Molecular iodination

The possibility that a number of iodinations with iodine and iodine monochloride may involve the molecular species rather than the positive species are formerly believed has been considered under positive iodination (see pp. 92-97). Briefly, iodination rates have been found to be proportional to [I-]-2 and [I ] 1, generally with a mixed dependence, and the former term has been interpreted as involving iodination by hypoiodous acid (or more likely, positive iodine) and the latter term attributed to iodination by molecular iodine. 

Additional kinetic evidence supporting molecular iodine as an iodinating species is sparse. Li325 found that the iodination of tyrosine in acetate buffers at 25 °C showed the mixed inverse dependence on iodide ion concentration noted above, so that part of the reaction appeared to involve the molecular species. Subsequently, Doak and Corwin326 found that the kinetics of the iodination of (N-Me)-4-carboethoxy-2,5-dimethyl- and (N-Me)-5-carboethoxy-2,4-dimethyl-pyrroles in phosphate buffers in aqueous dioxane at 26.5 °C obeyed equation (162), viz. 

The observed rate coefficient will be less than the true rate coefficient since some iodine is converted to triiodide ion through the equilibrium for which AT[IJ ] = DUDn, ie- 2 = D/Pzl which reduced to kota = k2Kf(K+[l ]) 

The kinetics of iodination of azulene with iodine in dilute aqueous sodium iodide at 25 °C followed the equation 

The zinc chloride-catalysed iodination of aromatics by iodine monochloride in acetic acid at 25.2 °C is first-order in each reagent329. The relative rates are given in Table 76 and the reactivities of the higher substituted aromatics are less 

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X resonance series of molecular iodine J. Chem. Phys. 40 1934—44... 

The 4,5-dihalogenothiazoles are obtained by cyclization-halogenation reactions as show in scheme 12 (3). 2-Acetamido-4,5-diiodothiazole has been obtained by Hurd and Wehrmeister (80). The triiodothiazole can be prepared by iodination by molecular iodine of the mercuric complex of 2-iodothiazole following the Travagli method (81). 

Iodine reacts with hydrocarbons to form iodine compounds, but compared to the other halogens, the equiUbria are unfavorable because the displacement step with the iodine atom is endothermic, requiring 4066.3 J (971.9 cal) for methane and 799.9 J (191.2 cal) for toluene. Hydrogen iodide can be used to reduce an alkah iodide to hydrocarbon plus molecular iodine. 

Molecular iodine is not a very powerful halogenating agent. Only very reactive aromatics such as anilines or phenolate anions are reactive toward iodine. Iodine monochloride can be used as an iodinating agent. The greater electronegativity of the... 

Treatment of the solvent-free chromatogram with iodine vapor or by dipping in or spraying with iodine solution (0.5 — 1%) is a rapid and economical universal method of detecting lipophilic substances. Molecular iodine is enriched in the chromatogram zones and colors them brown. 

Emetine andcephaeline, the two major alkaloids of ipecacuanha, begin to fluoresce after treatment with iodine vapor [254], The molecular iodine, which acts as a quencher, must be removed by heating in the drying cupboard or on a hotplate... 

Molecular iodine oxidatively adds to 156 (M = Rh, Ir) to yield 157 (M = Rh, Ir X = I), the species containing a metal-metal bond (970M4718). 

Lithium 1,2,4-triazolate with [Rh2( j,-Ph2PCH2PPh2)(CO)2( j.-Cl)]PFj. gives the A-framed complex 177 (L=L = CO) (86IC4597). With one equivalent of terf-butyl isocyanide, substitution of one carbon monoxide ligand takes place to yield 177 (L = CO, L = r-BuNC), whereas two equivalents of rerr-butyl isocyanide lead to the product of complete substitution, 177 (L = L = r-BuNC). The starting complex (L = L = CO) oxidatively adds molecular iodine to give the rhodium(II)-rhodium(II) cationic species 178. 

Another route to the amido complexes originates from [(>j-Tp )W(CO) (PhC=CMe)(OTf)l and benzylamine and yields [(i -Tp )W(CO)(PhC=CMe) (NHCH2Ph)] (96JA6916). The latter can be protonated with tetrafluoroboric acid to give the amine derivative [(> -Tp )W(CO)(PhC=CMe)(NH2CH2Ph)](Bp4), and this process can be reversed by -butyllithium. Hydride abstraction by silver tetrafiuoroborate, molecular iodine, or PhsCPEe leads to the cationic imine derivatives [(> -Tp )W(CO)(PhC=CMe)(HN=CHPh)]". -Butyllithium deproto-nates the product and gives the neutral azavinylidene species [(> -Tp )W(CO) (PhC=CMe)(N=CHPh)]. The latter with silver tetrafiuoroborate forms the cationic nitrile species [(j -Tp )W(CO)(PhC=CMe)(N=CPh)](Bp4). 

Of the four halogens, iodine is the weakest oxidizing agent. Tincture of iodine, a 10% solution of I2 in alcohol, is sometimes used as an antiseptic. Hospitals most often use a product called povidone-iodine, a quite powerful iodine-containing antiseptic and disinfectant, which can be diluted with water to the desired strength. These applications of molecular iodine should not delude you into thinking that the solid is harmless. On the contrary, if I2(s) is allowed to remain in contact with your skin, it can cause painful bums that are slow to heal. 

For the sake of simplicity, however, the equations in this book will usually be written in terms of molecular iodine rather than the tri-iodide ion. 

Iodination of 4 with molecular iodine in the presence of mercuric oxide formed the 3-iodo derivative (52JA4951 66CJC2283). Iodine in tetrahydro-furan oxidatively cyclized /3-(3-benzo[b]thienyl)-a-mercaptoacrylic acids rather than iodinating the thiophene ring [70JCS(C)2431]. 

Introduction of iodine by Sandmeyer processes has been discussed [84AHC(35)83], Direct electrophilic iodination is also observed in the 3-position with reagents such as iodine monochloride, iodine-iodic acid-acetic acid, or molecular iodine. With excess reagent, or when C-3 is blocked, 6-iodination follows [84AHC(35)83 84MI15]. 

Careful observations of the course of iodo-de-diazoniation demonstrate that the detailed pathway of such reactions is still relatively complex. For instance, after adding a solution of KI to a solution of an arenediazonium salt, normally molecular iodine appears to be formed first, followed by a precipitate and evolution of N2. Carey and Millar (1960) isolated the salt ArNJIj- on adding iodide to the diazo-nium salt. Ion pairs (ArNjHlg-), suggested as primary products by Meyer et al. (1979), were identified for diazonium halides (Cl- and Br-) by Israel et al. (1983) as 1 1 complexes on the basis of JOB analyses of visible spectra (Benesi-Hildebrand method). Iodides were, however, not included in that investigation. 

The values of kH/kD for the uncatalysed and catalysed reactions were 4.36 and 4.47 respectively, yet the isotope effect is not necessarily diminished on reducing the concentration of iodide ion to zero and by the arguments elaborated above (p. 95) this implies that molecular iodine is not the iodinating species and that this species is formed in some pre-equilibrium, the function of the base being to form the species and not to remove the proton. This argument assumes, as does the previous discussion of the effect of iodide ion concentration on isotope effects, that a minute concentration of I- is insufficient to compete effectively with the reaction involving proton loss. 

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