Reaction, endergonic reversible

This base-catalyzed aldol addition is an equilibrium reaction, and all steps of this reaction are reversible. The free enthalpy of reaction AG " of such aldol reactions is close to zero. In fact, AG° is negative only if there are many H atoms among the substituents R1, R2, and R3 of the two reacting components (structures in Figure 13.44, bottom). Otherwise, the formation of the aldol adduct is endergonic because of the destabilization due to the van-der-Waals repulsion between these substituents. A base-catalyzed aldol addition between two ketones, therefore, is never observed. 

On the other hand, it is well known that inputs of light energy can cause the occurrence of endergonic, reversible, and clean reactions. In the last two decades, the outstanding progress made by supramolecular photochemistry has led to the design and construction of photochemically driven molecular devices and machines which work without formation of waste products. 

If AG is equal to 0, the process is at equilibrium, and there is no net flow either in the forward or reverse direction. When AG = 0, A.S = H/T, and the enthalpic and entropic changes are exactly balanced. Any process with a nonzero AG proceeds spontaneously to a final state of lower free energy. If AG is negative, the process proceeds spontaneously in the direction written. If AG is positive, the reaction or process proceeds spontaneously in the reverse direction. (The sign and value of AG do not allow us to determine how fast the process will go.) If the process has a negative AG, it is said to be exergonic, whereas processes with positive AG values are endergonic. 

The reversal of the thermal decomposition of 6 to ethylene and vinylacetylene cannot be utilized to generate 6, since, according to a quantum-chemical analysis, the reaction is slightly endergonic and requires a large free activation enthalpy (0.9 and 42 kcal mol-1, respectively) [59]. The intramolecular variant of this process as well as the addition of typical dienophiles of the normal Diels-Alder reaction to divinylace-tylenes will be discussed at the end of Section 6.3.3. 

It was already mentioned [reactions (8) and (9) and the associated text, p. 94] that the first situation in which a radical ion of a spin trap was suggested to be involved (Crozet et al., 1975) was the reaction between an alkyl iodide and a thiolate ion in the presence of TBN [2], This compound is reduced reversibly at -1.25 V, and with °(RS /RS ) around 0.2 V reaction (8) is endergonic by 1.4 eV, not a favourable precondition for an ET reaction. Therefore, it is likely that some other mechanism is responsible for the observations made. 

In contrast to thermal electron-transfer processes, the back-electron transfer (BET) (kbet) in the PET is generally exergonic as well. The apparent contradiction can be resolved by the cyclic process excitation-electron transfer-back-electron transfer in which the excitation energy is consumed. The back-electron transfer is not the formal reverse reaction of the photoinduced-electron-transfer step and so not necessarily endergonic. This has different influences on PET reactions. On the one hand, BET is the reason for energy consumption and low quantum yields. On the other hand, it can cause more complex reaction mechanisms if the... 

This simple model predicts that the observed kinetics is determined by the rate of fragmentation only when the reverse process is much slower than counterdiffusion (i.e. when k f under activation control. On the other hand, for an endergonic fragmentation it is expected that k f k and /Cobs = The reaction now is described as a pre-equilibrium... 

Figure 9.8 Endergonic and reversible electrocyclic reactions obeying Woodward-Hoffman rule, (a) Valence isomerization, (b) cycloaddition, (c) sigmatropic effect, and (d) norbomadiene to quadricyclene conversion.

When AG of a reaction is negative, the reaction is exergonic and tends to go toward completion when AG is positive, the reaction is endergonic and tends to go in the reverse direction. When two reactions can be summed to yield a third reaction, the AG for this overall reaction is the sum of the AGs of the two separate reactions. This provides a way to couple reactions. 

The rates of electron transfer reactions can be well predicted provided that the electron transfer is a type of adiabatic outer-sphere reaction and the free energy change of electron transfer and the reorganization energy (A) associated with the electron transfer are known [5-9], In other words, in an adiabatic outer-sphere electron transfer reaction, the rate of electron transfer is automatically determined once the pair of reactants is fixed. Moreover, the rate of reversible electron transfer, which should be exergonic and thereby thermodynamically favorable, is usually very fast, since the endergonic electron transfer, which would be slow, results in no net electron transfer because of facile back electron transfer. Thus, there would seem to be no need of a catalyst to accelerate further the electron transfer reaction, which is already fast enough. 

The reverse reaction, the radical-ion coupling, is expected to have low activation energy if the cleavage reaction itself is endergonic (see above). The many known examples of SRN1 reactions [110] attest to the facility of that process. Indeed, in several cases the radical-anion coupling was shown to be diffusion limited [111]. Thus, for the purpose of kinetic analysis the reversibility of cleavage cannot be dismissed a priori. In several systems where the possibility of reversibility has been explicitly tested [29, 99, 102, 106] none was found. 

To extend the empirical tunneling expressions above to consider endergonic reactions, we can assume for simplicity that the forward and reverse electron transfers can be related by a temperature and Boltzman constant-dependent equilibrium constant. 

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