Homolytic

Other liquid inorganic compounds show the auto-dissociation characteristic of water and liquid ammonia for example, dinitrogen tetroxide (p. 231), as well as undergoing the more familiar homolytic dissociation... 

Configuration correlation diagram for homonuclear case in which homolytic bond cleavage is energetically favored. 

State correlation diagram for heteronuclear case in which both heterolytic bond cleavage products are energetically favored relative to homolytic cleavage. 

TABLE 111-35. ISOMER RATIOS ANP RELATIVES RATES FOR HOMOLYTIC 2-THlAZOLYLATTON OF ALKYLBENZENES (207)... 

TIVITIES (fc), AND fjf RATIOS, FOR HOMOLYTIC THIAZOL-2-YLATION AND 5-SUBSTITUTED THlAZOL-2-YLATlON OF ALKYLBENZENES n 76)... 

Some of the evidence indicating that alkyl substituents stabilize free radicals comes from bond energies The strength of a bond is measured by the energy required to break It A covalent bond can be broken m two ways In a homolytic cleavage a bond between two atoms is broken so that each of them retains one of the electrons m the bond... 

We assess the relative stability of alkyl radicals by measuring the enthalpy change (AH°) for the homolytic cleavage of a C—H bond m an alkane... 

The energy required for homolytic bond cleavage is called the bond dissociation energy (BDE) A list of some bond dissociation energies is given m Table 4 3... 

Step 1 Homolytic dissociation of a peroxide produces alkoxy radicals that serve as free radical initiators... 

Homolytic cleavage (Section 4 16) Dissociation of a two electron covalent bond in such a way that one electron is re tamed by each of the initially bonded atoms... 

Any one of these expressions gives the rate of initiation Rj for the particular catalytic system employed. We shall focus attention on the homolytic decomposition of a single initiator as the mode of initiation throughout most of this chapter, since this reaction typifies the most widely used free-radical initiators. Appropriate expressions for initiation which follows Eq. (6.6) are readily derived. 

Inorg anic Compounds. Hydrogen chloride reacts with inorganic compounds by either heterolytic or homolytic fission of the H—Cl bond. However, anhydrous HCl has high kinetic barriers to either type of fission and hence, this material is relatively inert. 

Free-Radical Formation. Hydrogen peroxide can form free radicals by homolytic cleavage of either an O—H or the O—O bond. 

Diall l Peroxydicarbonates. Some commercially available diaLkyl peroxydicarbonates and their corresponding 10-h half-life temperatures (deterrnined in trichloroethylene solutions) are Hsted in Table 7 (45). These peroxides are active at low temperatures and initially undergo homolytic cleavage to produce alkoxycarbonyloxy radical pairs that may subsequendy decarboxylate to produce alkoxy radicals ... 

Other Hydroperoxides. Several hydrotrioxides including alkyl hydrotrioxides, R—OOOH, have been reported (63,64). There is strong spectroscopic evidence that a-cumyl hydrotrioxide [82951-48-2] is produced in the low temperature ozonization of cumene. Homolytic decomposition of a-cumyl hydrotrioxide in cumene/acetone-hindered phenol resulted in cumyl alcohol as the only organic product (65). Based on the... 

The alkylation of pyridine [110-86-1] takes place through nucleophiUc or homolytic substitution because the TT-electron-deficient pyridine nucleus does not allow electrophiUc substitution, eg, Friedel-Crafts alkylation. NucleophiUc substitution, which occurs with alkah or alkaline metal compounds, and free-radical processes are not attractive for commercial appHcations. Commercially, catalytic alkylation processes via homolytic substitution of pyridine rings are important. The catalysts effective for this reaction include boron phosphate, alumina, siHca—alurnina, and Raney nickel (122). 

The piefeiied route depends upon the avajlabihty of a hydrogen atom in the beta-position to the thiol group. In other words, a-toluenethiol (in toluene) decomposes to give 1,2-diphenylethane and hydrogen sulfide, via the homolytic route, whereas 2-methyl-2-propanethiol decomposes to give 2-methyl-1-propene and hydrogen sulfide. 

Initiation. Free-radical initiators are produced by several processes. The high temperatures and shearing stresses required for compounding, extmsion, and molding of polymeric materials can produce alkyl radicals by homolytic chain cleavage. Oxidatively sensitive substrates can react directly with oxygen, particularly at elevated temperatures, to yield radicals. 

Propagation. Propagation reactions (eqs. 5 and 6) can be repeated many times before termination by conversion of an alkyl or peroxy radical to a nonradical species (7). Homolytic decomposition of hydroperoxides produced by propagation reactions increases the rate of initiation by the production of radicals. 

Autoca.ta.Iysis. The oxidation rate at the start of aging is usually low and increases with time. Radicals, produced by the homolytic decomposition of hydroperoxides and peroxides (eqs. 2—4) accumulated during the propagation and termination steps, initiate new oxidative chain reactions, thereby increasing the oxidation rate. 

Thermally induced homolytic decomposition of peroxides and hydroperoxides to free radicals (eqs. 2—4) increases the rate of oxidation. Decomposition to nonradical species removes hydroperoxides as potential sources of oxidation initiators. Most peroxide decomposers are derived from divalent sulfur and trivalent phosphoms. 

Photochemical Reactions. The photochemistry of chlorine dioxide is complex and has been extensively studied (29—32). In the gas phase, the primary photochemical reaction is the homolytic fission of the chlorine—oxygen bond to form CIO and O. These products then generate secondary products such as chlorine peroxide, ClOO, chlorine, CI2, oxygen, O2, chlorine trioxide [17496-59-2] CI2O2, chlorine hexoxide [12442-63-6] and... 

Photochromism Based on Dissociation Processes. Both heterolytic and homolytic dissociation processes can result in the generation of a photochromic system. An example of an heterolytic process is the reversible formation of triphenylmethyl cation, by photolysis of... 

The classical example of a photochromic process involving an homolytic dissociation is the formation of a red-purple free radical by photolysis of bis(2,4,5-triphenyliniida2ole) [63245-02-3] C 2B3qN (2 )-... 

Homolytic acylation of ethyl pyridazine-4-carboxylate is a convenient general method for preparation of 4-acylpyridazines (Scheme 42) (79M365). 

An interesting method for the substitution of a hydrogen atom in rr-electron deficient heterocycles was reported some years ago, in the possibility of homolytic aromatic displacement (74AHC(16)123). The nucleophilic character of radicals and the important role of polar factors in this type of substitution are the essentials for a successful reaction with six-membered nitrogen heterocycles in general. No paper has yet been published describing homolytic substitution reactions of pteridines with nucleophilic radicals such as alkyl, carbamoyl, a-oxyalkyl and a-A-alkyl radicals or with amino radical cations. 

It is estimated that thiophene reacts with phenyl radicals approximately three times as fast as benzene. Intramolecular radical attack on furan and thiophene rings occurs when oxime derivatives of type (112) are treated with persulfate (8UCS(Pt)984). It has been found that intramolecular homolytic alkylation occurs with equal facility at the 2- and 3-positions of the thiophene nucleus whereas intermolecular homolytic substitution occurs mainly at position 2. 

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