Solvation by alcohol

Anion solvation in alcohol clusters has been studied extensively (see Refs. 135 and 136 and references cited therein). Among the anions that can be solvated by alcohols, the free electron is certainly the most exotic one. It can be attached to neutral alcohol clusters [137], or a sodium atom picked up by the cluster may dissociate into a sodium cation and a more or less solvated electron [48]. Solvation of the electron by alcohols may help in understanding the classical solvent ammonia and the more related and reactive solvent water [138], By studying molecules with amine and alcohol functionalities [139] one may hope to unravel the essential differences between O- and N-solvents. One should note that dissociative electron attachment processes become more facile with an increasing number of O—H groups in the molecule [140],... 

This stability of the ions to decompose to 02 is probably caused by solvation by alcohol and 02 molecules. This would give more stability than expected from reaction energetics alone. 

Adducts of alkali metal salts prepared in anhydrous alcoholic media generally retain very little alcohol of solvation after being dried under vacuum at room temperature (see Table I). The unusual ability of adducts of D-glucitol to retain alcohol is probably due largely to the great ability of D-glucitol itself to retain solvent. Adducts of alkaline-earth metal salts, however, are more strongly solvated by alcohol than adducts of alkali metal salts. For example,21 lactose CaClj 4 MeOH is relatively stable at 60° at atmospheric pressure under vacuum (< 19 mm. of Hg), a molecule releases only two of the four molecules of methanol. From aqueous alcoholic media, adducts of alkaline-earth metal salts tend to crystallize as hydrates. 

This type of coordination stabilizes a crown conformation of the cyclohexa-siloxane ring. There are, in addition, two sodium cations attached at the top and the bottom of the sandwich in a crown ether-type complexation mode, with each sodium ion bonded to six oxygens of the rings, 9, and further solvated by alcohol molecules (not shown). The macrocyclic siloxane acts as both endo and exo receptors and the formation of the supramolecular architecture involves both ionic self-assembly and crown-ether-type ion recognition. 

In media such as water and alcohols fluoride ion is strongly solvated by hydro gen bonding and is neither very basic nor very nucleophilic On the other hand the poorly solvated or naked fluoride 10ns that are present when potassium fluoride dis solves m benzene m the presence of a crown ether are better able to express their anionic reactivity Thus alkyl halides react with potassium fluoride m benzene containing 18 crown 6 thereby providing a method for the preparation of otherwise difficultly acces sible alkyl fluorides... 

The effect of alkyl substitution on alcohol acidity is due primarily to solvation of the alkoxide ion that results from dissociation. The more readily the alkoxide ion is solvated by water, the more stable it is, the more its formation is energetically favored, and the greater the acidity of the parent alcohol. For example, the oxygen atom of an unhindered alkoxide ion, such as that from methanol, is stericallv accessible and is easily solvated by water. The oxygen... 

It should be born in mind, however, that the activation parameters calculated refer to the sum of several reactions, whose enthalpy and/or entropy changes may have different signs from those of the decrystalUzation proper. Specifically, the contribution to the activation parameters of the interactions that occur in the solvent system should be taken into account. Consider the energetics of association of the solvated ions with the AGU. We may employ the extra-thermodynamic quantities of transfer of single ions from aprotic to protic solvents as a model for the reaction under consideration. This use is appropriate because recent measurements (using solvatochromic indicators) have indicated that the polarity at the surface of cellulose is akin to that of aliphatic alcohols [99]. Single-ion enthalpies of transfer indicate that Li+ is more efficiently solvated by DMAc than by alcohols, hence by cellulose. That is, the equilibrium shown in Eq. 7 is endothermic ... 

The second group of studies tries to explain the solvent effects on enantioselectivity by means of the contribution of substrate solvation to the energetics of the reaction [38], For instance, a theoretical model based on the thermodynamics of substrate solvation was developed [39]. However, this model, based on the determination of the desolvated portion of the substrate transition state by molecular modeling and on the calculation of the activity coefficient by UNIFAC, gave contradictory results. In fact, it was successful in predicting solvent effects on the enantio- and prochiral selectivity of y-chymotrypsin with racemic 3-hydroxy-2-phenylpropionate and 2-substituted 1,3-propanediols [39], whereas it failed in the case of subtilisin and racemic sec-phenetyl alcohol and traws-sobrerol [40]. That substrate solvation by the solvent can contribute to enzyme enantioselectivity was also claimed in the case of subtilisin-catalyzed resolution of secondary alcohols [41]. 

The photochemical isomerization 41B — 41A was observed in nonpolar solvents (83AJC1603). Equilibrium 1) was attained in CD3OD solution after several days, but following concentration and cooling of the solution, only the cyclic isomer 41B, solvated by methanol, was isolated in the solid state. Alcoholic solvents effectively quench the photochemical isomerization. 

The redox potential values of all metal atoms, except alkaline and earth-alkaline metals [60], are higher than that of °(H20/eaq) = —2.87 V he- However, some complexed ions are not reducible by alcohol radicals under basic conditions and thus ii°(M L/ M°L)< —2.1 Vnhe (Table 2). The results were confirmed by SCF calculations of Ag L and Ag L structures associated with the solvation effect given by the cavity model for L = CN [61] or NH3 [47], respectively. 

For substitution at a carbonyl carbon, the nucleophilicity order is not the same as it is at a saturated carbon, but follows the basicity order more closely. The reason is presumably that the carbonyl carbon, with its partial positive charge, resembles a proton more than does the carbon at a saturated center. That is, a carbonyl carbon is a much harder acid than a saturated carbon. The following nucleophilicity order for these substrates has been de-termmined 321 Me2C=NO- > EtO" > MeO > OH" > OAr- > N-f > F" > H20 > Br" I". Soft bases are ineffective at a carbonyl carbon.322 In a reaction carried out in the gas phase with alkoxide nucleophiles OR solvated by only one molecule of an alcohol R OH, it was found that both RO and R O" attacked the formate substrate (HCOOR") about equally, though in the unsolvated case, the more basic alkoxide is the better nucleophile.323 In this study, the product ion R"0 was also solvated by one molecule of ROH or R OH. 

In weakly solvating solvents interionic interactions between organic molecular ions can lead to fixation of the ionic end of the molecule. The Tl values of pertinent and neighboring 13C nuclei become smaller. In contrast, strongly solvating solvents such as water and alcohols inhibit interionic interaction and lead to an enhanced mobility of the ions solvated by ion-dipole interactions 13C spin-lattice relaxation is consequently slower in such solvents. Thus the T, values of n-butylammonium trifluoroacetate increase with the polarity of the solvent, as shown in Table 3.19 [148]. 

Comparison of the properties of metal alkoxides with their structures permits a conclusion that the polymeric nature does not always lead to chemical inertness. The major role appears to be played by the nature of the M-OR bonding. Solubility in alcohols and liquid ammonia of the methoxides of alkaline and alkaline earth metals and that in hydrocarbons ofthe isopropoxides of K, Rb, Cs (isostructural with the corresponding methoxides), and also M(OC2H4OMe)n, M = Pb, Bi indicates the easy oligomerization due to solvation or chelation. At the same time the methoxides and ethoxides of Al, Cr, Fe, and so on, forming the strongest covalent bonds in the [MOs/6] octahedra (and not prone to solvation in alcohols), appear almost inert. They can be dissolved only due to complexation or partial destruction with formation ofoxobridges. 

The alkoxides of both oxidation states were obtained by metathesis of germanium halides (chlorides and iodides) with alkali alkoxides [1488, 1142, 857, 1535]. The yields can be increased by application of GeCl4 solvates with Py or NH3 or amines [3, 222] (method 5) and also by alcohol interchange of ethoxides (method 6) or alcoholysis of Ge[Si(NR3)2]2 [568, 1543] (method 4). The application of alkali alkoxides in the preparation of Ge(OR)4is possible in contrast to that of analogous derivatives of Sn(IV) and Zr due presumably to the much lower stability of corresponding alkoxogermanates the intermediate products of the corresponding reactions because of stability — of the tetrahedral coordination for Ge. The direct electrochemical preparation of Ge(OEt)4... 

Product selectivities were also reported for solvolyses of p-mcthoxybcnzoyl chloride in aqueous MeOH, EtOH, 2,2,2-trifluoroethanol, PrOH, IVOil, and Bu OH at 25, 35, and 45 °C. The S values were small and depended significantly on the alcohol cosolvent, varying from 1.3 in MeOH to 0.1 in Bu OH, but depended only slightly on the solvent composition and temperature. As S adjusts the product ratios for changes in bulk solvent compositions, it was concluded that preferential solvation by either alcohol or water at the reaction site was not a major factor influencing rates or products. Logarithms of rate of solvolyses of p-mcthoxybcnzoyl chloride correlated well with Kosower Z values (based on solvatochromism).24... 

Scheme 3.3 Hydrogen peroxide dimer solvated by a chiral alcohol...

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