Iodine atoms, reactions

The calculations assumed that Reactions 1 to 12 occur in the spur while reactions of iodine atoms (Reactions 13 to 19) are neglected. 

Kelley and Rentzepis [297] have recently studied the recombination of iodine atoms in liquid and fluid xenon over times to 150 ps after photolysis. The iodine molecule can be biphotonically dissociated through the state to produce geminate pairs with larger initial separations. Some degree of spin relaxation of excited iodine atoms ( Pi/2) produced by biphotonic excitation may occur and reduce the probability of recombination. There is also evidence that the 11 state of I2 may be collisionally predissociated and that recombination may be more rapid than the rate of vibrational relaxation of the excited 12 state in polyatomic solvents (see also ref. 57). Despite these complications, several workers have attempted to model the time dependence of the recombination (or survival) probability of iodine atom reactions. The simple diffusion equation analysis of recombination probabilities [eqn. 

D. M. Golden and S. W. Benson, Free-radical and Molecule Thermochemistry from Studies of Gas-phase Iodine-atom Reactions , Chem. Rev., 1969, 69, 125. Handbook of Chemistry and Physics, 50th Edition , ed. R. C. West, The Chemical Rubber Co., Cleveland, 1969. 

The most widely used reactions are those of electrophilic substitution, and under controlled conditions a maximum of three substituting groups, e.g. -NO2 (in the 1,3,5 positions) can be introduced by a nitric acid/sul-phuric acid mixture. Hot cone, sulphuric acid gives sulphonalion whilst halogens and a Lewis acid catalyst allow, e.g., chlorination or brom-ination. Other methods are required for introducing fluorine and iodine atoms. Benzene undergoes the Friedel-Crafts reaction. ... 

Although the transition to difhision control is satisfactorily described in such an approach, even for these apparently simple elementary reactions the situation in reality appears to be more complex due to the participation of weakly bonding or repulsive electronic states which may become increasingly coupled as the bath gas density increases. These processes manifest tliemselves in iodine atom and bromine atom recombination in some bath gases at high densities where marked deviations from TronnaF behaviour are observed [3, 4]. In particular, it is found that the transition from Lto is significantly broader than... 

Consequently, the reaction yield F in figure B2.5.15 is shown as a fiinction of the fluence, F. At the end of a laser-pulse sequence with a typical fluence F 3 J cm, practically 100% of the CF I is photolysed. As described in section B2.5.4.3. the product-level distribution of the iodine atoms fonned in this type of reaction can be detemiined... 

He Y, Pochert J, Quack M, Ranz R and Seyfang G 1995 Dynamics of unimolecular reactions induced by monochromatic infrared radiation experiment and theory for C F XI—> C F X + I probed with hyperfine-, Doppler- and uncertainty limited time resolution of iodine atom infrared absorption J. Chem. Soc. Faraday Discuss. 102 275-300... 

Cyclizations involving iodine-atom transfers have been developed. Among the most effective examples are reactions involving the cyclization of 6-iodohexene derivatives. The 6-hexenyl radical generated by iodine-atom abstraction rapidly cyclizes to a cyclo-pentylmethyl radical. The chain is propagated by iodine-atom transfer. 

The reactivities of the substrate and the nucleophilic reagent change vyhen fluorine atoms are introduced into their structures This perturbation becomes more impor tant when the number of atoms of this element increases A striking example is the reactivity of alkyl halides S l and mechanisms operate when few fluorine atoms are incorporated in the aliphatic chain, but perfluoroalkyl halides are usually resistant to these classical processes However, formal substitution at carbon can arise from other mecharasms For example nucleophilic attack at chlorine, bromine, or iodine (halogenophilic reaction, occurring either by a direct electron-pair transfer or by two successive one-electron transfers) gives carbanions These intermediates can then decompose to carbenes or olefins, which react further (see equations 15 and 47) Single-electron transfer (SET) from the nucleophile to the halide can produce intermediate radicals that react by an SrnI process (see equation 57) When these chain mechanisms can occur, they allow reactions that were previously unknown Perfluoroalkylation, which used to be very rare, can now be accomplished by new methods (see for example equations 48-56, 65-70, 79, 107-108, 110, 113-135, 138-141, and 145-146)... 

The first representatives of this group of compounds, 1,5-benzotelluroazepinones 57, have been prepared in 17% yield by the reaction between 2-iodopropyolanilides and NaHTe (98H631). The reaction proceeds, most probably, as nucleophilic substitution of the iodine, resulting in telluroles 58 and the subsequent nucleophilic addition of a hydrotelluride group to the triple bond. An alternative mechanism involving initial addition of NaTeH to the triple bond followed by the nucleophilic substitution of the iodine atom was mled out because the anilides PhNHCOC=CR do not react with NaTeH under the conditions at which the heterocycles 57 were obtained. Neither of the adducts PhNHCOCH=C(R)TeH or [PhNHCOCH=C(R)Te]2 was isolated. 

To ascertain the possibility of inserting more than one acetylenic moiety into the pyrazole ring, the replacement of two and three iodine atoms in the appropriate halides by different alk-l-ynes was carried out. To increase the total rate, the cross-coupling of diiodopyrazoles and triiodopyrazole was performed with higher initial concentrations of the reactants than for the monoiodides. The reaction of diiodopyrazoles with the acetal was completed for the most part in 40 h, and in 64 h in the case of triiodopyrazole. The yields of the di- and triacetals reached 70-90% (Table XTTT). 

In reaction (19) the iodine shown on the left has an oxidation number of zero. After the reaction, some of the iodine atoms have oxidation number +5 and some —1. In other words, the iodine oxidation number has gone both up and down in the reaction. This is an example of selfoxidation-reduction, sometimes called disproportionation. It is a reaction quite typical of, but not at all restricted to, the halogens. 

With the iodine atom in its proper place, provisions for construction of the C9-C10 bond by an aldol reaction could be made (see Scheme 44). To this end, oxidative cleavage of the para-methoxy-benzyl ether in 181 with 2,3-dichloro-5,6-dicyano-l,4-benzoqui-none (DDQ) in CH2CI2-H2O furnishes a primary alcohol that can... 

On the basis of all these results and his own investigations on chloro- and bromo-de-diazoniations (Galli, 1981), Galli proposed in 1988 that iodo-de-diazoniation, after formation of the aryl radical in the initiation reaction (Scheme 10-22) follows three coupled iodination chain reactions based on the formation of the I2 molecule and the If anion in the step shown in Scheme 10-23, namely iodine atom (I ) addition (Scheme 10-24), and iodine abstraction from I2 and If in Schemes 10-25 and 10-26 respectively. Aryl radicals and iodine molecules are regenerated as indicated in Scheme 10-27. The addition of iodide ion to aryl radicals forming the radical anion [Arl] -, as in Scheme 10-28, is considered an unlikely pathway, as that reaction has been found to be reversible (Lawless and Hawley, 1969 Andrieux et al. 1979). 

Reactions that proceed photochemically do not necessarily involve observations of an excited state. Long before observations are made, the excited state may have dissociated to other fragments, such as free radicals. That is, the lifetime of many excited states is shorter than the laser excitation pulse. This statement was implied, for example, by reactions (11-46) and (11-47). In these systems one can explore the kinetics of the subsequent reactions of iodine atoms and of Mn(CO)s, a 17-electron radical. For instance, one can study... 

The cyclization involves a nucleophilic attack of the malonic ester car-banion on the carbonyl carbon atom of the aldehyde, and the substituted malonic ester carbanion attacks the electron-deficient carbon atom bearing the iodine atom, or in the reverse order, to give 119. The hydroxyl group generated in the first step of the reaction attacks the carbon atom, giving the pyranose product. 

These reactions result in iodine atom transfer and introduce a potential functional group into the product. The trialkylborane method of radical generation can also be used in conjunction with either tri-n-butyl stannane or fnT-(trimethylsilyl)silane, in which case the product is formed by hydrogen atom transfer. 

Reaction conditions have been developed in which the cyclized radical can react in some manner other than hydrogen atom abstraction. One such reaction is an iodine atom transfer. The cyclization of 2-iodo-2-methyl-6-heptyne is a structurally simple example. 

In this reaction, the trialkylstannane serves to initiate the chain sequence but it is present in low concentration to minimize the rate of hydrogen atom abstraction from the stannane. Under these conditions, the chain is propagated by iodine atom abstraction. 

The fact that the cyclization is directed toward an acetylenic group and leads to formation of an alkenyl radical is significant. Formation of a saturated iodide could lead to a more complex product mixture because the cyclized product could undergo iodine atom transfer and proceed to add to a second unsaturated center. Vinyl iodides are much less reactive and the reaction product is unreactive. Owing to the potential... 

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