Reactions of Bronsted-Lowry Acids and Bases

A Br0nsted-Lowry acid-base reaction results in transfer of a proton from an acid to a base. These acid-base reactions, also called proton transfer reactions, are fnndamental to the study of organic chemistry. 

Consider, for example, the reaction of the acid H-A with the base B. In an acid-base reaction, one bond i broken and one is form,  

Recall from Section 1.5 that a curved arrow shows the movement of an electron pair. The tail of the arrow always begins at an electron pair and the head points to where that electron pair moves.  

This movement of electrons in reactions can be illustrated using curved arrow notation. Because two electron pairs are involved in this reaction, two curved arrows are needed. Two products are formed. 

Keep in mind two other facts about this general reaction  

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The definition of Bronsted-Lowry acids and bases is based on the transfer of a proton (H ). An acid is defined as a proton donor, while a base is defined as a proton acceptor. As an example, consider the following acid-base reaction ... 

For each of the following reactions, identify the Bronsted-Lowry acids and bases ... 

Another significant difference between definitions is that Bronsted-Lowry acids and bases need not be molecular substances. There are a variety of reactions in which ions donate or accept protons. In the sample below, note how the cyanide ion (CN-) acts as a base by accepting a proton and the bicarbonate ion (HC03 ) acts as an acid by donating a proton ... 

Before continuing on to the last definition of acids and bases, it will be helpful to consider the definitions for strong and weak acids within the context of the Bronsted-Lowry model of acids and bases. The definitions are really an extension of the Arrhenius ideas. In the Arrhenius definitions, strong acids and bases were those that ionize completely. Most Bronsted-Lowry acids and bases do not completely ionize in solution, so the strengths are determined based on the degree of ionization in solution. For example, acetic acid, found in vinegar, is a weak acid that is only about 1 percent ionized in solution. That means that when acetic acid, HC2H302, is placed in water, the reaction looks like ... 

The Lewis concept deals with the behavior of electron pairs in chemical reactions. The same electron pairs we looked at when we discussed molecular geometry (see Chapter 7) can be involved in many reactions. Substances that can form a covalent bond by accepting an electron pair from another substance are known as Lewis acids. Substances that can form a covalent bond by donating an electron pair to another substance are known as Lewis bases. Be careful that you don t mix these up with the Bronsted-Lowry acids and bases. It is easy to do since the words donate and accept are used, except they are associated with the opposite species (Bronsted-Lowry acids donate protons, while Lewis acids accept electron pairs). 

The Bronsted-Lowry system allows us to describe all of these reactions as acid-base Objective 26 reactions. They are repeated below, with the Bronsted-Lowry acids and bases labeled. 

Butanoic acid, CFf3CFf2CFf2C02ff, is a monoprotic weak acid that is responsible for the smell of rancid butter. Write the formula for the conjugate base of this acid. Write the equation for the reaction between this acid and water, and indicate the Bronsted-Lowry acid and base for the forward reaction. (The acidic hydrogen atom is on the right side of the formula.)... 

Perhaps you will not be surprised, then, you to learn that an even more general model of acids and bases was proposed by American chemist G. N. Lewis (1875-1946). Recall that Lewis developed the electron-pair theory of chemical bonding and introduced Lewis structures to keep track of the electrons in atoms and molecules. He applied his electron-pair theory of chemical bonding to acid-base reactions. Lewis proposed that an acid is an ion or molecule with a vacant atomic orbital that can accept (share) an electron pair. A base is an ion or molecule with a lone electron pair that it can donate (share). According to the Lewis model, a Lewis acid is an electron-pair acceptor and a Lewis base is an electron-pair donor. Note that the Lewis model includes all the substances classified as Bronsted-Lowry acids and bases and many more. 

In 1923, Gilbert Newton Lewis defined an acid as an electron pair acceptor and a base as an electron pair donor. This definition is even more inclusive than the previous one because it includes all Bronsted-Lowry acids and bases as a subset and provides the foundation for the field of coordination chemistry. A coordination compound is the product of a Lewis acid-base reaction, such as the one shown in Equation (14.11) and Figure 14.5, in which the metal ion (Lewis acid) and ligand (Lewis base) are held together by a coordinate covalent bond. 

Bronsted-Lowry acids and bases donate or accept protons. A Lewis acid or base accepts or donates a pair of electrons. Key Terms Bronsted-Lowry acid Bronsted-Lowry base Bronsted-Lowry acid-base reaction 1 lie polyprotic acid diprotic acid triprotic acid Ul IvO Lewis base Lewis acid-base reaction... 

The extent of the reaction between a Bronsted-Lowry acid and base depends on the relative strengths of the acids and bases involved. 

Water as the solvent is essential for the acid-base setting reaction to occur. Indeed, as was shown in Chapter 2, our very understanding of the terms acid and base at least as established by the Bronsted-Lowry definition, requires that water be the medium of reaction. Water is needed so that the acids may dissociate, in principle to yield protons, thereby enabling the property of acidity to be manifested. The polarity of water enables the various metal ions to enter the liquid phase and thus react. The solubility and extent of hydration of the various species change as the reaction proceeds, and these changes contribute to the setting of the cement. 

With the renaissance in alkene chemistry engendered by the rising versatility of olefin metathesis in both fine chemical and commodity production, new methods for alkene isomerization are of increasing interest and importance. Alkene isomerization can be performed using Bronsted-Lowry acid or base catalysis (1). However, these reactions are limited to substrates which tolerate carbanionic or carbocation intermediates, and are susceptible to undesired side reactions. 

The Arrhenius theory accounts for the properties of many common acids and bases, but it has important limitations. For one thing, the Arrhenius theory is restricted to aqueous solutions for another, it doesn t account for the basicity of substances like ammonia (NH3) that don t contain OH groups. In 1923, a more general theory of acids and bases was proposed independently by the Danish chemist Johannes Bronsted and the English chemist Thomas Lowry. According to the Bronsted-Lowry theory, an acid is any substance (molecule or ion) that can transfer a proton (H + ion) to another substance, and a base is any substance that can accept a proton. In short, acids are proton donors, bases are proton acceptors, and acid-base reactions are proton-transfer reactions ... 

In the reverse reaction, H30+ acts as the proton donor (acid) and A- acts as the proton acceptor (base). Typical examples of Bronsted-Lowry acids include not only electrically neutral molecules, such as HC1, HN03, and HF, but also cations and anions of salts that contain transferable protons, such as NH4+, HS04 , and HC03-. 

According to the Arrhenius theory, acids (HA) are substances that dissociate in water to produce H + (aq). Bases (MOH) are substances that dissociate to yield OH aq). The more general Bransted-Lowry theory defines an acid as a proton donor, a base as a proton acceptor, and an acid-base reaction as a proton-transfer reaction. Examples of Bronsted-Lowry acids are HC1, NH4+, and HSO4- examples of Bronsted-Lowry bases are OH-, F-, and NH3. 

From this you can see that the cation from the salt comes from the base and the anion comes from the acid. Salts can act as Bronsted-Lowry acids or bases to produce solutions that are acidic or basic. The salts react with water in a reaction known as hydrolysis to yield either a conjugate acid and a hydroxide ion or a conjugate base and a hydrogen (hydronium) ion. If you know the origins of the components of a salt, you can make some predictions about the pH of the solution formed from a hydrolysis of a salt ion. 

A Bronsted-Lowry acid is any substance that is capable of donating a proton, whereas a Bronsted-Lowry base is any substance that is capable of accepting a proton. The loss of a proton by an acid gives rise to an entity that is a potential proton acceptor and thus a base it is called the conjugate base of the parent acid. Examples of acids reacting with bases are given in Table 1.16. The reactions listed in Table 1.16 are spontaneous in the direction that favors production of the weaker acid and base. Compounds that may act as bases and acids are referred to as amphoteric. 

The Br0nsted-Lowry definition of acids and bases does not replace the Arrhenius definition, but extends it. The Bronsted-Lowry definition of acids and bases requires you to take a closer look at the reactants and products of an acid-base reaction. In this case, acids and bases are not easily defined as having hydronium and hydroxide ions. Instead, you are asked to look and see which substance has lost a proton and which has gained the very same proton that was lost. 

Many organic compounds can act as weak Bronsted-Lowry acids or bases. Their reactions involve the transfer of H+ ions, or protons (Section 10-4). Like similar reactions of inorganic compounds, these acid-base reactions of organic acids and bases are usually fast and reversible. Consequently, we can discuss the acidic or basic properties of organic compounds in terms of equilibrium constants (Section 18-4). 

Because both dihydrogen phosphate and hydrogen carbonate (and other substances like them) can be either Bronsted-Lowry acids or bases, they cannot be described as a Bronsted-Lowry acid or base except with reference to a specific acid-base reaction. For this reason, the Arrhenius definitions of acids and bases are the ones used to categorize isolated substances on the stockroom shelf A substance generates either hydronium ions, hydroxide ions, or neither when added to water, so it is always either an acid, a base, or neutral in the Arrhenius sense. Hydrogen carbonate is an Arrhenius base because it yields hydroxide ions when added to water. Dihydrogen phosphate is an Arrhenius acid because it generates hydronium ions when added to water. 

A more general definition was suggested independently by Bronsted and Lowry . Proton-transfer reactions are considered as responsible for boijh the self-ionization of the amphoteric solvent molecules and for most acid-base reactions in their solutions. Acids and bases are defined as proton donors and proton acceptors respectively and acid-base reactions are regarded as being due to proton transfer reactions (protolysis). The most significant difference from the Arrhenius definition is that the proton itself is neither acid nor base. Even the solvent molecules can act either as acids or bases, a phenomenon which is responsible for the autoprotolysis of the pure liquid solvents. 

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