Homolytic breaking

Thermal cracking takes place without a catalyst at temperatures up to 900 °C. The exact processes are complex, although they undoubtedly involve radical reactions. The high-temperature reaction conditions cause spontaneous homolytic breaking of C-C and C-H bonds, with resultant formation of smaller fragments. We might imagine, for instance, that a molecule of butane... 

The short lifetimes of carbon-centered monoradicals are generally reduced in the case of diradicals due to their propensity to form covalent bonds. It has been suggested that stable diradicals may be observable from highly strained bicyclic molecules where the TS for inversion is a diradical. Unfortunately, only persistent diradicals have been obtained in this way. Akin to this approach, in a recent attempt to generate the oxyallyl diradical, Sorensen and co-workers synthesized two substituted bicyclobutanones hoping to stretch and homolytically break the central bond using bulky substituents, which would also stabilize the diradical. Though the bicyclobutanones did not yield the desired oxyallyl derivative, the X-ray structures showed... 

Compounds I are formed by homolytic breaking of the C-C bond on the butyl chain. 

Let s first see how DFT handles a case where Hartree-Fock with its cavalier treatment of electron correlation fails badly homolytic breaking of a covalent bond (Section 5.4.1). Consider the reaction... 

Bond dissociation energies, described in Chapter 5, measure the energy required to homolytically break a bond. They are not the same as dissociation enthalpies, which measure the ability of a compound to dissociate heterolytically. Bond dissociation energies can be used to calculate dissociation enthalpies in the gas phase if other quantities are also known. 

The homolytic fragmentation of a C-H bond in the coordination sphere of the acceptor metal ion may occur via a transfer of the hydrogen to the oxygen ion at -2 oxidation state. This is a concerted action with homolytic breaking of metal - oxygen bond which transfers one electron to the metal. Without any hypothesis about the nature of the metal-oxygen bond one can write the reaction with formation of a rt-alkyl complex (as usually admitted) or a 5-alkyl complex. 

We will also briefly deal with coenzyme B this is a cobalt(III) complex that, by interacting with a number of proteins, produces an R—-CH2 radical by homolytic breaking of the Co—C bond as follows ... 

Radicalar polymerizations are very commonly used. In this case, the active site is an atom bearing a unpaired electron (free bond). The initiator is an organic material which can spontaneously split by homolytic breaking and in this way can produce radicals able to attack the monomer and thus to initiate the process. Actually, as initiators, one uses peroxides (R -O-O-R"), hydroperoxides (R -O-OH) (for instance hydrogen peroxide H202, or cumene hydroperoxide) and aliphatic azoics (R -N=R"). For each initiator, there is a domain of temperature often narrow (about 10 degrees) in which the homolytic decomposition occurs with an acceptable velocity. 

Many of homolytical reactions can occur upon pyrolysis of bound heptanol and phenyl-methanol, but only a portion of these processes was modeled using PM3 method with Cl (3 x 3) (Table 37.8). The obtained reaction energies (difference between the total energies of the reactants and the products) show that homolytical breaking of Si-OR, SiO-CR, or SiOC-CR bonds can... 

We saw earlier that in the photochemical reactions, an initiation step occurs due to absorption of a photon, favoring the homolytic breaking and forming two free radicals. In the liquid phase, these radicals are neighbors and can recombine themselves or combined with other radicals or molecules. 

The effect of this delocalization can be seen in the energy required to homolytically break a C—H bond in various organic molecules. The more stable the radical product, the more easily the bond is broken. This is easily seen in the series methyl > primary > secondary > tertiary in Table 5.3.2.2. 

For octahedral complexes the energy of activation of the reaction involving homolytic breaking of the M —C bond decreases depending upon the electronic structure in the following manner ... 

Imagine homolytically breaking each different type of C—H bond in the compound. This would produce the following radicals ... 

Further rates of homolytic breaking, equation (1), of the Cr-C bond in [(H20)5Cr-R] have been obtained. They seem to depend not only on steric effects but also on the stabilization of -R by resonance. The rates of reaction of R with [(H20)5CrR] are equal to or greater than 10 s . ... 

Upon thoroughly blending two homopolymers, such compatibilizers can sometimes be generated. Indeed, such a mechanical mixing can cause the homolytic breaking of a bonds and generate macromolecular free radicals that can give rise to block copolymers by random recombination. 

This is in fact a type of homolytic ion dissociation that is classified separately because, unlike the homolytic breaking previously shown, this dissociation does not originate from molecules. 

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