Nucleophile of water

The efficiency of this injection system depends upon the reaction conditions. 02) which traps the first formed radical 6, reduces the yield of enol ether 8. Therefore, we are using our assay under anaerobic conditions. Also, the pH of the solution influences the product ratio because it changes the nucleophilicity of the water. Figure 1 shows how the efficiency of the electron transfer is reduced as the pH value increases from 5.0 to 7.0 [5]. This is in accord with an increase of the nucleophilicity of water, which traps the radical cation (7—>9+10) in competition to the electron transfer step (7—>8). 

Some of the relevant data are collected in Table XXXII. From these data we note that the apparent nucleophilicity of water is lower than that of OH" by a factor of 55.5 k2/kx = 5.3 x 107. The equilibrium constant for the ionization of the hydroxyl group of adduct 161 is K a. 

Because of the enhanced nucleophilicity of water formed under reaction conditions, these hydrolysis products predominate in weaker acids (FSO3H, HF, H2S04). 

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... 

As noted above, the nucleophilicity of water allows it to enter into reactions that cause the degradation of biological macromolecules, including DNA and proteins. Analogous problems are associated with the assembly of biopolymers. In water, the assembly of nucleosides from component sugars and nucleobases, the assembly of nucleotides from nucleosides and phosphate, and the assembly of oligonucleotides from nucleotides are all thermodynamically uphill in water. 

The anodic coupling of aryl ethers is reviewed in Ref. 49, Aryl ethers are more selectively coupled than phenols, for the following reasons The carbon-oxygen coupling is made impossible, and the orr/ o-coupling and the oxidation to quinones become more difficult. A mixture of trifluoroacetic acid (TEA) and dichloromethane proved to be the most suitable electrolyte [50]. TEA enhances the radical cation stability and suppresses the nucleophilicity of water. Of further advantage is the addition of alumina or trifluoroacetic anhydride [51]. Table 4 summarizes representative examples of the aryl ether coupling. 

Attack as a Nucleophile. In the Cu(II)-catalyzed hydrolysis of the acetyl ester of 2-pyridinecarboxaldoxime, the metal-bound water molecule makes nucleophilic attack at the carbonyl carbon of the bound ester (H), instead of the kinetically equivalent attack by an external water molecule at the ester linkage bound by the metal ion (43). The basicity and nucleophilicity of water would decrease upon coordination to metal ions. Efficient nucleophilic attack by the Cu(II)-bound water in H is attributable to the general base assistance from another water molecule and to the efficient intramolecular reaction between the nucleophile and the ester. 

It can increase the rate of a hydrolysis reaction by increasing the nucleophilicity of water, as in C. 

The orf/zo-carboxylate group is an intramolecular general-base catalyst that increases the nucleophilicity of water, thereby increasing the rate of formation of the tetrahedral intermediate. 

A metal ion can increase the rate of a reaction by making a reaction center more susceptible to receiving electrons, by making a leaving group a weaker base, or by increasing the nucleophilicity of water. An electrophilic catalyst is a metal ion with the same catalytic effect as a proton. 

A metal-ion cofactor acts as a Lewis acid in a variety of ways to help an enzyme catalyze a reaction. It can coordinate with groups on the enzyme, causing them to align in a geometry advantageous for reaction it can help bind the substrate to the active site of the enzyme it can form a coordination complex with the substrate to increase its reactivity or it can increase the nucleophilicity of water at the active site (Section 24.5). An enzyme that has... 

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. 

Hexokinase can also catalyze the transfer of the terminal phosphoryl of ATP to water i.e., it acts as an ATPase but at a rate 5x10 times slower than for the above reaction. The basicity and nucleophilicity of water, versus the C-6 hydroxyl of glucose, are sufficiently similar to suggest that there would be no marked differences in rate. Therefore the explanation for the rate difference is that glucose induces a conformational change that establishes the correct active-site geometry in the enzyme, whereas a water molecule is too small to do so. 

Traces of water ((H2O) 10 M) in DCM or NM could inhibit the electropolymerization of benzene and biphenyl. This effect was partially attributed to the nucleophilicity of water which reacts with the radical-cation as soon as it is formed. When a small amount of a dehydrating agent (P2O5 or CuC ) was added to the medium, the electropolymerization process could occur again [113]. 

In the kinetically controlled process, serine and cysteine proteases catalyze acyl transfer from activated carboxy components to various acceptors. The initially formed acyl-enzyme intermediate can be competitively deacylated by water (hydrolysis) or by an amine nucleophile (aminolysis). The yield of the peptide bond formation depends on two factors (1) the relative rate of hydrolysis and aminolysis, which is determined by the nucleophilicity of water versus that of the nucleophile and (2) the molar ratio of water and the nucleophile [62]. The ratio between aminolysis and hydrolysis of the acyl-enzyme intermediate is generally... 

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