Brpnsted Lowry Acids and Bases

Charged species such as OH and NH2 are used as salts, with cations such as LE, Na , or to balance the negative charge. These cations are called counterions or spectator ions, and their identity is usually incoiLsequential. For this reason, the counterion is often omitted. 

Compounds like H O and CH3OH that contain both hydrogen atoms and lone pairs may be either an acid or a base, depending on the particular reaction. These fundamental principles are true no matter how complex the compound. For example, the addictive pain reliever 

Problem 2.1 a. which compounds are Bronsted-Lowry acids HBr, NH3, CCi4  

Problem 2.2 Although morphine has several basic sites, only three are indicated in the structure in Section 2.1. Label all additional basic sites in morphine. 

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

In 1923, Brpnsted and Lowry defined acids and bases on the basis of the transfer of protons. A Brpnsted-Lowry acid is any species that can donate a proton, and a Brpnsted-Lowry base is any species that can accept a proton. These definitions also include all the Arrhenius acids and bases because compounds that dissociate to give H30+ are proton donors, and compounds that dissociate to give OH are proton acceptors. (Hydroxide ion accepts a proton to form H20.) 

In addition to Arrhenius acids and bases, the Brpnsted-Lowry definition includes bases that have no hydroxide ions, yet can accept protons. Consider the following examples of acids donating protons to bases. NaOH is a base under either the Arrhenius or Brpnsted-Lowry definition. The other three are Brpnsted-Lowry bases but not Arrhenius bases, because they have no hydroxide ions. 

When a base accepts a proton, it becomes an acid capable of returning that proton. When an acid donates its proton, it becomes a base capable of accepting that proton back. One of the most important principles of the Br0nsted-Lowry definition is this concept of conjugate acids and bases. For example, NH4 and NH3 are a conjugate acid-base pair. NH3 is the base when it accepts a proton, it is transformed into its conjugate acid, NH4. Many compounds (water, for instance) can react either as an acid or as a base. Here are some additional examples of conjugate acid-base pairs. 

The strength of a Brpnsted-Lowry acid is expressed as it is in the Arrhenius definition, by the extent of its ionization in water. The general reaction of an acid (HA) with water is the following  

Because they span such a wide range, acid-dissociation constants are often expressed on a logarithmic scale. The pKa of an acid is defined just like the pH of a solution as the negative logarithm (base 10) of Ka. 

Identify conjugate acid-base pairs for Brpnsted-Lowry acids and bases. 

In 1923, J. N. Br0nsted in Denmark and T. M. Lowry in Great Britain expanded the definition of acids and bases to inclnde bases that do not contain OH ions. A Br0nsted-Lowry acid can donate a hydrogen ion, H, to another substance, and a Br0nsted-Lowry base can accept a hydrogen ion. 

A Br0nsted-Lowry acid is a substance that donates H . 

A Br0nsted-Lowry base is a substance that accepts H.  

A free hydrogoi ion, H, does not actually exist in water. Its attraction to polar water molecules is so strong that the bonds to a water molecule and forms a hydronium ion, HsO . 

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Reactions (a) and (c) are reactions between Brpnsted-Lowry acids and bases the stronger acid and stronger base are identified. Reactions (b) and (d) occur between Lewis acids and bases. 

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 6) 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 Brpnsted-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 (Brpnsted-Lowry acids donate protons, while Lewis acids accept electron pairs). 

How to determine protonation modes in reaction centers of enzymes is a very important issue in biochemistry [1-8], The protonation is obviously related to the catalytic activities of active side chains of amino residues the protonation and deprotonation to side chains of charged acids such as Glu, Asp, Arg, Lys, and His yield Brpnsted-Lowry acids and bases, catalyzing various chemical reactions. Also in metalloenzymes and these model systems, protonations are often critical parts of the reaction mechanisms. For instance, in (1) the water-oxidizing center (WOC) in photosystem II [9-14], (2) the Mn dimeric center in catalase [15-21], and (3) the Mn center in Mn superoxide dismutases (MnSODs)[22,23], the following reactions proceed, respectively ... 

Define and identify Brpnsted-Lowry acids and bases and identify conjugate acid-base pairs. (Section 16.2)... 

Define and recognize the structures of simple Brpnsted-Lowry acids and bases as well as simple Lewis acids and bases. 

A t5q)ical general chemistry course introduces Lewis acids (electron pair acceptors) and Lewis bases (electron pair donors) as well as Brpnsted-Lowry acids and bases. Note that defining acids as electron pair acceptors and bases as electron pair donors bypasses the need for ionization in water and simply focuses on transfer of electrons from one species to another. Apart from acids and bases, electron transfer is the fundamental requirement for a chemical reaction. A Lewis base must be electron rich in order to donate electrons. Likewise, a Lewis add must be electron deficient. In this book, Lewis bases will be confined to molecules that contain oxygen, nitrogen, a halogen atom, and sometimes sulfur or phosphorus. The structures of each Lewis base will be discussed in the context of each chapter. There are, however, some simple guidelines for identifying Lewis acids. ... 

The distinction between amphoteric and amphiprotic will be clearer after reading Chapter 18. Lewis theory is a more general acid-base theory that includes all Brpnsted-Lowry acids and bases. 

If you took a good look at the information just presented, you might think that I made a couple of mistakes. Did you notice that I included water (HzO) and ammonia (NH3) as examples of both Brpnsted-Lowry acids and Brpnsted-Lowry bases This is no error. Water and ammonia are examples of amphoteric substances. An amphoteric substance is a substance that acts as an acid in some cases and as a base in other cases. Look at the following sets of reactions to see examples of these substances acting as both acids and bases. 

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

Reagents that are classified as bases can be used with both Lewis and Brpnsted-Lowry acids and one application is the elimination of alkyl halides (E2 reactions sec. 2.9.A) in the presence of a base. Elimination involves conversion of a saturated moiety containing a leaving group to a molecule with a multiple bond. An example of this latter transform is ... 

Johannes Nicolaus Brpnsted (1879-1947). Danish chemist. In addition to his theory of acids and bases, Brbnsted worked on thermodynamics and the separation of mercury isotopes. In some texts, Brpnsted adds and bases are called Brpnsted-Lowry adds and bases. Thomas Martin Lowry (1874-1936). Enghsh chemist. Brpnsted and Lowry developed essentially the same add-base theory independently in 1923. 

The fundamental difference between a Brpnsted-Lowry acid and a Lewis acid is that the former is a proton that accepts electrons and the latter is any other atom that accepts electrons. The fundamental difference between a Brpnsted-Lowry base and a Lewis base is that the former donates electrons to a proton and the latter donates electrons to another atom. 

Blood contains several substances that minimize changes in its acidity by reacting with either an acid or a base. One of these is the hydrogen phosphate ion, HP04. Write one equation showing this species acting as a Brpnsted—Lowry acid and another in which the species acts as a Brpnsted— Lowry base. 

Given the formula of a Brpnsted-Lowry acid and the formula of a Br0nsted-Lowry base, write the net ionic equation for the reaction between them. 

COAL 9 Given the formulas of a potential Brpnsted-Lowry acid and a Brpnsted-Lowry base, write the equation for the possible proton-transfer reaction between them. 

Identify the reactant that is a Brpnsted-Lowry acid and the reactant that is a Brpnsted-Lowry base in each of the following ... 

A more general theory of acids and bases was devised independently by Johannes Br0n sted (Denmark) and Thomas M Lowry (England) m 1923 In the Brpnsted-Lowry approach an acid is a proton donor, and a base is a proton acceptor The reaction that occurs between an acid and a base is proton transfer... 

The same G N Lewis who gave us electron dot formulas also suggested a way of think mg about acids and bases that is more general than the Brpnsted-Lowry approach Where Brpnsted and Lowry viewed acids and bases as donors and acceptors of protons (positively charged) Lewis took the opposite view and focused on electron pairs (negatively charged) According to Lewis an acid is an electron pair acceptor and a base is an electron pair donor... 

This chapter sets the stage for all of the others by reminding us that the relationship between structure and properties is what chemistry is all about It begins with a review of Lewis structures moves to a discussion of the Arrhenius Brpnsted-Lowry and Lewis pictures of acids and bases and the effects of structure on acidity and basicity... 

This IS a very useful relationship You should practice writing equations according to the Brpnsted-Lowry definitions of acids and bases and familiarize yourself with Table 1 7 which gives the s of various Br0n sted acids... 

This more general view of acids and bases is named the Bronsted-Lowry theory after the two scientists who proposed it, J. N. Brpnsted and T. M. Lowry. 

At the microscopic level, the Arrhenius theory defines acids as substances which, when dissolved in water, yield the hydronium ion (H30+) or H+(aq). Bases are defined as substances which, when dissolved in water, yield the hydroxide ion (OH). Acids and bases may be strong (as in strong electrolytes), dissociating completely in water, or weak (as in weak electrolytes), partially dissociating in water. (We will see the more useful Brpnsted-Lowry definitions of acids and bases in Chapter 15.) Strong acids include ... 

The hydrogen ion accepts the lone pair of electrons from the ammonia to form the ammonium ion. The hydrogen ion, because it accepts a pair of electrons, is the Lewis acid. The ammonia, because it donates a pair of electrons, is the Lewis base. This reaction is also a Brpnsted-Lowry acid-base reaction. This illustrates that a substance may be an acid or a base by more than one definition. All Brpnsted-Lowry acids are Lewis acids, and all Brpnsted-Lowry bases are Lewis bases. However, the reverse is not necessarily true. 

In this reaction, hydrochloric acid (HCl) donates a proton to water (H2O), making it the Brpnsted-Lowry acid. Water, which accepts the proton, is the Bronsted-Lowry base. This makes hydronium (H3O+) the conjugate acid and chloride (Cl ) the conjugate base. Water can act as the base in this reaction and as an acid in the example problem because it s composed of both a hydrogen ion and a hydroxide ion therefore, it can either accept or donate a proton. 

Brpnsted-Lowry acids are proton donors, and Br0nsted-Lowry bases are proton acceptors. An acid increases the concentration of H,0+ in aqueous solution, and a base increases the concentration of OH. An acid-base pair related through the gain or loss of a single proton is described as conjugate. When a proton is transferred... 

Section 1.17 The Lewis definitions of acids and bases provide for a more general view of acid-base reactions than either the Arrhenius or Brpnsted-Lowry picture. A Lewis acid is an electron-pair acceptor. A Lewis base is an electron-pair donor. The Lewis approach incorporates the Brpnsted-Lowry approach as a subcategory in which the atom that accepts the electron pair in the Lewis acid is a proton. 

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