Driving equilibria

Instability of R2C(or)2 in acid solution. Driving equilibria in a chosen direction by the use of acid, solvent etc. 

The nature of the solvent used in reactions often has a profound effect on how the reaction proceeds. Often we are limited in our choice of solvent by the solubilities of the reactants and products—this can also be to our advantage when trying to separate products, for example, in ether extractions. We have seen so far in this chapter that THF is a good solvent for lithiations because it coordinates to Li, that water is a good solvent for hydrolyses of carboxylic acids because it is a reagent and because it dissolves the carboxylate anion, and that alcohols are a good solvents in reactions such as transesterifications where mass action is needed to drive equilibria over towards products. 

Perhaps the most commonly encountered equilibrium reactions are those involving water as a reactant or product. Driving such equilibria by using excess water (e.g. hydrolysis reactions) is easy, but driving equilibria by removing water (e.g. in ester or acetal formation) can be more difficult. An excellent device for the continuous removal of water from a reaction mixture is the Dean-Stark trap (Fig. 9.27). 

On a small scale, simply placing some activated sieves in the reaction flask is a convenient means of removing water. This method is effective in driving equilibria and is also used to promote reactions which are adversely affected by water. 

Highly Increased Number of Effective Collisions Active Site Preorganization of Solvation and General Acid/Base Catalysts Avoiding High-Energy Intermediates Electrostatic Catalysis by Metal Ions Covalent Catalysis by Enzyme-Bound Electrophiles and Nucleophiles Coupling ATP Hydrolysis to Drive Equilibria... 

As an example, consider reaction (6.1). Here, the Cu2+aq ion has water ligands in its coordination sphere substituted by a stronger ligand, in this case ammonia, to form [Cu(NH3)4]2+. Multiple ligand substitution is assisted by the use of an excess of the incoming ligand to drive equilibria towards the fully-substituted compound. 

The extra w initially present in Slurries will drive equilibrium (1) towards C02 and H2. This effect is somewhat counteracted by the reduction in detonation temp because of the quenching action of w contained in the undetonated Slurry, Lower temps mildly favor an equilibrium shift to the left... 

The close-coupling of reaction and separation units for driving equilibrium reactions to complete conversion, though... 

Diluents or reagents present in excess used to drive equilibrium... 

This imbalance drives equilibrium away from the vapor phase and into the liquid phase and lowers the... 

When E+ is a proton, equilibrium C lies to the left and routes A and B are followed. Examples are given in Table 13(a). When R is phenyl, the combination of / -silyl hyperconjugative stabilization and carbonium ion stabilization by the benzene ring are sufficient to drive equilibrium C to the left, and substitution with retention of configuration dominates, as shown in Table 13(b). 

The facile dissociation of [Fe(H20)g] means that its aqueous solutions must be stabilized by the addition of acid, which (by Le Chatelier s principle) drives equilibrium 6.36 to the left-hand side. 

Solid dissolves because high [CH drives equilibrium L-R. 

After a nonvolatile solute is added to a liquid solvent, only a fraction of the molecules at a liquid-gas interface are now volatile and capable of escaping into the gas phase. The vapor consists of essentially pure solvent that is able to condense freely. This imbalance drives equilibrium away from the vapor phase and into the liquid phase and lowers the vapor pressure by an amount proportional to the solute particles present. 

In an RD process, two operations are coupled and run simultaneously. One operation is chemical synthesis and the other is separation by distillation, enabling a shift in composition to drive equilibrium-limited reactions to higher conversion. In this chapter we summarize some of our published papers and extend the work to other applications of polymer carrier composite materials and components. 

Removal of ethylene drives equilibrium toward high-molecular-weight polymer... 

Alkanolamides based on DEA are tertiary amides and are not as stable as MEA amides such that a significant amount of esters can remain in equilibrium with the amide. The ester amines and esteramides of DEA have undesirable performance properties, but these components can be reduced by utilizing an excess of DEA to drive equilibrium toward the amide form. The most common version is prepared by the reaction of 2 moles of DEA with 1 mole of coconut fatty add or ester to give the Kritchevsky or Ninol-type DEA amide, which is liquid at room temperature. Products made with slightly more than a 1 1 molar ratio of DEA to fatty acid or ester are referred to as superamides and at reaction temperatures of 140-160°C, the mixture contains high level of ester components and free amine. However, with sufficient time at storage temperatures <50°C, the composition will increase in amide and decrease in ester and free-amine components and thus can be aged into specification for free DEA and ester content. 

In situ separation may be used to drive equilibrium reactions to completion. The use of adsorptive reactors for irreversible series reactions has not received much attention. Selectivity towards the intermediate product in an irreversible series reaction can be enhanced by removal of the intermediate product[l,2] or by reactant storage on the sorbent[3]. 

Reactive distillation. To avoid conversion of product into unwanted components or to drive equilibrium reactions forward, reactive distillation is used to distill out one of the products immediately as it is formed. A conventional equilibrium process consists of a reactor and a distillation column. One of the products is removed in the column, and the bottom is recycled (see Fig. 6.22). In reactive distillation the column is located on top of the reactor, obviating the need for a reboiler (see Fig. 6.23). Reaction of an organic chlorine compound with an aqueous caustic solution can be carried out in a reactive distillation column, which will combine functions such as dissolution of... 

Schauble, E.A. (2007) Role of nuclear volume in driving equilibrium stable isotope fractionation of mercury. 

Favorable acid-base reaction of step 2 drives equilibrium of step 1... 

One can, in fact, drive the piston in both directions from the equilibrium value / = (p = p ) and construct a... 

Here p is the chemical potential just as the pressure is a mechanical potential and the temperature Jis a thennal potential. A difference in chemical potential Ap is a driving force that results in the transfer of molecules tlnough a penneable wall, just as a pressure difference Ap results in a change in position of a movable wall and a temperaPire difference AT produces a transfer of energy in the fonn of heat across a diathennic wall. Similarly equilibrium between two systems separated by a penneable wall must require equality of tire chemical potential on the two sides. For a multicomponent system, the obvious extension of equation (A2.1.22) can be written... 

Biological reactions in vivo rarely operate under conditions even remotely approaching those of reversibility. For a living organism, the rate of a process is usually more important than the attainment of equilibrium, and large driving... 

To generate characteristic velocities and bring a molecular system to equilibrium at the simulation temperature, atoms are allowed to interact with each other through the equations of motion. Eor isothermal simulations, a temperature bath scales velocities to drive the system towards the simulation temperature. Scaling occurs at each step of a simulation, according to equation 28. 

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