Acidic conditioning

The chromates of the alkali metals and of magnesium and calcium are soluble in water the other chromates are insoluble. The chromate ion is yellow, but some insoluble chromates are red (for example silver chromate, Ag2Cr04). Chromates are often isomorph-ous with sulphates, which suggests that the chromate ion, CrO has a tetrahedral structure similar to that of the sulphate ion, SO4 Chromates may be prepared by oxidising chromium(III) salts the oxidation can be carried out by fusion with sodium peroxide, or by adding sodium peroxide to a solution of the chromium(IIl) salt. The use of sodium peroxide ensures an alkaline solution otherwise, under acid conditions, the chromate ion is converted into the orange-coloured dichromate ion ... 

Note that here chlorine is oxidising the manganate(Vl) to man-ganate(VlI) under more acid conditions, the latter oxidises chloride to chlorine, p. 103). 

In this oxidation state, iron is quite readily oxidised by mild oxidising agents, and hence in many of the reactions it is a mild reducing agent. For acid conditions... 

We chose benzyli dene acetone (4.39, Scheme 4.11) as a model dienophile for our studies. The uncatalysed Diels-Alder reaction of this compound with cyclopentadiene is slow, justifying a catalytic approach. Reaction of 4.39 with paraformaldehyde and dimethyl amine under acidic conditions in an aqueous ethanol solution, following a literature procedure, produced the HCl salt of 4.42 (Scheme 4.11). The dienophile was liberated in situ by adding one equivalent of base. 

Finally, in the last step, the chelating auxiliary had to be removed Ideally, one would like to convert 4.54 into ketone 4.55 via a retro Mannich reaction. Unfortunately, repeated attempts to accomplish this failed. These attempts included refluxing in aqueous ethanol under acidic and basic conditions and refluxing in a 1 1 acetone - water mixture in the presence of excess paraformaldehyde under acidic conditions, in order to trap any liberated diamine. Tliese procedures were repeated under neutral conditions in the presence of copper(II)nitrate, but without success. 

Fortunately, under moderately acidic conditions, in the presence of acetone and paraformaldehyde, 4.54 undergoes an elimination reaction similar to that described in Scheme 4.13, producing oc,(3-... 

The significance of the possible diprotonation of water under extremely acidic conditions directly affects the question of acid strength achievable in superacidic systems. The leveling effect mentioned above limits the acidity of any system to that of its conjugate acid. Thus, in... 

Nitration is almost always carried out under acidic conditions. If the compound being nitrated is basic, the problem arises of deciding whether the free base or its conjugate acid is being nitrated, or if both of these species are reacting. 

Indoles can also be alkylated by conjugate addition under alkaline conditions. Under acidic conditions, alkylation normally occurs at C3 (see Section 11.1). Table 9.1 includes examples of alkylation by ethyl acrylate, acrylonitrile, acrylamide and 4-vinylpyridine. 

Alkylation can also be accomplished with electrophilic alkenes. There is a dichotomy between basic and acidic conditions. Under basic conditions, where the indole anion is the reactive nucleophile, A-alkylation occurs. Under acidic conditions C-alkylation is observed. The reaction of indole with 4-vinylpyri-dine is an interesting illustration. Good yields of the 3-alkylation product are obtained in refluxing acetic acid[18] whereas if the reaction is done in ethanol containing sodium ethoxide 1-alkylation occurs[19]. Table 11.2 gives some examples of 3-alkylation using electrophilic alkenes. 

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]. 

The nucleophUic reactivity in neutral medium has been used extensively to prepare various thioethers of thiazole (122). In acidic medium, alkylation may be performed with alcohols (123, 124). An unexpected reaction encountered was the decarboxylation of 2-mercapto-4-methyl-5-thiazolecarboxyhc acid (60) when treated with butyl alcohol under acidic conditions (Scheme 27) (123). Reaction between A-4-thiazoline-2-thione... 

In conclusion, it appears that in neutral or weakly acidic conditions only the methyl in the 2-position shows pseudoacidic behavior. The same conclusion can be drawn from the base-induced hydrogen-metal exchange reactions discussed in Section III.5.B. 

With arylthioamides except for some nitrothiobenzamides (101), yields are usually higher than those obtained above, due to the increased stability of these amides under acidic conditions (3), Rj = Ph, yield 70 to 82% (264, 285, 336, 483, 578, 641). In this case, cyclizations are carried out several hours to reflux, in absolute alcohol, in the presence of melted sodium acetate and few drops of piperidine. 

Under acidic conditions, a thiosemicarbazone intermediate (145) has been isolated, this can be cyclized in alcohol either into the corresponding 2-hydrazinothiazole (142) in the presence of benzaldehyde or into 1,3,4-thiadiazine (146) in the absence of benzaldehyde (Scheme 71) (375, 397, 408). 

After the reaction is complete, the solution is acidified with H2SO4. Under the now acidic conditions U is oxidized to by Mn02. 

Not all of the hydrogens in phenol are equally reactive. Under acid conditions the quinoid structure... 

Under acidic conditions, furfuryl alcohol polymerizes to black polymers, which eventually become crosslinked and insoluble in the reaction medium. The reaction can be very violent and extreme care must be taken when furfuryl alcohol is mixed with any strong Lewis acid or Brn nstad acid. Copolymer resins are formed with phenoHc compounds, formaldehyde and/or other aldehydes. In dilute aqueous acid, the predominant reaction is a ring opening hydrolysis to form levulinic acid [123-76-2] (52). In acidic alcohoHc media, levulinic esters are formed. The mechanism for this unusual reaction in which the hydroxymethyl group of furfuryl alcohol is converted to the terminal methyl group of levulinic acid has recendy been elucidated (53). 

Fig. 22. Mechanism for the hydrolysis of tert-huty acetate under strongly acidic conditions.

This procedure may result in a concentration of cumene hydroperoxide of 9—12% in the first reactor, 15—20% in the second, 24—29% in the third, and 32—39% in the fourth. Yields of cumene hydroperoxide may be in the range of 90—95% (18). The total residence time in each reactor is likely to be in the range of 3—6 h. The product is then concentrated by evaporation to 75—85% cumene hydroperoxide. The hydroperoxide is cleaved under acid conditions with agitation in a vessel at 60—100°C. A large number of nonoxidising inorganic acids are usehil for this reaction, eg, sulfur dioxide (19). 

Treatment with hydrogen peroxide converts butenediol to 2,3-epoxy-l,4-butanediol (105) or gives hydroxylation to erythritol [149-32-6], C H qO (106). Under strongly acidic conditions, tetrahydro-3,4-furanediol is the principal product (107). 

Hydrolysis in neutral aqueous solutions proceeds slowly at room temperature and more rapidly at acidic conditions and elevated temperatures. The hydrolysis—esterification reaction is reversible. Under alkaline conditions hydrolysis is rapid and irreversible. Heating the alkaline hydrolysis product at 200—250°C gives 4,4 -oxydibutyric acid [7423-25-8] after acidification (148). 

Reaction with Chlorine. Polyacrylamide reacts with chlorine under acid conditions to form reasonably stable A/-chloroamides. The polymers are water soluble and can provide good wet strength and wet web strength in paper (60). 

Combination Flame Retardant—Durable Press Performance. Systems using THPC, urea, and TMM can be formulated to give fabrics which combine both flame-retardant performance and increased wrinkle recovery values (80). Another system employs dimethylol cyanoguanidine with THPC under acidic conditions (115). Both of these systems lead to substantial losses in fabric tensile and tearing strength. 

Flame-Retardant Treatments For Wool. Although wool is regarded as a naturally flame-resistant fiber, for certain appHcations, such as use in aircraft, it is necessary to meet more stringent requirements. The Zirpro process, developed for this purpose (122,123), is based on the exhaustion of negatively charged zirconium and titanium complexes on wool fiber under acidic conditions. Specific agents used for this purpose are potassium hexafluoro zirconate [16923-95-8] [16923-95-8] K ZrF, and potassium hexafluoro titanate [16919-27-0], K TiF. Various modifications of this process have been... 

Pyrohydrolysis and strong acidic conditions can be a source of toxicity owiag to Hberated HF. ... 

Formaldehyde condenses with itself in an aldol-type reaction to yield lower hydroxy aldehydes, hydroxy ketones, and other hydroxy compounds the reaction is autocatalytic and is favored by alkaline conditions. Condensation with various compounds gives methylol (—CH2OH) and methylene (=CH2) derivatives. The former are usually produced under alkaline or neutral conditions, the latter under acidic conditions or in the vapor phase. In the presence of alkahes, aldehydes and ketones containing a-hydrogen atoms undergo aldol reactions with formaldehyde to form mono- and polymethylol derivatives. Acetaldehyde and 4 moles of formaldehyde give pentaerythritol (PE) ... 

Methylamines are formed by heating formaldehyde with primary or secondary amines or their salts under acid conditions (61) ... 

Friedel-Crafts acylation using nittiles (other than HCN) and HCI is an extension of the Gattermann reaction, and is called the Houben-Hoesch reaction (120—122). These reactions give ketones and are usually appHcable to only activated aromatics, such as phenols and phenoHc ethers. The protonated nittile, ie, the nitrilium ion, acts as the electrophilic species in these reactions. Nonactivated ben2ene can also be acylated with the nittiles under superacidic conditions 95% trifluoromethanesulfonic acid containing 5% SbF (Hg > —18) (119). A dicationic diprotonated nittile intermediate was suggested for these reactions, based on the fact that the reactions do not proceed under less acidic conditions. The significance of dicationic superelectrophiles in Friedel-Crafts reactions has been discussed (123,124). 

Ketones and aldehydes react with ethylene glycol under acidic conditions to form 1,3-dioxolanes (cychc ketals and acetals) (eq. 7). 

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