Bases amphoteric compounds

Acid amides have weakly amphoteric properties, and thus give salts such as CjHsCONHj.HCl with strong acids, and salts of the type C HsCONHNa with strong bases. These compounds have to be prepared at low temperatures to avoid hydrolysis, and are difficult to isolate. The mercury derivatives can, however, usually be readily prepared, because mercuric oxide is too feebly basic to cause hydrolysis of the amide, and the heavy mercuric derivatives crystallise well. 

Amphoteric metal or metalloid hydroxides can act either as an acid or as a base. Such compounds include... 

Amphoteric. Compounds having the capability of acting as an acid or a base. Amino acids are amphoteric—their molecules contain both an acid group (-COOH) and a basic group (-NH2). 

Amphoteric compounds are compounds that may function as either acid or base, depending upon conditions. We have already met this concept in Section 4.5.4, where simple alcohols and amines have two pATa values according to whether the compound loses or gains a proton. Of course, with alcohols and amines, acidity and basicity involve the same functional group. Other amphoteric compounds may contain separate acidic and basic groups. Particularly... 

In 1896 Pinnow and Samann reported that diazotization of o-amino-benzamidoxime (17) gave the amphoteric compound (18) as the sole reaction product. Structural assignment was based on chemical evidence which was erroneously interpreted, and reinvestigation of this reaction by Parnell has established that the correct structure for this... 

Tetracycline antibiotics are closely related derivatives of the polycyclic naphtha-cenecarboxamide. They are amphoteric compounds with characteristic dissociation constants corresponding to the acidic hydroxyl group at position 3 (pK about 3.3), die dimethylamino group at position 4 (pK, about 7.5), and the hydroxyl group at position 12 (pK about 9.4). In aqueous solutions of pH 4-7, tetracyclines exist as dipolar ions, but as the pH increases to 8-9 marked dissociation of the dimethylamine cation occurs. They are soluble in acids, bases, and alcohols but are quite insoluble in organic solvents such as chloroform. Their ultraviolet spectra show strong absorption at around 270 and 360 nm in neutral and acidic solutions. Tetracyclines are readily transformed into fluorescent products in the presence of metal ions or under alkaline conditions. 

PROTON AFFINITY OF ACIDS AND BASES The division between acids and bases is not a very sharp one it has been remarked already that the borderline between the two groups is formed by the amphoteric compounds, that have both weak acid and weak basic properties, and thus behave as acids in the presence of strong bases and as bases in the presence of strong acids. To a... 

This is an amphoteric compound because it behaves as both an acid and a base in the Bronsted definition. As an acid, it can donate an H+ and becomes the conjugate base ... 

The dipolar ion has both an acidic group, the ammonium cation, and a basic group, the carboxylate anion, so it can act as either an acid or a base. Such compounds are termed amphoteric. The species that is present in aqueous solution depends on the pH. In the pH range near neutral the amino acid is present in the form of the dipolar ion. In acidic solution the carboxylate group becomes protonated and the amino acid is present as a cation, whereas in basic solution the ammonium group gives up a proton and the molecule exists as an anion ... 

Amphoteric compound (Section 26.2) A compound that can act as either an acid or a base. 

Basolo, F. Theories of Acids, Bases, Amphoteric Hydroxides, and Basic Salts as Applied to the Chemistry of Complex Compounds, in Bailar and Busch s Chemistry of Coordination Compounds, 416-447, Reinhold Publishing Co., New York (1956). 

There is a separate scale for bases, but it seems silly to have two different scales, the basic pKB and the familiar pKa, when one will do and so we will stick to pK However, to avoid any misunderstandings that can arise from amphoteric compounds like ammonia, whose pKa is around 33. we will either say ... 

Ampholyte — A substance that can react both as an acid and as abase is called an ampholyte, or amphoteric compound. Usually this property refers to the - Bronsted acid-base theory. An example is HCOj which can act as a proton acceptor and as a proton donator. An ampholyte can be a zwitterion, as in case of amino acids in the range between pH = pJCai and pH = pfCa2, they exist as [+(H3N)HRC-COO-]. 

We call compounds, like amino acids, that can act as acids or bases depending on their environment amphoteric compounds. The body uses this dual property in the stomach, as the protein stomach juices and lining can react to mop up any adverse effects of overeating or drinking (within reason). If the system is overloaded you might have to help a bit by taking a Rennie or stomach powder to help restore the correct acidity to the stomach. 

There are about 4-5 surface hydroxy groups per square nanometer on anatase and these are of two different types. One type is basic in nature and can be exchanged by F". This type of site is responsible for the adsorption of acidic species onto the surface and initiates the adsorption of base sensitive compounds such as Mo02(acac)2. The second type of surface hydroxide is acidic. It interacts strongly with base and initiates the adsorption of acid sensitive compounds such as Fe(acac)3.2 This amphoteric nature of titania is illustrated by the zeta potential versus pH curve shown in Fig. 9.9. Cationic catalyst precursers are adsorbed at pH values greater than 7-8 and anionic species from solutions having a pH less than about 5. 

The chemical nature and related physicochemical properties largely govern the distribution and elimination, which refers to biotransformation (metabolism) and excretion, of antimicrobial agents. The majority of antimicrobial agents are weak organic electrolytes, either weak acids (penicillins, cephalosporins, sulphonamides) or weak bases (aminoglycosides, lincosamides, macrolides, diaminopyrimidines, metronidazole), while fluoroquinolones, tetracyclines and rifampin are amphoteric compounds, and chloramphenicol and its... 

We have shown in the section on hydrolysis included in the first chapter that the reaction of salts derived from weak acids and weak bases is never strongly acidic or strongly alkaline, unless the difference between the acidic and basic dissociation constants is too large. The same explanation, applied to the acidic and basic functions of ampholytes, accounts for the fact that solutions of amphoteric compounds never react very strongly acid or alkaline. 

Cocaine metabolizes in the body to several compounds. The NIDA drug confirmation is based on the metabolite, benzoylecgonine. Benzoylecgonine is an amphoteric compound (contains both an amine and a carboxylic acid) that will isolate by mixed-mode SPE (Fig. 8.6). 

Amino acids show basic properties because of the amino group (— NH2) and acidic properties because of the carboxyl group (— COOH), so they are amphoteric compounds. Amino acids act as a base when they react with an acid and act as an acid when they react with a base, in both cases a salt is formed. 

There are several types of compounds that one might attempt typically to isolate from a crude extract using an ion-exchange method strong and weak acids and bases, as well as amphoteric compounds. As both adsorption and elution processes will generally involve pH adjustments, it is wise first to determine the stability of the compound of interest at various pH values, as mentioned earlier. With information on the stability of the compound of interest at hand, one can then choose an appropriate ion-exchange process. 

This table lists the dissociation (ionization) constants of over 1070 organic acids, bases, and amphoteric compounds. All data apply to dilute aqueous solutions and are presented as values of p7C, which is defined as the negative of the logarithm of the equilibrium constant K for the reaction... 

A general method for estimating dissociation values, given a set of solubility and pH measurements for tyrosine and 2,8-dihydroxyadenine, is presented. Equations are derived extending solubility, pH and dissociation constant relationships from weak acids and bases to polyprotic, amphoteric compounds. Included in ttie estimation procedure is a subroutine for approximating ttiermodynamic dissociation constants."... 

P-Lactams are sensitive to acids and bases, and this sensitivity varies with the nature of the sidechain. The maximum stability of monobasic compounds such as Pen G is exhibited in the pH range 6-7, whereas for ampicillin (an amphoteric compound), the maximum stability occurs at its isoelectric point of pH 5. The highly susceptible f)-lactam nitrogen is prone to attack by nucleophiles such as methanol. Furthermore, this nucleophilic attack is accelerated by acid catalysis and application of heat. They are also readily isomerized in an acidic environment. f)-lactams are typically extracted with water and/or polar organic solvents from solid matrices. 

The first chapter of the book deals with enzyme-like eatalysis by synthetic polymers - catalysis by polymeric acids and bases, amphoteric polyelectrolytes and nonionic polymers. Because coordination compounds of metal ions with macromolecular ligands are interesting with regard to bioinorganic chemistry, this book elucidates some problems involving the catalysis by water-soluble polymer-metal complexes. Ester hydrolysis, hydrogen peroxide decomposition, oxidation of disubstituted phenols, hydroquinones, mercaptoalcohols and other types of reaction are chosen as model processes. A section devoted to interfacial catalysis is also included. 

Lipophilicity profiles of amphoteric drugs are more complicated than partitioning of simple acids and bases. A comprehensive review of the issues encountered in the determination of logF/Z) of amphoteric compounds has been published by Pagliara et al. ... 

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