Poly protic acids

Potentiometric acid-base titrations are particularly useful for the analysis of mixtures of acids or poly-protic acids (or bases) because often, discrimination between the endpoints can be made. An approximate numerical value for the dissociation constant of a weak acid or base can be estimated from potentiometric titration curves. In theory, this quantity can be obtained from any point along the curve, but it is most easily derived from the pH at the point of halfneutralization. 

Stepwise addition of equal volumes also suitable for mixtures of poly-protic acids/bases Automatic microcomp.-contr. poten-tiometric titrator for student demon-strations... 

In some cases, we may want to know the concentrations of the anions formed by a poly-protic acid. Consider the following generic polyprotic acid H2X and its ionization steps ... 

This chapter examines three types of titration curves, representing three types of acid-base reactions a strong acid with a strong base, a weak acid with a strong base (or vice versa), and a poly-protic acid with a base. 

Thickeners. Thickeners are added to remover formulas to increase the viscosity which allows the remover to cling to vertical surfaces. Natural and synthetic polymers are used as thickeners. They are generally dispersed and then caused to swell by the addition of a protic solvent or by adjusting the pH of the remover. When the polymer swells, it causes the viscosity of the mixture to increase. Viscosity is controlled by the amount of thickener added. Common thickeners used in organic removers include hydroxypropylmethylceUulose [9004-65-3], hydroxypropylceUulose [9004-64-2], hydroxyethyl cellulose, and poly(acryHc acid) [9003-01-4]. Thickeners used in aqueous removers include acryHc polymers and latex-type polymers. Some thickeners are not stable in very acidic or very basic environments, so careful selection is important. 

Adequate amounts of the (macro)initiator, monomer, and thiourea were added to the vial and dissolved in anhydrous dichloromethane (DCM) or amylene stabilized chloroform (TCM). Typically, the mass concentration of the solid reagents, i.e. lactide monomer and macroinitiator, was around 10 %. After stirring the solutions for a couple of minutes the DBU was added with an accurate micropipette. The vials were sealed and the reaction solution was stirred at room temperature. Once the reaction time had elapsed the DBU was neutralized by adding equimolar amounts of benzoic acid. Then the polymer was precipitated into cold methanol, collected by filtration, and dried under vacuum at 80 °C for 24 h. To reduce the risk of poly(lactic acid) degradation in the protic methanol, fast filtration is essential. This was achieved by allowing the precipitate to sediment and removing the excess methanol before filtration. 

While the original Scholl conditions called for both a Lewis and protic acid, research by Kovacic and Kyriakis into the role of oxidizing agents (that facilitate the formation of radical cations) in the Scholl reaction led to the observation that benzene (12) when reacted in the presence of a heterogeneous mixture of anhydrous aluminum chloride and copper chloride afforded poly(para-phenylene) (13). It is important that Ihis reaction was conducted under mild conditions (25-35 °C) and was complete in only 2 h. As discoverers of one of the first Scholl reactions conducted under mild conditions, Kovacic and Kyriakis have greatly increased the synthetic utility of this reaction. 

Polymerizations of unsaturated monomers, such as olefins, vinyl monomers, or acetylenes, exclusively proceed as KC reaction sequences. With exception of crystalline polyethylene, the resulting polymers are thermodynamically unstable, because the corresponding cyclohexanes or benzenes are more stable, but equilibration catalysts do not exist. As soon as polymer backbones include heteroatoms, equilibration reactions become possible, and many polymers may be prepared by KC and by TC polymerizations. An interesting case is synthesis and polymerization of oxiranes. Their synthesis from )S-bromoalcohols and their polymerization via anionic or coordination insertion catalysts represent KC reactions. Yet, heating of oxiranes or poly(alkylene oxides)s with strong protic acids yields the thermodynamically more stable 1,4-dioxanes. 

This reversal has been demonstrated by both product and kinetic studies. In the absence of solvent nucleophilic assistance and of substituents favouring P-bromo-carbonium ion intermediates, the ionization of CTCs to bromonium (poly)bromide has been shown to occur not only for congested olefins, but more generally for "normal olefins both in aprotic chlorinated hydrocarbons and in protic solvents like acetic acid and methanol. 

The nature of the solvent plays an important role in the reactions between carboxy-containing compounds and diazoalkanes. Protic solvents of the R-OH type can react with diazoalkanes in the presence of carboxylic acids to form the corresponding ethers Hence, in reactions with polymers dissolved in alcohols or alcohol-containing mixtures (poly(acrylic and methacrylic acids) etc.) the yield of the main reaction can decrease to 30—50% (with respect to ADM) 1 Consequently, when polymers soluble in aprotic solvents undergo anthryhnethylation, the reaction should be carried out in such solvents as benzene or toluene. 

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