Reaction with active hydrogen

Phosphoms halides are subject to reactions with active hydrogen compounds and result in the elimination of hydrogen halide. They are convenient reagents in the synthesis of many esters, amides, and related compounds. However, because the involved hydrogen halide frequendy catalyzes side reactions, it is usually necessary to employ a hydrogen halide scavenger to remove the by-product. 

Glacial acetic acid, pure or mixed with other solvents, is one of the most attractive solvents for the titration of amines. Commercial acetic acid containing not more than 1% of water (Karl Fischer titration check) can be used in normal practice for the highest accuracy, however, the water content must be lowered to about 0.01% by addition of acetic anhydride and standing for 24 h not more than the stoichiometric amount of acetic anhydride should be used in order to avoid possible reactions with active hydrogen-containing analyte components such as primary or secondary amines or alcohols. A similar procedure is followed in the preparation of perchloric acid titrant from the commercial... 

The application of N—iV-linked pyridinium salts to induce reaction with active hydrogen compounds in the 4-position is illustrated in Scheme 23. 

Concerning phosgene reactions with active hydrogen substrates [Scheme 5] ... 

Reaction with active-hydrogen compounds [1, 96, before references]. Hauser11 reports that n-butyllithium can afford dilithio salts from active-hydrogen compounds when use of potassium amide in liquid ammonia is ineffective. Thus acetanilide (1) reacts with n-butyllithium in THF or ether to form a dilithio derivative formulated as (2). This disalt on alkylation with benzyl chloride gives (3). 

The self-addition reactions of isocyanates do not usually proceed as readily as reactions with active hydrogen compounds [10]. 

As regards phosgene reactions with active hydrogen substrates, mechanistic studies were indicative of substrate activation as a result of the nucleophilidty of chloride anion in the case of Q" C1 type catalysts (e.g. quaternary ammonium chloride). The mechanism of nucleophilic assistance by these catalysts can be ra-... 

Solventless, multifunctional isocyanates are available based on aromatic diphenyl methene-4-4 -di-isocyanate (MDI) or aliphatic hexamethylene di-isocyanate (I dDI), although due to toxicity reasons, HMDI itself would not be used, but lower volatility adducts may be an alternative. In dried coatings, the water dispersible isocyanates undergo the usual reaction with active hydrogen groups. See Figure 7-18. (Also see isocyanate Chapter for more details on isocyanate reactions ). 

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