Hydrogen-Alkene Titration

The notion of turnover numhera was raised by Boudart, but Carberry has emphasised Chat generally we attempt to measure the number of catalyst sites (by chemisorption or titration) rather than the wore relevant number of sites c f catalysis and these are unlikely to be equal. Possibly the alkene titration using reactant cyclohexene would assess the summation of active sites as the active surface area under reaction conditions. An interesClng alternative approach is to use pulsed isothermal alkane titrations Co deduce the numbers of different surface Ptg sites Involved later In hydrogenation. If such a partial deconvolution of total average turnover numbers is really possible, then potentially it Is excemely useful. 

Oxidation indices, 656-72 peroxide determination, 762-3 peroxide value, 656, 657-64 colorimetry, 658-61 definition, 657 direct titration, 657 electrochemical methods, 663-4 IR spectrophotometry, 661-3 NIR spectrophotometry, 663 UV-visible spectrophotometry, 658-61 secondary oxidation products, 656, 665-72 tests for stability on storage, 664-5, 672 thermal analysis, 672 Oxidative amperometiy, hydroperoxide determination, 686 Oxidative cleavage alkenes, 1094-5 double bonds, 525-7 Oxidative couphng, hydrogen peroxide determination, 630, 635 Oxidative damage... 

To overcome problems associated with the removal of iodobenzene and its derivatives formed upon fluorination of arylalkenes and arylalkynes with (difluoroiodo)arenes, polymer-supported (difluoroiodo)arenes were proposed.139 With these agents, the separation procedures are reduced to filtration of the iodinated polymer. For this purpose popcorn polystyrene is io-dinated and then transformed into the difluoroiodide by treatment with xenon difluoride in the presence of hydrogen fluoride in dichloromelhane at 25 C. The amount of active fluorine bonded to iodine atoms on the polymer support is estimated by iodometric titration. The reactions with phenyl-substituted alkenes result in rearranged gew-difluorides. The procedure provides the same fluorination products as with (difluoroiodo)benzenc (see Section 4.13.) but in much higher yields, e.g. PhCF2CH2Ph (96%), PhCF2CH(Me)Ph (95%). PhCH2CF2H (86%), and l,l-difluoro-2-phenylcyclopentanc (91 %). 

Alkene hydrogenation was also suggested to test for mass transfer effects during liquid-phase hydrogenations236,237. The method is based on the linear poisoning of hydrogen addition to alkenes (cyclohexene and apopinene) by CS2. When the active sites of Pd or Pt catalysts are titrated with CS2 the decrease in rate is linear unless mass transfer limitations occur. 

Variants on this procedure are possible. Carbon monoxide has been used in place of hydrogen,but although this eliminates some problems, it often creates others. Chemisorbed hydrogen has been titrated with alkenes these reactions may be followed using frontal elution and gas chromatography to detect products, or pulse-wise. Chemisorbed hydrogen also reacts almost instantly with excess deuterium " ... 

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