Active hydrogen compounds acidity

Active Hydrogen Compound Acid Halide Product ... 

Developments in aliphatic isocyanates include the synthesis of polymeric aliphatic isocyanates and masked or blocked diisocyanates for appflcafions in which volatility or reactivity ate of concern. Polymeric aliphatic isocyanates ate made by copolymerizing methacrylic acid derivatives, such as 2-isocyanatoethyl methacrylate, and styrene [100-42-5] (100). Blocked isocyanates ate prepared via the reaction of the isocyanate with an active hydrogen compound, such as S-caprolactam, phenol [108-95-2] or acetone oxime. 

According to this mechanism, it is the free amine, not the salt that reacts, even in acid solution and the active-hydrogen compound (in the acid-catalyzed process) reacts as the enol when that is possible. This latter step is similar to what happens in 12-4. There is kinetic evidence for the intermediacy of the iminium ion (13). 

Compounds Z are active hydrogen compounds such as amines, anhydrides, and acids [30]. The curing reaction can also involve homopolymerization catalyzed by Lewis acids or tertiary amines. In most cases, reactions catalyzed by... 

Acylation of active hydrogen compounds followed by cleavage 0-109 Reduction of p-keto sulfoxides 0-110 Acylation of carboxylic acid salts followed by cleavage... 

However, 2,4,6-trisubstituted pyrylium salts with certain active methyl and methylene compounds undergo ring fission and subsequent cyclization to benzenoid products. 2,4,6-Triphenylpyrylium ion (261 Z = O) in this way forms 2,4,6-triphenylnitrobenzene (299) with nitromethane and the substituted benzoic acid (300) with malonic acid, the latter reaction involving a decarboxylation. In reactions of this type, 1,3-oxazinium salts react with active hydrogen compounds to give pyridines (Scheme 25). 

Instead of carboxylic acids, other carbonyl compounds can be used acid halides, esters, amides, etc. The commonly accepted general mechanism for these reactions consists of the initial nucleophilic addition of an active hydrogen compound to the electron-poor carbonyl carbon atom of the R COOI I molecule, with the formation of a metastable intermediate that can undergo a subsequent elimination reaction ... 

Free radical initiators or active hydrogen compounds such as amines or alcohols are not very effective initiators for the polymerization of lactones. Polyesters of low molecular weight are produced by these techniques. For example, copolymerization of various lactones in the presence of water at 200 °C proceeded via a hydrolysis followed by the polycondensation reaction of the hydroxy acid, giving low molecular weight products [67-69]. Low molecular weight (=10,000) tri-block copolymer (CL-b-EO-b-CL) has been prepared from e-CL and poly(ethylene glycol) (Mn=10 3) by carrying out the polymerization at 165 °C for several hours in the absence of catalysts [70]. 

The polyaddition reaction is the most commonly used type of reaction for the cure of epoxy resins. The curing agents used in this type of reaction have an active hydrogen compound, and they include amines, amides, and mercaptans. With this reaction mechanism, the most important curing agents for adhesives are primary and secondary amines containing at least three active hydrogen atoms and various di- or polyfunctional carboxylic acids and their anhydrides. 

Step-Growth Polymerization. Only a relative few of the dozens of active hydrogen compounds that undergo reactions with the epoxy ring find widespread use in epoxy structural adhesives. The most common are amines, acid anhydrides, phenols, thiols, and carboxylic acids. 

Certain pyrones react with active hydrogen compounds in a quite different way they display their alkenic character by participating in Michael reactions, thus coumarin itself gives 316 with malonate anion, and coumarins 317 react with arylboronic acids catalyzed by rhodium giving 318 in greater than 99% ee <2005OL2285>. 

The reactants are dissolved in a pair of immiscible liquids, usually water and a hydrocarbon or chlorinated hydrocarbon. The aqueous phase contains the diol, diamine, dimercaptan, or other active hydrogen compound, an acid acceptor like... 

Interfacial polycondensations can also be carried out in vapor-liquid systems. Reaction takes place at the interface between an aqueous solution of a bifunctional active hydrogen compound and the vapor of diacid chloride. Interfacial condensation is commercially important in the synthesis of polycarbonates (1-52). Polymerizations based on diacids are always less expensive than those that use diacid chlorides. In the polycarbonate case, however, the parent reactant, carbonic acid, is not suitable and the derived acid chloride, phosgene (COCI2), must be used. 

Addition reaction of active hydrogen compounds to carbon — carbon mble bond is one of the important clasps of asymmetric thesis in enzymatic reactions. Examples are the addition of water and ammonia to form S-malic acid and S-aq>artic... 

Some of the most familiar reactions falling into the polycondensation class are those leading to polyamides derived from dicarboxylic acids and diamines, polyesters from glycols and dicarboxylic acids, polyurethanes from polyols and polyisocyanates, and polyureas from diamines and diisocyanates. Similar polymer formations utilizing bifunctional acid chlorides with polyols or polyamines also fall into this class. The condensations of aldehydes or ketones with a variety of active hydrogen compounds such as phenols and diamines are in this group. Some of the less familar polycondensation reactions include the formation of polyethers from bifunctional halogen compounds and the sodium salts of bis-phenols, and the addition of bis-thiols to diolefins under certain conditions. 

The effect of the solvent on the kinetics of organic reactions has been studied in detail for the reactions of a host of model compounds, and for some polymerizations. Several generalizations are readily apparent. The most powerful solvents, i.e, those giving the fastest reaction rate, are often those which may be considered to function as mild catalysts themselves. The dipoles in h hly polar solvents such as dimethyl formamide or dimethyl sulphoxide may actually function as weak acids or bases, catalysing the reactions. In contrast, solvents of intermediate polarity may complex the active hydrogen compound to such an extent (at least at low to moderate temperatures) that they shield it from reaction, thus retarding the reaction. Solvents of very low polarity may permit faster reactions in these cases. For example, the reaction between... 

Only few studies of the stereochemistry of anodic oxidative dimerization of active hydrogen compounds (carbon acids) and their salts (carbanions) have been reported. 

This reaction shows the addition of an active hydrogen compound, HA, across the N=C double bond of an isocyanate. Isocyanates also react with water and the initial product is carbamic acid which is unstable, losing carbon dioxide, and generates an amine that reacts with the excess isocyanate to produce urea as follows ... 

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