Iodine reaction + halogen atoms

The standard entropy change for the atom-molecule reactions is in the range 5-20 mole and the halogen molecule dissociation has an eiiU opy change of about 105 e.u. The halogen molecule dissociation energy decreases from chlorine to iodine, but the atom-molecule reactions become more endothermic from chlorine to iodine, and this latter effect probably influences the relative contributions to the mechanism from chain reaction and biinolecular reaction. 

The promoting effect of Reaction 5 should be strongest with chlorine and weakest with iodine atoms since the activation energy of this reaction with respect to the different halogen atoms is (2) ... 

Other bimolecular reactions of complex systems, such as those of benzene and iodine and acid-base reactions, have also been studied. Currently, we are examining the inelastic and reactive collision of halogen atoms with polyatomics (e.g., CH3I). Other groups at the National Institutes of Science and Technology and at the University of Southern California have studied a new class of reactions O + CH4 — [CH3OH] — CH3 + OH and H + ON2 — HO + N2 or HN + NO. 

In bromopentafluorobenzene and pentafluoroiodobenzene the main reaction with methoxide ions is the replacement of fluorine in the 4-position relative to the halogen atom (bromine and iodine) that gives products 8 and 9.85 Replacement at the 2-position occurs to a smaller extent and decreases along the series chlorine > bromine > iodine hydrogen.85,86... 

The mechanism of this reaction shows that excitation of the substrate gave an n,n triplet state, but this excited state was unable to dissociate the carbon-iodine bond. This was demonstrated by showing that the n,n triplet state, when sensitized by chrysene, did not produce coupling products. Probably, the reaction occurred in an excited a,a triplet state mainly localized on the carbon-iodine bond, and the interaction between this triplet state and aromatic compounds led to homolytic cleavage of the C-I bond with the formation of both a 5-thienyl radical and a complex between the aromatic compound and the halogen atom. The formation of this complex was demonstrated by the presence of a short-lived transient with Amax = 510 nm, showing a second-order decay kinetics and a half-life of ca. 0.4 (is in laser flash photolysis. The thienyl radical thus formed... 

Finally, the oxidation of DMS by halogen atoms and halogen oxides is considered. Following a model of iodine chemistiy in the marine boundary layer by Chameides and Davis (3D) which indicated concentrations of the IO radical in the 0.1 -10 pptv range, the localized oxidation of DMS in the marine environment by IO could be significant (see the chapters by Barnes et al.). This became apparent after recent laboratory studies of the reaction... 

The title reactions offer a possibility for exchanging the halogen atom in aryl halides (Hal = Cl, Br, I) first with a metal (MgHal, Li) and then with an electrophile. It is generally easier to introduce bromine than chlorine or iodine into aromatic compounds. Accordingly, functionalizations of aryl bromides are the preparatively most important examples of the title reaction. 

To facilitate the discussion in the following sections we start by summarizing the main halogen reaction cycles that are of importance for the troposphere. In the following the species X represents chlorine, bromine, or iodine. If two halogens occur in the same reaction we use X for the first and Y for the second halogen atom. 

This reaction is given also by bromine and iodine, as well as by all substances containing halogen atoms in the molecule. The sensitivity in these cases depends only on the quantity of halogen present in the substance under test. 

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