Abstraction Addition

The relative propensity of radicals to abstract hydrogen or add to double bonds is extremely important. In radical polymerization, this factor determines the significance of transfer to monomer, solvent, etc. and hence the molecular weight and end group functionality (Chapter 6). It also provides one basis for initiator selection (Section 3.2.1). 

CpHg-CjHs 343 /-C3H7-C3H5 335 NC4H9-C2H5 326 allyl-C2H5 299 

The hydrogen abstraction addition ratio is generally greater in reactions of heteroatoin-centered radicals than it is with carbon-centered radicals. One factor is the relative strengths of the bonds being formed and broken in the two reactions (Table 1.6). The difference in exothermicity (A) between abstraction and addition reactions is much greater for heteroatom-centered radicals thait it is for carbon-centered radicals. For example, for an alkoxy as opposed to an alkyl radical, abstraction is favored over addition by ca 30 kJ mof . The extent to which this is reflected in the rates of addition and abstraction will, however, depend on the particular substrate and the other influences discussed above. 

A number of studies have found that increasing nucleophilicity of the attacking radical favors abstraction over addition to an unsaturated system (benzene ring or double bond). Bertrand and Surzur surveyed the literature on the 

However, the situation is not as clear-cut as it might at first seem since a variety of other factors may also contribute to the above-mentioned trend. Abuin ef a/. pointed out that the transition state for addition is sterically more demanding than that for hydrogen-atom abstraction. Within a given scries (alkyl or alkoxy), the more nucleophilic radicals are generally the more bulky (i.e. steric factors favor the same trends). It can also be seen from Table 1.6 that, for alkyl radicals, the values of D decrease in the series primary secondary tertiary (i.e. relative bond strengths favor the same trend). 

---

Double bond migration occurs either by the Jt-allyl mechanism (abstraction-addition) or by the Horiuti-Polanyi mechanism (addition-abstraction). Pd is thought to favor Jt-allyl and Pt Horiuti-Polanyi mechanisms. 

Scheme S. Hydrogen exchange in benzene by an abstraction-addition mechanism involving dissociative chemisorption of the reactant [Farkas and Farkas (32)].

Careful reading of references (85-40), and of recorded discussion where this exists, indicates that authors who favor exchange by an addition-abstraction mechanism seldom reject the alternative entirely. Indeed, since evidence from subsection B supports the abstraction-addition mechanism, it may well be that both mechanisms operate simultaneously when molecular deuterium is present, and that only when one predominates can telling experimental evidence be obtained. 

To summarize, the use of heavy water as a deuterium source has provided a wealth of experimental information. Evidence for the associative ir-adsorption of benzene [species (I) J is secure (2). Evidence for hydrogen exchange in the benzene ring by an abstraction-addition mechanism is less well established, partly because of uncertainties that surround the mode of chemisorption and reaction of water at metal surfaces. Nevertheless, it would be wrong to deny that Scheme 6 is consistent with a large body of experimental work. 

Scheme 7. Hydrogen exchange in benzene by double abstraction-addition, benzene being initially associatively chemisorbed [Moyes et al. (4)].

A variety of 7r-allyl complexes are possible, including the transannular ones. It should be noted that, although 1,3-cyclooctadiene is thermodynamically the most stable diene, the stability of the 1,5 complex provides the driving force for the rearrangement. The 1,4 diene complex does not appear to be an intermediate, suggesting that the hydride abstraction-addition sequence is not straightforward. 

It is generally agreed that alkenyl hydroperoxides are primary products in the liquid-phase oxidation of olefins. Kamneva and Panfilova (8) believe the dimeric and trimeric dialkyl peroxides they obtained from the oxidation of cyclohexene at 35° to 40° to be secondary products resulting from cyclohexene hydroperoxide. But Van Sickle and co-workers (20) report that, The abstraction/addition ratio is nearly independent of temperature in oxidation of isobutylene and cycloheptene and of solvent changes in oxidations of cyclopentene, tetramethylethylene, and cyclooctene. They interpret these results to support a branching mechanism which gives rise to alkenyl hydroperoxide and polymeric dialkyl peroxide, both as primary oxidation products. This interpretation has been well accepted (7, 13). Brill s (4) and our results show that acyclic alkenyl hydroperoxides decompose extensively at temperatures above 100°C. to complicate the reaction kinetics and mechanistic interpretations. A simplified reaction scheme is outlined below. 

Over the alumina-supported metals, the activity sequences for isomerisation and hydroisomerisation were observed to be similar. With these catalysts, the results were interpreted in terms of a H-atom addition-abstraction mechanism involving an adsorbed but-2-yl intermediate the importance of the alumina support as a source of hydrogen atoms to initiate the isomerisation was stressed [130,133]. It was suggested that, with platinum black, isomerisation occurred by both an addition—abstraction and an abstraction—addition mechanism, the relative contributions of each depending upon the experimental conditions [135]. 

In the isomerisation of the tetra-substituted olefin 3,4-dimethylhex-3-ene over palladium—alumina [146], it has been shown that double bond migration is a necessary precursor to cis—trans isomerisation. This has been interpreted as showing that the mechanism involves a series of elementary steps, each of which is stereospecific, although no definite conclusions were drawn as to whether an addition—abstraction or an abstraction-addition mechanism was involved. 

In propagation many types of reactions are involved including H abstraction, addition, radical decomposition, and radical isomerization. 

If there is competition between addition and H-abstraction, addition is always preferred. As a consequence, H-abstraction from the sugar moiety is a very minor process in DNA and related compounds (Das et al. 1985). 

Table II summarises the parameters employed in the standard simulations. Cloudless conditions are assumed throughout. The modelling runs performed are listed in Table III. A lower rate coefficient of 4.2 x 10 n cm3 molecule 1 s 1 and a 50 50 abstraction/addition ratio for the OH/DMS reaction were adopted in run b . In run e the triple plume was considered as a single one with corresponding larger initial cross-section.

Atom-abstractive addition of Cpji YbOEt2 with alkyl and aryl halides give Cp YbX and Cp YbX2 where X = halide [149]. The side products of the reaction are Cp H, R-R, RH. 

Hot atom chemistry. The subject of hot atom chemistry finds itself here because, together with molecular beam and photochemical methods, nuclear recoil processes have been used extensively as hot atom sources. These reactions lead variously to abstraction, addition and displacement processes both with saturated species such as CH4 and unsaturated species. Table 14 shows various sources of hot atoms. 

---