Alkyl iodides, photolysis

The gas phase photodecomposition of higher alkyl iodides has received relatively little attention and the contribution made by hot alkyl radicals is uncertain. Schindler and Wijnen6 have compared the rate of production of ethane during the photolysis of pure ethyl iodide, with that in the presence of HI and I2. They assume that in the presence of HI, all the ethyl radicals produced in the primary process form ethane, whereas in the presence of I2 ethane is produced only through reaction of hot ethyl radicals with the parent molecule. Since the rate of production of ethane in the presence of HI is approximately fifty times greater than in the presence of I2, they estimate that <2% of the ethyl radicals produced in the primary process react as hot radicals. 

The radicals were generated in the photolysis of the appropriate alkyl iodide in the presence of excess C02 to minimize hot radical and wall effects. The analysis was identical to that of Christie s work with CH3I, previously described (Sect. VII-B). The ratios found for k2i/ (k22 + k23) were 7, 11, and 22, respectively, for C2H5, n-C3H7, and z-C3H7 radicals. Lower limits for reaction (24) are known, and thus minimum values for k22 + k23 could be estimated (Table 7-3). 

The most precise work has been done by Christie and her coworkers.91,92,94,96 In thorough and cleverly designed experiments, they were able to sort out the various processes that occur. The technique most commonly used by them to study the chain reaction was the photolysis of a few torr of an alkyl iodide in the presence of 0-15 torr of NO. The amount of RNO formed is twice the I2 produced. If the RNO pressure is kept small its dimerization is minimized. After the RNO is produced, additional NO is introduced, and the reaction monitored by... 

Diem et al. investigated the photolysis of adamantyl iodide as a po ible source of carbocationic initiation for the polymerisation of isobutene. Althou no polymer was obtained with this simple tystem (or with f-butyl iodide as photolyte), addition of iodine scavengers such as zinc, zinc fodide or both together gave some polymerisation, indicating that the carbocations produced in the photolysis of alkyl iodides possess a modest initiating power if generated in the presence of isobutene. 

The importance of hot radical reactions in ethyl and higher alkyl iodides is not clear, although there is some evidence of the participation of these reactions during photolysis both in solution and in the gas phase . 

Other photolysis studies have been reported for ethyl iodide -propyl iodide and isopropyl iodide in the gas phase, and also in liquid and solu-tion . Solution and liquid phase studies have also been reported for butyl ° , pentyl and cetyl iodides ". The primary process in the photolysis of alkyl iodides has been further discussed by Donovan and Husain in reference to the formation of excited halogen atoms. 

Products resulting from photolysis of alkyl iodides indicate that according to Scheme 58 homolysis of the C—bond is followed by electron transfer, which results in an ion pair in a solvent cage (Kropp, 1984). In the case of norbornyl iodide (171) and other bridgehead iodides, bridgehead cations re-... 

Alkyl halides (RX X = Cl, I) are an important source of halogens in the atmosphere. The major tropospheric sinks of these compounds are photolysis (RBr, RI) and reaction with OH radicals. In the case of alkyl iodides (RI) relative kinetic studies of their OH reactions in photoreactors are complicated by fast reactions with the 0( P) atoms generated by the photochemical OH radical sources. Figure 1 below shows a In-ln plot of the kinetic data from an experiment performed in a large photoreactor to determine the OH rate coefficient for the reaction OH + CH3CH2CH2I relative to OH + ethene using the photolysis of methyl nitrite (CH3ONO) as the OH radical source. A recent example of the implementation of the relative kinetic technique for the determination of OH radical rate coefficients in a photoreactor can be found in Olariu et al. (2000). 

A good linear correlation with zero intercept was obtained. The value of the rate coefficient obtained for the reaction of 0( P) with 1-iodopropane is in excellent agreement with the absolute value obtained by Teruel et al. (2004) using pulsed laser photolysis-resonance Fluorescence (PLP-RF) which demonstrates the validity of the applied method for the determination of rate coefficients for the reactions of (sP) with alkyl iodides. 

Irradiation of primary alkyl bromides or iodides affords typically a mixture of reduction and elimination products. For example, a radical pair produced by photolysis of the halooctanes 469 undergoes in the initial stage either competing diffusion and hydrogen abstraction from the solvent molecules (to give 470) or electron transfer giving rise to elimination product (471) a favoured process in case of alkyl iodides (Scheme 6.229).1336 Formation of the nucleophilic substitution product 472 is not observed, because solvent does not participate in carbocation generation. 

The addition of primary, secondary or tertiary alkyl radicals to an oc./l-unsaturated dinitrile, bearing a chiral substituent at the attacked carbon atom, affords the two isomeric nitriles with modest selectivity2. The radicals are generated via photolysis of the corresponding alkyl thiohydroxamates or from alkyl iodides using sonication. 

Much the most carefully investigated photodissociation laser is the iodine laser from the photodissodation of alkyl iodides, preferentially CF3I or i— C3F7I. This laser, reported first by Kasper and Pimentel in 1964, uses the excitation of iodine to the 52Pi/2 state in the photolysis of the iodides according to the following much simplified scheme. 

The differential reactivity of allyltributyltin towards electron-rich versus electron-poor radicals means that it is possible to carry out reaction sequences in which multiple carbon-carbon bonds are formed in a single transformation. The first example of such a sequence was reported by Mizuno and Otsuji [24], They showed that reaction of alkyl iodides with electron-deficient alkenes such as l,l-dicyano-2-phenylethene 33 and allyltributyltin gives good yields of three-component coupling products 34 (Scheme 6). In this process, an electron-rich alkyl radical 35 generated either by photolysis or by AIBN-mediated initiation undergoes selective addition to the electron-deficient alkene. Addition to the alkene occurs selectively since this process is much faster than addition of the alkyl radical to allyltributyltin. How-the resulting adduct radical 36 is now electron deficient, so it adds to allyl-... 

The radical carbonylation of an alkyl iodide in the presence of Kim s sulfonyl oxime ethers [58, 59, 60] provides a new type of multicomponent coupling reaction where plural radical Cl synthons are consecutively combined [61]. In the transformation, allyltin was used to serve as a trap of benzenesulfonyl radical which converts sulfonyl radical to a tin radical, thus creating a chain. Scheme 14 illustrates such an example, where the product was easily dehydroxylated to give the corresponding tricarbonyl compound on treatment with zinc/AcOH. The radical acylation reaction by Kim s sulfonyl oxime ethers can be conducted under irradiation with the addition of hexamethylditin. This is an alternative path for achieving a similar transformation without the use of photolysis equipment. Scheme 15 illustrates several examples where carbon monoxide and Kim s sulfonyl oxime ethers are successfully combined to create new tandem radical reaction sequences [61],... 

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