Solutions hydroxide

In alkaline or neutral solution, hydroxide ions are the oxidation product ... 

The reaction of the proposed intermediate in the nonsticky collision mechanism proceeds very rapidly with hydroxide—when it doesn t have to substitute very fast. And in fact, in competition experiments that Olcott and I did with hydroxide and all the other possible reactants in solution, hydroxide was 100% efficient in capturing the aquo complex intermediate. 

Making a solution more basic speeds up reactions in which alcohols act as nucleophiles because it increases the concentration of the alkoxide ion, which is more nucleophilic than the alcohol itself. The same thing happens in hydrolysis reactions. The hydrolysis of esters is fast in either acidic or basic solutions. In basic solution, hydroxide is a better nucleophile than water. 

Shilov et al studied the rate of oxidation of formate ions in phosphate and carbonate buffers, and showed that the reaction with molecular chlorine is negligible in solutions of pH > 6. At 20 °C the rate of reaction with hypochlorous acid is constant in the range pH 5.5-7, then it decreases with increase of pH, and becomes negligible at pH 13. The kinetics are second-order with respect to hypochlorous acid, and first with respect to formate ions. In alkaline solution hydroxide ion catalysis is apparent viz. 

In less acidic (or more basic) solutions, hydroxide or oxide bridges between metal atoms form, the high positive charge promotes more hydrogen ion dissociation, and a large aggregate of hydrated metal hydroxide precipitates. A possible first step in this process is... 

Poly(methylmethacrylate) is a well-known single component positive photoresist, whereas a diazoqui-none ester and a phenolic novolak resin is a two component positive photoresist. As anticipated, radiation will render the scission or rupture the main and side chain of the polymer making it soluble in an alkaline developing solution. Hydroxide solutions such as KOH and tetramethylammonium hydroxide are commonly used as the developer for positive photoresist. 

FIGURE 17.4 The mechanism of hydration of an aldehyde or ketone in basic solution. Hydroxide ion is a catalyst it is consumed in the first step, and regenerated in the second. 

The aqueous chemistry of particularly Al(III), Fe(III) and Cr(III) involves formation of strong M—O bonds, and in basic aqueous solution, hydroxide species (or unreactive oligomers) usually precipitate preferentially and rapidly because many added ligands are strong bases that cause a rise in solution pH. The behaviour of Fe(III) in a weakly basic aqueous solution is a good example, and can be represented simplistically by (6.12). 

In a basic solution, hydroxide ion removes a proton from the a-carbon of the keto tautomer. The anion that is formed has two resonance contributors a carbanion and an enolate ion. The enolate ion contributes more to the resonance hybrid because the negative charge is better accommodated by oxygen than by carbon. Protonation on oxygen forms the enol tautomer, whereas protonation on the a-carbon reforms the keto tautomer. 

Oxygen dissolved in liquid alkali metals also reacts with other non-metals, which may be present in the solutions. Hydroxides are formed with dissolved hydrogen. Carbonate is one of the reaction products formed in solutions containing oxygen and carbon. 

Figure 1 and Table 2 highlight the fact that three reaction paths are available for the precipitation of the M(II) hydroxide phase (i.e., paths AP2, BP2, and BP4) and three for the M(III) hydroxide (i.e., paths AP3, APS, and BP3). Depending on the particular metal salt solution, hydroxide formation may be effected by base addition only (paths AP2, AP3), acidification only (paths BP2 and BP3), base addition accompanied by metal oxidation (path APS), and acidification accompanied by metal reduction (path BP4). 

Classically, a base is a substance capable of liberating hydroxide ions, OH , in water solution. Hydroxides of the alkali metals (Group lA) and alkaline earth metals (Group 2A),suchasLiOH, NaOH, KOH, Ca(OH)2,and Ba(OH)2,are the most common inorganic bases. Water solutions of bases are cdlled alkaline solutions or basic solutions. Some of the characteristic properties commonly associated with bases include the following ... 

Alkyl chlorides are electrophiles, since chlorine is electronegative and chloride is a good leaving group. Sodium carbonate is a weak base in aqueous solutions, hydroxide is generated which is a good base as well as a good nucleophile. 

Silica, If present, separates with the lead sulfate as does tungsten, niobium, tantalum, barlvun and less completely strontliim and calcium. Bismuth, antimony and silver contaminate the lead to some extent. The copreclpltatlon of antimony Is decreased by addition of tartaric acid. Addition of alcohol to the sulfate solution decreases the solubility of lead sulfate but Increases contamination by silver, bismuth or calcium. The normal separation based on the sulfate Involves dissolution of the lead sulfate In ammonium acetate solution. Hydroxide... 

If it were not solubilized, the surface oxide layer formed in Eq. 9 would protect the underlying elemental silicon from further oxidation. This is indeed the case at low pH values (pH < 7), where the dissolution of silica is quite slow. However, in basic solutions hydroxide ions attack and dissolve the surface oxide via Eq. 10, revealing a fresh Si surface. 

In reactions in aqueous solutions water, hydrogen ion, and hydroxide ion may come into action as reactants or products. For example, in an acid solution hydrogen ion may be either a reactant or a product, and water may also be either a reactant or a product in the same reaction. In acid solutions hydroxide ion exists only in extremely low concentration, and would hardly be expected to enter into the reaction. Hence water and hydrogen ion may enter into the reaction now under consideration. 

Hydrolysis of p-chloronitrobenzene with KOH to p-nitrophenol is presented in almost every textbook as an example of the S r reaction [3, 4]. However, nowhere there is an information that this is a slow secondary reaction, which proceeds at elevated temperatures, whereas at low temperatures in liquid ammonia solution, hydroxide anions supplied by KOH add rapidly at positions occupied by hydrogen. Subsequent oxidation of the resulting a -adducts with oxygen gives 2-nitro-5-chlorophenol, the product of ONSH [8]. Thus, ONSH that proceeds below -35°C is obviously a much faster reaction than S Ar of chlorine because the latter takes place only at an elevated temperature of, for example, -f140°C (Scheme 11.3). 

Marx D, Chandra A, Tuckerman ME (2010) Aqueous basic solutions hydroxide solvation, structural diffusion, and comparison to the hydrated proton. Chem Rev 110 2174—2216... 

The ammonia electrode operates by the diffusion of dissolved ammonia from the sample through a highly gas-permeable membrane into an internal proprietary filling solution. Diffusion proceeds until a reversible equilibrium is established between the ammonia level of the sample and the level in the filling solution. Hydroxide ions are formed in the filling solution by the ammonia-water reaction... 

---