The Acid-Base Reaction

This chapter provides our first detailed examination of a chemical reaction, the acid-base reaction or transfer of a proton. Although acid—base reactions are simple, they are very important in oiganic chemistry because more complicated reactions often involve one or more proton transfer steps. In addition, an important purpose of this chapter is to introduce many concepts about reactions in general. Much of what we learn about the acid-base reaction is applied to other reactions in later chapters. 

There are several definitions of acids and bases. According to the Bronsted-Lowrv definition, an acid is a proton donor and a base is a proton acceptor. Any compound that has a hydrogen can potentially act as a Bronsted-Lowry acid (although the strength of the acid can vary enormously). Therefore, H—A is used as a general representation for an acid. To accept a proton, most bases 

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In this reaction, a proton is transferred from the acid to the base. The unshared pair of electrons on the base is used to form the new bond to the proton while the electrons of the H—A bond remain with A as an unshared pair. Previously, curved arrows have been used to show electron reorganization in resonance structures. Organic chemists also use these arrows to show electron movement in reactions. The arrows are a kind of bookkeeping device that helps us keep track of electrons as the Lewis structures of the reactants are converted to the Lewis structures of the products. Remember, an arrow always points from where the electrons are to where they are going. It does not point from where the hydrogen (or other atom) is to where it is going. 

Explain whether ammonia, NH3, can act as an acid, a base, or both. 

1 According to the Bronsted-Lowry definition, an acid is a proton donor and a base is a proton acceptor. Compounds that have both a hydrogen and an unshared pair of electrons can potentially react as either an acid or a base, depending on the reaction conditions. For example, water, has both unshared pairs of electrons and hydrogens. Therefore it can act like an acid or a base. 

3 A conjugate base is formed by removing a proton from the acid, leaving the electrons of the bond as an unshared pair on the base. The conjugate base has one unit less of positive charge than the acid. 

4 The reaction of an acid with a base is in equilibrium with the conjugate acid and the conjugate base. Remember, in an acid-base reaction the conjugate acid is the protonated form of the corresponding base and the conjugate base is the deprotonated form of the corresponding acid. 

5 According to the Lewis acid-base definition, an acid is an electron pair acceptor and a base is an electron pair donor. 

7 The acidity constant, K, is a measure of the acidic strength of a compound. The pKg is by definition -log Kg. As the strength of an acid increases, its Kg increases and its pKg decreases. For example, CjHj in (b) has a larger pKg than water. Therefore it is a weaker acid than water. 

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In Section 1 9 we introduced curved arrows as a tool to systematically generate resonance structures by moving electrons The mam use of curved arrows however is to show the bonding changes that take place in chemical reactions The acid-base reactions to be discussed in Sections 1 12-1 17 furnish numer ous examples of this and deserve some preliminary comment... 

The acid-base reactions that occur after the amide bond is broken make the overall hydrolysis irreversible m both cases The amine product is protonated m acid the car boxylic acid is deprotonated m base... 

Most of the reactions of ester enolates described so far have centered on stabilized eno lates derived from 1 3 dicarbonyl compounds such as diethyl malonate and ethyl ace toacetate Although the synthetic value of these and related stabilized enolates is clear chemists have long been interested m extending the usefulness of nonstabilized enolates derived from simple esters Consider the deprotonation of an ester as represented by the acid—base reaction... 

This type of cement has been further improved by the substitution of -hexyl van ill ate [84375-71-3] and similar esters of vanillic acid [121 -34-6] and/or syringic acid [530-57 ] for eugenol (93—95). These substituted cements are strong, resistant to dissolution, and, unlike ZOE and EBA cements, do not inhibit the polymerization of resin-base materials. Noneugenol cements based on the acid—base reaction of zinc and similar oxides with carboxyhc acids have been investigated, and several promising types have been developed based on dimer and trimer acids (82). 

In the amine regenerator, the rich amine solution is heated to reverse the acid-base reaction that takes place in the contactor. The heat is supplied by a steam reboiler. The hot, lean amine is pumped from the bottom of the regenerator and exchanges heat with the rich amine in the lean-rich exchanger and a cooler before returning to the contactor. 

Look closely at the acid-base reaction in Figure 2.5, and note how it is shown. Dimethyl ether, the Lewis base, donates an electron pair to a vacant valence orbital of the boron atom in BF3, a Lewis acid. The direction of electron-pair flow from the base to acid is shown using curved arrows, just as the direction of electron flow in going from one resonance structure to another was shown using curved arrows in Section 2.5. A cuived arrow always means that a pair of electrons moves from the atom at the tail of the arrow to the atom at the head of the arrow. We ll use this curved-arrow notation throughout the remainder of this text to indicate electron flow during reactions. 

The experimental setup for a titration is shown in Figure 4.7. The flask contains vinegar, a water solution of a weak organic add called acetic acid. A solution of sodium hydroxide of known concentration is added from a buret. The net ionic equation for the acid-base reaction that occurs is... 

For which of the following pairs would this be the correct prototype equation for the acid-base reaction in solution If it is not correct, write the proper equation for the acid-base reaction between the pair. 

The discussion of acid-base titrations in Chapter 4 focused on stoichiometry. Here, the emphasis is on the equilibrium principles that apply to the acid-base reactions involved. It is convenient to distinguish between titrations involving—... 

Treatment of a carboxylic acid with the strong base NaOH harms the sodium salt of the acid. With acetic acid, the acid-base reaction is... 

Scheme 5-14 may be called a two-dimensional system of reactions, in contrast to Scheme 5-1 which consists of a one-dimensional sequence of two acid-base equilibria. In Scheme 5-14 the (Z/E) configurational isomerism is added to the acid-base reactions as a second dimension . The real situation, however, is yet more complex, as the TV-nitrosoamines may be involved as constitutional isomers of the diazohydroxide. In order not to make Scheme 5-14 too complex the nitrosoamines are not included, but are shown instead in Scheme 5-15. The latter also includes the addition reactions of the (Z)- and ( )-diazoates (5.4 and 5.5) to the diazonium ion to form the (Z,Z)-, (Z,E)- and (2 2i)-diazoanhydrides (5.6, 5.7 and 5.8) as well as proto-de-nitrosation reactions (steps 10, 11 and 12). This pathway corresponds to the reverse reaction of diazotization, as amine and nitrosating reagent (nitrosyl ion) are formed in this reaction sequence.

First, the simple thermodynamic description of pe (or Eh) and pH are both most directly applicable to the liquid aqueous phase. Redox reactions can and do occur in the gas phase, but the rates of such processes are described by chemical kinetics and not by equilibrium concepts of thermodynamics. For example, the acid-base reaction... 

C06-0067. When 10.00 mL of a solution of a strong acid is mixed with 100.0 mL of a solution of a weak base in a coffee-cup calorimeter, the temperature falls from 24.6 °C to 22.7 °C. Determine q for the acid -base reaction, assuming that the liquids have densities of 1.00 g/mL and the same heat capacity as pure water. 

In this chapter the nature of the cementitious bond and the acid-base reaction will be discussed. 

The cement-forming reaction is a special case of an acid-base reaction so that concepts of acid, base and salt are central to the topic. In AB cement theory, we are concerned with the nature of the acid-base reaction and how the acidity and basicity of the reactants are affected by their constitution. Thus, it is appropriate at this stage to discuss the various definitions and theories available. 

The setting reaction for the great majority of acid-base cements takes place in water. (The exceptions based on o-phenols are described in Chapter 9.) This reaction does not usually proceed with formation of a precipitate but rather yields a substance which entrains all of the water used to prepare the original cement paste. Water thus acts as both solvent and component in the formation of these cements. It is also one of the reaction products, being formed in the acid-base reaction as the cements set. 

It may be, because of the slowness of the acid-base reaction in resin glass polyalkenoate cements, that free poly(acrylic acid) is available for a longer period than in conventional glass polyalkenoate cements, for the... 

C) The Bronsted-Lowry or proton theory interprets the acid-base reaction as a mere proton exchange between the acid (proton donor) and the base (proton acceptor) however, the Lewis theory or electron theory interprets the reaction as a donation and acceptance of a lone pair of electrons, where the... 

In an early work by Mertz and Pettitt, an open system was devised, in which an extended variable, representing the extent of protonation, was used to couple the system to a chemical potential reservoir [67], This method was demonstrated in the simulation of the acid-base reaction of acetic acid with water [67], Recently, PHMD methods based on continuous protonation states have been developed, in which a set of continuous titration coordinates, A, bound between 0 and 1, is propagated simultaneously with the conformational degrees of freedom in explicit or continuum solvent MD simulations. In the acidostat method developed by Borjesson and Hiinenberger for explicit solvent simulations [13], A. is relaxed towards the equilibrium value via a first-order coupling scheme in analogy to Berendsen s thermostat [10]. However, the theoretical basis for the equilibrium condition used in the derivation seems unclear [3], A test using the pKa calculation for several small amines did not yield HH titration behavior [13],... 

As we have seen earlier in the case of proton transfer reactions such as occurs between HCl(g) and NH3(g), water is not necessary for the acid-base reaction to take place. This is also true of the reactions between the acidic oxides of nonmetals and the basic oxides of metals. In many cases, they react directly as illustrated in the following equations ... 

The acid-base reactions are (a) and (k) only, in which an acid reacts with a base to give a salt and water. In all acid-base reactions, no oxidation or reduction is involved. 

Perform the limiting reactant testing for the acid-base reaction. 

The mixture fraction as defined above describes turbulent mixing in the reactor and does not depend on the chemistry. However, by comparing Eqs. (45) and (46), we can note that they have exactly the same form. Thus, for the acid-base reaction, the mixture fraction is related to rA—B by... 

The acid-base reaction is a simple example of using the mixture fraction to express the reactant concentrations in the limit where the chemistry is much faster than the mixing time scales. This idea can be easily generalized to the case of multiple fast reactions, which is known as the equilibrium-chemistry limit. If we denote the vector of reactant concentrations by and assume that it obeys a transport equation of the form... 

The microscopic transport equations for the reaction-progress variables can be found from the chemical species transport equations by generalizing the procedure used above for the acid-base reactions (Fox, 2003). If we assume that Fa Fb Fc, then the transport equations are given by... 

As with the acid-base reaction, Eq. (68) implies that A and B cannot coexist at any point in the flow. Using this infinite-rate approximation, we need only solve transport equations for and Y2, where the source term for Y2 is now... 

The relative stabilities of the dioxides, sesquioxides and monoxides for first period transition metals are given in Figure 7.11(c). The stability of the higher oxidation state oxides decreases across the period. As we will discuss later, higher oxidation states can be stabilized in a ternary oxide if the second metal is a basic oxide like an alkaline earth metal. The lines in Figure 7.11(c) can in such cases be used to estimate enthalpies of formation for unstable oxidation states in order to determine the enthalpy stabilization in the acid-base reactions see below. Finally, it should be noted that the relative stability of the oxides in the higher oxidation states increases from the 3d via 4d to the 5d elements, as illustrated for the Cr, Mo and W oxides in Figure 7.11(d). 

The chromophore in hydrangeas is delphinidin (X), which is a member of the anthrocyanidin class of compounds. Compound X reminds us of phenol (VII), indicating that delphinidin is also a weak acid. In fact, all pH indicators are weak acids or weak bases, and the ability to change colour is a visible manifestation of the indicator s ability to undergo reversible changes in structure. In the laboratory, only a tiny amount of the pH indicator is added to the titration solution, so it is really just a probe of the solution pH. It does not participate in the acid-base reaction, except insofar as its own structure changes with the solution pH. 

Determine which species take part in the acid-base reaction. The bicarbonate anion (HC03) is basic, and the H+ from the hydrobromic acid is acidic. 

The detection of proton is based on the acid-base reaction of a chro-mophore or fluorophore that are weak acid or base. 

An acid in the Br0nsted-Lowery theory is an H+ donor and a base is an H+ acceptor. In the Brpnsted-Lowery acid-base theory, there is a competition for an H+. Consider the acid-base reaction between acetic acid, a weak acid, and ammonia, a weak base ... 

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