Basic solutions hydroxide ion

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. 

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. 

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. 

Note that this equation shows that the reaction occurs in acid solution. This is important because in acid solution, hydrogen ions and water molecules are abundant and able to participate as either reactants or products in redox reactions. When a redox reaction occurs in basic solution, hydroxide ions (OH ) and water molecules are abundant and free to react. 

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. 

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... 

Titration curves for strong bases are derived in an analogous way to those for strong acids. Short of the equivalence point, the solution is highly basic, the hydroxide ion concentration being numerically related to the analytical molarity of the base. The solution is neutral at the equivalence point and becomes acidic in the region beyond the equivalence point then the hydronium ion concentration is equal to the analytical concentration of the excess strong acid. 

The Bronsted-Lowry definitions also explain why carbonate salts such as sodium carbonate (washing soda) dissolve in water to give basic solutions. Carbonate ion removes a hydrogen ion from a water molecule, which leaves behind a hydroxide ion ... 

Bases are defined as proton or hydrogen ion acceptors. Most bases are chemieals that dissociate to produce hydroxide ions (OH ). A substance that has a lower hydrogen ion concentration than hydroxide ion concentration is considered a base, so the addition of hydroxide ions to a solution makes the solution more basic. Since hydroxide ions (OH ) act as a base and accept hydrogen ions (H ), water is formed. Therefore, hydroxide ions tend to neutralize substances. 

Because the solution is slightly basic, a hydroxide ion concentration slightly larger than 10 M is predicted. A hydronium ion concentration slightly less than 10 M is also predicted. The answers agree with these predictions. 

We recall that hydroxide ion is not basic enough to form an enolate ion in high concentration. Alkoxide anions cannot provide a high concentration of the enolate ion either. With these bases, the enolate ion is not the predominant basic species in solution. Hydroxide ion or alkoxide ion would substitute for the leaving group and give an alcohol or ether. 

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 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... 

Strong and Weak Bases Just as the acidity of an aqueous solution is a measure of the concentration of the hydronium ion, H3O+, the basicity of an aqueous solution is a measure of the concentration of the hydroxide ion, OH . The most common example of a strong base is an alkali metal hydroxide, such as sodium hydroxide, which completely dissociates to produce the hydroxide ion. 

Another important parameter that may affect a precipitate s solubility is the pH of the solution in which the precipitate forms. For example, hydroxide precipitates, such as Fe(OH)3, are more soluble at lower pH levels at which the concentration of OH is small. The effect of pH on solubility is not limited to hydroxide precipitates, but also affects precipitates containing basic or acidic ions. The solubility of Ca3(P04)2 is pH-dependent because phosphate is a weak base. The following four reactions, therefore, govern the solubility of Ca3(P04)2. 

Although reasonably stable at room temperature under neutral conditions, tri- and tetrametaphosphate ions readily hydrolyze in strongly acidic or basic solution via polyphosphate intermediates. The hydrolysis is first-order under constant pH. Small cycHc phosphates, in particular trimetaphosphate, undergo hydrolysis via nucleophilic attack by hydroxide ion to yield tripolyphosphate. The ring strain also makes these stmctures susceptible to nucleophilic ring opening by other nucleophiles. 

Cadmium Hydroxide. Cd(OH)2 [21041-95-2] is best prepared by addition of cadmium nitrate solution to a boiling solution of sodium or potassium hydroxide. The crystals adopt the layered stmcture of Cdl2 there is contact between hydroxide ions of adjacent layers. Cd(OH)2 can be dehydrated to the oxide by gende heating to 200°C it absorbs CO2 from the air forming the basic carbonate. It is soluble ia dilute acids and solutions of ammonium ions, ferric chloride, alkah haUdes, cyanides, and thiocyanates forming complex ions. 

Addition of hydride ion from the catalyst gives the adsorbed dianion (15). The reaction is completed and product stereochemistry determined by protonation of these species from the solution prior to or concurrent with desorption. With the heteroannular enolate, (13a), both cis and trans adsorption can occur with nearly equal facility. When an angular methyl group is present trans adsorption (14b) predominates. Protonation of the latter species from the solution gives the cis product. Since the heteroannular enolate is formed by the reaction of A" -3-keto steroids with strong base " this mechanism satisfactorily accounts for the almost exclusive formation of the isomer on hydrogenation of these steroids in basic media. The optimum concentration of hydroxide ion in this reaction is about two to three times that of the substrate. 

An artificial fruit beverage contains 12.0 g of tartaric acid, H2C4H406, to achieve tartness. It is titrated with a basic solution that has a density of 1.045 g/cm3 and contains 5.00 mass percent KOH. What volume of the basic solution is required (One mole of tartaric acid reacts with two moles of hydroxide ion.)... 

We see that the existence of the stable bicarbonate ion, HCO foqj produces the chemical species, OH (aq) in common with solutions of the hydroxides. We can postulate that OH (ag) accounts for the slippery feel and bitter taste of the basic solutions. The stability of bicarbonate ion also explains the removal of acid properties through reaction (26). 

The extent of hydrolysis of (MY)(n 4)+ depends upon the characteristics of the metal ion, and is largely controlled by the solubility product of the metallic hydroxide and, of course, the stability constant of the complex. Thus iron(III) is precipitated as hydroxide (Ksal = 1 x 10 36) in basic solution, but nickel(II), for which the relevant solubility product is 6.5 x 10 l8, remains complexed. Clearly the use of excess EDTA will tend to reduce the effect of hydrolysis in basic solutions. It follows that for each metal ion there exists an optimum pH which will give rise to a maximum value for the apparent stability constant. 

These resins are similar to the sulphonate cation exchange resins in their activity, and their action is largely independent of pH. Weakly basic ion exchange resins contain little of the hydroxide form in basic solution. The equilibrium of, say,... 

Solutions which prevent the hydrolysis of salts of weak acids and bases. If the precipitate is a salt of weak acid and is slightly soluble it may exhibit a tendency to hydrolyse, and the soluble product of hydrolysis will be a base the wash liquid must therefore be basic. Thus Mg(NH4)P04 may hydrolyse appreciably to give the hydrogenphosphate ion HPO and hydroxide ion, and should accordingly be washed with dilute aqueous ammonia. If salts of weak bases, such as hydrated iron(III), chromium(III), or aluminium ion, are to be separated from a precipitate, e.g. silica, by washing with water, the salts may be hydrolysed and their insoluble basic salts or hydroxides may be produced together with an acid ... 

Self-Test 12.2A An alkaline (basic) solution of hypochlorite ions reacts with solid chromium(III) hydroxide to produce aqueous chromate ions and chloride ions. Write the net ionic equation for the reaction. 

Iron(Il) hydrogen phosphite, FeHPO, is oxidized by hypochlorite ions in basic solution. The products are chloride ion, phosphate ion, and iron(lll) hydroxide. Write the balanced equation for each half-reaction and the overall equation for the reaction. 

The oxidation is slow in acidic solution but rapid in basic solution, where insoluble iron(III) hydroxide, Fe(OH)3, is precipitated. Although [Fe(H20)6]3+ ions are pale purple and Fe3 1 ions give amethyst its purple color, the colors of aqueous solutions of iron(III) salts are dominated by the conjugate base of [Fe(H20)g]3+, the yellow [Fe0H(H20)d2+ ion ... 

---