Condensation reactions formaldehyde

Frensch and Vdgtle have recently appended three crown ether units to the cyclo-triveratrylene unit . Note that Hyatt had previously prepared the open-chained relatives of this structure (see Sect. 7.3 and Eq. 7.6). Whereas Hyatt prepared the cyclo-triveratrylene skeleton and then appended polyethyleneoxy arms to it, Frensch and Vogtle conducted the condensation reaction (formaldehyde/HCl) on the preformed benzocrown. Thus benzo-15-crown-5 was converted into the corresponding tris-crown (IS) (mp 203.5—205.5°) in 4% yield. The yield was somewhat higher for the condensation of benzo-18-crown-6, but in both cases, yield ranges were observed. These species formed 1 3 (ligand/salt) complexes with sodium and potassium ions. 

Sadly, this simple mechanism cannot be correct because, when the P-hydroxy ketone is made in other ways, it is not converted into Mannich product under the reaction conditions. What else can happen Amines are nucleophiles and react rapidly with carbonyl compounds in acid to give iminium ions (p. 795). It is this imini-um ion that is the real participant in the Mannich condensation reaction. Formaldehyde is more reactive than the ketone, so the iminium ion is preferentially... 

The Mannich Reaction involves the condensation of formaldehyde with ammonia or a primary or secondary amine and with a third compound containing a reactive methylene group these compounds are most frequently those in which the methylene group is activated by a neighbouring keto group. Thus when acetophenone is boiled in ethanolic solution with paraformaldehyde and dimethylamine hydrochloride, condensation occurs readily with the formation of... 

Indole (I) condenses with formaldehyde and dimethylamine in the presence of acetie acid (Mannich reaction see Section VI,20) largely in the 3-position to give 3 dimethylaminomethylindole or gramine (II). The latter reaets in hot aqueous ethanol with sodium cyanide to give the nitrile (III) upon boiling the reaction mixture, the nitrile undergoes hydrolysis to yield 3-indoleaeet-amide (IV), part of which is further hydrolysed to 3-indoleacetic acid (V, as sodium salt). The product is a readily separable mixture of 20 per cent, of (IV) and 80 per cent, of (V). 

Scheme 55) (235. 236). -The product obtained (77) is probably formed via the protonated form of the thiazole, whose reactivity is treated in Section IV, 1. The light-yellow leucobase (77) is reported to be oxidized by PbOj to the red-black carbinol (78) (236). This condensation reaction is also successful when benzaidehyde is replaced by formaldehyde, bis(2-amino-4-phenylthiazolyl-5)methane (79 i beine obtained (Scheme 56) (237). 

Rhoda.mines, Rhodamines are commercially the most important arninoxanthenes. If phthalic anhydride is used in place of formaldehyde in the above condensation reaction with y -dialkylarninophenol, a triphenyknethane analogue, 9-phenylxanthene, is produced. Historically, these have been called rhodamines. Rhodamine B (Basic Violet 10, Cl45170) (17) is usually manufactured by the condensation of two moles of y -diethylaminophenol with phthahc anhydride (24). An alternative route is the reaction of diethylamine with fluorescein dichloride [630-88-6] (3,6-dichlorofluoran) (18) under pressure. 

Acetylation of acetaldehyde to ethyUdene diacetate [542-10-9], a precursor of vinyl acetate, has long been known (7), but the condensation of formaldehyde [50-00-0] and acetic acid vapors to furnish acryflc acid [97-10-7] is more recent (30). These reactions consume relatively more energy than other routes for manufacturing vinyl acetate or acryflc acid, and thus are not likely to be further developed. Vapor-phase methanol—methyl acetate oxidation using simultaneous condensation to yield methyl acrylate is still being developed (28). A vanadium—titania phosphate catalyst is employed in that process. 

The cyanoacryhc esters are prepared via the Knoevenagel condensation reaction (5), in which the corresponding alkyl cyanoacetate reacts with formaldehyde in the presence of a basic catalyst to form a low molecular weight polymer. The polymer slurry is acidified and the water is removed. Subsequendy, the polymer is cracked and redistilled at a high temperature onto a suitable stabilizer combination to prevent premature repolymerization. Strong protonic or Lewis acids are normally used in combination with small amounts of a free-radical stabilizer. 

An important synthetic process for forming a new carbon—carbon bond is the acid-catalyzed condensation of formaldehyde with olefins (Prins reaction) ... 

Many of the chemical reactions used to modify lignosulfonates are also used to modify kraft lignins. These include ozonation, alkaline—air oxidation, condensation with formaldehyde and carboxylation with chloroacetic acid (100), and epoxysuccinate (101). In addition, cationic kraft lignins can be prepared by reaction with glycidjiamine (102). 

The most important commercial chemical reactions of phenol are condensation reactions. The condensation reaction between phenol and formaldehyde yields phenoHc resins whereas the condensation of phenol and acetone yields bisphenol A (2,2-bis-(4-hydroxyphenol)propane). PhenoHc resins and bisphenol A [80-05-7] account for more than two-thirds of U.S. phenol consumption (1). 

The bulk of 4-methylphenol is used in the production of phenoHc antioxidants. The alkylation of 4-methylphenol with isobutylene under acid catalysis yields 2-/ f2 -butyl-4-methylphenol [2409-55-4] and 2,6-di-/ f2 -butyl-4-methylphenol [128-37-0]. The former condenses with formaldehyde under acid catalysis to yield 2,2 -methylene bis(6-/ f2 -butyl-4-methylphenol) [119-47-1], which is widely used in the stabilization of natural and synthetic mbber (43). The reaction of 2-/ l -butyl-4-methylphenol with sulfur dichloride yields 2,2 -thiobis(6-/ l -butyl-4-methylphenol) [90-66-4]. 

ButylatedPhenols and Cresols. Butylated phenols and cresols, used primarily as oxidation inhibitors and chain terrninators, are manufactured by direct alkylation of the phenol using a wide variety of conditions and acid catalysts, including sulfuric acid, -toluenesulfonic acid, and sulfonic acid ion-exchange resins (110,111). By use of a small amount of catalyst and short residence times, the first-formed, ortho-alkylated products can be made to predominate. Eor the preparation of the 2,6-substituted products, aluminum phenoxides generated in situ from the phenol being alkylated are used as catalyst. Reaction conditions are controlled to minimise formation of the thermodynamically favored 4-substituted products (see Alkylphenols). The most commonly used is -/ fZ-butylphenol [98-54-4] for manufacture of phenoHc resins. The tert-huty group leaves only two rather than three active sites for condensation with formaldehyde and thus modifies the characteristics of the resin. 

In addition to the above possible mechanisms the possibility of reaction at w-positions should not be excluded. For example, it has been shown by Koebner that o- and p-cresols, ostensibly difunctional, can, under certain conditions, react with formaldehyde to give insoluble and infusible resins. Furthermore, Megson has shown that 2,4,6-trimethylphenol, in which the two ortho- and the one para-positions are blocked, can condense with formaldehyde under strongly acidic conditions. It is of interest to note that Redfam produced an infusible resin from 3,4,5,-trimethylphenol under alkaline conditions. Here the two m- and the p-positions were blocked and this experimental observation provides supplementary evidence that additional functionalities are developed during reaction, for example in the formation of quinone methides. 

Mannich reaction is the condensation between formaldehyde, ammonia, or a primary or secondary amine (preferably as the hydrochloride), and a compound containing at least one active hydrogen atom... 

The study of PF polymerization is far more difficult than that of methylolation due to the increased complexity of the reactions, the intractability of the material, and a resulting lack of adequate analytical methods. When dealing with methylolation, we saw that every reactive ring position had its own reaction rate with formaldehyde that varied with the extent of prior reaction of the ring. Despite this rate sensitivity and complexity, all reactions kinetics were second-order overall, first-order in phenol reactive sites and first-order in formaldehyde. This is not the case with the condensation reactions. 

Alkaline co-condensation to yield commercial resins and the products of reaction obtained thereof [93,94] as well as the kinetics of the co-condensation of mono methylol phenols and urea [104,105] have also been reported [17]. Model reactions in order to prove an urea-phenol-formaldehyde co-condensation (reaction of urea with methylolphenols) are described by Tomita and Hse [98,102, 106] and by Pizzi et al. [93,104] (Fig. 1). 

The same chemical mechanisms and driving forces presented for phenol-formaldehyde resins apply to resorcinol resins. Resorcinol reacts readily with formaldehyde to produce resins (Fig. 2) which harden at ambient temperatures if formaldehyde is added. The initial condensation reaction, in which A-stage liquid resins are formed, leads to the formation of linear condensates only when the resorcinol/formaldehyde molar ratio is approximately 1 1 [119]. This reflects the reactivity of the two main reactive sites (positions 4 and 6) of resorcinol [120]. However, reaction with the remaining reactive but sterically hindered site (2-positiori) between the hydroxyl functions also occurs [119]. In relation to the weights of resorcinol-formaldehyde condensates which are isolated and on a molar basis, the proportion of 4- plus 6-linkages relative to 2-linkages is 10.5 1. However, it must be noted that the first-mentioned pair represents two condensa-... 

The condensation reaction of resorcinol with formaldehyde, on an equal molar basis and under identical conditions, also proceeds at a rate which is approximately 10 to 15 times faster than that of the equivalent phenol-formaldehyde system [16-18,123]. The high reactivity of the resorcinol-formaldehyde system renders it impossible to have these adhesives in resol form. Therefore, only resorcinol-formaldehyde novolaks, i.e. resins not containing methylol groups can be produced. All the resorcinol nuclei are linked together through methylene... 

The Eschweiler reaction (formaldehyde and formic acid) has been used for the pyr-A-methylation of l-methyl-l,2,3,4-tetrahydro-j8-carboline, as has formaldehyde and hydrogen in the presence of Raney nickel. Tetrahydroharmine has been reported to react with benzaldehyde to yield a condensation product, C33H36N403, which is presumably a pyr-iV-substituted derivative (318). It is not known whether a similar condensation product of benzaldehyde with harm-aline is a C- or V-substituted derivative of harmaline. Conden-... 

The condensation reaction of a CH-acidic compound—e.g. a ketone 3—with formaldehyde 1 and ammonia 2 is called the Mannich reaction, the reaction products 4 are called Mannich bases. The latter are versatile building blocks in organic synthesis, and of particular importance in natural products synthesis. 

A rather more complex amino alcohol side chain is accessible by a variation of the Mannich reaction. Taking advantage of the acidic proton in acetylenes, propargyl acetate (62) is condensed with formaldehyde and dimethylamine to give the acetylated amino... 

Phenol-formaldehyde resins are the oldest thermosetting polymers. They are produced by a condensation reaction between phenol and formaldehyde. Although many attempts were made to use the product and control the conditions for the acid-catalyzed reaction described by Bayer in 1872, there was no commercial production of the resin until the exhaustive work by Baekeland was published in 1909. In this paper, he describes the product as far superior to amber for pipe stem and similar articles, less flexible but more durable than celluloid, odorless, and fire-resistant. ° The reaction between phenol and formaldehyde is either base or acid catalyzed, and the polymers are termed resols (for the base catalyzed) and novalacs (for the acid catalyzed). 

A variety of methylols are possible due to the availability of six hydrogens in melamine. As with urea formaldehyde resins, polymerization occurs by a condensation reaction and the release of water. 

Resole syntheses entail substitution of formaldehyde (or formaldehyde derivatives) on phenolic ortho and para positions followed by methylol condensation reactions which form dimers and oligomers. Under basic conditions, pheno-late rings are the reactive species for electrophilic aromatic substitution reactions. A simplified mechanism is generally used to depict the formaldehyde substitution on the phenol rings (Fig. 7.21). It should be noted that this mechanism does not account for pH effects, the type of catalyst, or the formation of hemiformals. Mixtures of mono-, di-, and trihydroxymethyl-substituted phenols are produced. 

Condensation reactions between two hydroxymethyl substituents eliminate water to form ether linkages (Fig. 7.23a) or eliminate both water and formaldehyde to form methylene linkages (Fig. 7.23b). Ether formation is favored under neutral or acidic conditions and up to 130°C above which formaldehyde departs and methylene linkages are generated. The methylene linkage formation reaction, which eliminates water and fortualdehyde, is more prevalent under basic conditions. Condensation reactions between hydroxymethyl groups and reactive... 

As the reactions proceed, the disappearance of phenol is delayed due to competition for reaction with formaldehyde between phenol and the faster reacting hydroxymethyl-substituted phenols. Competition also exists between formaldehyde substitution reactions and condensation reactions between rings. Condensation reactions between two ortho-hydroxymethyl substituents are the least favorable condensation pathway. Depending on the reaction conditions, substitutions occur... 

The initial step of the polymerisation process is reaction of the amine groups with formaldehyde to generate methylol units, as illustrated in Reaction 1.9. Further heating of the polymer then leads to a variety of reactions. For example, the methylol groups can undergo self-condensation (Reaction 1.10). 

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