Medium dilution reactions

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). 

If there is no possibility to maintain a constant temperature by manipulating the temperature of the cooling medium the reaction can be slowed down by diluting the reaction mixture and/or the catalyst. After some components of the reaction mixture have been consumed to a sufficient extent and the reaction becomes too slow, more catalyst or reactants can be added to complete the reaction with the rate of heat generation not yet exceeding that of heat removal. This is the normally used semibatch operation. 

Multifect GC from Genencor, Rochester, NY. The enzyme solutions were characterized by the following activities endo-l,4-(3-xylanase, (3-xylosidase, acetylesterase, a-L-arabinofuranosidase, and filter paper activity (FPase, which describes the overall cellulolytic activity). The dilutions were calculated to provide 100 U of xylanase/g of total xylose in the reaction medium. Hydrolysis reactions were stopped by boiling for 5 min to inactivate enzymes and clarified by centrifugation prior to analysis. A blank OCL sample was assayed identically to enzyme-treated OCL using water instead of the enzyme solution. Control experiments were carried out for each enzyme, in which buffer replaced the OCL. All the hydrolysis trials were performed in duplicate. 

Similarly, in the so-called crab-like cyclisation reaction of an a-chloro-acetamide with secondary amines15 which takes place in the presence of sodium carbonate in acetonitrile (Scheme 1.10), the product diamide possesses three distinctive nitrogen sites. Such a reaction may be carried out under medium dilution, when yields of around 50% are common. If the reactants are added slowly using syringe pumps and the reaction is undertaken at high dilution, then the yield improves to 80%. 

The saturated aza-oxa crown ethers were first synthesised as intermediates in the synthesis of the nitrogen cryptands.1 The reaction conditions used for the formation of these macrocycles involved the high-dilution technique. In this versatile method, a diamine and a diacid chloride are simultaneously added in the presence of triethylamine to a large volume of solvent, usually toluene, over an extended period of time. The major product from such a reaction is the [1+1] cyclised product, although the [2+2] adduct can often be isolated as well, in low yield. Whilst this method is still sometimes used,2,3 particularly for cryptand synthesis (Chapter 5), it has been superseded by methods that are more convenient and which proceed under medium dilution. 

The following procedure is representative of a direct [1+1] cyclisation reaction using a toluenesulfonamide at medium dilution.3... 

This reagent can also be used for the oxidation of methylated aryl derivatives to the corresponding aryl aldehyde, in what has become known as the Etard reaction. Tillotson and Houston found that the Etard reaction is catalyzed by small amounts of alkene, added to or present in the reaction medium. The reaction involves addition of chromyl chloride to a carbon disulfide or carbon tetrachloride solution of the arene. A dark brown, insoluble, and explosive intermediate usually precipitates. Dilute sulfurous acid is added to decompose the precipitate to the aldehyde. Toluene is converted to benzaldehyde and ethylbenzene was oxidized to phenylacetaldehyde with this reagent. 

It is supposed that similar to the ROMP of cycloolefins, initiated by bis-phosphine ruthenium carbenes (e.g. 4), one of the phosphines dissociates from the ruthenium center during RCM to free a coordination site where the olefin can bind and undergo a metathesis reaction. However, because of the dilute reaction medium to prevent the molecules to undergo a ROMP reaction, the active catalyst, which is now a four-coordinate species, is not stabilized enough to have an infinite lifetime. The lifetimes of 11 and 12 are much longer than of previous ruthenium carbene catalysts, because the pyridyl alkoxide ligands in 11 and 12 remain bonded to the metal, whereas the pyridyl ligands, e.g. in 9 and 10 are lost after the first metathesis reaction. 

A full report has appeared on the use of intramolecular Reformatsky reactions to prepare macrolides. Yields with this approach are in the range 35—68%. Further examples of the template approach (4, 134) to macrolides have been discussed as yet the yields are <40% using medium dilution. ... 

The study of the decondensation kinetics of the silicates is not easy. The NMR of Na reveals that when a solid sodium silicate is put into solution, the first step is the integral rehydration of the sodium that passes into the aqueous solvent. Afterward, there is progressive decondensation of the silicic polymers or oligomers. The decondensation reactions take place in the washing liquor, which is a dilute aqueous medium. The reactions evolve very rapidly, and the kinetics are difficult to measure by NMR. That is why other techniques, like the silico-molybdic complex, are used [16]. With this method, the quantity of silicic monomers in a solution resulting from the decondensation of the larger silicic species can be measured. The monomers formed react with the molybdic acid in controlled conditions to give a yellow complex that can be easily titrated by spectrophotometry. 

Regarding the influence of the anhydride content (PjO.) in PPA, when the reactions were carried out in the presence of PjOj [4], by keeping all reaction variables constant (temperature, time, monomer concentration, molar dilution) and then adding different amounts of P2O5 to the medium does not lead to any significant improvement of the reproducibility. Addition of P Oj increased further the reaction medium viscosity, reaction mixture cannot be stirred in a reaction apparatus, decreasing homogenization. 

Primary aromatic amines and their N-mono- and N,N-dialkyl derivatives can be coupled with diazonium salts in a slightly acid medium. The reaction can be carried out using stabilized diazonium salts (which do not contain free nitrous acid), for example, with p-nitrobenzenediazonium fluoroborate (17) or with a salt with 1-naphthalenesulfonate. The reaction takes place in dilute acetic acid in the presence of sodium acetate. Recently, 4-azo-benzenediazonium fluoroborate was proposed as the reagent, and dimethyl sulfoxide or dimethylformamide (18, 19) as the reaction medium in which intensely colored di-cations of bis-azo dyes can be formed. 

To hydrolyse an ester of a phenol (e.g., phenyl acetate), proceed as above but cool the alkaline reaction mixture and treat it with carbon dioxide until saturated (sohd carbon dioxide may also be used). Whether a solid phenol separates or not, remove it by extraction with ether. Acidify the aqueous bicarbonate solution with dilute sulphuric acid and isolate the acid as detailed for the ester of an alcohol. An alternative method, which is not so time-consuming, may be employed. Cool the alkaline reaction mixture in ice water, and add dilute sulphuric acid with stirring until the solution is acidic to Congo red paper and the acid, if aromatic or otherwise insoluble in the medium, commences to separate as a faint but permanent precipitate. Now add 5 per cent, sodium carbonate solution with vigorous stirring until the solution is alkaline to litmus paper and the precipitate redissolves completely. Remove the phenol by extraction with ether. Acidify the residual aqueous solution and investigate the organic acid as above. 

The nitric acid used in this work contained 10% of water, which introduced a considerable proportion of acetic acid into the medium. Further dilution of the solvent wnth acetic acid up to a concentration of 50 moles % had no effect on the rate, but the addition of yet more acetic acid decreased the rate, and in the absence of acetic anhydride there was no observed reaction. It was supposed from these results that the adventitious acetic acid would have no effect. The rate coefficients of the nitration diminished rapidly with time in one experiment the value of k was reduced by a factor of 2 in i h. Corrected values were obtained by extrapolation to zero time. The author ascribed the decrease to the conversion of acetyl nitrate into tetranitromethane, but this conversion cannot be the explanation because independent studies agree in concluding that it is too slow ( 5.3.1). 

The reaction is carried out at low temperature in aqueous medium and then allowed to stand overnight (221). Ammonium thiocarbamate is prepared from a cold saturated solution of ammonium thiocyanate, which is gradually added to dilute sulfuric acid at 25°C. The liberated carbonyl sulfide is passed into a saturated solution of alcoholic ammonia at about 10°C (221). The fairly low yield indicates that the reaction has not been greatly developed. 

Unlike the addition of concentrated sulfuric acid to form alkyl hydrogen sulfates this reaction is carried out m a dilute acid medium A 50% water/sulfuric acid solution is often used yielding the alcohol directly without the necessity of a separate hydrolysis step Markovmkov s rule is followed... 

According to Le Chatelier s principle, a system at equilibrium adjusts so as to mini mize any stress applied to it When the concentration of water is increased the system responds by consuming water This means that proportionally more alkene is converted to alcohol the position of equilibrium shifts to the right Thus when we wish to pre pare an alcohol from an alkene we employ a reaction medium m which the molar con centration of water is high—dilute sulfuric acid for example... 

The reaction mixture is diluted with 250 ml of water, the mixture is transferred to a 2 liter flask using methanol as a wash liquid, and the organic solvents are distilled at 20-25 mm using a rotary vacuum evaporator. The product separates as a solid and distillation is continued until most of the residual toluene has been removed. The solid is collected on a 90 cm, medium porosity, fritted glass Buchner funnel and washed well with cold water. After the material has been sucked dry, it is covered with a little cold methanol, the mixture is stirred to break up lumps, and the slurry is kept for 5 min. The vacuum is reapplied, the solid is rinsed with a little methanol followed by ether, and the material is air-dried to give 9.1 g (85%), mp 207-213° after sintering at ca. 198°. Reported mp 212-213°. The crude material contains 1.0-1.5% of unreduced starting material as shown by the UV spectrum. Further purification may be effected by crystallization from methanol. 

After completion of the reaction, the mixture is diluted with water, extracted with ether and the residue from the ether phase purified by chromatography and/or recrystallization. If the substrate contains aromatic protons, the reduction procedure is repeated in protic medium to back exchange deuteriums incorporated into the aromatic ring. 

Thioketals are readily prepared by reaction of saturated 3-ketones with thiols or dithiols in the presence of boron trifluoride or hydrogen chloride catalysts. Selective protection of the 3-ketone in the presence of a 6-ketone is possible by carrying out the reaction in diluted medium. Similarly, 3-ketones react selectively with monothiols " " or with bulky dithiols in the presence of 6-, 7-, 11- and 12-ketones. 

Thioketals are readily formed by acid-catalyzed reaction with ethane-dithiol. Selective thioketal formation is achieved at C-3 in the presence of a 6-ketone by carrying out the boron trifluoride catalyzed reaction in diluted medium. Selective protection of the 3-carbonyl group as a thioketal has been effected in high yield with A" -3,17-diketones, A" -3,20-diketones and A" -3,l 1,17-triones in acetic acid at room temperature in the presence of p-toluenesulfonic acid. In the case of thioketals the double bond remains in the 4,5-position. This result is attributed to the greater nucleophilicity of sulfur as compared to oxygen, which promotes closure of intermediate (66) to the protonated cyclic mercaptal (67) rather than elimination to the 3,5-diene [cf. ketal (70) via intermediates (68) and (69)]." " ... 

That the rates of many reactions are markedly dependent upon the acidity or alkalinity of the reaction medium has been known for many decades. In this section, the kinetic analysis of reactions in dilute aqueous solution in which pH is the accessible measure of acidity is presented in sufficient detail to allow the experimentalist to interpret data for most of the systems likely to be encountered and to extend the treatment to cases not covered here. This section is based on an earlier discussion.The problem has also been analyzed by Van der Houwen et al. "... 

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