Hydroxide-ion

Uncharged pyridines are resistant to hydroxide ion at usual temperatures. Pyridine itself reacts with hydroxide ions under extreme conditions (KOH-air, 300°C) to give 2-pyridone, the stable tautomer of 2-hydroxypyridine, which is formed by oxidation of the initial adduct. As is expected, this reaction is facilitated by electron-withdrawing groups and fused benzene rings quinoline and isoquinoline form 

2-quinolone and 1-isoquinolone, respectively, rather more readily. However, y-hydroxylation is more difficult. Thus, acridine is hydroxylated with KOH at 300°C to give 9-acridone in 28% yield (72KGS1673). 

Increasing numbers of nitrogen atoms increase not only the kinetic susceptibility toward attack but also the thermodynamic stability of the adducts. Reversible covalent hydration of C = N bonds has been observed in a number of heterocyclic compounds (76AHC(20)117). Pyrimidines with electron-withdrawing groups and most quinazolines show this phenomenon of covalent hydration . Thus, in aqueous solution the cation of 5-nitropyrimidine exists as (164) and quinazoline cation largely as (165). These cations possess amidinium cation resonance. The neutral pteridine molecule is covalently hydrated in aqueous solution. Solvent isotope effects on the equilibria of mono- (166) and dihydration (167) of neutral pteridine as followed by NMR are near unity (83JOC2280). The cation of 1,4,5,8-tetraazanaphthalene exists as a bis-covalent hydrate (168). 

The following factors help stabilize covalent hydrates (65AHC(4)33)  

On irradiation uracil and related pyrimidines undergo photohydration across the 5,6-bond. 

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Table XI-1 (from Ref. 166) lists the potential-determining ion and its concentration giving zero charge on the mineral. There is a large family of minerals for which hydrogen (or hydroxide) ion is potential determining—oxides, silicates, phosphates, carbonates, and so on. For these, adsorption of surfactant ions is highly pH-dependent. An example is shown in Fig. XI-14. This type of behavior has important applications in flotation and is discussed further in Section XIII-4.

One anomaly inmrediately obvious from table A2.4.2 is the much higher mobilities of the proton and hydroxide ions than expected from even the most approximate estimates of their ionic radii. The origin of this behaviour lies in the way hr which these ions can be acconmrodated into the water structure described above. Free protons cannot exist as such in aqueous solution the very small radius of the proton would lead to an enomrous electric field that would polarize any molecule, and in an aqueous solution the proton inmrediately... 

When either hydrogen ions or hydroxide ions participate in a redox half-reaction, then clearly the redox potential is alTected by change of pH. Manganate(Vir) ions are usually used in well-acidified solution, where (as we shall see in detail later) they oxidise chlorine ions. If the pH is increased to make the solution only mildly acidic (pH = 3-6), the redox potential changes from 1.52 V to about 1.1 V, and chloride is not oxidised. This fact is of practical use in a mixture of iodide and chloride ions in mildly acid solution. manganate(VII) oxidises only iodide addition of acid causes oxidation of chloride to proceed. 

The hydroxides M (OH)2 are generally less soluble and are of lower base strength. The Group I hydroxides are almost unique in possessing good solubility—most metal hydroxides are insoluble or sparingly soluble hence sodium hydroxide and, to a lesser extent potassium hydroxide, are widely used as sources of the hydroxide ion OH" both in the laboratory and on a large scale. 

Aqueous ammonia can also behave as a weak base giving hydroxide ions in solution. However, addition of aqueous ammonia to a solution of a cation which normally forms an insoluble hydroxide may not always precipitate the latter, because (a) the ammonia may form a complex ammine with the cation and (b) because the concentration of hydroxide ions available in aqueous ammonia may be insufficient to exceed the solubility product of the cation hydroxide. Effects (a) and (b) may operate simultaneously. The hydroxyl ion concentration of aqueous ammonia can be further reduced by the addition of ammonium chloride hence this mixture can be used to precipitate the hydroxides of, for example, aluminium and chrom-ium(III) but not nickel(II) or cobalt(II). 

The liberated iodine is titrated with standard sodium thiosulphate(Vr) solution after acidification to remove the hydroxide ions. 

Cobaltill) hydroxide is obtained as a precipitate when hydroxide ion is added to a solution containing cobalt(II) ions. The precipitate is often blue, but becomes pink on standing it dissolves in excess alkali to give the blue [CofOH) ion, and in slightly alkaline solution is easily oxidis by air to a brown solid of corttposition Co "0(OH). 

The reactions of aqueous solutions of nickel(II) salts with hydroxide ions, with excess ammonia, with sulphide ion and with dimethyl-glyoxime (see above) all provide useful tests for nickel(II) ions. 

The reaction of Cd (aq) with sulphide ion, to give yellow CdS, and with hydroxide ion to give the white CdfOHlj, soluble in ammonia, provide two useful tests. 

Mercury(II) oxide, HgO, occurs in both yellow and red forms the yellow form is precipitated by addition of hydroxide ion to a solution containing mercury(II) ions, and becomes red on heating. Mercury(II) oxide loses oxygen on heating. 

In the strongly basic medium, the reactant is the phenoxide ion high nucleophilic activity at the ortho and para positions is provided through the electromeric shifts indicated. The above scheme indicates theorpara substitution is similar. The intermediate o-hydroxybenzal chloride anion (I) may react either with a hydroxide ion or with water to give the anion of salicyl-aldehyde (II), or with phenoxide ion or with phenol to give the anion of the diphenylacetal of salicylaldehyde (III). Both these anions are stable in basic solution. Upon acidification (III) is hydrolysed to salicylaldehyde and phenol this probably accounts for the recovery of much unreacted phenol from the reaction. 

The mechanism of the reaction probably involves the production, by into -action of the aldehyde with hydroxide ions, of two reducing anions, the first (I) more easily than the second (II). Either of these anions may transfer a hydride ion to a carbonyl carbon atom in another aldehyde molecule ... 

The production of benzyl benzoate from benzaldehyde, which may be isolated under special conditions (low temperature and absence of excess of alkali), is explained by assuming that when some benzyloxide ions (CgHj—CHjO s RCH O ) are formed in the alkaline solution, these can replace hydroxide ions thus ... 

A probable mechanism for this rearrangement postulates the intermediate formation of a hydroxide-ion addition complex, followed by the migration of a phenyl group as an anion ... 

Perhaps the most extensively studied catalytic reaction in acpreous solutions is the metal-ion catalysed hydrolysis of carboxylate esters, phosphate esters , phosphate diesters, amides and nittiles". Inspired by hydrolytic metalloenzymes, a multitude of different metal-ion complexes have been prepared and analysed with respect to their hydrolytic activity. Unfortunately, the exact mechanism by which these complexes operate is not completely clarified. The most important role of the catalyst is coordination of a hydroxide ion that is acting as a nucleophile. The extent of activation of tire substrate througji coordination to the Lewis-acidic metal centre is still unclear and probably varies from one substrate to another. For monodentate substrates this interaction is not very efficient. Only a few quantitative studies have been published. Chan et al. reported an equilibrium constant for coordination of the amide carbonyl group of... 

Hydroxide ion lies below phenol m Table 1 7 hydrogen carbonate ion lies above phe nol The practical consequence of the reactions shown is that NaOH is a strong enough base to convert phenol to phenoxide ion but NaHCOs is not... 

Clearly the two reactions are analogous and demonstrate that the reaction between hydroxide ion and hydrogen bromide is simultaneously a Brpnsted acid-base reaction and a Lewis acid Lewis base reaction Br0nsted acid-base reactions constitute a sub category of Lewis acid Lewis base reactions... 

Recall that the carbon atom of carbon dioxide bears a partial positive charge because of the electron attracting power of its attached oxygens When hydroxide ion (the Lewis base) bonds to this positively polarized carbon a pair of electrons in the carbon-oxygen double bond leaves carbon to become an unshared pair of oxygen... 

According to the Arrhenius definitions an acid ionizes m water to pro duce protons (H" ) and a base produces hydroxide ions (HO ) The strength of an acid is given by its equilibrium constant for ionization m aqueous solution... 

Step 3 Carbon migrates from boron to oxygen displacing hydroxide ion Carbon migrates with the pair of electrons m the carbon-boron bond these become the electrons m the carbon-oxygen bond... 

Hughes and Ingold interpreted second order kinetic behavior to mean that the rate determining step is bimolecular that is that both hydroxide ion and methyl bromide are involved at the transition state The symbol given to the detailed description of the mech anism that they developed is 8 2 standing for substitution nucleophilic bimolecular... 

Which of these two opposite stereochemical possibilities operates was determined in experiments with optically active alkyl halides In one such experiment Hughes and Ingold determined that the reaction of 2 bromooctane with hydroxide ion gave 2 octanol having a configuration opposite that of the starting alkyl halide... 

The graphic that opened this chapter IS an electrostatic po tential map of the Sn2 transi tion state for the reaction of hydroxide ion with methyl chloride... 

They found that the rate of hydrolysis depends only on the concentration of tert butyl bromide Adding the stronger nucleophile hydroxide ion moreover causes no change m... 

Hydrogen sulfide ion HS and anions of the type RS are substantially less basic than hydroxide ion and react with both primary and secondary alkyl halides to give mainly substitution products... 

Although acetylene and terminal alkynes are far stronger acids than other hydro carbons we must remember that they are nevertheless very weak acids—much weaker than water and alcohols for example Hydroxide ion is too weak a base to convert acety lene to its anion m meaningful amounts The position of the equilibrium described by the following equation lies overwhelmingly to the left... 

IS a two step process m which the first step is rate determining In step 1 the nucleophilic hydroxide ion attacks the carbonyl group forming a bond to carbon An alkoxide ion is the product of step 1 This alkoxide ion abstracts a proton from water m step 2 yielding the gemmal diol The second step like all other proton transfers between oxygen that we have seen is fast... 

The role of the basic catalyst (HO ) is to increase the rate of the nucleophilic addi tion step Hydroxide ion the nucleophile m the base catalyzed reaction is much more reactive than a water molecule the nucleophile m neutral solutions... 

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