Reaction, endergonic irreversible

If Wr is really large, this reaction proceeds as an irreversible one up to AG, = 0. This border between exergonic and endergonic electron transfer is located at... 

Recently,the electron-transfer theory was extended in order to incorporate the slow and reversible chemically induced electron-exchange reactions, as observed for the fluorescer-catalyzed chemiluminescent decomposition of a-peroxylactones. It was argued that electron transfer is complete in the transition state for such a slow and irreversible endergonic electron-transfer reaction, but that the typically small slopes (— a/RT where a is about 0.3) of the In (intensity) vs. oxidation potential plot was due to the fact that only a fraction (a) of the total free-energy change manifests itself in the activation energy. 

The E° value for FeCpCC H ) /" is about - 2 V vs SCE. Thus with L = PMe3, the ET responsible for the disproportionation found in the presence of NaPF is about isoergonic but with P(OMe)3, it is endergonic by about 0.25 V (24 kJ Mole ) and nevertheless driven by the follow-up double ion-pair exchange and ensuing irreversible reactions. ... 

In its overall effect the new cycle is a reversal of the oxidative tricarboxylic add cycle, but endergonic in nature and hence includes two reactions (a-ketoglutarate synthase and citrate lyase) that can by-pass the irreversible steps of the KLrebs cycle. Photosynthetic bacteria also contain pyruvate synthase that brings about the direct synthesis of pyruvate from acetyl-CoA, COg and reduced ferredoxin, as well as PEP synthase that catalyzes the direct synthesis of PEP from pyruvate and ATP. 

In general, the normal DA reaction mechanism is a domino process that is initialized by the polar reaction between the diene and the dienophile to give the primary cicloadduct. These DA reactions have a two-step non-intermediate mechanism characterized by the nucleophilic attack on the non-substituted methylene of the diene to the electrophilically activated position of the dienophile. The subsequent ring-closure affords the primary cicloadduct. This behavior makes the reaction to be regioselective. The latter concerted elimination of the nitrous acid from the primary cicloadduct yields the precursor of the final aromatic product. Spite of the large activation free energy associated with the DA reaction and the endergonic character of formation of the primary cicloadduct, the irreversible extrusion of the nitrous acid make feasible thermodynamically the domino reaction. 

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