Covalent active species reactivities

For the discussion of the reactivity of covalent active species, see Section II.B.6.C.)... 

Whereas in Wmethyl-4,6-dimethylpyrimidinium ion the covalent addition takes place at C-6, it is assumed that 30 also undergoes covalent hydrazina-tion at C-6. However, the formation of dimer 32 shows the high sensitivity of C-2 in 30 for addition of nucleophiles, and it leads to the daring suggestion that it is the resonance-stabilized ylide 31 that probably is the active species undergoing addition at C-2 (Scheme III. 19). It was calculated (80UP1) that the reactivity at C-2 in the N-ylide 31 is greater than that at C-2 in the Waminopyrimidinium salt 30. 

The surest way to inhibit an enzyme is to block the active site irreversibly by chemical reaction with some active species to form a covalent bond. Thus, iodoacetate will irreversibly inactivate thiol proteases by forming the stable carboxymethyl mercaptan. lodoacetate is of course non-selective (many other enzymes would be inactivated), toxic (many sensitive sites would be alkylated) and moreover the drug itself is unstable due to its very reactivity. 

In order to prevent erroneous correlations of structure and reactivity, only active species of the same type should be compared. For example, carbenium ions with electron-donating substituents are more stable and less reactive than those with electron-withdrawing substituents, whereas covalent species with electron-donating substituents ionize much easier than those with electron-withdrawing substituents. Both factors result in a higher concentration of the more stabilized carbenium ion, which may lead to a higher apparent or overall rate constant for electrophilic addition of carbenium ions to a standard alkene, contrary to the order of reactivity... 

The degenerative nature of propagation results in reformation of the same active species, but with monomer consumption and chain growth. Although the monomer s thermodynamic polymerizability is independent of the mechanism, the mechanism and structure of the active species determines the rate of monomer conversion. The structure of the active species involved in carbocationic polymerizations was discussed in Section II detailed information on the reactivities of model species was presented in Chapter 2, with the conclusion that covalent precursors do not react directly with alkenes, but must first ionize to sp2-hybridized carbenium ions. Only the resulting carbenium ions can add to double bonds. 

Resulting polymers containing active species (ionic or covalent) at both ends may be converted into telechelics containing various reactive group by reaction with suitable nucleophiles [266], e.g. ... 

The term bare has been chosen here to define carbenium ions or radical cations which are formed without the corresponding anion. The difference in reactivity between these ecies and the free ions present in cationic systems involving fully dissociated entities such as carbenium ion salts, resides of course in the absence of all interactions involving the anion, namely ion pairing, coll se to give a covalent bond, etc. Most techniques available for the production of bare cations involve the expulsion of an electron from amono-mer molecule, i.e. ph3 cally-induced initiation. The present chapter deals with these techniques and their major contributions to the understanding of the chemistry and properties of active species in cationic polymerisation. 

Correlations of structures and reactivities for anionic and cationic ring-opening polymerization are reviewed. The following topics are discussed chemical structure of active species and their isomerism, determination of active centers concentration, covalent vs ionic growth and correlations between structures of active centers or monomers and their reactivities. 

These data indicate that the rate constants of reactions between various monomers and model active species, both ionic and covalent are the function of monomer basicity on the other hand no correlation between ring strain and reactivity was found (44). 

In direct analogy to the Michaelis-Menten mechanism for reaction of enzyme with a substrate, the inactivator, I, binds to the enzyme to produce an E l complex with a dissociation constant K. A first-order chemical reaction then produces the chemically reactive intermediate with a rate constant k. The activated species may either dissociate from the active site with a rate constant to yield product, P, or covalently modify the enzyme ( 4). The inactivation reaction should therefore be a time-dependent, pseudo-first-order process which displays saturation kinetics. This is verified by measuring the apparent rate constant for the loss of activity at several fixed concentrations of inactivator (Fig. lA). The rate constant for inactivation at infinite [I], itj act (a function of k2, k, and k4), and the Ki can be extracted from a double reciprocal plot of 1/Jfcobs versus 1/ 1 (Fig. IB) (Kitz and Wilson, 1962 Jung and Metcalf, 1975). A positive vertical... 

In contrast to radical polymerizations in which there is only one type of propagating species, ionic polymerizations may involve several active species, each with different reactivities and/or lifetimes. As outlined in Scheme 2, ionic polymerizations may potentially involve equilibria between covalent dormant species, contact ion pairs, aggregates, solvent separated ion pairs, and free ions. Although ion pairs involving alkali metal countercations can not collapse to form covalent species, group transfer polymerization apparently operates by this mechanism. In anionic polymerizations, free ions are much more reactive than ion pairs although the dissociation constants are quite small = 10 ) [5]. 

Another important commercial utilization of cotton etherification is in coloration of fabrics with reactive dyes [338 340]. Reactive dyes contain chromophoric groups attached to moieties that have functions capable of reaction with cotton cellulose by nucleophilic addition or nucleophilic substitution to form covalent bonds. In the nucleophilic addition reaction, an alkaline media transforms the reactive dye to an active species by converting the sulfatoethyl-... 

In spite of the living character of the anionic polymerization of PL, it was revealed that in the case of p-substituted p-lactones or medium-size cyclic esters (LA and CL) this process suffers from various side reactions such as chain transfer to monomer, racemization, segment exchange, or formation of macrocydic esters. " Finally, it has been concluded that in the so-called coordination-insertion (pseudoanionic) polymerization, initiated with covalent metal alkoxides (R MtOR c. where Mt is Zn, Al, Sn, Ti, etc.), these side reactions can be kinetically suppressed or even eliminated. Correlation of the selectivity parameters, defined as the ratio of rate constants of propagation and transfer (fep/fetr). with the reactivity of active species (fep) showed that these polymerizing systems conform to the reactivity-selectivity principle. ... 

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