Hydroxylic solvents ionization

With regard to SN2 reactions, the solvent can affect profoundly the reactivity of a given nucleophile. Thus anions such as Cl and CNe, which are weakly nucleophilic in hydroxylic solvents and in poor ionizing solvents such as 2-propanone (acetone), become very significantly nucleophilic in polar aprotic solvents such as (CH3)2SO. The reason is that for salts such as NaCl and NaCN the aprotic solvent preferentially solvates the cation, leaving the anion relatively bare. This dissociation of the anion from the cation together with its poor solvation makes the anion abnormally reactive as a nucleophile. 

There are five papers on carbamate chemistry of interest.6,84-87 The mechanism of the reaction in MeCN of /V-mcthyI-/V-phcnyIcarbamoyI chlorides (94) with benzyl-amines is believed to be Sf2 based on Hammett p values, a cross-interaction constant pxy of —0.14, ka/ko values for the /V-dcutcriatcd benzylamines all <1, and low activation enthalpies.84 The aminolysis of /7-nitrophcnyl /V-phenylcarbamatcs in acetonitrile involving the T1 (7) was discussed earlier.6 Solvolysis-decomposition of N- -adamantyl-/V-/7-tolylcarbamoyl chloride (95) in hydroxylic solvents involves a facile slow ionization (SWl mechanism) giving a cation which eliminates ArNCO to... 

Rates of decomposition do not vary much with the halide ion. The reactivity order falls between that which is usally observed in the gas phase or in aprotic media (Cl>Br>I) and that which is observed in hydroxylic solvents (Clfirst-order decomposition may occur in more ionizing media, for example CD3CN or PhNC>2, in which dissociation leads to the incursion of a second-order component. 

The 13C-NMR spectral analysis of ajaconine in nonionic and ionic solvents indicated that in hydroxylic solvents the ether linkage of ajaconine ionizes and covalent solvation takes place (87). This observation accounted for the formation of the Schiff salt, with the resultant high pA a value (11.8) of ajaconine in aqueous solution—behavior which parallels that of atisine (pKa 12.5) and veatchine (pA a 11.5). These results suggested that ajaconine may be rearranged by refluxing in an ionic solvent to a compound in which the C-7-C-20 ether linkage is absent. 

Ammonia is well known to be a strong nucleophile. The nucleophilic substitution reactions in the ionized clusters of X-benzene (X = F, Cl) have been widely studied (Brutschy 1989, 1990 Brutschy et al. 1988, 1991 Mayeama and Mikami 1988, 1990, 1991 Riehn et al. 1992). All these experiments clearly show a specific behaviour not encountered in the hydroxylic solvents depending on the substituted aromatic and the cluster size, two types of substitution reaction can be detected. The first type leads to ionic amines by substitution of X by the -NH2... 

Water, alcohols, and carboxylic acids are polar protic solvents able to form hydrogen bonds (hydroxylic solvents). They solvate both cations and anions well. A nucleophilic reagent such as bromide ion must be accompanied by a cation, say, the sodium ion, and hydroxylic solvents dissolve salts such as NaBr by hydrogen bonding to the anion and electron donation to the cation. This is solvation by a polar protic solvent. These solvents do not ionize the salt, which already exists in the solid state as ions they separate and solvate the ions already present. 

Solvolysis in Aqueous Systems. In weakly acidic hydroxylic solvents, nitrite ion is a better leaving group than acetate ion, and l,li-adducts typically follow an aromatization path that leads to the formation of an aryl acetate and the elements of nitrous acid. Figure 2. Kinetic studies of solvolysis of the adducts derived from hemimellitene reveal the characteristics of El eliminations. Rates of solvolysis of both the cis and trans-isomers can be correlated to measures of solvent ionizing power such as Grunwald-Winstein Y values. Figure if. Both isomers... 

A solvent isotope effect is observed when the rate constant or the equilibrium constant for a process changes when a solvent is replaced with an isotopically substituted solvent. Most often solvent isotope effect studies involve a hydroxylic solvent in which the OH group is replaced with an OD group. For example, the value of Kh/Kd for ionization of phenylacetic add in H2O versus ionization of phenylacetic acid-O-d in D2O is 3. " Schowen has indicated three ways for solvent isotope effects to occur ... 

Many substances which behave as acids in hydroxylic solvents exhibit basic properties in sulphuric acid. Thus most carboxylic acids are strong bases, forming the ion RCOOHJ, though reaction is incomplete for strong acids such as di- and tri-chloroacetic acids, which are thus weak bases in this solvent. Nitro-compounds, sulphones, and sulphonic acids also behave as weak bases, and it is in fact difficult to find substances which are soluble in sulphuric acid without detectable ionization. Since cryoscopic measurements lead only to the total number of solute particles, it is not possible to obtain quantitative measurements of base strength over a wide range, especially since there are complications caused by the self-dissociation of the solvent, and interionic effects, though small, must be taken into account. 

Ionization of the carbon-bromine bond forms a resonance-stabilized 2° allylic car-bocation. Acetic acid is a poor nucleophile, which reduces the likelihood of an S j2 reaction. Further, acetic acid is a moderately polar protic (hydroxylic) solvent that favors Sjjl reaction. From this analysis, we predict that this reaction occurs by an 8, 1 mechanism and both enantiomers of the product are observed. 

Internal and External Phases. When dyeing hydrated fibers, for example, hydrophUic fibers in aqueous dyebaths, two distinct solvent phases exist, the external and the internal. The external solvent phase consists of the mobile molecules that are in the external dyebath so far away from the fiber that they are not influenced by it. The internal phase comprises the water that is within the fiber infrastmcture in a bound or static state and is an integral part of the internal stmcture in terms of defining the physical chemistry and thermodynamics of the system. Thus dye molecules have different chemical potentials when in the internal solvent phase than when in the external phase. Further, the effects of hydrogen ions (H" ) or hydroxyl ions (OH ) have a different impact. In the external phase acids or bases are completely dissociated and give an external or dyebath pH. In the internal phase these ions can interact with the fiber polymer chain and cause ionization of functional groups. This results in the pH of the internal phase being different from the external phase and the theoretical concept of internal pH (6). 

Ionizing radiations (a, ft and y) react unselectively with all molecules and hence in the case of solutions they react mainly with the solvent. The changes induced in the solute due to radiolysis are consequences of the reactions of the solute with the intermediates formed by the radiolysis of the solvent. Radiolysis of water leads to formation of stable molecules H2 and H2O2, which mostly do not take part in further reactions, and to very reactive radicals the hydrated electron eaq, hydrogen atom H" and the hydroxyl radical OH" (equation 2). The first two radicals are reductants while the third one is an oxidant. However there are some reactions in which H atom reacts similarly to OH radical rather than to eaq, as e.g. abstraction of an hydrogen atom from alcohols, addition to a benzene ring or to an olefinic double bond, etc. 

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