Rate determining with acetylene

With cation-exchanged zeolites [304], the first-order kinetics [eqn. (18)] is explained by the degeneration of the Langmuir—Hinshelwood equation for monomolecular transformation of adsorbed acetylene in the rate-determining step... 

The reactions of sydnones with acetylenic esters are generally thermally induced concerted processes which are allowed on orbital symmetry considerations.8 On the basis of Huisgen s mechanistic study,805 the conversion of 71 into 73 proceeds through the formation of 72 in a slow rate-determining step, followed by rapid loss of carbon dioxide, giving the pyrazole 73. 

A quantitative comparison can be based on catalytic activity, i.e., on the rate of formation of adsorbed acetaldehyde (AA,a). Fig. 2 shows the integrated intensity of the typical AA,a bands on Cd-forms of MOR, LTA, Y,FAU, and C,HEU as a function of time in contact with acetylene at 10 kPa and 298 K (for the sake of clarity, the corresponding data for the other Cd-zeolites are not plotted). The formation rates of AA,a decrease with time because of saturation of the surface with acetaldehyde and by-products (as it can be seen in Fig.l). When initial slopes of absorbance vs. time curves are determined and the values are divided by the number of Cd ions in the sample, relative rates of formation of adsorbed acetaldehyde on one Cd " ion (ar AA,a / Ncd ), i.e., the relative intrinsic activities of Cd ions in different zeohtes can be obtained. These initial rates are plotted against the Si/Al ratio in different zeohtes in Fig. 3. Accordingly, three groups of zeohtes can be distinguished, for which the activity increases with Si/Al ratio in different degrees. 

Alkyl-substituted acetylenes can react with HCI by either the Ad S or the Ad 2 mechanism. The Ad S mechanism leads to anti addition. The preference for one or the other mechanism depends on the reactant structure and the reaction conditions. Added halide ion promotes the Ad S mechanism and increases the overall rate of reaction. " Reaction of 4-octyne with TFA in CH2CI2 containing O.l-l.OMBr leads mainly to Z-4-bromo-4-octene by an anti addition. The presence of Br greatly accelerates the reaction as compared to reaction with TFA alone, indicating the involvement of the Br in the rate-determining protonation step. " ... 

The reaction of silastannation of acetylene catalyzed by a Pd complex involves the oxidative addition of the H3Si-SnH3 to the catalyst Pd(PR3)2, followed by coordination of acetylene to the catalyst accompanied with dissociation of one of the PR3 bondi and reductive elimination of the product. The rate-determining step of the reaction is the insertion of substituted acetylene RCCH into the Pd-Sn or Pd-Si bond, the activation energy of which increases in the order of R CN < OCH3 < H < CH3. 

In a study of the effects of substituents upon reactivity, it was found that electron-withdrawing groups retarded the reaction while electron-releasing groups assisted it . It is probable that the rate-determining stage, which involves one molecule of peracid and one molecule of acetylene , has similar requirements to that of the analogous reaction with olefins. 

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