Reactions in Acidic Solution

In acid solutions, generally a proton or other Lewis acid attaches to an electron-rich site, giving a carbocation that may lead to a series of other carbocations, often ending finally with loss of the Lewis acid or a proton. The carbocations are frequently stabilized by resonance with much of the charge residing on an oxygen atom. 

It is possible to write different steps within this proposed mechanism, and it is likely that other equihhria exist, particularly the protonation of the other carbonyl groups [27]. These equilibria do not affect the formation of the final product however. 

Aluminum trichloride polarizes the carbonyl group leading to ringopening of the four membered ring. A 1,2-hydride shift followed by 

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Solids, gases, and liquids are identified all other species are aqueous. Reduction reactions in acidic solution are written using H instead of HsO. Reactions may be rewritten by replacing H with HsO and adding one molecule of H2O to the opposite side of the reaction for each H thus... 

Aminophenols have been detected in waste water by investigating uv absorptions at 220, 254, and 275 nm (87). These compounds can also be detected spectrophotometricaHy after derivatization at concentrations of 1 part per 100 million by reaction in acid solution with /V-(1-napbtby1)etby1enediamine [551-09-7] (88) or 4-(dimethylainino)ben2aldehyde [100-10-7] (89), and the Schiff base formed can be stabilized in chloroform by chelation to increase detection limits (90). 

The main anodic reaction in acid solutions is given in Reaction 7.1 iron is dissolved at exactly the rate of the cathodic process ... 

Amino- and 6-chloro-pteridine also give this reversible ringopening reaction in acid solution. 

Write balanced equations for the following reactions in acid solution. 

Induced chain reactions. In acidic solution, oxalic acid reacts very slowly with chlorine, but iron(ll) reacts rapidly." The net reactions are these ... 

Ketones can react with themselves in a process known as aldol condensation. The mechanism for this reaction in acidic solution is shown here. Write the overall reaction, identify any intermediates, and determine the role of the hydrogen ion. 

Another way to make (32) is to carry out the reaction in acid solution when the regioselectivity is reversed. 

C19-0069. Balance and calculate the standard potential for the following reaction in acidic solution ... 

The oxides often are nonstoichiometric (with an excess or dehcit of oxygen). Many oxides are semiconducting, and their conductivity can be altered by adding various electron donors or acceptors. Relative to metals, the applications of oxide catalysts in electrochemistry are somewhat limited. Cathodic reactions might induce a partial or complete reduction of an oxide. For this reason, oxide catalysts are used predominantly (although not exclusively) for anodic reactions. In acidic solutions, many base-metal oxides are unstable and dissolve. Their main area of use, therefore, is in alkaline or neutral solutions. 

Examples are known of hydrocoupling between methyl acrylate and ketones in both protic and aprotic solvents. Reaction in acid solution is thought to involve reduction of the protonated ketoneto a radical, which adds to acrylate. In aprotic solvents, the ketone is more difficult to reduce and electron addition occurs on methyl acrylate. Modest yields of coupling product, dimethylbutanolide, are obtained from acetone and methyl acrylate in dimethylformamide [134]. Better results are obtained by reduction of methyl acrylate and an exces of the carbonyl compound in dimethyIformamide in the presence of chlorotrimethylsilane [135]. This process is useful for the synthesis of butenolides and some examples are given in Table 3.8. 

The most important reactions of hydrogen peroxide involve oxidation-reduction. It is a powerful oxidizing agent, both in acidic and basic solutions. Usually a 30% aqueous solution of H2O2 is used in oxidation. While oxidation with H2O2 in basic solution is fast, it is slow in acid solution. The half-reaction in acidic solution is ... 

The formation of NO was demonstrated by carrying out the reaction in acid solution in the presence of sulfanilic acid and o -naphthylamine, neither of which reacts with the Mo complexes. The formation of the known red azo dye is diagnostic of NOJ formation. The stoichiometry was confirmed by determining the N2 evolved (equation 17). 

Oxazepam (251) is reduced polarographically in acid solution in a four-electron wave and in alkaline solution in a two-electron wave 335 the four-electron reaction in acidic solution leads to a reductive removal of the hydroxyl group and a saturation of the C=N bond. In weak alkali, preparative reduction gives 4 > n > 2, and polarograms taken during the reduction show the presence of an intermediate more easily reducible than 251 and also oxidizable in an anodic reaction. Anodic oxidation of one form of the intermediates yields 251. 

Several trihydroxo-bridged rhodium(III) complexes are known, but their hydrolysis reactions have been studied only briefly. The tacn complex undergoes a rapid two-stage reaction in acidic solution to give [Pg.149]

Exercise 15-33 The oxidation of (CH3)2CDOH is one seventh as fast as the oxidation of (CH3)2CHOH using potassium permanganate in acidic solution. What does this tell us about the mechanism of the reaction in acidic solution ... 

To summarize, balancing a redox reaction in acidic solution by the oxidation-number method is a six-step process, followed by a check of the answer (Figure 4.3.)... 

Worked Example 4.10 shows how the oxidation-number method is used to balance a reaction carried out in basic solution. The procedure is exactly the same as that used for balancing a reaction in acidic solution, but OH- ions are added as the final step to neutralize any H+ ions that appear in the equation. This simply reflects the fact that basic solutions contain negligibly small amounts of H+ but large amounts of OH-. 

For each of the following reactions in acidic solution, predict the products and write a balanced net ionic equation ... 

Yates and coworkers have examined the mechanism for photohydration of o-OH-8. The addition of strong acid causes an increase in the rate of quenching of the photochemically excited state of o-OH-8, and in the rate of hydration of o-OH-8 to form l-(o-hydroxyphenyl)ethanol. This provides evidence that quenching by acid is due to protonation of the singlet excited state o-OH-8 to form the quinone methide 9, which then undergoes rapid addition of water.22 Fig. 1 shows that the quantum yields for the photochemical hydration of p-hydroxystyrene (closed circles) and o-hydroxystyrene (open circles) are similar for reactions in acidic solution, but the quantum yield for hydration of o-hydroxystyrene levels off to a pH-independent value at around pH 3, where the yield for hydration of p-hydroxystyrene continues to decrease.25 The quantum yield for the photochemical reaction of o-hydroxystyrene remains pH-independent until pH pAa of 10 for the phenol oxygen, and the photochemical efficiency of the reaction then decreases, as the concentration of the phenol decreases at pH > pAa = 10.25 These data provide strong evidence that the o-hydroxyl substituent of substrate participates directly in the protonation of the alkene double bond of o-OH-8 (kiso, Scheme 7), in a process that has been named excited state intramolecular proton transfer (ESIPT).26... 

An aldol reaction in acid solution ensures that the more substituted enol is formed and the aldehyde is by far the most electrophilic of all the carbonyl groups. The Diels-Alder reaction gives the free acid 30 which was resolved with a chiral amine and each enantiomer used for a different part of the B12 molecule. The slightly unusual reagent Cr(VI) was used for the alkene cleavage and acetal formation occurred spontaneously under the acidic conditions. 

Different materials can be used as oxidants in fuel cells. Yet it is desirable, if possible, to use 02 from air at one electrode in every earthbound fuel cell because this avoids the necessity of carrying a second fuel for the cathodic reaction. Hence, the cathodic reduction of 02 has a special importance in electrochemical reactors. The overall reaction in acid solution (see Sec. 7.10) is... 

Also seen in the diagram is that some intermediate oxidation states are prone to disproportionation. Thus Mn(VI) undergoes the following reaction in acid solution ... 

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