Solvent destabilization

Solvent destabilization of the molecular structure of organic materials can be quantified by simple proton nuclear magnetic resonance ( H NMR) measurements at ambient temperatures. Such measurements have shown that up to 60% of a coal s molecular structure can be destabilized by pyridine and, by the same token, that at least 40% Is Impervious to this solvent (15-18). 

I) reduction initially decreased in magnitude from —780 to —580 mV in going from n = 0 to n = 2 (versus Ag wire quasi-reference, 100 mM Bu4NC104 in DMF (dimethyl formamide), GCE). It then was reported to increase in magnitude to —830 mV for n — 3 and —880 mV for n — A. However, from the narrative, it was unclear why the authors were able to measure a thermodynamic redox potential as they commented on the broadness of their CV data. At least for the first few points, the trend in the redox potential follows the established paradigm. As the charge upon reduction decreases, the relatively less polar environment of the dendrimer compared to the solvent destabilizes the reduced form less than the oxidized form. 

As might be anticipated, the majority of solvent effects of the structures of pyridines relate to conformational changes and, where appropriate, this has been mentioned above. It may be further exemplified by the conformational changes observed on addition of alcohols and fluorinated alcohols to solutions of the pyridinium salts such as 51 in water <2000JA738>. Helical coils are favored by the fluorinated alcohols as the co-solvent destabilizes the exposed hydrophobic side chains in other conformations along with favoring the helical conformation entropically. The former effect is less marked in nonfluorinated alcohols. 

A polar solvent solvates the anionic nucleophile and slows the reaction down. A nonpolar solvent destabilizes the starting materials more than it destabilizes the transition state and speeds up the reaction. There is another reason for using acetone tor this particular reaction. Nai is very soluble in acetone but NaBr is rather insoluble. The NaBr product precipitates out of solution which helps to drive the reaction over to the right. 

The Type 3 SN2 reaction between Cl- + CH3SHf is interesting because it represents a formal anion-cation recombination through substitution. Because charges are annihilated in forming the transition state, polar solvents will significantly destabilize product formation. Fortunately, the loss in solvation of the two ions is compensated for by electrostatic attractions in bringing the two reactant species into contact. Therefore, the outcome of an SN2 reaction of Type 3 depends on the balance of Coulomb stabilization and solvent destabilization. The reactant and product diabatic states are defined as follows in MOVB theory ... 

In media that destabilizes caibonium ions, eg, hydiocaibon solvents, Ciiegee-type leaiiangements occur (66,69), eg, for tertAmX tiityl peroxide [7664-86-0]. ... 

The presence of polysulfonates in petroleum sulfonates used in lube formulations has a destabilizing effect on the formulation stabiUty and function of the sulfonate in motor oils, etc. Special techniques are utilized to help reduce the carryover of residual sludge components, including the use of hydrocarbon solvents such as hexane or heptane to faciUtate separation of sludge, often with centrihigation. Other desludging procedures include water wash, H2SO4 wash, clay percolation, and filtration. 

Other techniques aimed at improving grease recovery (and often attempting also to improve the scouring process itself) have included solvent degreasing of the wool (52,53), solvent extraction of the Hquor or sludge (178), aeration (179,180), and physical and chemical destabilization (175). 

Recently Alan Fersht, Cambridge University, has developed a protein engineering procedure for such studies. The technique is based on investigation of the effects on the energetics of folding of single-site mutations in a protein of known structure. For example, if minimal mutations such as Ala to Gly in the solvent-exposed face of an a helix, destabilize both an intermediate state and the native state, as well as the transition state between them, it is likely that the helix is already fully formed in the intermediate state. If on the other hand the mutations destabilize the native state but do not affect the energy of the intermediate or transition states at all, it is likely that the helix is not formed until after the transition state. 

Saturated hydrocarbons show a slight stabilizing effect on the initial state but a very destabilizing effect on the transition state, consistent with arguments based on solvent polarity. 

The Self-Consistent Reaction Field (SCRF) model considers the solvent as a uniform polarizable medium with a dielectric constant of s, with the solute M placed in a suitable shaped hole in the medium. Creation of a cavity in the medium costs energy, i.e. this is a destabilization, while dispersion interactions between the solvent and solute add a stabilization (this is roughly the van der Waals energy between solvent and solute). The electric charge distribution of M will furthermore polarize the medium (induce charge moments), which in turn acts back on the molecule, thereby producing an electrostatic stabilization. The solvation (free) energy may thus be written as... 

A study of the relative stability of tra 5-2-azidothiazole 32b with respect to the corresponding cis conformer 32a showed that 32b is destabilized in the gas phase but appears to be the more stable conformer in solution (Scheme 23). The stability increases with increasing polarity of the solvent [98JA4723]. 

What about solvent Do solvents have the same effect in S>g 1 reactions that they have in S j2 reactions The answer is both yes and no. Yes, solvents have a large effect on S l reactions, but no, the reasons for the effects on S jl and SN2 reactions are not the same. Solvent effects in the SN2 reaction are due largely to stabilization or destabilization of the nucleophile reactant. Solvent effects in the Sjsjl reaction, however, are due largely to stabilization or destabilization of the transition state. 

FIGURE 2.3. The energetics of a heterolytic bond cleavage reaction in a polar solvent. The specific example shown corresponds to the CH3OCH3— CH3 + CH30 reaction in water. The energy of the covalent state does not include the effect of the solvent on this state, but a more consistent treatment (e.g., eq. (2.21) should account for the polarization of the solvent toward the charges of the ionic state. This would result in destabilization of H31. 

Intramolecular hydrogen-bonds can increase the stability of certain conformations. For example, dianhydrides that contain fJ-L-Sorp or ct-D-Frup in the 5C2 conformation have the C-4 hydroxyl group in a 1,3-diaxial relationship with 0-2, which permits the formation of an intramolecular hydrogen bond. This might, in part, offset the destabilizing influence of three or two axial substituents, respectively. This effect is decreased in hydrogen-bonding solvents. 

Solubility limits depend on the stabilization generated by solute-solvent interactions balanced against the destabilization that occurs when solvent-solvent interactions are dismpted by solute. Thus, we must examine intermolecular interactions involving water and alcohol molecules. 

---