Acids as solvents

Using sulpholan and acetic acid as solvents competitive nitrations were performed with solutions containing 75% and 30% of mixed acid (table 4.1, columns h, i and /, g, respectively). In the former the concentration of nitronium ions was substantial [c. 5-7 % by weight), whereas in the latter the concentration was below the level of spectroscopic detection. 

The argument for the S 2 process, when the transition from acetic acid as solvent to nitric acid as solvent is considered, is less direct, for because of the experimental need to use less reactive compounds, zeroth-order nitration has not been observed in nitric acid. It can be estimated, however, that a substance such as nitrobenzene would react about 10 faster in first-order nitration in nitric acid than in a solution of nitric acid (7 mol 1 ) in acetic acid. Such a large increase is understandable in terms of the S z mechanism, but not otherwise. 

Most cellulose acetate is manufactured by a solution process, ie, the cellulose acetate dissolves as it is produced. The cellulose is acetylated with acetic anhydride acetic acid is the solvent and sulfuric acid the catalyst. The latter can be present at 10—15 wt % based on cellulose (high catalyst process) or at ca 7 wt % (low catalyst process). In the second most common process, the solvent process, methylene chloride replaces the acetic acid as solvent, and perchloric acid is frequentiy the catalyst. There is also a seldom used heterogeneous process that employs an organic solvent as the medium, and the cellulose acetate produced never dissolves. More detailed information on these processes can be found in Reference 28. 

Another method that appears to have commercial potential is the ozonolysis of cyclooctene. Ozonolysis is carried out using a short chain carboxyHc acid, preferably propanoic acid, as solvent. The resultant mixture is thermally decomposed in the presence of oxygen at about 100°C to give suberic acid in about 60—78% yield (38—40). Carboxylation of 1,6-hexanediol using nickel carbonyl as catalyst is reported to give suberic acid in 90% yield (41). 

The reaction is generally performed between 0 and 100 °C with the majority of the reactions being mn at reflux. Polar protic solvents such as methanol, ethanol, isopropanol, and water are commonly used as solvents. Addition of acid or use of acetic acid as solvent generally helps push sluggish reactions. The use of P-ketoesters as the dicarbonyl partner occasionally requires added base for cyclization to occur to form the pyrazolone. When using alkyl hydrazine salts, base may be required to deprotonate the hydrazine for the reaction to take place. 

Cyclizations, which failed to occur under thermal conditions, have been forced by using strong acids as solvent. Such cyclizations required careful temperature control in order to cyclize while maintaining the 3-carboxyl substituent. ... 

Another decarboxylation reaction that employs lead tetraacetate under milder conditions, has been introduced by Grob et alJ In that case A-chlorosuccinimide is used as chlorinating agent and a mixture of A,A-dimethylformamide and acetic acid as solvent. 

The Prins reaction often yields stereospecifically the and-addition product this observation is not rationalized by the above mechanism. Investigations of the sulfuric acid-catalyzed reaction of cyclohexene 8 with formaldehyde in acetic acid as solvent suggest that the carbenium ion species 7 is stabilized by a neighboring-group effect as shown in 9. The further reaction then proceeds from the face opposite to the coordinating OH-group " ... 

The solubility of the resulting product may dictate the choice of solvent. Reductive alkylation of norepinephrine with a series of keto acids proceeded smoothly over platinum oxide in methanol-acetic acid mixtures. However, when n = 4 or 5, the product tended to precipitate from solution, making catalyst separation difficult. The problem was circumvented by using glacial acetic acid as solvent 38). 

Proton Transfers in Various Solvents. The Autoprotolysis of Methanol. Formic Acid as Solvent. The Sulfate Ion. Autoprotolysis of Formic Add. The Urea Molecule. Sulfuric Add and Liquid Ammonia as Solvents. 

Formic Acid as Solvent. In Tables 9 and 12 data were given for the proton transfer... 

The Sulfate Ion. In Fig. 36 we see that the vacant level of the (SO ) ion in aqueous solution lies only 0.13 electron-volt above the occupied level of HCOOH. If the interval has a comparable value when sulfate ions are present in formic acid as solvent, the thermal agitation should transfer a large number of protons from solvent HCOOH molecules to the (SO4)" ions. This was found to be the case when Na2SC>4 was dissolved in pure formic acid. Such a transfer of protons from molecules of a solvent to the anions of a salt is analogous to the hydrolysis of the salt in aqueous solution and is known as solvolysis, as mentioned in Sec. 76. In a 0.101-molal solution of Na2SC>4 in formic acid the degree of the solvolysis was found to be 35 per cent.1... 

Furthermore, since in Sec. 121 we found the value J = 0.36 electron-volt for the proton transfer (211), this gives the occupied proton level of the (HCOOII2)+ ion a position at (0.52 — 0.36) = 0.16 electron-volt above that of the (H30)+ ion in formic acid as solvent. This is shown in Fig. 65, where, for comparison, a diagram for proton levels in aqueous solution has been included, the level of the (H30)+ ion in aqueous solution being drawn opposite to the level of the same ion in formic acid solution. This choice is quite arbitrary, but was made in order to show more clearly that we may expect that one or more acids that are strong... 

The CH3COOH2 ion so formed can very readily give up its proton to react with a base. A weak base will, therefore, have its basic properties enhanced, and as a consequence titrations between weak bases and perchloric acid can frequently be readily carried out using acetic acid as solvent. 

Reference electrodes are usually a calomel or a silver-silver chloride electrode. It is advisable that these be of the double-junction pattern so that potassium chloride solution from the electrode does not contaminate the test solution. Thus, for example, in titrations involving glacial acetic acid as solvent, the outer vessel of the double junction calomel electrode may be filled with glacial acetic acid containing a little lithium perchlorate to improve the conductance. 

With acetic acid as solvent 68 is still the major product (Scheme 32). The minor product (69) probably forms in preference to the 3,5-isomer because the quinoline free base is reacting the high yield of 68 can be rationalized in terms of a 1,4- or 1,2-addition product that is rapidly bromi-nated at C-3. The 6- and 8-positions substitute more slowly [62JCS283, 62JCS291 77HC(32-1)319]. Both the 6- and the 8-bromoquinolines were 3-brominated under neutral conditions (62JOC1318). 

With trifluoroacetic acid as solvent, toluene and o-xylene gave second-order kinetics and for the activation energy for toluene was 12.7 (from data at 1.6 and 25.2 °C), i.e. considerably less than for the zinc chloride-catalysed reaction in acetic acid330. 

The first kinetic study used chloromethyl methyl ether as chlorinating reagent and acetic acid as solvent, viz. reaction (188)381... 

The present procedure describes conditions, which allow for the formation of 5-bromoisoquinoline in good yield and high purity using easily available and inexpensive starting materials. In order to obtain the desired product, it is important to ensure careful temperature control to suppress the formation of 8-bromoisoquinoline, which is difficult to remove. By choosing sulfuric acid as solvent for the bromination, a convenient one-pot procedure to prepare 5-bromo-8-nitroisoquinoline, without prior isolation of 5-bromoisoquinoline, has been developed. Finally, the method can easily be scaled up from grams to kilograms of the title compounds. 

One of the simplest ways to prepare a chitin gel is to treat chitosan acetate salt solution with carbodiimide to restore acetamido groups. Thermally not reversible gels are obtained by AT-acylation of chitosans N-acetyl-, N-propionyl- and N-butyryl-chitosan gels are prepared using 10% aqueous acefic, propionic and bufyric acid as solvents for treatment with appropriate acyl anhydride. Both N- and 0-acylation are found, but the gelation also occurs by selective AT-acylation in the presence of organic solvents. 

Fig. 14. Equipment for the hydrothermal method, with hydrohalic acids as solvents.

Another catalytic system which has been successfully applied to the autoxidation of substituted toluenes involves the combination of Co/Br" with a quaternary ammonium salt as a phase transfer catalyst (ref. 20). For example, cobalt(II) chloride in combination with certain tetraalkylammonium bromides or tetraalkylphosphonium bromides afforded benzoic acid in 92 % yield from toluene at 135-160 °C and 15 bar (Fig. 19). It should be noted that this system does not require the use of acetic acid as solvent. The function of the phase transfer catalyst is presumably to solubilize the cobalt in the ArCH3 solvent via the formation of Q + [CoBr]. ... 

The oxidation of sulphoxides containing aromatic groups such as methyl phenyl sulphoxide and diphenyl sulphoxide proceeds at 20-30 °C in low yields in the presence of sulphuric acid as solvent. However the product is usually contaminated with compounds containing nitro groups in the aromatic nucleus, as indicated in equation (6). 

The reaction conditions necessary to obtain a good yield of the title compound (a difficult isomer), and to avoid hazards during the nitration of resorcinol, are critical and strict adherence to those specified is essential. The necessary 80% white fuming nitric acid must be completely free from oxides of nitrogen and nitrous acid, and procedures for this are detailed. Then the temperature dining addition of the diacetate must be kept between -10 and 0°C by regulating the rate of addition. The alternative use of 80% sulfuric acid as solvent for the 80% nitric acid (5 equiv.) is preferred as more reliable, but both methods have led to violent exothermic decomposition, accompanied by fume-off, after an induction period. In any event, the explosive 2,4,6-trinitroresorcinol ( styphnic acid ) is produced as a by-product. 

Addition of fresh platinum oxide catalyst to a hydrogenation reaction in acetic acid caused immediate explosion. Several similar incidents, usually involving acetic acid as solvent, are known to the author. 

In order to make the Biginelli protocol amenable to an automated library generation format, utilizing the integrated robotic interface of the instrument, attempts were made to dissolve most of the building blocks used in solvents compatible with the reaction conditions. Since many of the published protocols employ either ethanol or acetic acid as solvents in Biginelli-type condensations, a 3 1 mixture of acetic acid... 

The Friedlander reaction is the acid- or base-catalyzed condensation of an ortho-acylaniline with an enolizable aldehyde or ketone. Henichart and coworkers have described microwave-assisted Friedlander reactions for the synthesis of indoli-zino[l,2-b]quinolincs, which constitute the heterocyclic core of camptothecin-type antitumor agents (Scheme 6.238) [421], The process involved the condensation of ortho-aminobenzaldehydcs (or imines) with tetrahydroindolizinediones to form the quinoline structures. Employing 1.25 equivalents of the aldehyde or imine component in acetic acid as solvent provided the desired target compounds in 57-91% yield within 15 min. These transformations were carried out under open-vessel conditions at the reflux temperature of the acetic acid solvent. 

In addition to the data already discussed on acids or Lewis acids as solvents, some data are available for solvents in which the interpretation in terms of molecular complexing is less obvious. For example, the ionization of trityl chloride has been compared spectroscopically in nitromethane, nitroethane, and 2-nitropropane.198 Unfortunately the absorption band broadens as the solvent is changed, rendering a quantitative interpretation difficult. In the author s laboratory two... 

In pulp and paper processing, anthraquinone (AQ) accelerates the delignification of wood and improves liquor selectivity. The kinetics of the liquid-phase oxidation of anthracene (AN) to AQ with NO2 in acetic acid as solvent has been studied by Rodriguez and Tijero (1989) in a semibatch reactor (batch with respect to the liquid phase), under conditions such that the kinetics of the overall gas-liquid process is controlled by the rate of the liquid-phase reaction. This reaction proceeds through the formation of the intermediate compound anthrone (ANT) ... 

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