Acidic solution examples

A traditional method for such reductions involves the use of a reducing metal such as zinc or tin in acidic solution. Examples are the procedures for preparing l,2,3,4-tetrahydrocarbazole[l] or ethyl 2,3-dihydroindole-2-carbox-ylate[2] (Entry 3, Table 15.1), Reduction can also be carried out with acid-stable hydride donors such as acetoxyborane[4] or NaBHjCN in TFA[5] or HOAc[6]. Borane is an effective reductant of the indole ring when it can complex with a dialkylamino substituent in such a way that it can be delivered intramolecularly[7]. Both NaBH -HOAc and NaBHjCN-HOAc can lead to N-ethylation as well as reduction[8]. This reaction can be prevented by the use of NaBHjCN with temperature control. At 20"C only reduction occurs, but if the temperature is raised to 50°C N-ethylation occurs[9]. Silanes cun also be used as hydride donors under acidic conditions[10]. Even indoles with EW substituents, such as ethyl indole-2-carboxylate, can be reduced[ll,l2]. 

Calculate the concentrations of all species in a polyprotic acid solution, Example 10.14. 

The general rule is that if the anion X- is an effective base—that is, if HX is a weak acid—the salt MX will show increased solubility in an acidic solution. Examples of common anions that are effective bases are OH-, S2-, CO32-, C2042-, and Cr042-. Salts involving these anions are much more soluble in an acidic solution than in pure water. 

Quality. The zinc dust of commerce is a fine powder which passes 150 mesh. It contains some matter insoluble in hydrochloric acid solution. Examples of analyses are shown as follows Made by manufacturer A metallic zinc 69.625, total zinc 8. 69% made by B metallic zinc 77 91% total zinc 86.79% made by C metallic zinc 81.02, total zinc 87.13% ... 

The Lewis definition provides a way to organize the chemistry of the main group oxides. The oxides of most of the nonmetals are acid anhydrides, which react with an excess of water to form acidic solutions. Examples are... 

Chelates of metal ions with alkyl- and arylphosphoric and thiophosphoric acids can be extracted into chloroform and other solvents [25,26]. Such systems enable one to separate, by means of extraction procedures, many metals from strongly acid solutions. Examples of such reagents are di-(2-ethylhexyl)phosphoric acid (HDEHP) and di-n-butyldithiophosphoric acid (formulae 1.9 and 1.10). HDEHP is a viscous liquid (density 0.98), slightly soluble in water, but readily soluble in benzene, hexane, and MIBK. 

Some inhibitors used in neutral environments protect the metal by adsorbing onto the surface as described for acid solutions. Examples include benzoates, tartrates or salycilates among other. Because neutral solutions contain a low concentration of protons the inhibitor can have an effect on the pH at the metal surface, in addition to blocking parts of the surface or changing the corrosion mechanism. Metal and oxide surfaces that are in contact with an aqueous solution are covered by adsorbed water. Any other species, inhibitors or electrolyte anions, that adsorb on the surface must replace one or several water molecules [23]. We can therefore write the adsorption reaction of an inhibitor in the following general way ... 

Let us reexamine the simplest case, strong acid solution. Example 1 in Chapter 2. In this 0.100 M HCl solution, it is not exact to set H equal to the molarity of HCl. The OH cannot be zero, therefore water must produce some H and OH. Call this amount y M. Now the complete situation is expressed by... 

Some organic compounds form coloured products if they are oxidized by nitrate ions in acid solution. Examples are brucine (Jenkins and Medsker, 1964), diphenylbenzidine Atkins, 1954), reduced strychnidine Dal Pont, 1962), diphenylamine Isaeva, 1958) and resorcinol Costa, 1951). However, no reliable trouble-free method has been developed based on this technique. Another spectrophotometric method based on the formation of nitrosyl chloride in seawater containing 50 % concentrated sulphuric acid has been developed by Armstrong (1963), but this method cannot be recommended because of the difficulties connected with handling concentrated sulphuric acid solutions at sea, and because it is relatively insensitive. 

As an example, the reduction of pennanganate m acid solution follows the equation... 

Arsenic present only in traces (in any form) can be detected by reducing it to arsine and then applying tests for the latter. In Marsh s test, dilute sulphuric acid is added dropwise through a thistle funnel to some arsenic-free zinc in a flask hydrogen is evolved and led out of the flask by a horizontal delivery tube. The arsenic-containing compound is then added to the zinc-acid solution, and the delivery tube heated in the middle. If arsenic is present, it is reduced to arsine by the zinc-acid reaction, for example ... 

Ozone is very much more reactive than oxygen and is a powerful oxidising agent especially in acid solution (the redox potential varies with conditions but can be as high as + 2.0 V). Some examples are 1. the conversion of black lead(ll) sulphide to white lead(II) sulphate (an example of oxidation by addition of oxygen) ... 

Compare this reaction with (2) of the oxidising examples, where iron(II) is oxidised to iron(III) in acid solution change of pH, and complex formation by the iron, cause the complexed iron(III) to be reduced.)... 

These are similar to those of the alkali metals but are rather less soluble in water. However, calcium sulphide, for example, is not precipitated by addition of sulphide ions to a solution of a calcium salt, since in acid solution the equilibrium position... 

These are practically insoluble in water, are not hydrolysed and so may be prepared by addition of a sufficient concentration of sulphide ion to exceed the solubility product of the particular sulphide. Some sulphides, for example those of lead(II), copper(II) and silver(I), have low solubility products and are precipitated by the small concentration of sulphide ions produced by passing hydrogen sulphide through an acid solution of the metal salts others for example those of zincfll), iron(II), nickel(II) and cobalt(II) are only precipitated when sulphide ions are available in reasonable concentrations, as they are when hydrogen sulphide is passed into an alkaline solution. 

Although sulphur dioxide, as a gas, is a reducing agent in the sense that it unites with oxygen, free or combined (for example in dioxides or peroxides) most of its reducing reactions in aqueous solution are better regarded as reactions of sulphurous acid (in acid solution), or the sulphite ion (in alkaline solution). 

In strongly acid solution, substances which are normally reducing agents reduce sulphur dioxide solution or sulphites, for example iron(II) and zinc ... 

Most covalent halides are hydrolysed by water (carbon tetrachloride being a notable exception, p. 195) to give acidic solutions, by either method (a) (example FeClj) or method (b) (example BCI3) ... 

Hence manganate(VII) is used in acid solution to oxidise, for example,... 

The oxide MnO is obtained by heating the earbonate MnCOj, Oxidation of manganesedli in aqueous acid solution requires a strong oxidising agent, for example... 

Hydrated cobalt III) sulphate, Co2(S04)3. JSHjO is obtained when cobalt(II) sulphate is oxidised electrolytically in moderately concentrated sulphuric acid solution it is stable when dry but liberates oxygen from water. Some alums, for example KCo(S04)2.12H,0 can be obtained by crystallisation from sulphuric acid solutions. In these and the sulphate, the cation [CofHjO) ] may exist it is both acidic and strongly oxidising. 

Amino acids may be prepared by the action of a large excess of concentrated ammonia solution upon a-chloro- or a-bromo-acids the presence of a considerable amount of ammonium carbonate often increases the yield of monoamino acid, for example ... 

Picrates, Many aromatic hydrocarbons (and other classes of organic compounds) form molecular compounds with picric acid, for example, naphthalene picrate CioHg.CgH2(N02)30H. Some picrates, e.g., anthracene picrate, are so unstable as to be decomposed by many, particularly hydroxylic, solvents they therefore cannot be easily recrystaUised. Their preparation may be accomplished in such non-hydroxylic solvents as chloroform, benzene or ether. The picrates of hydrocarbons can be readily separated into their constituents by warming with dilute ammonia solution and filtering (if the hydrocarbon is a solid) through a moist filter paper. The filtrate contains the picric acid as the ammonium salt, and the hydrocarbon is left on the filter paper. 

Similarly aniline CgHjNHj is converted into benzonltrlle C3H5CN. Hydrolysis of the nitrile with so um hydroxide solution, followed by acidification, 3uelds the corresponding acid, for example ... 

The monoarylurea may be prepared directly from the amine by heating it in aqueous solution with an equivalent quantity of alkali cyanate and excess of acetic acid, for example ... 

By oxidation of the methyl derivative of an aromatic hydrocarbon with a solution of chromic anhydride in acetic anhydride and acetic acid. The aldehyde formed is immediately converted into the (/m-diacetate, which is stable to oxidation. The diacetate is collected and hydrolysed with sulphuric acid, for example ... 

Nitro compounds are reduced in acid solution (for example, by tin and hydrochloric acid) to the corresponding primary amines, which may be detected as described under II ... 

Separations based upon differences in the chemical properties of the components. Thus a mixture of toluene and anihne may be separated by extraction with dilute hydrochloric acid the aniline passes into the aqueous layer in the form of the salt, anihne hydrochloride, and may be recovered by neutralisation. Similarly, a mixture of phenol and toluene may be separated by treatment with dilute sodium hydroxide. The above examples are, of comse, simple apphcations of the fact that the various components fah into different solubihty groups (compare Section XI,5). Another example is the separation of a mixture of di-n-butyl ether and chlorobenzene concentrated sulphuric acid dissolves only the w-butyl other and it may be recovered from solution by dilution with water. With some classes of compounds, e.g., unsaturated compounds, concentrated sulphuric acid leads to polymerisation, sulphona-tion, etc., so that the original component cannot be recovered unchanged this solvent, therefore, possesses hmited apphcation. Phenols may be separated from acids (for example, o-cresol from benzoic acid) by a dilute solution of sodium bicarbonate the weakly acidic phenols (and also enols) are not converted into salts by this reagent and may be removed by ether extraction or by other means the acids pass into solution as the sodium salts and may be recovered after acidification. Aldehydes, e.g., benzaldehyde, may be separated from liquid hydrocarbons and other neutral, water-insoluble hquid compounds by shaking with a solution of sodium bisulphite the aldehyde forms a sohd bisulphite compound, which may be filtered off and decomposed with dilute acid or with sodium bicarbonate solution in order to recover the aldehyde. 

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