogen abstraction is observed in the addition of bromotrichloromethane because bromine-atom abstraction is sufficiently rapid to prevent effective competition by the intramolecular hydrogen abstraction. 

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. 

Acyl radicals can fragment with toss of carbon monoxide. Decarbonylation is slower than decarboxylation, but the rate also depends on the stability of the radical that is formed. For example, when reaction of isobutyraldehyde with carbon tetrachloride is initiated by t-butyl peroxide, both isopropyl chloride and isobutyroyl chloride are formed. Decarbonylation is competitive with the chlorine-atom abstraction. 

Most of the free-radical mechanisms discussed thus far have involved some combination of homolytic bond dissociation, atom abstraction, and addition steps. In this section, we will discuss reactions that include discrete electron-transfer steps. Addition to or removal of one electron fi om a diamagnetic organic molecule generates a radical. Organic reactions that involve electron-transfer steps are often mediated by transition-metal ions. Many transition-metal ions have two or more relatively stable oxidation states differing by one electron. Transition-metal ions therefore firequently participate in electron-transfer processes. 

Experiments in which radical scavengers are added indicate that a chain reaction is involved, because the reaction is greatly retarded in the presence of the scavengers. The mechanism shown below indicates that one of the steps in the chain process is an electron transfer and that none of the steps involves atom abstraction. The elimination of nitrite occurs as a unimolecular decomposition of the radical anion intermediate, and the SrnI mechanistic designation would apply. 

One of the most common reactions of photoexcited carbonyl groups is hydrogen-atom abstraction from solvent or some other hydrogen donor. A second common reaction is cleavage of the carbon-carbon bond adjacent to the carbonyl group ... 

The intermediates which are generated are free radicals. The hydrogen-atom abstraction can be either intramolecular or intermolecular. Many aromatic ketones react by hydrogen-atom abstraction, and the stable products are diols formed by coupling of the resulting a-hydroxyben2yl radicals ... 

The efficiency of reduction of benzophenone derivatives is greatly diminished when an ortho alkyl substituent is present because a new photoreaction, intramolecular hydrogen-atom abstraction, then becomes the dominant process. The abstraction takes place from the benzylic position on the adjacent alkyl chain, giving an unstable enol that can revert to the original benzophenone without photoreduction. This process is known as photoenolization Photoenolization can be detected, even though no net transformation of the reactant occurs, by photolysis in deuterated hydroxylic solvents. The proton of the enolic hydroxyl is rapidly exchanged with solvent, so deuterium is introduced at the benzylic position. Deuterium is also introduced if the enol is protonated at the benzylic carbon by solvent ... 

Intramolecular hydrogen-atom abstraction is also an important process for acyclic a,/ -unsaturated ketones. The intermediate diradical then cyclizes to give the enol of a cyclobutyl ketone. Among the by-products of such photolyses are cyclobutanols resulting from alternative modes of cyclization of the diradical intermediate ... 

Spin density surface for the most stable radical formed by hydrogen atom abstraction from a model of a-tocopherol shows delocalization of the unpaired electron. 

Examine the energies of radicals resulting from hydrogen atom abstraction in 3-ethylpentane. Which radical is the lowest energy Is there a relationship between the CH bond lengths in 3-ethylpentane and the stabilities of the radicals resulting from bond dissociation Elaborate. 

Draw resonance structures for the possible radicals resulting from hydrogen atom abstraction from toluene. Which would you anticipate to be the most stable Why Compare energies for the different radicals (radical A, radical B,. ..). Is the lowest-energy radical that which you anticipated Are any of the alternatives significantly better than any of the others Explain your reasoning. 

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