1,2-Dioxetanes iodometric titration

As already mentioned in connection with the determination of purity, iodometric titration is useful in the identification of dioxetanes. Unfortunately this simple and convenient method is not specific since most peroxides release iodine from acidic potassium iodide. If the necessary care is taken, iodometry is quantitative and thus an excellent purity criterion. However, the sterically hindered dioxetanes (9) do not titrate well. 

For the experimental determination of the 0, it is necessary to quantify the light output of the direct chemiluminescent process. The experimental definition of the direct chemiluminescence quantum yield is given in Eq. 36, that is, the initial rate of photon production (/q ) per initial rate of dioxetane decomposition k )[D]o). Alternatively, the total or integrated light intensity per total dioxetane decomposed can be used. The /t )[Z)]o term is readily assessed by following the kinetics of the chemiluminescence decay, which is usually first order. Thus, from a semilogarithmic plot of the emission intensity vs. time, the dioxetane decomposition rate constant kjj is obtained and the initial dioxetane concentration [Z)]o is known,especially if the dioxetanes have been isolated and purified. In those cases in which the dioxetanes are too labile for isolation and purification, [/)]o is determined by quantitative spectroscopic measurements or iodometric titration. 

Iodometric titration is one of the most direct methods for identifying 1,2-dioxetanes, since we are specifically analyzing for peroxide content. When the necessary precautions are taken to avoid thermal decomposition, many 1,2-dioxetanes titrate quantitatively.12 However, the adamantylidene (17) does not react with iodide ion since it is too sterically hindered.28 The product of I- reduction of la is the corresponding diol.12... 

The thermal decompositions are first order and usually unimolecular. A variety of experimental methods can. be used to follow the rates, which include direct chemiluminescence of the excited carbonyl product (A),14 50,93,96 activated chemiluminescence by energy transfer of the excited carbonyl to an efficient fluorescer (B),14c,l2a,94 9< dioxetane consumption or carbonyl product formation by NMR spectroscopy (C),l2M4b c iodometric titration of the cyclic peroxide (DVJ, Ub,c and infrared spectroscopy of a-peroxylactone consumption or carbonyl product formation (E).2,22,38 The method of choice depends on the particular system, but usually several techniques can be employed. 

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See also in sourсe #XX -- [ ]

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