Reduced second-order state observer

Alkynes are reduced to rranj-alkenes in excellent yields by Cr S04 in aqueous DMF at room temperature (equation 32). ° Terminal alkynes are reduced to terminal alkenes in excellent yield (equation 33). The reduction of alkynes is second order in Cr . The proposed mechanism for the reduction involves rate-determining attack by Cr" on a 1 1 alkyne-Cr complex as is illustrated in equation (34). ° In order to account for the observed stereochemistry, the reduction is proposed to occur via the transition state structure (91). 

First-order decay rate constants of transients that are also subject to second-order reactions cannot be determined reliably, even when attempts are made to correct for the observable second-order contributions. A typical example is the triplet state half-life of anthracene in degassed solution, which appears to be somewhere in the range 20 (ts 1 ms on most instruments (the values are reproducible on a given instrument but vary widely between different setups and solvent purification methods). The lifetime of triplet anthracene in solution rises to 25 ms when measures are taken to reduce quenching by diffusional encounters with the parent molecule (self-quenching) with other molecules in the triplet state (triplet triplet annihilation) and with impurity quenchers such as dioxygen.200 Hence lifetimes of transients that are prone to second-order decay contributions, such as long-lived triplet states or radicals, should always be considered as lower limits. 

Unlike alkenes, the unsaturated azo group in N=N or C=N derivatives possess an inplane lone electron pair in an n-orbital. As a result, both the n,Jt and Tt.Tt excited states can be involved in two limiting mechanisms of photoisomerization.1062 1080 1081 The first mechanism is a 180° rotation (twist) about the former double bond in conjunction with a reduced bond order, similar to that observed in alkenes (Scheme 6.157). In the second case, an in-plctne inversion proceeds due to rehybridization of one nitrogen without... 

Gunnarsson et al. 166) have measured the rate of reduction of T q)es 1 and 3 Cu2+ in the presence of excess EDTA by several substrates, including ascorbate, l oth reactions were found to be well behaved second-order processes in which no saturation was observed even when substrate was present at a concentration several-fold greater than its steady state Km value. The rate of reduction of Types 1 and 3 were identical suggesting either a complete equihbration between the two electron acceptors or identical rates of reduction by substrate. When less than stoichiometric amounts of substrate (ascorbate) were present, however, the two chromophores were reduced at significantly different rates 172). 

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