Protic Acids and Bases

Understanding the behavior of acids and bases is essential to every branch of science having anything to do with chemistry. In analytical chemistry, we almost always need to account for the effect of pH on analytical reactions involving complex formation or oxidation-reduction. pH can affect molecular charge and shape—factors that help determine which molecules can be separated from others in chromatography and electrophoresis and which molecules will be detected in some types of mass spectrometry. 

In aqueous chemistry, an acid is a substance that increases the concentration of H30+ (hydronium ion) when added to water. Conversely, a base decreases the concentration of H,0+. We will see that a decrease in H3Of concentration necessarily requires an increase in OH concentration. Therefore, a base increases the concentration of OH in aqueous solution. 

The word protic refers to chemistry involving transfer of H+ from one molecule to another. The species H+ is also called a proton because it is what remains when a hydrogen atom loses its electron. Hydronium ion, H30+, is a combination of H+ with H20. Although H,0 is a more accurate representation than H+ for the hydrogen ion in aqueous solution, we will use H30+ and H+ interchangeably in this book. 

Brpnsted and Lowry classified acids as proton donors and bases as proton acceptors. HC1 is an acid (a proton donor) and it increases the concentration of H30+ in water  

The Brpnsted-Lowry definition does not require that H30+ be formed. This definition can therefore be extended to nonaqueous solvents and even to the gas phase  

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Similar expressions have been obtained for the particular cases of mono-protic acids and bases, diprotic acids and bases, and zwitterions (207, 208), and in each case the data conformed well to Eq. (111). It has also been shown (207) that the acid dissociation constants can be determined by using reversed phase chromatography. The pIK, values of 10 aromatic acids calculated from chromatographic data by employing Eq. (91) were... 

A) This Appendix deals with protic acids and bases, called Bronsted-Lowry acids. Similar statements can be made about Lewis acids but they are not the focus of this discussion. 

Triprotic Acids and Bases, and Beyond The treatment of a diprotic acid or base is easily extended to acids and bases having three or more acid-base sites. For a triprotic weak acid such as H3PO4, for example, we can treat H3PO4 as if it was a mono-protic weak acid, H2P04 and HP04 as if they were intermediate forms of diprotic weak acids, and P04 as if it was a monoprotic weak base. 

Equivalent Weights Acid-base titrations can be used to characterize the chemical and physical properties of matter. One simple example is the determination of the equivalent weighf of acids and bases. In this method, an accurately weighed sample of a pure acid or base is titrated to a well-defined equivalence point using a mono-protic strong acid or strong base. If we assume that the titration involves the transfer of n protons, then the moles of titrant needed to reach the equivalence point is given as... 

The third group of solvents comprises the hydrocarbons and their halogen derivatives. They are not of interest for electrochemistry, since the solubilities and dissociation of salts, acids, and bases in them are low. Systems with protic or aprotic polar solvents are used in practice and have been investigated widely. 

Bronsted acid-base theory — In 1923, Bron-sted and, independently of him, Lowry published essentially the same theory of acids and bases which can be applied not only to water as a solvent but also to all other - protic solvents, as well as to proton transfer reactions in gases. An acid is defined as a proton donor, i.e.,... 

Protection of carboxylic acids. The 2-(2-pyridyl)ethyl group is useful for protection of amino acids. The esters are obtained by reaction with 1 and DCC/1-hydroxybenzo-triazole. The group provides higher solubility in protic solvents, and is stable to both acids and bases. It is removed by treatment with methyl iodide and a weak base. 

Taking into account the fact that the solvation of ambident anions in the activated complex may differ considerably from that of the free anion, another explanation for the solvent effect on orientation, based on the concept of hard and soft acids and bases (HSAB) [275] (see also Section 3.3.2), seems preferable [366]. In ambident anions, the less electronegative and more polarizable donor atom is usually the softer base, whereas the more electronegative atom is a hard Lewis base. Thus, in enolate ions, the oxygen atom is hard and the carbon atom is soft, in the thiocyanate ion the nitrogen atom is hard and the sulfur atom is soft, etc. The mode of reaction can be predicted from the hardness or softness of the electrophile. In protic solvents, the two nucleophilic sites in the ambident anion must interact with two electrophiles, the protic solvent and the substrate RX, of which the protic solvent is a hard and RX a soft acid. Therefore, in protic solvents it is to be expected that the softer of the two nucleophilic atoms (C versus O, N versus O, S versus N) should react with the softer acid RX. 

Base-catalyzed enol formation occurs by an acid-base reaction between catalyst and carbonyl compound. The carbonyl compound acts as a weak protic acid and donate. one of its a hydrogens to the base. The resultant anion—an enolate ion—is then reprotonated to yield a neutral compound. Since the enolate ion is a resonance hybrid of two forms, it can be proto-nated either on the a carbon to regenerate the keto tautomer or on oxygen to give the enol tautomer (Figure 22.2, p. 905). 

Two general classes of ionic liquids which are easily distilled are protic ionic liquids (PILs) formed by reaction between a volatile acid and base [86], and dialkylammo-nium carbamate salts such as dimethylammonium dimethylcarbonate (DIMCARB) [72, 73, 206], An example of a distillable PIL is methylpyrrolidinium acetate formed from the exchange of a proton between acetic acid and methylpyrrolidine [86], DIMCARB, a low-cost ionic liquid produced by mixing C02 with dimethylamine, has been used as a reaction medium for the synthesis of monoarylidene and macro-cyclic compounds, where in each case, the product was separated by distillation of the ionic liquid [72, 73, 206],... 

Usually these reactions have been studied in water or in other protic solvents such as the alcohols. Thus, the acid-base properties of the solvent are important in determining the relative strength of acids and bases which are solutes in water. This leads to the definition of two other types of proton transfer reaction, namely, the protolysis reaction,... 

The major advantage of the updated definition was that it was not limited to aqueous solution. This definition of acids and bases allowed chemists to explain a great number of reactions that took place in protic or aprotic solvents tliat were not water, and it also allowed for gaseous and solid phase reactions (altliough those reactions are more rare). 

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