Unfavorable reaction

On the other hand, under superacidic conditions, alkanes are readily alkylated via front-side CJ-iasertion by carbocationic alkylating agents. The direct alkylation of the tertiary C—H CJ-bond of isobutylene with isobutane has been demonstrated (71). The stericaHy unfavorable reaction of tert-huty fluoroantimonate with isobutane gave a Cg fraction, 2% of which was 2,2,3,3-tetramethylbutane ... 

Copolymers with butadiene, ie, those containing at least 60 wt % butadiene, are an important family of mbbers. In addition to synthetic mbber, these compositions have extensive uses as paper coatings, water-based paints, and carpet backing. Because of unfavorable reaction kinetics in a mass system, these copolymers are made in an emulsion polymerization system, which favors chain propagation but not termination (199). The result is economically acceptable rates with desirable chain lengths. Usually such processes are mn batchwise in order to achieve satisfactory particle size distribution. 

Heat Sensitivity. The heat sensitivity or polymerization tendencies of the materials being distilled influence the economics of distillation. Many materials caimot be distilled at their atmospheric boiling points because of high thermal degradation, polymerization, or other unfavorable reaction effects that are functions of temperature. These systems are distilled under vacuum in order to lower operating temperatures. For such systems, the pressure drop per theoretical stage is frequently the controlling factor in contactor selection. An exceUent discussion of equipment requirements and characteristics of vacuum distillation may be found in Reference 90. 

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

What typically happens for an energetically unfavorable reaction to occur is that it is "coupled" to an energetically favorable reaction so that the overall free-energy change for the two reactions together is favorable. To understand what it means for reactions to be coupled, imagine that reaction 1 does not occur to any reasonable extent because il has a small equilibrium constant and is energetically unfavorable that is, the reaction has AG > 0. 

The energy released in catabolic pathways is used in the electron-transport chain to make molecules of adenosine triphosphate, ATP. ATP, the final result of food catabolism, couples to and drives many otherwise unfavorable reactions. 

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. 

There are two main issues concerning the chemistry of the reaction and the separation. One is how to separate the hydriodic acid and sulfuric acid produced by the Bunsen reaction. The other is how to carry out the hydrogen iodide (HI) decomposition section, where the presence of azeotrope in the vapor-liquid equilibrium of the hydriodic acid makes the energy-efficient separation of HI from its aqueous solution difficult, and also, the unfavorable reaction equilibrium limits the attainable conversion ratio of HI to a low level, around 20%. 

The free energy of a favorable chemical reaction can be used to make an unfavorable reaction happen. 

A thorough kinetic and thermodynamic analysis of this model system (small positive or negative enthalpies of formation are canceled by more negative entropies of formation) led Karlin s group to conclude that the stability of dioxygen binding is driven by favorable enthalpies, but unfavorable reaction entropies preclude observation of Cu2-02 at room temperatures.412... 

LLB, a so-called heterobimetallic catalyst, is believed to activate both nucleophiles and electrophiles.162 For the hydrophosphonylation of comparatively unreactive aldehydes, the activated phosphite can react with only the molecules precoordinated to lanthanum (route A). The less favored route (B) is a competing reaction between Li-activated phosphite and unactivated aldehyde, and this unfavored reaction can be minimized if aldehydes are introduced slowly to the reaction mixture, thus maximizing the ratio of activated to inactivated aldehyde present in solution. Route A regenerates the catalyst and completes the catalysis cycle (Fig. 2-9). 

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). 

Very favorable reactions (ones that progress on their own) produce a lot of product, so they have values much larger than 1. Very unfavorable reactions (ones that require an input of energy) convert very little reactant into product, so they have values between 0 and 1. In a reaction with = 1, the amount of product equals the amount of reactant at equilibrium. 

Favorable reactions possess negative values for AG, and unfavorable reactions possess positive values for AG. Energy must be added to drive an unfavorable reaction forwcird. If the AG for a set of reaction conditions is 0, the reaction is at equilibrium. 

In this step, the well-established dehydrogenated C2H3(a) species is ethyl-idyne, CCH3. The hydrogenation of ethylidyne to give ethane is clearly an unfavored reaction on a Pt(lll) surface unless enough adsorbed hydrogen is available from the presence of H2 in the gas phase. 

Figure 18. The reaction cavity model as presented by Cohen. Reaction cavity before and after the reaction is shown as full lines. Transition state requirements for a reaction are shown as broken lines. Case I represents a favorable and case II an unfavorable reaction. [Reproduced with permission from M. D. Cohen, Angew. Chem. Int. Ed. Eng. 14, 386 (1975).]...

There is one further consideration. The value of kcJKM cannot be at the diffusion-controlled limit for a reaction that is thermodynamically unfavorable. This point stems from the Haldane equation (Chapter 3, section H), which states that the equilibrium constant for a reaction in solution is given by the ratio of the values of kctA/KM for the forward and reverse reactions. Clearly, kcat/KM for an unfavorable reaction cannot be at the diffusion-controlled limit, since kQ.JKM for the favorable reverse reaction would have to be greater than the diffusion-controlled limit to balance the Haldane equation. The value of kcM/KM for an unfavorable reaction is limited by the diffusion-controlled limit multiplied by the unfavorable equilibrium constant for the reaction. 

Enzyme-product complementarity changes the equilibrium constant for highly unfavorable reactions. The formation of Tyr-AMP from Tyr and ATP in solution is a very unfavorable reaction. The equilibrium constant for equation 15.4 (= [Tyr-AMP][PPj]/[Tyr][ATP])... 

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. 

This postulated sequence of events may leave you wondering why the enzyme speeds up the hydrolysis, especially because the sequence proceeds through an energetically unfavorable reaction, the formation of a carboxylic anhydride from an amide and a carboxylic acid ... 

Biochemical reactions frequently require energy. The most common source of chemical energy used is adenosine triphosphate (ATP). The splitting of a phosphate from the ATP molecule can provide the energy needed to make an otherwise unfavorable reaction proceed in the desired direction. 

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