Chlorination, homolytic

However, in the presence of chlorendic anhydride, the decomposition of diethoxyacetophenone occurs only by Norrish I type cleavage, yielding benzoyl chloride, acetophenone and benzll. No Norrish Type II cleavage products were observed. The 24 hour photolysis of chlorendic anhydride in acetone using diethoxyacetophenone as the photoinitiator shows carbon-chlorine homolytic bond cleavage and the generation of chlorendic radicals as evidenced by the appearance of benzoyl chloride, chloro-... 

Reactions in which only bond breaking occurs are always endothermic. The energy required to break the covalent bonds of hydrogen or chlorine homolytically is exactly equal to that evolved when the separate atoms combine to form molecules. In the bond cleavage reaction, however, A/T is positive ... 

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... 

They argued that pre-equilibria to form Cl+ or S02C1+ may be ruled out, since these equilibria would be reversed by an increase in the chloride ion concentration of the system whereas rates remained constant to at least 70 % conversion during which time a considerable increase in the chloride ion concentration (the byproduct of reaction) would have occurred. Likewise, a pre-equilibrium to form Cl2 may be ruled out since no change in rate resulted from addition of S02 (which would reverse the equilibrium if it is reversible). If this equilibrium is not reversible, then since chlorine reacts very rapidly with anisole under the reaction condition, kinetics zeroth-order in aromatic and first-order in sulphur chloride should result contrary to observation. The electrophile must, therefore, be Cli+. .. S02CI4- and the polar and non-homolytic character of the transition state is indicated by the data in Table 68 a cyclic structure (VII) for the transition state was considered as fairly probable. 

It is thought that the chlorination proceeds through a ir-com-plex between cupric chloride and anthracene, and that this complex then undergoes homolytic dissociation. Hence aromatic rings subject to attack by chlorine atoms can be chlorinated in this way. Thus one can convert pyrene to 1-chloropyrene (90% yield), but phenanthrene is not chlorinated. Analogous procedures using cupric bromide lead to 9-bromoanthracene (99% yield) and 1-bromopyrene (94% yield).7... 

When a covalent bond breaks to produce radicals, i.e. one electron of the bond pair goes to each atom, homolytic fission has occurred. These highly reactive chlorine radicals attack the methane molecules. 

In CfE Higher Chemistry, you came across free radicals when we considered the mechanism of the substitution reaction between methane and chlorine In the presence of ultraviolet light. You will recall that the initiation step In the mechanism Is the homolytic fission of chlorine molecules to generate chlorine free radicals. 

Chlorine and bromine react under favourable conditions (activation by other substituent, appropriate nucleophile). Iodine may also be photosubstituted by nucleophiles but is easily induced to enter into homolytic reaction pathways. One has to bear in mind that the heavier substituents (iodine, bromine and even chlorine) increase the rate of intersystem crossing which, depending on the conditions, increases or decreases the quantum yield. 

Chlorine dioxide gas is unstable and can rapidly decompose at high concentrations. It also decomposes rapidly to chlorine and oxygen with exposure to mild heat. Chlorine dioxide will decompose upon exposure to sunlight (Vogt et al. 1986). The gas-phase absorption spectrum for chlorine dioxide is the same as in aqueous solution (Kaczur and Cawfield 1993). The primary photochemical reaction of CIO2 in the gas phase corresponds to homolytic scission of one of the chlorine-oxygen bonds (i.e., C102 CIO +... 

Eisch [66AHC(7)1] considers that the relatively weak bonds in fluorine and iodine predispose these halogens to homolytic processes more than with chlorine and bromine. 

Thiepane (35) has been converted to 2-acetoxythiepane (137) by a homolytic mechanism using f-butyl peracetate in the presence of a copper(I) ion catalyst (67JCS(C)1130). Similarly, a-chlorination of thiepane (35) by N- chlorosuccinimide (NCS) to yield 2-chlorothiepane (132) probably occurred by a free radical pathway (Scheme 27) (69JHCU5). 

The enthalpy A// (Section 3.8) is seldom related to A// of the reaction. In this reaction, however, AH and AH are identical. In simple homolytic dissociations of this type, the free radicals formed have the same enthalpy as does the transition state. On this basis alone, iodine, having the smallest AH and AH, should react fastest. Similarly, chlorine, with the largest AH and AH, should react slowest. But the actual order of reaction rates is... 

There are virtually no reports of homolytic reactions on the triazolo-pyridines. Unsuccessful attempts have been made to treat triazolopyridine (1) with methyl radicals,25 and a free radical mechanism is suggested as a possibility in the replacement of the methylthio group by chlorine (Section IV,C).208... 

Perhaps the best-known photoaddition reaction of benzene is that with chlorine to produce hexachlorocyclohexane (3.37). of which one steroisomer is widely used as an active component in insecticides. However, this reaction does not involve the excited state of benzene chlorine absorbs light and cleaves homolytically to give chlorine atoms, which then attack the ground state of benzene, leading to overall addition. 

Homolytic dissociations. The dissociation of a diatomic molecule is probably the most simple of all chemical reactions thus the chlorine molecule Cl2 leads to atomic chlorine Cl ... 

In terms of tons of chemicals per year, this is by far the most important industrial process in which a light-induced reaction plays a key role. The photochemical reaction itself is extremely simple, being the homolytic dissociation of molecular chlorine, Cl2, into Cl atoms,... 

A free radical chain reaction proceeds through a succession of free radicals. In the photochemical chlorination of an alkane, the initiating step is the homolytic lission of chlorine molecules to produce chloroalkanc molecules and chlorine free radicals. These two reactions constitute the propagating step. However, the chlorine free radicals may also combine to form chlorine molecules or react with the alkane free radicals to form chloroalkane molecules. Both of these reactions constitute terminating steps of the chain reaction. Il should be noted, however, that the foregoing sequence cannot take place in the dark. Exposure to light allows the series of reactions then to proceed rather violently. 

Free radical (homolytic) reactions involve species with an unpaired electron. The ultraviolet-light-initiated reaction of methane with chlorine is an example (Scheme 4). 

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