Unimolecular heterolysis

The process of formation of a coordinate-covalent bond the reverse of unimolecular heterolysis. 3. The number of ligands surrounding a central atom. See also... 

Any process that generate one or more ions by (a) a unimolecular heterolysis of a neutral molecular entity into two or more ions or (b) a heterolytic substitution reaction involving neutral molecules. See also Ionization Energy Dissociation... 

A long series on the kinetics and mechanism of the unimolecular heterolysis of commercial haloorganic compounds has continued in a study of the effect of bromide salts and lithium perchlorate on the ionization rate of benzhydryl bromide in y-butyrolactone and acetone.122 The verdazyl indicator method was applied. The nature of special and normal salt effects has also been discussed.123... 

Solvent effects on the kinetics and mechanism of unimolecular heterolysis of commercial organohalogen compounds have been investigated.9-11 The reaction rate is satisfactorily correlated by parameters for polarity, electrophilicity, and cohesion of the solvent, whereas the solvent nucleophilicity and polarizability exert no effect. 

Hydrolysis and Other Reactions and Features.- The rate limiting step in the acid-catalysed hydrolysis of alkyl a- and g-D-fructo-furanosldes has been shown to be the unimolecular heterolysis of... 

The reactions involved are unimolecular, and the cyclohexenyl derivative 3 undergoes solely the spontaneous heterolysis while both spontaneous heterolysis and ligand coupling occur with the iodane 14. The relative contributions of the two reactions of 14 depend on the solvent polarity. The results summarized in Table I show that the iodonium ion and the counteranion are in equilibrium with the hypervalent adduct, X3-iodane. The equilibrium constants depend on the identity of the anion and the solvent employed, and the iodane is less reactive than the free iodonium ion as the k /k2 raios demonstrate. Spontaneous heterolysis of 3 occurs more than 100 times as fast as th t of the adduct 14 as observed in methanol the leaving ability of the iodonid group is lowered by association by more than 100 times. 

A further support for the identification of the species responsible for the unimolecular conductance increase in terms of a chlorine-containing radical is the fact that in a blank experiment, i.e. one in which chloride is left out, a slow conductance increase is not observed and the overaU conductance yield is only half of that in the presence of chloride. Since in isobutene-saturated aqueous solution the lifetime of SOi is only 90 ns due to its rapid reaction with the alkene (as determined by optical experiments at 450 nm) [46], the non-observability of a unimolecular conductance increase means that the rate constant for heterolysis of the SO adduct to isobutene is 10 s" (cf. Eq. 32) ... 

SnCl (presumably in the selvedge) and apparently from successive losses of Cl (free gas phase). In these cases, heterolysis of the substrate (e.g., to give SnCl and SnClj+) followed by solvation in the selvedge and unimolecular fragmentation via elimination of stable neutral molecules accounts for much of the behavior observed in FAB. 

There is some confusion about the various theoretical and experimental distinctions between the Sjj 1 and SN 2 mechanisms. Since molecularity is the number of molecules necessarily undergoing covalency change during the rate determining step (Ingold, 1969), the unimolecular reaction (SN 1) involves rate determining heterolysis of the R—X bond (kx, Fig. 2) without assistance from nucleophilic attack. This definition is independent of the nature of the first intermediate, which in many cases is probably a contact ion pair rather than a free (i.e. symmetrically solvated) carbocation (see... 

Evidence was then obtained that protonation of the oxygen atom is a rapid, equilibrium-controlled process - and that the slow, rate-determining, heterolysis step is unimolecular these facts are consistent with both A-1 mechanisms (see equation 3) and eliminate aU others. (A third mechanism has, however, been proposed by Rydon and coworkers for the hydrolysis of some aryl glycosides it is discussed in Section IV,2 see pp. 79-81). 

It is found that esters which are known to undergo unimolecular alkyl-oxygen heterolysis will alkylate hydrogen peroxide and alkyl hydroperoxides (3, 4) for example, sodium 1,2,3,4-tetrahydro-l-naphthyl phthalate in 90% hydrogen peroxide rapidly yields 1,2,3,4-tetrahydro-l-naphthyl hydroperoxide ... 

In very strongly acidic solution, a unimolecular ifiechanism involving acyl-oxygen cleavage of the conjugate acid can operate.This mechanism is the result of decreased availability of nucleophilic water in the strongly acidic medium. The products of heterolysis are the alcohol (which is subsequently protonated) and an acylium ion ... 

In the first step of an El reaction (elimination, unimolecular see Scheme 10.15) there is a heterolysis of a C-LG bond to form a carbenium ion. Carbenium ions are prone to rearrangement, and so rearranged products are always possible in an El reaction. The second step involves deprotonation of an adjacent hydrogen by any base, shown here as water. The initial heterolytic cleavage is as in an SnI reaction, but now an elimination occurs due to the deprotonation step after heterolysis. 

Heterolytic reactions (2.1.2) and (2.1.3) are basically different. Reaction (2.1.2) requires an entering ligand that is a nucleophile, i.e., a Lewis base. Such substitutions are called nucleophilic and carry the well-known symbol Sf,j. They can be unimolecular (S l) or bimolecular (8 2), as already discussed in section 1.7. On the other hand, heterolysis as in (2.1.3) leaves the metal center with an electron pair. Such substitutions are called electrophilic and carry the symbol Sg. Analogously to nucleophilic substitutions (S l and Sf,j2), electrophilic substitutions can be unimolecular or bimolecular, and carry symbols Sgl or Sg2. 

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