Chlorine radicals bonding

Most chlorofluorocarbons are hydrolytically stable, CCI2F2 being considerably more stable than either CCl F or CHCI2F. Chlorofluoromethanes and ethanes disproportionate in the presence of aluminum chloride. For example, CCl F and CCI2F2 give CCIF and CCl CHCIF2 disproportionates to CHF and CHCl. The carbon—chlorine bond in most chlorofluorocarbons can be homolyticaHy cleaved under photolytic conditions (185—225 nm) to give chlorine radicals. This photochemical decomposition is the basis of the prediction that chlorofluorocarbons that reach the upper atmosphere deplete the earth s ozone shield. 

Initiation Irradiation with ultraviolet light begins the reaction by breaking the relatively weak Cl-Cl bond of a small number of Cl2 molecules to give a few reactive chlorine radicals. 

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. 

A study by Abu-Raqabah and Symons (Abu-Raqabah, A. Symons, M. C. R., J. Am. Chem. Soc., 1990, 112, 8614) has characterized the pyridine-chlorine atom three-electron bonded species Py <— Cl by ESR and UV spectroscopy. In an earlier paper, Breslow and co-workers (Breslow, R. Brandi, M. Hunger, J. Adams, A. D., J. Am. Chem. Soc., 1987,109, 3799) considered ring acylated pyridine-chlorine radicals to be n radicals and anticipated special stability for the 4-carboalkoxypyridine-chlorine radical. 

Polar effects can also be important in atom transfer reactions. 4 In an oft-cited example (Scheme 13), the methyl radical attacks the weaker of the C—H bonds of propionic acid, probably more for reasons of bond strength than polar effects. However, the highly electrophilic chlorine radical attacks the stronger of the C—H bonds to avoid unfavorable polar interactions. As expected, the hydroxy hydrogen remains intact in both reactions. 

Sometimes reactions take place that involve the movement of single electrons rather than pairs of electrons. Such reactions are called radical reactions. For example, a chlorine molecule can be split into two chlorine radicals on treatment with light. One of the original bonding electrons ends up on one chlorine radical and the second bonding electrons ends up on the other chlorine radical. The movement of these single electrons can be illustrated by using half curry arrows rather than full curly arrows ... 

The radical relay process also works with other template types. Thus, the thioether unit in 92 directed chlorination of C-14 by S02C12 [165]. Also, the sulfur in the thiox-anthone template of 93 directed the radical relay process to C-9 [166]. The thiophene sulfur in 94 was able to direct chlorination to C-9 in all three attached steroids [167]. In all these cases, an intermediate is formed with a chlorine atom bonded to sulfur. 

When a chlorine radical collides with a methane molecule, it abstracts (removes) a hydro- Propagation Formation of products gen atom from methane. One of the electrons in the C — H bond remains on carbon while with regeneration of reactive the other combines with the odd electron on the chlorine atom to form the H—Cl bond, intermediates. 

Transition states have high energies because bonds must begin to break before other bonds can form. The following equation shows the reaction of a chlorine radical with methane. The transition state shows the C — H bond partially broken and the H—Cl bond partially formed. Transition states are often enclosed by brackets to emphasize their transient nature. 

Although chlorination shows a preference for a substitution (the a position is the benzylic carbon bonded to the benzene ring), the chlorine radical is too reactive to give entirely benzylic substitution. Mixtures of isomers are often produced. In the chlorination of ethylbenzene, for example, there is a significant amount of substitution at the (3 carbon. 

Fig. 8-14. Example of the oxidation of cellulose by chlorine radicals. After oxidation at C-1, the glycosidic bond is cleaved with formation of a gluconic acid end group. R denotes cellulose chain.

How can we explain the ratios of products that are formed The key is to look at the relative stabilities of the radicals involved in the reaction and the strengths of the bonds that are formed and broken. First, the chlorination of propane. A chlorine radical, produced by photolysis, can abstract either a primary hydrogen atom, from the end of the molecule, or a secondary hydrogen atom, from the middle. For the first process, we have these energy gains and losses. 

RhCl(PPh3) 3 The chlorine radical (Cl ) accepts an electron from rhodium metal (electronic configuration Ad1,5s2) to give Cl and Rh+. The chloride ion then donates two electrons to the rhodium ion to form a dative or a coordinate bond. Each PPh3 donates a lone pair of electrons on the phosphorus atom to the rhodium ion. The total number of electrons around rhodium is therefore 8 + 2 + 3X2=16, and the oxidation state of rhodium is obviously 1 +. The other way of counting is to take the nine electrons of rhodium and add one electron for the chlorine radical and six for the three neutral phosphine ligands. This also gives the same electron count of 16. 

Initiation The initiation step starts oflF the reaction by producing a small number of reactive radicala. In the present cose, the relatively weak Cl-CL bond is hnmolytically broken by irradiation with ultraviolet light. Two reactive chlorine radicals ore produced ... 

Homolytic bond cleavage has been reported for the reaction between CuCl2 and thiourea (thu) which leads to formation of a Cu(I)-thiourea complex and chlorine radicals, which are converted into free chlorine 20) ... 

Thus, the Cl-Cl bond AH° = 58 kcal/mol), which is weaker than either the C-C or C- H bond in ethane (AH° = 88 and 98 kcal/mol, respectively), is broken to form two chlorine radicals. 

A chlorine radical can also abstract either a 1° or a 2° hydrogen from propane, generating either a 1° radical or a 2° radical. Calculating A//° using bond dissociation energies reveals that both reactions are exothermic. 

In its relative reactivity toward toluene, ethylbenzene and cumene the more highly substituted 1-methyl-2,2-diphenylcyclopropyl radicaP , derived from the decomposition of the precursor diacyl peroxide, resembles the chlorine radical more than it does the phenyl radical (Table 3). Similarly, comparison of the relative reactivities of primary, secondary and tertiary aliphatic hydrogens toward chlorine atoms (1.0 3.6 4.2) and phenyl radicals (1.0 9.3 44) with the relative reactivities of the C-H bond in the methanol/ethanol/2-propanol series toward the 1-methyl-2,2-diphenylcyclopropyl radical (1.0 2.4 15) further confirms the low selectivity of the cyclopropyl radical. Again, this radical resembles the chlorine atom in its reactivity more than it does the phenyl radical. 

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