Solvent delocalization

Fig. 11. Apparent B-solvent delocalization for amino-silica column as a function of B-solvent surface coverage (S) (---) Eq. (40) data of Fig. 10.

Fig. 13, C-solvent delocalization (as on silica for mixtures B/C, where B is benzene (A) O, ethyl ether Q, ethyl acetate V, acetone 0, tetrahydrofuran (B) D, acetonitrile V. isopropanol 0, ethanol. Data taken from Snyder and Glajch (/6). 

An interesting case are the a,/i-unsaturated ketones, which form carbanions, in which the negative charge is delocalized in a 5-centre-6-electron system. Alkylation, however, only occurs at the central, most nucleophilic position. This regioselectivity has been utilized by Woodward (R.B. Woodward, 1957 B.F. Mundy, 1972) in the synthesis of 4-dialkylated steroids. This reaction has been carried out at high temperature in a protic solvent. Therefore it yields the product, which is formed from the most stable anion (thermodynamic control). In conjugated enones a proton adjacent to the carbonyl group, however, is removed much faster than a y-proton. If the same alkylation, therefore, is carried out in an aprotic solvent, which does not catalyze tautomerizations, and if the temperature is kept low, the steroid is mono- or dimethylated at C-2 in comparable yield (L. Nedelec, 1974). 

In fee absence of fee solvation typical of protic solvents, fee relative nucleophilicity of anions changes. Hard nucleophiles increase in reactivity more than do soft nucleophiles. As a result, fee relative reactivity order changes. In methanol, for example, fee relative reactivity order is N3 > 1 > CN > Br > CP, whereas in DMSO fee order becomes CN > N3 > CP > Br > P. In mefeanol, fee reactivity order is dominated by solvent effects, and fee more weakly solvated N3 and P ions are fee most reactive nucleophiles. The iodide ion is large and very polarizable. The anionic charge on fee azide ion is dispersed by delocalization. When fee effect of solvation is diminished in DMSO, other factors become more important. These include fee strength of fee bond being formed, which would account for fee reversed order of fee halides in fee two series. There is also evidence fiiat S( 2 transition states are better solvated in protic dipolar solvents than in protic solvents. 

The formation of acyl halide-Lewis acid complexes have been observed by several methods. For example, both 1 1 and 1 2 complexes of acetyl chloride, with AICI3 can be observed by NMR spectroscopy. The existence of acylium ions has been demonstrated by X-ray diffraction studies on crystalline salts. For example, crystal structure determinations have been reported for /i-methylphenylacylium and acetylium ions as SbFg salts. There is also a good deal of evidence from NMR measurements which demonstrates that acylium ions can exist in nonnucleophilic solvents. " The positive charge on acylium ions is delocalized onto the oxygen atom. This delocalization is demonstrated in particular by the short O—C bond lengths in acylium ions, which imply a major contribution from the structure having a triple bond ... 

The NMR spectra of some 1- and 2-substituted perimidinium salts 139 are discussed and assigned. Several 2-substituted derivatives possess and H NMR spectra that reflect relatively slow prototropic tautomerism due to hydrogen bonding with a solvent, intramolecular hydrogen bonding, or enhanced delocalization of the nitrogen lone pair (88MRC191). 

The n values were high for all of the ionic liquids investigated (0.97-1.28) when compared to molecular solvents. The n values result from measuring the ability of the solvent to induce a dipole in a variety of solute species, and they will incorporate the Coulombic interactions from the ions as well as dipole-dipole and polarizability effects. This explains the consistently high values for all of the salts in the studies. The values for quaternary ammonium salts are lower than those for the monoalkylammonium salts. This probably arises from the ability of the charge center on the cation to approach the solute more closely for the monoalkylammonium salts. The values for the imidazolium salts are lower still, probably reflecting the delocalization of the charge in the cation. 

The H and 13CNMR spectra of various cyclopentazepines have been recorded (Tables 1 and 2), as has the HNMR spectrum of 7V,7V-dimethylcyclopent[e]azepin-l-amine.68 A detailed analysis of geminal and long-range 13C-H coupling constants for cyclopent[c]azepine is also available.87 The HNMR spectra of 9//-pyrrolo[l,2-a]azepin-9-one (8b) and its fully delocalized cation have been recorded in various solvents.7... 

More recently considered candidates are large molecular anions with delocalized anionic charge, which offer low lattice energies, relatively small ion-ion interaction, and hence sufficient solubility and relatively large conductivity. Delocalization of the charge is achieved by electron-with drawing substituents such as -F or - CF3. Furthermore, these anions show a good electrochemical stability to oxidation. In contrast to Lewis acid-based salts they are chemically more stable with various solvents and often also show excellent thermal stability. 

If the smaller monomer, capable of forming the cation with least charge delocalization, is indicated as monomer 1 and monomer 2 is characterized in an opposite manner, then the ratio r2/r, decreases during the transition into a more polar solvent. 

The di-n-propyl cyclopropenyl cation failed to photolyze either in aqueous acid or organic solvents, with or without sensitizers. A possible explanation in the discrepancy between the triphenyl system and this one lies in the calculated energy differences between the cations and their corresponding radicals. In the triphenyl system this energy difference is 0-5 3 or 16 kcal/mol, while in the di-n-propyl case it is 1 00)3 or 32 kcal/ mol, based on calculated delocalization energies for the two species. 

Evidence is provided by this analysis that (a) structural considerations discriminate among at least four practical classes of pi delocalization behavior, each of which has limited generality (b) the blend of polar and pi delocalization effect contributions to the observed effect of a substituent is widely variable among different reaction or data sets (the contributions may be opposite as well as alike in direction), depending upon structural considerations and the nature of the measurement (c) solvent may play an important role in determination of the observed blend of effects (d) it is the first three conditions which lead to the deterioration of the single substituent parameter treatment as a means of general and relatively precise description of observed electronic substituent effects in the benzene series. 

It is interesting to note in the latter connection that nucleophilic substitution transition states in which there apparently is not strong delocalization of pi electron density into the substituent tend to fall into the <7 type (cf. reactions 18 and 19 of Table VIII). In set 18, there are two ortho nitro groups which apparently take up much of the pi charge (thus it is unavailable to X), whereas in set 19, the positive piperidinium center may cause (perhaps with assistance from the NH hydrogen bonding permitted by the aprotic solvent) the... 

The facile thermal isomerization (17) of norbornadiene derivatives [71]-[77] to cycloheptatrienes in polar solvents has been explained in terms of the initial heterolytic cleavage of the strained C(l)-C(7) bond (Hoffmann and Hauser, 1965 Lemal et al., 1966 Hoffmann, 1971, 1985 Lustgarten and Richey, 1974 Hoffmann et al., 1986 Bleasdale and Jones, 1993). The resulting zwitterion intermediates are stabilized by the cation-stabilizing groups attached to C(7) and the cyclohexadienyl-type delocalization of the negative charge. 

Metals do not dissolve in water, because they contain extensive delocalized bonding networks that must be disrupted before the metal can dissolve. A few metals react with water, and several reacf with aqueous acids, but no metal will simply dissolve in water. Likewise, metals do not dissolve in nonpolar liquid solvents. 

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