Thermodynamically unfavorable reactions

A negative AE indicates an exothermic (thermodynamically favorable) reaction, while a positive AE an endothermic (thermodynamically unfavorable) reaction. 

ATP Allows the Coupling of Thermodynamically Unfavorable Reactions to Favorable Ones... 

Possibility of significant equilibrium shift in the desired direction. Thermodynamically unfavorable reactions become possible. 

The solid state and the surface chemistry of some of the solid Fe-phases impart to these oxides and sulfides the ability to catalyze redox reactions. Surface complexes and the solid phases themselves acting as semiconductors can participate in photoredox reactions, where light energy is used to drive a thermodynamically unfavorable reaction (heterogeneous photosynthesis) or to catalyze a thermodynamically favorable reaction (heterogeneous photocatalysis). 

Suggest a sequence of reasonable steps for this process on the basis that sulfides, RSR, are both excellent nucleophiles and good leaving groups. It is implicit that all steps will be enzyme-catalyzed, although the way the enzymes function is unknown. Each step must be energetically reasonable because enzymes, like other catalysts, cannot induce thermodynamically unfavorable reactions. 

Chemists can often make a thermodynamically unfavorable reaction proceed to some extent by manipulating experimental reaction parameters such as pressure, temperature, or concentrations of reactants. Biochemists generally cannot alter such reaction parameters because organisms function within limited concentration ranges and at essentially constant pressure and temperature. 

Note that the hydrolysis of two high-energy phosphate bonds in ATP provides the energy source for the reaction. The inorganic pyrophosphate, PPi, is subsequently broken down to two phosphate ions by inorganic pyrophosphatase. The action of this enzyme means that very little PPi remains in the cell, making the synthesis of the fatty acyl-CoA favored. This is an example of metabolic coupling, the process whereby a thermodynamically unfavored reaction is allowed because it shares an intermediate (in this case PPO with a favored one. 

The developed H+ concentration gradient plus an electric potential across the membrane supply the driving force for ATP synthesis from ADP and Pi, a thermodynamically unfavorable reaction catalyzed by ATP synthase (Karrasch and Walker, 1999). The latter is a mitochondrial enzyme located on, and spanning, the inner mitochondrial membrane. At least when in submitochondrial particles, ATP synthase saturation kinetics involve ADP positive site-site interactions in catalysis. One group has proposed that ADP saturation in vivo also shows site-site interactions ( , the interaction or Hill coefficient increasing from 1, meaning no interaction, to 2) however, others have not found this, so this issue at this time must be considered to remain unresolved. 

The implications of being able to increase the conversion of an equilibrium reaction by using a permselective membrane are several. First, a given reaction conversion may be attained at a lower operating temperature or with a lower mean residence time in a membrane reactor. This could also prolong the service life of the reactor system materials or catalysts. Second, a thermodynamically unfavorable reaction could be driven closer to completion. Thus, the consumption of the feedstock can be reduced. A further potential advantage is that, by being able to conduct the reaction at a lower temperature due to the use of a membrane reactor system, some temperature-sensitive catalysts may find new applications [Matsuda et al., 1993]. 

A Thermodynamically Unfavorable Reaction Can Be Driven by a Favorable Reaction... 

Even for thermodynamically favorable reactions, a large activation energy means that the reaction will be slow. For thermodynamically unfavorable reactions, even a fast reaction (with small activation energy) would be unlikely to occur. The rate of reaction depends on the activation energy, as in the Arrhenius equation... 

Under standard conditions, A cannot be spontaneously converted into B and C, because AG° is positive. However, the conversion of B into D under standard conditions is thermodynamically feasible. Because free-energy changes are additive, the conversion of A into C and D has a AG° of — 13kJmol ( —3kcalmoU ), which means that it can occur spontaneously under standard conditions. Thus, a thermodynamically unfavorable reaction can be driven by a thermodynamically favorable reaction to which it is coupled. In this example, the reactions are coupled by the shared chemical intermediate B. Thus, metabolic pathways are formed by the coupling of enzyme-catalyzed reactions such that the overall free energy of the pathway is negative. 

In the presence of light, chloroplasts are capable of driving thermodynamically unfavorable reactions. 

F2 and I2 are halogens, but they are not used as reagents in electrophilic addition reactions. Fluorine reacts explosively with alkenes, so the electrophilic addition of F2 is not a synthetically useful reaction. The addition of I2 to an alkene is a thermodynamically unfavorable reaction The vicinal diiodides are unstable at room temperature, decomposing back to the alkene and I2. 

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