The reactivity-selectivity principle RSP

The question of the relationship between reactivity and selectivity is usually discussed in terms of the reactivity-selectivity principle (RSP) in a series of related reactions the more reactive species tend to be less selective in their reactivity than less reactive ones. Let us consider two substances A and B that follow the same type of reaction by two different pathways X and 7 for example, one leading to an isomer 1 and the other to an isomer 2. 

Experimental selectivity in the abstraction of hydrogen atoms from alkanes at 350 K 

When one compares the selectivity of Br and Cl, the more reactive species (Cl) is clearly the less selective one. For example, bromine atoms abstract tertiary H-atoms 17(X) times faster than the primary H-atom. By comparison, chlorine radical is only 7 times faster. In fact, the electronic factor m for those two radicals has almost identical values (m close to 2) and RSP is obeyed. When changes in reactivity are not due only to changes in reaction energy (A °, A/7° or AG°), the postulates of physical organic chemistry are no longer verified. For example, Br and CHj have almost the same reactivity with respect to CH4. According to RSP one would expect that both radicals should have identical selectivities for H-atom abstractions. Nevertheless, Br is much more selective, because the electrophilicity 

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Selectivity versus Reactivity. The "reactivity-selectivity principle" (RSP) states that "in a set of similar reactions, the less reactive the reagent, the more selective it is in its attack" (451. This principle is widely held, but its generality has recently been questioned (28.46-481. The reactivity data assembled in Tables IV through VIII appear to provide an opportunity to examine the validity of the RSP, using log(kA/ka)sN as a measure of the selectivity of a haloaliphatic compound between nucleophiles A and B. 

In early work on methyl transfers (18, 19), we searched for the applicability of the reactivity-selectivity principle (RSP). Initially, small effects in this direction were found. However, with more data, counterexamples of comparable magnitude appeared (20). The only conspicuous loss of selectivity appeared in a reaction of (CH3)30+ with C6H5S , which was believed to be to some extent diffusionally limited (19). Thus, no convincing evidence was found for the RSP. 

The more reactive ylides discriminate less well among the aldehydes in competition experiments (p, KIE), in accordance with the reactivity-selectivity principle (RSP) (15b, 233). Thus, both the KIE and p values could be small. However, Yamataka et al. (223c) question the validity of the RSP in an ET sequence. Their arguments depend on the interpretation of KIE and model reactions for comparison, but it is not clear what experiment would be decisive. Objections to ET mechanisms based on the radical anion clock experiments discussed earlier may also not be decisive because of differences in the counterions. Futhermore, radical clock evidence against ET is available for aliphatic (not aromatic) aldehydes. 

Physical organic chemistry has been built around LFER, which is still a matter of active research from experiment to theoretical simulation [53,54]. We ourselves have also addressed these problems under the perspective of the ISM [10,36a,42]. Here we will only deal with the interpretation of the Brpnsted relations, the postulate of Hammond and the Reactivity-Selectivity Principle (RSP). 

Use the quadratic equation of Marcus to verify the Reactivity-Selectivity Principle (RSP) for a reaction with an intrinsic barrier of 10 kJ mol... 

There are many examples in which the RSP is followed, but there are also many examples corresponding to situations (i) and (iii). The RSP is in accord with intuitive feeling and certainly holds in the limiting case when reactivity is controlled by diffusion. However, the validity of the RSP is a matter of great controversy, and diverse opinions have been expressed, from declaring the reactivity-selectivity principle as a universal law up to virtually useless in practice as a general rule. 

Several chemical reactions verify the RSP, the more reactive species tend to be less selective in their reactivity. Nevertheless, we have shown [10] that this is valid when the reactivity is controlled by changes in AG otherwise exceptions occur and more reactive species can also be more selective than the less reactive ones. Table 8 illustrates the effect of n on the selectivity of radicals towards CH bonds. None the less, a is always a good parameter to assess selectivity, for reactions which obey RSP and even for reactions which have a behaviour opposed to such a principle. But literature reports also reaction families where no free-energy relations are observed as, for example, in cation-anion recombinations [57]... 

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