Acids and Bases The Arrhenius View

Acids and bases are a big part of organic chemistry, but the emphasis is much different from what you may be familiar with from your general chemistry course. Most of the attention in general chemistry is given to numerical calculations pH, percent ionization, buffer problems, and so on. Some of this returns in organic chemistry, but mostly we are concerned with the roles that acids and bases play as reactants, products, and catalysts in chemical reactions. We ll start by reviewing some general ideas about acids and bases. 

According to the theory proposed by Svante Arrhenius, a Swedish chemist and winner of the 1903 Nobel Prize in Chemistry, an acid is a substance that ionizes to give protons when dissolved in water. 

Acids differ in the degree to which they ionize. Those that ionize completely are called strong acids those that do not are weak acids. Likewise, strong bases ionize completely weak bases do not. 

The strength of a weak acid is measured by its acidity constant, which is the equilibrium constant for its ionization in aqueous solution. 

acetic acid with K = 1.8 x 10 has a of 4.7. The advantage of over is that it avoids exponentials. You are probably more familiar with but most organic chemists and biochemists use It is a good idea to be comfortable with both systems, so you should practice converting to and vice versa. 

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The Lewis definitions of acids and bases provide for a more general view of acid-base reactions than either the Arrhenius or Br0nsted-Lowry pic ture A Lewis acid is an electron pair acceptor A Lewis base is an electron pair donor The Lewis approach incorporates the Br0nsted-Lowry approach as a subcategory m which the atom that accepts the electron pair m the Lewis acid is a proton... 

Early studies of acids and bases were restricted to aqueous solutions and were made with an inadequate understanding of the peculiarities of water. The compounds were defined as substances which dissociated in water, the acids to give free hydrogen ions and the bases free hydroxide ions (Arrhenius, Ostwald, 1887). This definition does not express present views about aqueous solutions and cannot be applied to solutions in other solvents. Thermodynamics shows that free H+ ions cannot exist in appreciable concentration in water itself, and spectroscopy that the hydration of the proton is a strongly exothermic reaction. Accordingly the dissociation of an acid in water leads to often represented by H3O+ and referred to as the... 

The Arrhenius theory satisfactorily explains the behavior of many acids and bases. However, a substance such as ammonia, NH3, has basic properties but cannot be an Arrhenius base, because it contains no OH. The Brnnsted-Lowry theory explains this mystery and gives us a broader view of acid-base theory by considering the central role of the solvent in the dissociation process. 

The Lewis definitions of acids and bases provide for a more general view of acid-base reactions than either the Arrhenius or Br0nsted-Lowry picture. 

When Arrhenius formulated the classical views on acids and bases in 1887, his theory dealt exclusively with reactions in aqueous media [1]. He defined as acids all substances that upon reaction with water increase the concentration of hydro-nium ion in solution those that increase the hydroxyl ion (O H) concen-... 

The above description of acids and bases, in which H (aq) and OH (aq) ions are viewed as responsible for acidic and basic properties, respectively, and different acidic (or electrolytic) strengths are attributed to varying degrees of ionic dissociation, was developed by the Swedish chemist S. Arrhenius between 1880 and 1890. While very useful, this theory has some problems. The first problem has to do with the nature of the positive-charge carrier in aqueous solutions the second problem is that some substances can act as bases, even though they do not release OH (aq) ions. We will now consider both of these problems. 

Although the Arrhenius concept of acids and bases is useful, it is somewhat limited. For example, it tends to single out the OH ion as the source of base character, when other ions or molecules can play a similar role. In 1923, Johannes N. Brpnsted and Thomas M. Lowry independently noted that many reactions involve nothing more than the transfer of a proton (H ) between reactants, and they reaUzed that they could use this idea to expand the definitions of acids and bases to describe a large class of chemical reactions. In this view, acid—base reactions are proton-transfer reactions. 

The Swedish chemist Svante Arrhenius framed the first successful concept of acids and bases. He defined acids and bases in terms of the effect these substances have on water. According to Arrhenius, acids are substances that increase the concentration of H ion in aqueous solution, and bases increase the concentration of OH ion in aqueous solution. But many reactions that have characteristics of acid-base reactions in aqueous solution occur in other solvents or without a solvent. For example, hydrochloric acid reacts with aqueous ammonia, which in the Arrhenius view is a base because it increases the concentration of OH ion in aqueous solution. The reaction can be written... 

The view of acids and bases as we have been discussing them thus far is know as the Arrhenius theory, named after Svante Arrhenius, a Swedish scientist who introduced this theory in 1884. According to this theory, an acid is defined as a substance that releases hydrogen ions when dissolved in water. A base is defined as a substance that releases hydroxide ions when dissolved in water. All Arrhenius acids have the symbol for hydrogen first in the formula. See Tables 12.1 and 12.2 for examples. All Arrhenius bases have hydroxide ions in the formula. All bases listed in Table 12.3 are Arrhenius bases. 

The Arrhenius theory similarly attempts to define acids and bases as isolated species in solution giving rise, respectively, to hydrogen ions and hydroxide ions. While this view accounts for some properties of acids and bases and their reactions, it cannot begin to explain acidic and basic characteristics in non-aqueous media. Acidic and basic properties are, in fact, consequent upon interaction with the solvent and until such interaction has taken place the properties are not shown. 

Earlier definitions of acids and bases as species producing respectively hydrogen ions and hydroxyl ions are only valid in aqueous solution. The concept of Br nsted and Lowry, while extending the classical definition, does not exclude the treatment of Arrhenius for aqueous media. The extended view regards acids as proton donors and bases as proton acceptors regardless of whether substances concerned are ionic or neutral. A terse summary of this definition may be given as follows... 

Arrhenius and Ostwald played very important roles in the early studies on add-base catalysis, one century ago. Arrhenius contributed to the definition of acids and bases, and established the dependence between the rate constants and the temperature. Additionally, he also formulated an electrolytic theory of dissociation that ultimately led to him receiving the 1903 Nobel Prize in Chemistry. Ostwald proposed useful definitions of catalysis and classifications of catalysts, but he was unable to develop a satisfactory theory of these effects. This is not surprising, in view of the very limited knowledge of the mechanisms of catalysis at his time, and of the lack of understanding of how molecular properties can influence the rates of reactions. Nevertheless, his seminal work on catalysis was rewarded by him receiving the 1909 Nobel Prize in Chemistry. 

In the case of our initial and unsuccessful TLC attempts to enantioseparate the 5,/ -( )-ibuprofen and the 5,/ -( )-2-phenylpropionic acid antipodes [1-3], we kept our test samples for a longer period of time dissolved in 70% ethanol (and also in dichloromethane and physiological salt). Evidently, none of these solvents can be considered as a base or an acid, at least not in the spirit of the acid and base definitions introduced by Arrhenius. In other words, none of these solvents can catalyze or hamper transenantiomerization of the chiral APAs. However, the 70% ethanol solvent can easily be viewed as a weak ampholyte, able to simultaneously exert the catalytic and inhibiting effect on transenantiomerization of the chiral analytes considered. Perhaps, this perceptible ampholytic nature of 70% ethanol (combined with a change in viscosity of the APA solutions, as related to that of... 

The classic acid-base reaction between hydrogen ions and hydroxide ions to produce water can be viewed from the Arrhenius, Bronsted-Lowry, and Lewis definitions. Assign each reactant as an acid or a base under each definition and briefly justify your assignments. 

Ideas about adds and bases (or alkalis) date back to ancient times. The word acid is derived from the Latin acidus (sour). Alkali (base) comes from the Arabic al-qali, referring to the ashes of certain plants from which alkaline substances can be extracted. The acid-base concept is a major theme in the history of chemistry. In this section, we emphasize the view proposed by Svante Arrhenius in 1884 but also introduce a more modern theory proposed in 1923 by Thomas Lowry and by Johannes Bronsted. 

I remarked earlier that Arrhenius s vision was too limited because his view of acids, bases, and neutralization reactions depended on the presence of water. This restriction is removed in the proton transfer vision of neutralization reactions, as a proton can hop directly from an acid to a base without a solvent needing to be present. 

From a practical standpoint, acids can be identified by their sour taste, their ability to react with a variety of metals and carbonate minerals, and the effect they have on the colors of substances called acid-base indicators. Methyl red is an acid-base indicator that appears red in acidic environments and yellow otherwise (see Figure 5-10). From a chemist s point of view, however, an acid can be defined as a substance that provides hydrogen ions (H ) in aqueous solution. This definition was first proposed by Svante Arrhenius in 1884. 

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