Abstract reaction scheme

The sequential reactions in Eqs. 1.48 and 1.50 are special cases of the two abstract reaction schemes... 

This reaction is fundamentally different than transfer to polymer in several respects. It is an addition rather than a transfer (H-abstraction) reaction (Scheme 4.10). The reaction rate is dependent on the number of unsaturated chain ends rather than the number of repeat units in the chains so that its importance relative to propagation is controlled by the ratio [D) (]/[M] according to Eq. (31). 

Some transfer agents react by addition-fragmentation (Section 6.2.3) or abstraction-fragmentation mechanisms. Both of these processes involve the formation of a short-lived intermediate. The reaction scheme for addition-fragmentation can be summarized schematically as follows (Scheme 6.3). 

Stames el al.I7 have provided support for the above mechanism (Scheme 6.29) by determining the unsaturated chain ends (112) in low conversion PVAc by l3C NMR. They were able to distinguish (112) from chain ends that might have been formed if transfer involved abstraction of a vinylic hydrogen. The number of unsaturated chain ends (112) was found to equate with the number of -CH OAc ends suggesting that most chains arc formed by transfer to monomer. Stames et a . 13 also found an isotope effect k kD of 2.0 for the abstraction reaction with CTTpCHOiCCD as monomer. This result is consistent with the mechanism shown in Scheme 6.28 but is contrary to an earlier finding.174... 

On the basis of all these results and his own investigations on chloro- and bromo-de-diazoniations (Galli, 1981), Galli proposed in 1988 that iodo-de-diazoniation, after formation of the aryl radical in the initiation reaction (Scheme 10-22) follows three coupled iodination chain reactions based on the formation of the I2 molecule and the If anion in the step shown in Scheme 10-23, namely iodine atom (I ) addition (Scheme 10-24), and iodine abstraction from I2 and If in Schemes 10-25 and 10-26 respectively. Aryl radicals and iodine molecules are regenerated as indicated in Scheme 10-27. The addition of iodide ion to aryl radicals forming the radical anion [Arl] -, as in Scheme 10-28, is considered an unlikely pathway, as that reaction has been found to be reversible (Lawless and Hawley, 1969 Andrieux et al. 1979). 

Reaction step 5 in Scheme 3.1 can be rnled ont becanse the flnoranil ketyl radical (FAH ) reaches a maximum concentration within 100 ns as the triplet state ( FA) decays by reaction step 2 while the fluoranil radical anion (FA ) takes more than 500 ns to reach a maximum concentration. This difference snggests that the flnoranil radical anion (FA ) is being produced from the fluoranil ketyl radical (FAH ). Reaction steps 1 and 2 are the most likely pathway for prodncing the flnoranil ketyl radical (FAH ) from the triplet state ( FA) and is consistent with the TR resnlts above and other experiments in the literatnre. The kinetic analysis of the TR experiments indicates the fluoranil radical anion (FA ) is being prodnced with a hrst order rate constant and not a second order rate constant. This can be nsed to rnle ont reaction step 4 and indicates that the flnoranil radical anion (FA ) is being prodnced by reaction step 3. Therefore, the reaction mechanism for the intermolecular hydrogen abstraction reaction of fluoranil with 2-propanol is likely to predominantly occur through reaction steps 1 to 3. 

Scheme 3.1 Possible reaction steps in the hydrogen abstraction reaction of fluoranil with 2-propanol. Note FA= fluoranil, (CH3)2CHOH = 2-propanaol, FAH = fluoranil ketyl radical, FA = fluoranil radical anion.

Addition reaction of peroxide-generated macroalkyl radicals with the reactive unsaturation in MA is shown in reaction scheme 4. The functionalised maleic-polymer adduct (II, scheme 4) is the product of hydrogen abstraction reaction of the adduct radical (I, scheme 4) with another PP chain. Concomitantly, a new macroalkyl radical is regenerated which feeds back into the cycle. The frequency of this feedback determines the efficiency of the cyclical mechanism, hence the degree of binding. Cross-linking reaction of I occurs by route c ( scheme 4). 

Abstract. The photocatalytic oxidation cycle (POC) is that process in chalking in which the pigment participates. This paper has shown the chemical reaction scheme for the course of this process as well as the experimental results confirming this scheme. 

A truly co-catalytic effect of ionic liquids is observed with those ionic liquids displaying a certain latent or real Lewis-acid character. These ionic liquids are usually formed by the reaction of a halide salt with a Lewis acid (e. g. chloroaluminate or chlorostannate melts). In many examples, the Lewis acidity of an ionic liquid has been used to convert the neutral catalyst precursor into the active form of the catalyst by halide abstraction (see Scheme 7.1) [39]. 

The results from these experiments also allowed Hannon and Traylor to determine the primary and secondary hydrogen deuterium kinetic isotope effects for the hydride abstraction reaction. If one assumes that there is no kinetic isotope effect associated with the formation of 3-deutero-l-butene, i.e. that CH2=CHCHDCH3 is formed at the same rate (k ) from both the deuterated and undeuterated substrate (Scheme 25), then one can obtain both the primary (where a deuteride ion is abstracted) and the secondary deuterium... 

Since alkyllithium compounds and their carbanions have an isoelectronic structure with alkoxides, their reaction behavior with carbenes is expected to be similar to that of alkoxides, showing enhanced reactivity in both C-H insertion and hydride abstraction.35 In this reaction, the hydride abstraction cannot be followed by recombination and, therefore, can be differentiated from the insertion. Indeed, the reaction of alkyllithium compounds 70 or nitrile anions (see Section IV.B) with ethyl(phenylthio)carbenoid, which is generated by the reaction of 1-chloropropyl sulfide 69 with BuLi, takes place at the -position of 70 more or less in a similar manner giving both insertion product 71 and hydride abstraction products 72 and 73, respectively. This again supports a general rule C-H bonds at the vicinal position of a negatively charged atom are activated toward carbene insertion reactions (Scheme 22). 

Scheme 14 /3-Hydrogen abstraction reactions of alkyltitanium compounds.

The synthesis of menthol is given in the reaction scheme, Figure 5. 6. The key reaction [2] is the enantioselective isomerisation of the allylamine to the asymmetric enamine. It is proposed that this reaction proceeds via an allylic intermediate, but it is not known whether the allyl formation is accompanied by a base-mediated proton abstraction or hydride formation. 

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