Hydrogen atom, branching reactions

By ab initio MO and density functional theoretical (DPT) calculations it has been shown that the branched isomers of the sulfanes are local minima on the particular potential energy hypersurface. In the case of disulfane the thiosulfoxide isomer H2S=S of Cg symmetry is by 138 kj mol less stable than the chain-like molecule of C2 symmetry at the QCISD(T)/6-31+G // MP2/6-31G level of theory at 0 K [49]. At the MP2/6-311G //MP2/6-3110 level the energy difference is 143 kJ mol" and the activation energy for the isomerization is 210 kJ mol at 0 K [50]. Somewhat smaller values (117/195 kJ mor ) have been calculated with the more elaborate CCSD(T)/ ANO-L method [50]. The high barrier of ca. 80 kJ mol" for the isomerization of the pyramidal H2S=S back to the screw-like disulfane structure means that the thiosulfoxide, once it has been formed, will not decompose in an unimolecular reaction at low temperature, e.g., in a matrix-isolation experiment. The transition state structure is characterized by a hydrogen atom bridging the two sulfur atoms. 

An example of a backbiting reaction that creates the short chain branches is shown in Fig. 18.5. In this example the growing end of a polyethylene chain turns back on itself and abstracts a hydrogen atom from the carbon atom located four bonds away from the chain end, as shown in Fig. 18.5 a). Chain growth proceeds from the newly formed unpaired electron, leaving a pendant butyl group, as shown in Fig. 18.5 b). There are many variants of backbiting, which create a variety of short chain branches. 

Although theoretical estimates for neutral branching fractions of reactions such as 10 have been attempted, their reliability is suspect.32 In the absence of measurement, modelers have typically assumed equal branching fractions between channels in which one and two hydrogen atoms are separated from the molecular skeleton (e.g. 0.50 for reactions 10a and 10b). New assumptions based on recent experimental work have also been made.33... 

The great majority of experimental data (see Section III.A) indicate that the hydrogen-deuterium exchange reaction belongs to the class of acceptor reactions (i.e., reactions that are accelerated by electrons and decelerated by holes). This means that the experimenter, as a rule, remains on the acceptor branch of the thick curve in Fig. 8a, on which the chemisorbed hydrogen and deuterium atoms act as donors. Here a donor impurity must enhance the catalytic activity, while an acceptor impurity must decrease it. This is what actually occurs, as we have already seen (see Section III.A). 

The hydrogen-chlorine chain reaction has proved to be one of the most controversial systems yet studied. After thirty years of investigation Bodenstein43 was able to say in 1931 that every worker on the photochemical synthesis of HC1 had produced his own mechanism even as late as 1940 little positive information had been obtained. However, the accumulated techniques and experience had firmly established the importance of atom chain reactions. The mechanism of photo-initiation and propagation is the same as for the hydrogen bromide photosynthesis, a non-branching chain reaction... 

Elimination reactions (Figure 5.7) often result in the formation of carbon-carbon double bonds, isomerizations involve intramolecular shifts of hydrogen atoms to change the position of a double bond, as in the aldose-ketose isomerization involving an enediolate anion intermediate, while rearrangements break and reform carbon-carbon bonds, as illustrated for the side-chain displacement involved in the biosynthesis of the branched chain amino acids valine and isoleucine. Finally, we have reactions that involve generation of resonance-stabilized nucleophilic carbanions (enolate anions), followed by their addition to an electrophilic carbon (such as the carbonyl carbon atoms... 

Despite this superficial similarity, however, subtle differences between the behaviour of ionized amines and the analogous ionized alcohols and ethers remain. Thus, metastable ionized 2-butylamine loses 80% ethane in contrast, ionized 2-butanol eliminates both ethane (35%) and methane (40%)85. The latter reaction corresponds to loss of the smaller methyl group and an a-hydrogen atom from the larger ethyl substituent at the branch point. Methane loss does not occur from ionized amines with a methyl substituent on the -carbon, with the solitary exception of ionized isopropylamine which does expel methane (10%). However, ionized 3-hexylamine eliminates both ethane (35%) and propane (20%)85. 

Hydrogen atom abstraction can occur to a small extent, particularly with larger and more highly branched compounds. However, the contribution of this path is, overall, relatively small. For example, for the reaction with 3-methyl-l-butene, where there is a weaker allylic C-H bond, 5-10% of the reaction proceeds by abstraction at 1 atm in air (Atkinson et al., 1998). 

It has been known for some time [see Ref. (176) for earlier work] that if poly(vinyl alcohol), produced by hydrolysis of poly(vinyl acetate) is reacetylated, the PVAc so obtained has a lower MW than the original PVAc prior to hydrolysis, though the MW of the material is not lowered any further by subsequent cycles of hydrolysis and reacetylation. Various explanations had been advanced for this phenomenon Wheeler explained it as a consequence of the presence of branches joined to the main chain through ester linkages which would be broken on hydrolysis and not re-formed on reacetylation. These branches were ascribed to chain transfer reactions with acetate groups, either in the polymer, or in monomer molecule subsequently polymerized at their double bonds. Transfer reactions by attack on hydrogen atoms other than those in... 

Lewandowski and Ollis carried out thermodynamic calculations for the reaction of chlorine radicals with branched (toluene, xylenes) and unbranched (benzene) aromatic contaminants [51], On a branched aromatic contaminant, abstraction of the hydrogen atom on a methyl side group by a chlorine radical. 

The alkyl radical may also dissociate thermally to form an alkene and a smaller alkyl radical. The mechanism that is initiated by these reactions is chain propagating rather than chain branching and for this reason the overall oxidation rate of the fuel decreases. Also there is a change from OH to HO2 as the main chain carrier, and as we have seen, the HO2 radical is much less reactive than OH. The HO2 radical is formed both from alkyl + O2 hydrogen abstraction reactions such as (R69) and from recombination of hydrogen atoms with O2, H + O2 + M HO2 + M (R5). Under lean conditions any hydrogen atoms formed will primarily react with oxygen. At intermediate temperatures the reaction H + O2 O + OH (Rl) is still too slow to compete with (R5). 

Hydrogen atoms readily diffuse upstream of the flame front into the cooler unbumed region. At temperatures below about 750 K, the production of H02 dominates, but at the higher temperatures in the flame front, the chain-branching dominates. As the temperatures continue to rise, the chain-branching reaction equilibrates and the three-body reaction can... 

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