Hydroxyl attack

FIGURE 16.12 Catalysis of nitrophenylacetate hydrolysis by imidazole—an example of general base catalysis. Proton transfer to imidazole in the transition state facilitates hydroxyl attack on the substrate carbonyl carbon. 

Addition of new nucleotide units to the growing chain occurs in the 5 to C direction, and is catalysed by the enzyme DNA polymerase. The most important step is the addition of a 5 -mononucleoside triphosphate to the free 3 -hydroxyl group of the growing chain as the 3 -hydroxyl attacks the triphosphate and expels a diphosphate leaving group. 

The slope is indicative of the type of release mechanism. A slope of 0.5 indicates a diffusion-controlled release a slope of 1.0 indicates that a corrosion-related mechanism is operable.The diffusion release mechanism is characterized by surface adsorption, ion exchange, and migration. Chemical corrosion, or alteration of the silicate lattice, is characterized by hydroxyl attack on silicon or by hydrogen attack on bridging oxygens. 

Figure 81 Lactone formation from intramolecular hydroxyl attack on a carboxylic acid.

Base-catalyzed protiodedeuteriation of 5-deuterio-l,3-dimethyluracil (Scheme 10.4) probably involves the addition of water across the 5,6-bond, initiated by hydroxyl attack at C-6. While a similar mechanism may apply with 5-deuterio-l-methyluracil, direct exchange with the anion is also a possibility the anion is almost certainly implicated in the case of the 5-deuterio-3-methyl isomer (70JHC903). 

The UV irradiation of phenol in the presence of nitrite (pH around neutrality) yields 4-nitrosophenol, catechol, hydroquinone, 1,4-benzoquinone, 2-nitrophenol and 4-nitrophenol [44,62,78,79]. It is interesting to observe that nitrophenols do not form in appreciable amount upon irradiation of 1.0 x 10-3 M phenol and nitrite, while relevant nitrophenol formation can be observed for [N02 ] > 1.0 x 10-3 M [44,79]. The formation of catechol, hydroquinone and 1,4-benzoquinone is most likely due to the photoinduced generation of hydroxyl (reactions 17 and 18). The relevant processes are initiated by hydroxyl attack on phenol (H-Ph-OH) in the presence of oxygen ([78,79] HO - Ph - OH catechol or hydroquinone, O = Ph = O 1,4-benzoquinone) ... 

Hydroxylation of arenes. This reaction can be carried out conveniently with 30% H2O2 in pyridinium polyhydrogen fluoride at 0° (20-60% yield). In the case of alkylbenzenes predominate ortho-para substitution obtains. 1,2-Methyl shifts are sometimes observed. Loss of an alkyl group can occur to a slight extent by ipso hydroxyl attack. ... 

The oxidation of soot in fuel-rich premixed flames proceeds through hydroxyl attack, with one atom of carbon removed by each OH radical hitting the surface of the soot. The absolute level and composition dependence of the disappearance rate of soot show that OH is the important species. The oxidation was followed experimentally by laser light scattering from soot particles generated in one flame and passed directly into the gas supply of a larger hydrogen/air flame controlled at 1700-2200 K. 

Photo-oxidation of chlorinated ethanes (in the presence of Clj) produces COClj along with a variety of other chlorocarbonylated derivatives, see Table 3.6 [1926]. Chlorine atoms (generated by the photodissociation of Cl 2) initiate the oxidation of the ehlorinated ethenes. A hydrogen atom is abstracted (simulating hydroxyl attack in the ambient atmosphere) and... 

The example below refers to a recent study by pulse and gamma radiolysis [8]. A pH 6 has been generally used. Hydroxylation mainly concerns the aromatic ring of carbofuran (Fig. 4 ) in fact, the first intermediate shows the absorption of a cyclohexadienylic type radical in the region 280-330 nm.The hydroxylation attack to carbofuran... 

Schematic drawing showing the two transesterification steps of group I self-splicing. In the first step there is a nucleophilic attack by the guanosine cofactor on the phosphodiester bond at the 5 boundary of the intron. In the second transesterification, the free 3 hydroxyl attacks the phosphodiester bond at the downstream intron-exon border, thus causing the release of the intron as a linear RNA fragment and the covalent linkage of the two exons.

Alternatively, initial attack of the nitrogen dioxide end of ONOO" on phenol would produce phenoxyl radical and NOi followed by immediate hydroxyl attack to give either 2- or 4-hydroxyphenol (catechol or hydroqui-none, respectively). The higher yield of nitrated phenolic products relative to hydroxylated products suggests that the hydroxyl end of ONOO" generally serves as the primary oxidant which is precisely what one would predict given the relative redox potentials of hydroxyl radical and -NOi. 

The liberated secondary hydroxyl attacks die aldehyde, forming a hemiacetal. 

The dienol hydroxyl attacks one of the mixed anhydride cm-bonyls, producing the expulsion of the acetate. 

Thiamin is unstable in alkaline solutions (Figure 5.3A) in the pH range 8-10, hydroxyl attack on C-2 of the thiazolium ring leads to the relatively slow formation of a pseudobase intermediate (compound 2 in Figure 5.3) and the... 

Table 11.5 Second-order rate constants for hydroxyl attack on the chelate complex k and k ) and formation constants K j. and Xy ) of the complex in the absence and presence of sodium lauryl sulphate micelles at pH 5.00 and ionic strength 0.1 m...

The activator polyether, in this case dioxane, opens-up the cyanide-zinc lattice in a manner that permits coordination with oxirane, which displaces the activator polyether. This is followed by hydroxyl attack, in this case from water, and initiation of polymerization. Polymerization proceeds by rearward attack of alkoxide on oxirane, so that the polyether produced is terminated, in the case of propylene oxide, by secondary hydroxyl groups. 

The degradation mechanism by nucleophilic substitution (8 2) results from the hydroxyl attack of a-hydrogen on the ammonium (Fig. 5.34). Alcohol and amine are formed. 

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