Weak acid intermediate form

Some of the intermediate steps in the synthesis of a dipyrrylmethene have been studied chemically (36) and the spatial changes are similar to those postulated by Michaelis (93) for the triphenylmethane dyes. The condensation of the pyrrole-a-aidehyde (5) goes through an intermediate carbinol stage (4) in which the central carbon atom is bound tetrahedrally. The carbinol readily loses hydroxyl ion in the presence of very weak acid to form the planar carbonium ion 5), which represents one of the resonance forms of dipyrrylmethene usually written as (6),... 

Triprotic Acids and Bases, and Beyond The treatment of a diprotic acid or base is easily extended to acids and bases having three or more acid-base sites. For a triprotic weak acid such as H3PO4, for example, we can treat H3PO4 as if it was a mono-protic weak acid, H2P04 and HP04 as if they were intermediate forms of diprotic weak acids, and P04 as if it was a monoprotic weak base. 

We treat HP04 as the intermediate form of a diprotic weak acid... 

Chapters 1 and 2. Most C—H bonds are very weakly acidic and have no tendency to ionize spontaneously to form carbanions. Reactions that involve carbanion intermediates are therefore usually carried out in the presence of a base which can generate the reactive carbanion intermediate. Base-catalyzed condensation reactions of carbonyl compounds provide many examples of this type of reaction. The reaction between acetophenone and benzaldehyde, which was considered in Section 4.2, for example, requires a basic catalyst to proceed, and the kinetics of the reaction show that the rate is proportional to the catalyst concentration. This is because the neutral acetophenone molecule is not nucleophihc and does not react with benzaldehyde. The much more nucleophilic enolate (carbanion) formed by deprotonation is the reactive nucleophile. 

In 1882 Griess discovered that in aqueous acidic solution 4-diazobenzenesulfonate and 4-methylaniline react quantitatively to yield 4-toluenediazonium ion and 4-ami-nobenzenesulfonic acid. This phenomenon is called diazo migration or diazo exchange. We now know that it is a consequence of the tautomerism of the initially formed l-(4 -methylphenyl)-3-(4 -sulfophenyl)-triazene, as discussed above. Griess also found another reaction that could not be explained at his time, but which is based on the tautomerism of intermediate triazenes occasionally, the reaction of an arenediazonium salt with a primary aromatic amine in weakly acidic solution yields a mixture of two isomeric aminoazo compounds (Scheme 13-21). 

It is evident from the nature of the products, especially those formed with toluene present, that the photoreaction in weakly acidic medium involves incursion of a radical species. The complete suppression of reactions leading to the above products, in the presence of oxygen, strongly suggests that it is an excited triplet trityl ion which undergoes reaction. It is postulated that the primary photochemical process is the abstraction of a hydrogen atom by the triplet trityl ion to form the radical cation 90, which was proposed as an intermediate in the dimerization reactions carried out in strong acid (Cole, 1970). 

The reactor is designed to provide sufficient residence time (for recirculating liquid) for the reaction producing chlorate (started in the electrolyzers) to be completed. This involves further reaction of intermediates formed by the complex reactions in the electrolyzer, such as hypochlorite and hypochlorous acid, to produce chlorate. The reactor receives weak chlorate liquor from a crystallizer (not shown), fresh brine feed (also not... 

Schiff base org chem RR C=NR" Any of a class of derivatives of the condensation of aldehydes or ketones with primary amines colorless crystals, weakly basic hydrolyzed by water and strong acids to form carbonyl compounds and amines used as chemical intermediates and perfume bases, in dyes and rubber accelerators, and in liquid crystals for electronics, shif, bas ... 

Polynitroaliphatic alcohols are invaluable intermediates for the synthesis of energetic materials (see Section 1.11). The most important route to /i-nitroalcohols is via the Henry reaction where a mixture of the aldehyde and nitroalkane is treated with a catalytic amount of base, or the nitronate salt of the nitroalkane is used directly, in which case, on reaction completion, the reaction mixture is acidified with a weak acid. Reactions are reversible and in the presence of base the salt of the nitroalkane and the free aldehyde are reformed. This reverse reaction is known as demethylolation if formaldehyde is formed. 

Citric acid is a weak acid and loses hydrogen ions from its three carboxyl groups (COOH) in solution. The loss of a hydrogen ion from each group in the molecule results in the citrate ion, C3H50(C00)33. A citric acid molecule also forms intermediate ions when one or two hydrogen atoms in the carboxyl groups ionize. The citrate ion combines with metals to form salts, the most common of which is calcium citrate. Citric acid forms esters to produce various citrates, for example trimethyl citrate and triethyl citrate. 

An increased incidence of cancer of the paranasal sinuses was observed in workers at factories where isopropanol was manufactured by the strong-acid process. The risk for laryngeal cancer may also have been elevated in these workers. It is unclear whether the cancer risk was due to the presence of diisopropyl sulfate, which is an intermediate in the process, to isopropyl oils, which are formed as by-products, or to other factors, such as sulfuric acid. Epidemiological data concerning the manufacture of isopropanol by the weak-acid process are insufficient for an evaluation of carcinogenicity (lARC, 1987). 

Polynucleotides. These compounds are now generally prepared by use of a bifunctional phosphitylating agent such as o-chlorophenylphosphorodichloridite (6, 114-115) or methoxydichlorophosphine, CHjOPCl,. The intermediate nucleoside phosphites from these reagents tend to be unstable. This difficulty can be alleviated by use of I, which reacts with suitably protected nucleosides to form stable phosphoramidales 2 in good yield. These products can be activated for formation of a dinucleotide phosphite (4) by treatment with a weak acid such as N,N-dimethylaniline hydrochloride or l/Z-tetrazole (5). 

BF3 reacts smoothly, in inert solvents, with alkali metal sulfates and phosphates to give stable 2 1 and 3 1 complexes (Table 9), whereas the intermediate complexes that form with NOT, SO and COl- decompose to [BF4] and B203.55 Brownstein et al. have established114 that BF3 reacts easily with alkylammonium salts in CH2C12 or liquid S02. [BF3A]- complex anions are formed with the salts of strong acids (equation 17) whereas complexes with salts of weak acids easily undergo disproportionation (equation 18) and/or conversion into a 2 1... 

It must be stated that the use of ethanol as solvent in the nitrile reaction makes it difficult to understand the mechanism. At this stage of our present results, it is particularly not possible to determine if the ester is formed from a direct addition of ethanol on the starting nitrile or from the amide as intermediate. As shown in Table 1, the less acidic HY zeolite favours ester formation while in the case of the HMg zeolites the amide is preferably formed. Such results seem to indicate that the ester formation occurs over weak acidic sites whereas the hydration reaction needs stronger acidic sites. 

Hemi-orthoester intermediates have a weakly acidic hydrogen (from the 0-H group), and they have electron pairs on the oxygen atoms which can be pro-tonated in acidic medium. They can exist in three different ionic forms T+, T°, and T" depending on the acidity or the basicity of the medium. Since the pKa of a hemi-orthoester is about 10, it will exist in the T+ and the T° forms in acidic medium and essentially in the T° form in neutral. In slightly basic medium (pH =8-10), it will exist in the T° and the T" forms whereas in stronger basic medium (pH >11), only T" should be present. 

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