Reactions heterolytic

Many nitroxides are very stable under normal conditions, and heterolytic reactions can be carried out on other fimctional groups in the molecule without destroying the nitroxide group. Nitroxides are very useful in biochemical studies by virtue of being easily detected paramagnetic probes. ... 

Structure-reactivity relationships can be probed by measurements of rates and equiUbria, as was diseussed in Chapter 4. Direct comparison of reaction rates is used relatively less often in the study of radical reactions than for heterolytic reactions. Instead, competition methods have frequently been used. The basis of competition methods lies in the rate expression for a reaction, and the results can be just as valid a comparison of relative reactivity as directly measured rates, provided the two competing processes are of the same kinetic order. Suppose that it is desired to compare the reactivity of two related compounds, B—X and B—Y, in a hypothetical sequence ... 

The interaction between a solute species and solvent molecules is called solvation, or hydration in aqueous solution. This phenomenon stabilizes separated charges and makes possible heterolytic reactions in solution. Solvation is, therefore, an important subject in solution chemistry. The solvation of ions has been most thoroughly studied. 

The first two examples clearly indicate that slight modifications (02 to N2 atmosphere, unsubstituted to 4-nitrosubstituted benzenediazonium ion) can change a heterolytic reaction into a homolytic one. The changeover from heterolytic to homolytic dediazoniation will be discussed in more detail in Section 8.7 of this chapter. The ratio of products in the third example suggests, as indicated in Scheme 8-2, that a DN + AN substitution (A), a reaction via an aryne (B), a o-substitution of the type described by Kovacic and Gormisk (1966) (D), and a... 

Szele and Zollinger (1978 b) have found that homolytic dediazoniation is favored by an increase in the nucleophilicity of the solvent and by an increase in the elec-trophilicity of the P-nitrogen atom of the arenediazonium ion. In Table 8-2 are listed the products of dediazoniation in various solvents that have been investigated in detail. Products obtained from heterolytic and homolytic intermediates are denoted by C (cationic) and R (radical) respectively for three typical substituted benzenediazonium salts and the unsubstituted salt. A borderline case is dediazoniation in DMSO, where the 4-nitrobenzenediazonium ion follows a homolytic mechanism, but the benzenediazonium ion decomposes heterolytically, as shown by product analyses by Kuokkanen (1989) the homolytic process has an activation volume AF = + (6.4 0.4) xlO-3 m-1, whereas for the heterolytic reaction AF = +(10.4 0.4) x 10 3 m-1. Both values are similar to the corresponding activation volumes found earlier in methanol (Kuokkanen, 1984) and in water (Ishida et al., 1970). 

Heteroaromatic diazonium salts can also be used for Gomberg-Bachmann aryla-tions. Fukata et al. (1973) refluxed 3,5-dimethyl-4-diazopyrazole (10.27) in benzene and obtained 3,5-dimethyl-4-phenylpyrazole (10.28, 36%), biphenyl (10.29, 17%), 3,5-dimethylpyrazole (10.30, 12%), and pyrazolo[4,3-c]pyrazole (10.31, 15%). In nitrobenzene the three isomeric 3,5-dimethyl-4-(nitrophenyl)-pyrazoles were formed in the ratio o m p = 10 3 3. In the opinion of Fukata et al. this ratio and the course of the reaction indicate a homolytic process. The present author thinks that the data do not exclude a competitive heterolytic reaction with the pyrazolyl cation, because equal amounts of substitution of nitrobenzene in the 3- and 4-positions are not typical for a homolytic aromatic substitution. 

In any heterolytic reaction in which a new carbon-carbon bond is formed one carbon atoms attacks as a nucleophile and the other as an electrophile. The classification of a given reaction as nucleophilic or electrophilic is a matter of convention and is usually based on analogy. Although not discussed in this chapter, 11-12-11-28 and 12-14-12-19 are nucleophilic substitutions with respect to one reactant, though, following convention, we classify them with respect to the other. Similarly, all the reactions in this section (10-93-10-123) would be called electrophilic substitution (aromatic or aliphatic) if we were to consider the reagent as the substrate. 

Also in this case, a heterolytic reaction according to... 

Among the two sorts of interaction mentioned above, one of the reactants happens to be an electron-donor and the other an electron-acceptor, as is the case in most heterolytic reactions. In such cases the charge-transfer effect will perhaps predominate over the polarization effect. Even in homolytic interactions, the importance of the mutual charge transfer is not to be disregarded. 

Having a weak O—O bond, peroxides split easily into free radicals. In addition to homolytic reactions, peroxides can participate in heterolytic reactions also, for example, they can undergo hydrolysis under the catalytic action of acids. Both homolytic and heterolytic reactions can occur simultaneously. For example, perbenzoates decompose into free radicals and simultaneously isomerize to ester [11]. The para-substituent slightly influences the rate constants of homolytic splitting of perester. The rate constant of heterolytic isomerization, by contrast, strongly depends on the nature of the para-substituent. Polar solvent accelerates the heterolytic isomerization. Isomerization reaction was proposed to proceed through the cyclic transition state [11]. 

The rate constants of these reactions were found to be very close kA = 2.0 x 10 5 s 1 and kis = 2.2x 10 5 s 1 (//-nonane, 403 K). The competition between homolytic and heterolytic reactions influences the effectiveness of initiation. When the heterolytic isomerization of... 

Acids are well known as efficient catalysts of various heterolytic reactions (hydrolysis, esterification, enolyzation, etc. [225,226]). They catalyze the heterolytic decay of hydroperoxides formed during oxidation. For example, they catalyze the decomposition of cumyl hydroperoxide into phenol and acetone (important technological reaction) [5]. 

The stoichiometry of this reaction is usually close to unity [6-9]. Thus, cumyl hydroperoxide oxidizes triphenyl phosphite in the stoichiometry A[ROOH]/A[Ph3P] from 1.02 1 to 1.07 1, depending on the proportion between the reactants [6], The reaction proceeds as bimolecular. The oxidation of phosphite by hydroperoxide proceeds mainly as a heterolytic reaction (as follows from conservation of the optical activity of reaction products [5,11]). Oxidation is faster in more polar solvents, as evident from the comparison of k values for benzene and chlorobenzene. Heterolysis can occur via two alternative mechanisms... 

The concurrent slow homolytic reaction gives rise to free radicals [14]. The occurrence of the homolytic reaction can be revealed by the consumption of free radical acceptors [8,15], CL [16], or NMR spectroscopy [17,18]. The introduction of phosphite into the hydroperoxide-containing cumene causes an initiation, pro-oxidative effect related to the formation of free radicals [6]. The yield of radicals from aliphatic phosphites is much lower (0.01-0.02%) than that from aromatic phosphites (up to 5%) [17]. The homolytic reaction of phosphites with hydroperoxide has a higher activation energy than the heterolytic reaction, which results in the predominance of the former reaction at elevated temperatures. 

As there are fairly high concentrations of thiols in many tissues this reaction may occur whenever a nitrosothiol is used in ex vivo or in vivo experiments. Thus, the nitrosothiol used may be merely a storage vehicle for NO+ and NO+ passes in a heterolytic reaction to a tissue thiol, probably cysteine or glutathione. It is this nitrosothiol that cleaves homolytically to give the radical species NO. Thus the reaction... 

Molecules in heterolytic (polar) reactions form and break bonds by "coordination" and molecules in homolytic (nonpolar or free radical) reactions form and break bonds by "colligation."75 (Two more new terms ) Heterolytic reactions occur mostly in solutions, usually involving ion formation and electrophilic or nucleophilic reactions homolytic reactions occur mostly in gases and do not involve ions because less energy is required to distance the atoms into neutral radicals.76... 

B. P and tt,as Reactivity Indices To make a preliminary assessment of the significance of P and 71, in discussions of heterolytic reactions, we use the usual point charge model... 

A heterolytic reaction in which a nucleophilic reagent brings (i.e., attacks with) a free pair of electrons on a molecular entity. 

Similar relative reactivities for homolysis versus heterolysis have been observed in the fragmentation of l-NMOPh versus 1-NpOBz and 2-NMOPh versus l-NpOBz. In these cases, both the heterolytic and the homolytic pathways were observed. However, the heterolytic reactions were reported to be about 10 times faster than the homolytic reactions. 

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