Water relative nucleophilicity

Liotta and Grisdale (1975) have reported on the relative nucleophilicities of anions whose potassium salts were solubilized into acetonitrile by 18-crown-6 [3]. The results (Table 28) show the sequence Nj > OAc- > CN- > F- > Cl-x Br- > I- > SCN-, which is very different from the reactivity scale in water CN- > I- >SCN- > Nj > Br- > Cl- > OAc- > F-. Furthermore, the relative nucleophilicities in acetonitrile vary only by a factor of 30, whereas in water they differ by as much as a factor of 1000. The fact that gas-phase nucleophilicities also span a narrow range led the authors to conclude that anion solvation is much less important in acetonitrile than in water. The values recently reported by Lemmetyinen et al. (1978) for the relative nucleophilicities of anions towards methyl methanesulfonate in benzene show the same sequence as in protic solvents, however. The authors offered no explanation for this peculiar behaviour. 

Depending on the relative nucleophilicities, [Nu]50% ranges from micromolar to molar concentrations (Table 13.5). Although these values represent only order-of-magnitude estimates, they allow some important conclusions. First, in uncontaminated freshwa-ters (where bicarbonate typically occurs at about 10"3 M, chloride and sulfate occur at about 10 4 M, and hydroxide is micromolar or less, Stumm and Morgan, 1996), the concentrations of nucleophiles are usually too small to compete successfully with water in SN2 reactions involving aliphatic halides. Hence the major reaction will be the displacement of the halide by water molecules. In salty or contaminated waters, however, nucleophilic substitution reactions other than hydrolysis may occur Zafiriou (1975), for example, has demonstrated that in seawater ([CL] 0.5 M) an important sink for methyl iodide is transformation to methyl chloride ... 

Substantial rate accelerations are observed in these systems for base hydrolysis. Thus for the ethylenediamine complex (18) rate increases of 4x 104 for GlyOEt to 1.4 x 107 for ethyl picolinate are observed.85 These rate accelerations are consistent with the formation of carbonyl-bonded species (18). The effects with methyl L-cysteinate and methyl L-histidinate are much less marked as such ligands can give mixed ligand complexes which do not involve alkoxycarbonyl donors. Thus in the case of methyl L-histidinate the complex (20) is formed. For these latter two esters only relatively small rate accelerations of 20-100 occur. For the chelate ester complexes, the ratios of kcm/kH2o fail within the range 3.8 x 109 to 3.2 x 1011. Such values for the relative nucleophilicity of water and hydroxide ion are comparable with those previously noted for copper(II) complexes.82... 

Previous work has shown that the base hydrolysis of organonitriles is increased by a factor of ca. 108 on coordination to pentaamineruthenium(III) and by about 10s on coordination to Co111 and Rh111. A base-independent pathway is also observed with Rum, due to water attack on the coordinated nitrile.330 The relative nucleophilicities of hydroxide ion and water for attack on N-coordinated nitriles are ca. 109 (Table 22). 

Several reports on the effect of ionic liquids on Sn reactions have been published. The rate constants of the Sn2 reactions of several anions with n-hexyl and n-octyl mesylates in two ionic liquids, [hexmim]C104 and [hexmim]PF6 containing 2000 ppm of water, were compared with the reactivity in chlorobenzene, DMSO, and MeOH.74 The results indicated that the relative nucleophilicities in the ionic liquids were similar to those in the other solvents. The rates in the ionic liquids were generally faster than those in MeOH but slower than those in DMSO or chlorobenzene. 

It becomes complex if we try to compare anions having nucleophilic centres from different parts of the periodic table. In such a case, relative nucleophilicity does not match relative basicity. This is because the solvent used in a reaction has an important effect. In protic solvents like water or alcohol, the stronger nucleophiles are those which have a large nucleophilic centre, i.e., an atom lower down the periodic table (e.g. S" is more nucleophilic than O but is less basic). This is because protic solvents can form hydrogen bonds to the anion. The smaller the anion, the stronger the solvation and the more difficult it is for the anion to react as a nucleophilic. 

The problem of water in ionic ring-opening polymerization is much less critical than in vinyl polymerization, because, if present, water in the former system has to compete with relatively nucleophilic monomer present in large excess. Thus, certain polymerizations (e.g., polymerization of cyclic amine-conidine) can be conducted even in alcohols as solvents. 

Sometimes relative nucleophilicities change in going from a protic to an aprotic solvent. For example, the relative nucleophilicities of the halide ions in water are I >Br >C1 , whereas in dimethylformamide, the nucleophilicities are reversed, i.e., Cl"> Br > I . 

The observation that water attacks only the benzylic a-carbon of styrene oxide, whereas hydroxide ion attacks equally at both the a-carbon and /j-carbon indicates that the aromatic ring stabilizes the transition state for a-attack of water (relative to /i-attack) more than it does the transition state for a-attack of hydroxide ion. The transition state for attack of water must be a looser structure in which the nucleophile is less bonded to the benzylic carbon. 

Unequivocal evidence for associative activation within Cr(DMF)63+, A ion pairs in DMF is given in Table 8.1, which shows that azide ion attacks the Cr center some 100 times faster than solvent exchange and 650 times faster than bromide. This is of particular interest in that no comparable result can be obtained for the reaction of N3 with Cr(H20)63+ in water because of the Brpnsted basicity of azide that causes the reaction to proceed via HN3 and Cr(H20)50H2+, but in anhydrous DMF the relative nucleophilic power of anionic reagents toward Cr111 is clearly displayed N3- NCS > Cl > Br > C104, BPh4 . 

TS analysis of inosine hydrolysis. Experimental KIEs were determined for inosine hydrolysis under pre-steady state conditions because of the exceptionally slow dissociation of hypoxanthine (estimated K = 1.3 pM), which is rate-limiting under steady state conditions. Hypoxanthine binds to the other two PNP subunits with much lower affinity. KIEs in the ribosyl ring were typical of an AnDn mechanism. When the reaction was run in 20% methanol, the product ratio was 85 15 1-methylribose ribose. This is close to the ratio expected based on the relative nucleophilicities of MeOH and water, indicating that there is significant participation of the nucleophile in the reaction coordinate. As with the arsenolysis, the primary, 9- N KIE of 1.000 was anomalous. These KIEs were initially rationalized as indicating an internal equilibrium formation of ribose and hypoxanthine however, this is inconsistent with what is now known about the observable KIEs for stepwise reactions. 

The term nucleophilicity refers to the capacity of a Lewis base to participate in a nucleophilic substitution reaction and is contrasted with basicity, which is defined by the position of an equilibrium reaction with a proton donor, usually water. Nucleophilicity is used to describe trends in the rates of substitution reactions that are attributable to properties of the nucleophile. The relative nucleophilicity of a given species may be different toward various reactants and there is not an absolute scale of nucleophilicity. Nevertheless, we can gain some impression of the structural features... 

Reaction with 1.0 equivalent of 1-octanol (to 0.03 eq of 1-octene) gave a 98 2 mixture of alcohol/ether in 50% aq THE. The use of 10 equivalents of 1-octanol in 0.3 M THF in water gave a 52 48 mixture of alcohol/ether. Explain what the 52 48 mixture tells you about the relative nucleophilicity of water and 1-octanol. Explain why increasing the amount of 1-octanol leads to more ether product. 

PhCl/H20 medium are 3.4, 2.1, and 1.0, respectively [5,49]. The presence of hydrated water molecules tends to reduce the anion activation and vary the relative nucleophilicities of anions. For example, in the displacement reaction of n-octylmethanesulfonate and halides under homogeneous conditions, the order of relative nucleophilicities is Cl > Br > I whereas it is Br > I > CF under PTC conditions [5,49,53]. The desiccating salting-out effect provided by the presence of inorganic salt, especially concentrated 50% aqueous NaOH solution, reduces substantially the hydration of anions and ion pairs [9]. 

For methyl glycinate at 25°C, ko - is 7.6 x 10 M s and k for water attack is 4.3 x 10 s. The observed first order rate constant for water attack can be converted to a second order rate constant by dividing by the molar concentration of water OChk) = k/55.5 = 7.7 X 10 M s ). The ratio Icoh- hjo = 10" and is a measure of the relative nucleophilicities of hydroxide ion and water towards the copper complex. The ratio kou-/kg = 7.6 X lOVl.28 10 and this is within the normal range of rate enhancements (10 -10 fold) observed for copper(II) promoted hydrolysis of carboxylic esters where copper(II) interacts directly with the alkoxycarbonyl group of the ester. 

H H2o fall within the range 3.8 x 10 to 3.2 x 10". Such values for the relative nucleophilicity of hydroxide ion and water are quite comparable with those previously observed with copper(ll) complexes. ... 

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