Facilitated dissociation

In the discussion of the relative acidity of carboxylic acids in Chapter 1, the thermodynamic acidity, expressed as the acid dissociation constant, was taken as the measure of acidity. It is straightforward to determine dissociation constants of such adds in aqueous solution by measurement of the titration curve with a pH-sensitive electrode (pH meter). Determination of the acidity of carbon acids is more difficult. Because most are very weak acids, very strong bases are required to cause deprotonation. Water and alcohols are far more acidic than most hydrocarbons and are unsuitable solvents for generation of hydrocarbon anions. Any strong base will deprotonate the solvent rather than the hydrocarbon. For synthetic purposes, aprotic solvents such as ether, tetrahydrofuran (THF), and dimethoxyethane (DME) are used, but for equilibrium measurements solvents that promote dissociation of ion pairs and ion clusters are preferred. Weakly acidic solvents such as DMSO and cyclohexylamine are used in the preparation of strongly basic carbanions. The high polarity and cation-solvating ability of DMSO facilitate dissociation... 

As the deletion of S5a/Rpnl0 facilitates dissociation of the RC into base and lid, it is assumed to be located at the interface this is consistent with observations that it interacts with multiple subunits in the base and in the lid (Ferrell et al. 2000). Interestingly, S5a/Rpnl0 is the only subunit of the 19S complex that is also present in significant amounts in free monomeric form (Haracska and Udvardy 1995 van Nocker et al. 1996). 

The molecular details of the physiologically critical step of iron release from serum transferrin are unclear. The anion may play an important role in this process by providing a handle for modulating the affinity of transferrin for iron. If the anion were to be removed (perhaps by protonation followed by dissociation), then the binding of iron to transferrin would be greatly weakened, facilitating dissociation of the metal. It seems... 

More recent results support the view that Lewis acids, on the interaction with metal carbonyls, may facilitate dissociation of a ligand to leave a vacant site for alkene coordination.75 Photochemical generation of carbene species [Eq. (12.20)], the crucial step in photoinduced metathesis, is a complicated, multistep process 74,76,77... 

In order to explain the field effects observed for the cationic polymerizations, we have earlier proposed a kinetic scheme based on the two-state polymerization mechanism and on the field-facilitated dissociation hypothesis (11). Though the assumptions involved in the proposed interpretation turn out to be partly invalid in the light of the experimental data accumulated most recently (15), it is still necessary to give an outline of the scheme. We assumed that, by the initiation reaction between initiator molecules (C) and monomer molecules (M), active species of an ion-pair type (My) are produced, a portion of which dissociates into active species of a free ion type (Mf) and gegenions (C ). The propagation, monomer transfer and termination can be effected by the free ions and ion pairs. A dissociation equilibrium is established between the free ions and ion pairs, which can be characterized by a dissociation constant K. Then we have ... 

We condude this section by stating that the field-accelerating effect on copolymerizations and the change of the monomer reactivity ratio with the field can be accounted for in terms of the interpretation proposed for cationic homopolymerizations, namely the field-facilitated dissociation of the growing chain ends. We should note that the observed field influence on the copolymerization excludes the possibility of the electroinitiated polymerization mechanism. 

If we admit that the Onsager theory correctly accounts for the field-facilitated dissociation phenomenon occurring in the present system,... 

With accumulation of the experimental data, the field effects on polymerization reactions have become more and more definite. As the causes of the effects, we have suggested two factors, namely the field-facilitated dissociation of the ion-pair growing chain ends and the desolvation of the free-ion growing ends. Further detailed study is certainly required to determine unequivocally which of the above-mentioned factors is more influential. If the true cause or causes are discovered, we can be hopeful of finding much larger upgrading effects on polymerization reactions, which are interesting from the academic and practical points of view. 

The higher rate of dehydration of benzamide and nicotinamide appears to be due to electron withdrawing effect of the aromatic ring which facilitates dissociation of the C=0 bond. 

Another problem often encountered with poor transfer of proteins relates to the amount of SDS in the gel, which coats the proteins and improves mobility during electrotransfer. Nitrocellulose membranes are not sensitive to the amount of SDS in the gel and can even tolerate addition of SDS to the transfer buffer to encourage transfer of recalcitrant proteins. PVDF membranes are more sensitive to excess SDS, which can inhibit protein binding to the hydro-phobic membrane nevertheless, in a limited number of cases, addition of SDS to the transfer buffer may be beneficial. Low-retention PVDF membranes are particularly sensitive to excess SDS concentrations, whereas high-retention PVDF membranes are less so. Methanol is a common component of many transfer buffers because it facilitates dissociation of bound SDS from proteins. Therefore, if proteins transfer from the gel efficiently but do not bind well to PVDF membranes, the methanol concentration can be increased to 20% and/or the gel can be preequilibrated in transfer buffer for 15 to 30 min prior to transfer to reduce the SDS concen-... 

In the E. coli system, it is important to stop the in vitro translation reaction by rapid cooling on ice. The reaction is usually diluted severalfold in prechilled buffer containing the components for stabilization of the ribosomal complexes. In the E. coli system, the ribosomal complexes can be very efficiently stabilized by low temperature and by high Mg2+ concentrations (50 mM), and then used for affinity selection. It is believed that high Mg2+ condenses the ribosome by binding totherRNA, making it difficult for the peptidyl-tRNA to dissociate or be hydrolyzed. The low temperature probably slows down the hydrolysis of the peptidyl-tRNA ester bond, and perhaps also the thermal motions, which would facilitate dissociation of the peptidyl-tRNA. Such complexes are stable for up to several days. 

When a silylium cation is generated from a neutral silyl compound in solution, then both reactant and product will be solvated to some extent, i.e. solvation of a silylium ion does not necessarily take place as a consequence of its generation from a neutral silyl compound in solution. Often solvation facilitates dissociation of a neutral silyl compound into silylium cation and counterion. In this case, part of the solvation shell may be carried along with the developing silylium ion in form of a solvent complex R3Si(S)n+ that possesses completely different properties than a silylium ion. One has to check in such a case whether it is still justified to speak of dissociation into silylium ion and counterion. 

Controiied/Living Polymerizations with Added Salts The two approaches discussed above are primarily useful in nonpolar solvents (like toluene and n-hexane) where the interactions of carbocations with nucleophiles are strong and favored. In relatively polar solvents like methylene chloride, these methods often fail to give controlled polymerizations, most likely because the interaction is weaker between the growing carbocations and nucleophiles [whether they are built-in (counteranions) or externally added (esters, etc.)], which facilitates dissociation of the carbocation. The effect of solvent in the latter system, however, is much weaker. 

Adsorption of methyl mercaptan in moist conditions was performed on numerous samples of activated cartons of various origins. Methyl mercaptan adsorption was tested by a dynamic method. The amount of products of surface reaction was evaluated using thermal analysis. The results revealed that the main product of oxidation, dimethyl disulfide, is adsorbed in pores smaller than SO A. There is apparent competition for adsorption sites between water (moist conditions) and dimethyl disulfi. The comp ition is won by the latter molecule due to its strong adsorption in the carbon pore system. Althou dimethyl disulfide has to compete with water for the adsorption sites it can not be formed in a significant quantity without water. Water facilitates dissociation of methyl mercaptan and thus ensures the efficient removal process. 

In polymerizing systems, the resonance-stabilised fragments could act mainly as terminating species, but it remains a fact nonetheless, that their formation, simultaneous with a reactive, initiating radical, is a general feature of the known, efficient, photoinitiator systems presumably because their relative stability facilitates dissociation. This may help to explain why benzil... 

The correlation of these mechanisms with solvent polarity (Lewis basicity) is strongly supported by a study of the solvent effect (% E) on the EjZ product ratio of equation (95) at 25 °C dioxane (100), THF (100), THF +AlCl, (100), 2,5-dimethyl-tetrahydrofuran (55), EtjO (60), Et20 +AICI3 (60), O-Pr) (25) . The addition of a crown ether raised the yield in /-Pr20 to 70% E, presumably by facilitating dissociation and the ionic route a drop in reaction temperature to —25 °C lowered the yield in EtjO to 45% E, presumably by facilitating association and the aggregate route . ... 

Direct reaction of Mg and at elevated T and P yields MgHj. The rate is slow, and the process is difficult to carry to completion because the metal particles become coated with a dense layer of the hydride, through which the Hj must diffuse. If the T is raised to increase the diffusion rate, the dissociation P is also increased and, for the reaction to proceed, the ambient Hj P must exceed the dissociation P e.g., 40 MPa and over 500°C are employed To increase the rate and facilitate complete reaction, various catalysts, such as Ij CCl allyl iodide and Mglj are used. Intimate mixing of LaNij MgjCu or Fe " with Mg metal decreases the time needed to complete hydride formation e.g., the addition of 20% LaNij increases the percentage completion after 15 min (at 345°C) from ca. 12% for pure Mg, to over 80% for the mixture . These latter three catalysts provide an oxide-free surface to facilitate dissociative adsorption of the Hj. 

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