Taylor & Frauds Group
`
`CHEMISTRY
`An Acid—Base Approach
`
`CRC Press
`
`SYNGENTA EXHIBIT 1013
`
`Syngenta v. FMC, PGR2020-00028
`
`SYNGENTA EXHIBIT 1013
`Syngenta v. FMC, PGR2020-00028
`
`

`

`CRC Press
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`International Standard Book Number: 978-1-4200-7920-3 (Hardback)
`
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`Library of Congress Cataloging-in-Publication Data
`
`Smith, Michael B., 1946 Oct. 17-
`Organic chemistry: an acid-base approach / Michael B. Smith.
`p.cm.
`Includes bibliographical references and index.
`ISBN 978-1-4200-7920-3 (hardcover: all. paper)
`1. Chemistry, Organic--Textbooks. 2. Organic acids--Textbooks. 3. Chemical reactions--Textbooks.
`I. Title.
`
`QD251.3.S654 2011
`547--dc22
`
`Visit the Taylor & Francis Web site at
`http://www.taylorandfrancis.com
`
`and the CRC Press Web site at
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`
`2010026467
`
`Prefac
`Ackno
`The Au
`
`Chapti
`1.1
`1.2
`
`Chapt1
`Themt
`2.1
`2.2
`2.3
`
`2.4
`2.t
`2.t
`
`2.'i
`
`Chapt
`3.~
`3.~
`3.~
`3.L
`3.i
`3.l
`
`3.'.
`3.,
`
`Chap1
`Intro<
`4.
`
`4 ..
`4 ..
`4.
`4.
`4.
`
`4.
`4.
`
`Chap·
`5.
`5.
`5.
`5.
`5.
`
`

`

`·why Is an
`Acid-Base Thetne
`ltnportant?
`
`The operational theme of this book is that under(cid:173)
`standing acid and base chemistry will lead to
`a better understanding of organic chemistry.
`Applying acid-base principles to many reactions
`allows one to understand and predict the reac(cid:173)
`tion rather than simply memorizing it. To exam(cid:173)
`ine this premise, the acid-base reactions found in
`general chemistry must be reexamined to deter(cid:173)
`mine how they can be applied directly or when
`they must be modified. In most general chemistry
`books, an acid-base reaction is defined as one in
`which a proton is transferred from an acid to a
`base, and a Brr;msted-Lowry acid is a proton donor
`(HB), whereas a Br¢nsted-Lowry base is a proton
`acceptor (A-). The species formed when a proton is
`removed from an acid is the conjugate base (B-),
`and the species formed when a proton is added to
`a base is the conjugate acid, HA:
`,-------------------------------------------------------,
`s- (aq) + HA(aql
`; HB(aq) +
`A(aql ____
`;
`'
`'
`' --------------------------------------------------------
`To begin this chapter, you should know the fol(cid:173)
`lowing from a general chemistry course:
`
`----
`
`• Define and recognize .the. structures
`of simple Br~nsted-Lowry acids and
`bases as well as simple Lewis acids
`and bases.
`• Understand the definitions of conju(cid:173)
`gate acid and conjugate base.
`• Understand the fundamentals of acid(cid:173)
`base strength in aqueous media.
`
`

`

`20
`
`Organic Chemistry: An Acid-Base Approach
`
`Why Is an A(
`
`• Understand the role of water in acid-base equilibria.
`• Understand Ka and pKa and how to manipulate them.
`• Recognize classical mineral acids.
`• Recognize classical Br~nsted-Lowry bases.
`
`This chapter will review the principles of acid-base chemistry from general
`chemistry in order to make a link with modern organic chemistry. The chapter
`will introduce the theme of acid-base chemistry as a basis for understanding
`each chemical transformation where it is appropriate.
`When you have completed this chapter, you should understand the follow(cid:173)
`ing points:
`
`• There is an inverse relationship between Ka and pKa, and a large
`Ka or a small pKa is associated with a stronger acid.
`• It is important to put water back into the acid-base reaction for
`aqueous media.
`• The fundamental difference between a Br~nsted-Lowry acid
`and a Lewis acid is that the former is a proton that accepts elec(cid:173)
`trons and the latter is any other atom that accepts electrons.
`The fundamental difference between a Br~nsted-Lowry base
`and a Lewis base is that the former donates electrons to a pro(cid:173)
`ton and the latter donates electrons to another atom.
`• Curved arrows are used to indicate electron flow from a source
`of high electron density to a point of low electron density.
`• Acid· strength in an acid X-H is largely determined by the sta(cid:173)
`bility of the conjugate acid and base and the strength of the
`X-H bond. For example, HI is a stronger acid than HF due to
`a weaker X-H bond and charge dispersal in the larger anionic
`conjugate base. An acid such as perchloric acid is stronger than
`sulfuric acid due to charge dispersal in the conjugate base that
`results from resonance.
`• Base streng.th is. largely determined by the stability of the con(cid:173)
`jugate acid and base and the electron-donating ability of the
`basic atom. Water is a stronger acid than ammonia because
`hydroxide ion is more stable than the amide anion and the
`OH bond is weaker than the NH bond. Ammonia is a stron(cid:173)
`ger base than water because oxygen is more electronegative
`than nitrogen, and the ammonium ion is more stable than the
`hydronium ion.
`• Reaction of a Lewis acid and a Lewis base leads to an ate com(cid:173)
`plex as the product.
`• Acid-base reactions have biological relevance.
`
`In 1884, SE
`rial that ca
`(HCI) is dii
`to hydratec
`definition, :
`hydroxide i1
`sodium ion,
`of acids an<
`(England;
`194 7). Accoi
`and a base 1
`nitions men
`when an ac
`must have,
`take the for
`N, etc.).
`
`Write,
`sulfuri
`
`Accordir
`a proton tra
`
`Water is
`istry are al'
`placed in wi
`a conjugate
`(the chlorid,
`concentratic
`hydrogenio
`If the pH is
`
`If the c1
`mineral aci
`~ikewise, si
`include Na(
`and Ba(OH
`bases will ic
`Indeed, Wea
`
`

`

`-Base Approach
`
`Why Is an Acid-Base Theme Important?
`
`21
`
`·y from general
`ry. The chapter
`understanding
`
`and the follow-
`
`and a large
`
`reaction for
`
`Lowry acid
`.ccepts elec(cid:173)
`s electrons.
`Lowry base
`ns to a pro-
`1.
`om a source
`:msity.
`l by the sta(cid:173)
`:ngth of the
`1 HF due to
`·ger anionic
`rongerthan
`tte base that
`
`y of the con(cid:173)
`bility of the
`nia because
`ion and the
`t is a stron(cid:173)
`~tronegative
`ble than the
`
`an ate com-
`
`In 1884, Santa Arrhenius (Sweden; 1859-1927) defined an acid as a mate(cid:173)
`rial that can release a proton or hydrogen ion (H+). When hydrogen chloride
`(HCl) is dissolved in water, a solution is formed via ionization that leads
`to hydrated hydrogen ions and hydrated chloride ions. Using the original
`definition, a base (then called an alkali) is a material that can donate a
`hydroxide ion (-OH). Sodium hydroxide in water solution ionizes to hydrated
`sodium ions and hydrated hydroxide ions. This concept led to a definition
`of acids and bases as reported independently in 1923 by Thomas M. Lowry
`(England; 1874-1936) and Johannes Nicolas Bronsted (Denmark; 1879-
`1947). According to this definition, an acid is a material that donates a proton
`and a base is a material that can accept a proton1•2-the Br¢nsted-Lowry defi(cid:173)
`nitions mentioned before. If a high concentration of hydrogen ions is observed
`when an acid is added to water (an aqueous solution of the acid), that acid
`must have an ionizable hydrogen (known as a proton). Such acids typically
`take the form H-X, where Xis an atom other than carbon or hydrogen (0, S,
`N, etc.).
`
`2.1 Write out the structmres of hydrochloric acid, hydrobromfo acid,
`sulfuric acid, and nitric acid,
`
`According to Bronsted and Lowry, an acid-base reaction is defined in terms of
`a proton transfer. By this definition, the reaction of HCl in water is the following:
`
`---------------------------------------------------------,
`'
`+ HP(aq) !
`; HCl(aq) + H2O
`ff(aq)
`
`Water is the base in this reaction. Acid and base reactions in general chem(cid:173)
`istry are always done in water. When a Bronsted-Lowry acid such as HCl is
`placed in water, a proton is transferred to a water molecule (water is the base);
`a conjugate acid is formed (the hydronium ion H 3O+) as well as a conjugate base
`(the chloride ion). In neutral pure water (no acid is present), the hydrogen ion
`concentration is about 1.0 x 10-7 M (pH of 7). An increase in the concentration of
`hydrogen ions above 1.0 x 10-7 M gives an acidic solution, with a pH less than 7.
`If the pH is greater than 7, it is considered a basic solution.
`2.2 What is the conjugate base fo:rmed when HCl reacts with ammonia?
`
`If the criterion for acid strength is the extent of ionization in water, the
`mineral acids HCl, HBr, HI, H 2SO4 , HNO3 , and HC1O4 are all strong acids.
`Likewise, strong bases are extensively ionized in water; common strong bases
`include NaOH (soda lye), KOH (potash lye), LiOH, CsOH, Mg(OH)2 , Ca(OH)2 ,
`and Ba(OH)2 • Using the definitions given previously, weak acids and weak
`bases will ionize to a lesser extent in water relative to the strong acids or bases.
`Indeed, weak acids are solutes that react reversibly with water to form H 3O+ ions.
`
`

`

`22
`
`Organic Chemistry: An Acid-Base Approach
`
`There are two categories: (1) molecules containing an ionizable hydrogen atom
`such as nitrous acid (HN02), and (2) cations such as the ammonium ion (NH4 +).
`Weak bases are defined as solutes that react with water molecules to acquire
`a H+ ion and leave an hydroxide ion (HO-) behind. Once again, all definitions
`relate to ionization in water.
`An example of a weak base is the reaction of ammonia plus water to give
`the ammonium cation and hydroxide anion. (Amines will be discussed in
`Chapter 5, Section 5.6.2.) It is known that they are weak bases. Examples of
`weak acids that will be discussed in this book are acetic acid (CH3COOH),
`boric acid (H3B03), butanoic acid (CH3CH2CH2COOH), formic acid (HCOOH),
`hydrocyanic acid (HCN), uric acid (HC5H 3N40 3), and the fatty acid known as
`stearic acid ([CH2(CH2)15CH2COOH, an 18-carbon acid]). See Chapter 5 (Section
`5.9.3) for a definition of carboxylic acids such as these and the structures of
`these acids. Weak bases include ammonia (NH3) and amines such as meth(cid:173)
`ylamine (CH3NH2), triethylamine [(CH3) 3N], and pyridine (C 5H 5N) (discussed
`in Section 5.6.2 of Chapter 5). Once again, the relative strength of these acids
`and bases is defined by their reaction with water.
`
`2.3 In the reaction of nitric acid and KOH, which atom in nitric acid
`accepts the electron pair from the base, and which atom in KOH
`donates the electrons to that proton?
`
`2.4 If the ammonium ion is an acid that reacts with a suitable base,
`what is the conjugate base of this reaction?
`
`Several of the weak acids and the weak bases are carbon based (carbon
`atoms in the structure), although the structures may not yet be familiar. It is
`reasonable to assume that such acids and bases will play a prominent role in
`the chemistry to be discussed throughout this book. Two main points are to be
`made. First, many organic acids and bases are insoluble in water, so the clas(cid:173)
`sical Br¢nsted-Lowry definition does not formally apply. Second, a very weak
`acid can react with a particularly strong base, and a very weak base may react
`with a particularly strong acid. If an organic molecule is a very weak base,
`for example, an acid-base reaction occurs only if that base is treated with a
`very strong acid. The acid-base pair must be defined in order to evaluate the
`reaction. An acid-base equilibrium in general chemistry usually deals with
`strong acid-strong base, weak acid-strong base, or strong acid-weak base
`interactions.
`
`General chemistry defines acids and bases in terms of their ionization in water.
`The chemistry discussed in this book will show that many organic compounds
`are insoluble in water. Although the acetic acid and ethanol discussed in the
`
`Why Is an Aci1
`
`previous sect
`are complete]
`than water iE
`solvent is ty:r:
`Diethyl e
`dichlorometb
`tion in organ
`interacts wit
`Because mai
`a mixture of
`organic solvE
`The issue of
`The examplE
`
`Water re
`also reacts \
`acid than Fl
`anion, 2, ar
`than HCl, t
`when comp~
`indicates sc
`used for th£
`and HCOO:
`anion (HCC
`chloride ion
`Just as
`(CH3CH20I
`is the acid•
`acid, so the
`paring thn
`established
`the concept
`or base is iI
`solvent fror
`ConsidE
`than water
`rather thar
`
`

`

`Base Approach
`
`Why Is an Acid-Base Theme Important?
`
`23
`
`1ydrogen atom
`um ion (NH4+).
`1les to acquire
`all definitions
`
`; water to give
`~ discussed in
`s. Examples of
`:l (CH3COOH),
`tcid (HCOOH),
`acid known as
`1pter 5 (Section
`e structures of
`such as meth(cid:173)
`[5N) (discussed
`1 of these acids
`
`in nitric acid
`atom in KOH
`
`suitable base,
`
`1 based (carbon
`,e familiar. It is
`·ominent role in
`points are to be
`iter, so the clas(cid:173)
`rid, a very weak
`: base may react
`rery weak base,
`, treated with a
`· to evaluate the
`ially deals with
`acid-weak base
`
`1ization in water.
`ranic compounds
`discussed in the
`
`previous section are soluble in water, many other weak acids or very weak acids
`are completely insoluble, as are many bases or very weak bases. A solvent other
`than water is required to put such acids or bases into solution, and this "other"
`solvent is typically another organic compound.
`Diethyl ether (CH3CH20CH2CH3), hexane (CH3CH2CH2CH2CH2CH3), and
`dichloromethane (CH2Cl2) are common solvents. A study of an acid-base reac(cid:173)
`tion in organic solvents must begin with understanding how the organic solvent
`interacts with the acid, the base, the conjugate acid, and the conjugate base.
`Because many of the strong mineral acids are insoluble in organic solvents,
`a mixture of water and an organic solvent is used. In other words, a water(cid:173)
`organic solvent mixture is required to make the acid-base components soluble.
`The issue of solubility may force one to choose a different type of acid or base.
`The examples that follow illustrate this problem.
`
`2,5 What are the common mineral adds?
`
`HCl(aq) + H2O - - - - - -
`
`HP1aq) + Ci-(aq)
`
`0
`II
`H --- C ,
`1
`
`__.
`,,, H + H20 ...._ __ _
`(aq)
`
`0
`
`0
`II
`H ---C'
`2 o- (aq)
`
`+ HP1aq)
`
`Water reacts with HCl, and the organic acid known as formic acid (HCOOH, 1)
`also reacts with water as a weak acid, as shown. Formic acid is a much weaker
`acid than HCl. When 1 reacts with water, the conjugate base is the formate
`anion, 2, and the conjugate acid is the hydronium ion. If 1 is a weaker acid
`than HCl, the equilibrium for 1 + H 20 lies to the left in the reaction shown
`when compared to the reaction of HCl + H 20 in Section 2.1. Note the (aq) term
`indicates solvation by the solvent water. Note also that the term "reaction" is
`used for the acid-base equilibrium. The acid-base equilibria shown for HCl
`and HCOOH are chemical reactions that generate two products: the formate
`anion (HCOO-, 2) and the hydronium ion (from 1) or the hydronium ion and the
`chloride ion (from HCl).
`Just as water reacts with HCOOH or with HCl, water reacts with ethanol
`(CH3CH20H). As with HCl and with HCOOH, water is the base and ethanol
`is the acid in this reaction. Ethanol is known to be much weaker than formic
`acid, so the equilibrium for this reaction lies almost entirely to the left. By com(cid:173)
`paring three different acids for ionization in water, an order of acid strength is
`established: HCl > HCOOH >ethanol.This analysis is more or less identical to
`the concepts used in general chemistry, but with different acids. If an acid and/
`or base is insoluble in water, how is this analysis applicable? Simply change the
`solvent from water to an organic solvent!
`Consider the reaction of formic acid using diethyl ether (3) as a solvent rather
`than water. Note the (ether) term in the reaction, indicating solvation by ether
`rather than by water. The organic solvent diethyl ether (3) is also the base, just
`
`

`

`24
`
`Organic Chemistry: An Acid-Base Approach
`
`Why Is an Aci
`
`as water is the base in reactions with HCl or HCOOH (1). The basic atom in 3
`is the oxygen, and it reacts with the weak acid (formic acid) to give the formate
`anion in 4 (the conjugated base) and the protonated form of ether (5, an oxonium
`ion), which is the conjugate acid. The equilibrium lies to the left in the reaction
`because that formic acid is a weak acid in ether; however, the definition of strong
`or weak acid relating to dissociation in water no longer applies. It is the same
`reaction, in principle, but the solvent is different and that influences the position
`of the equilibrium. In fact, the equilibrium for formic acid lies further to the left in
`ether than in water, so formic acid is a weaker acid in ether than in water. This is
`an important statement because it suggests that the strength of the acid depends
`on the base with which it reacts, which is critical for establishing whether the
`equilibrium lies to the left or to the right.
`If HCl is placed in anhydrous (no water) methanol (6), where methanol is
`the solvent, the oxygen of the methanol is the only available base for reaction
`with the acid HCl. The oxygen of methanol reacts with HCl to form an oxonium
`ion 7 (the conjugate acid), with chloride ion as the counter-ion (the conjugate
`base). The term (ether) is used for solvation in the formic acid reaction to show
`the analogy to the water reaction, but the (methanol) designation is omitted
`for 6. Although it is understood that all components are solvated by the solvent
`methanol, the salvation term is usually omitted in organic chemistry and it will
`be omitted throughout this book. The important point of this reaction is the same
`as that for the formic acid reaction in ether. The definition for ionization
`in water does not apply, but ionization is still the key to the acid-base
`reaction regardless of solvent. This leads to the idea that an acid reacts with
`a base, regardless of solvent, but the extent of ionization changes with the solvent
`and with the base. In fact, the degree of ionization changes as the acid and the
`base are changed and also as the solvent is changed.
`
`2.6 In the reaction of 5 with water, indicate which Hin 5 is the add
`and which atom in water is the base.
`
`2.7 Which is the stronger base, water or hydroxide ion? Justify your
`answer.
`
`However,
`
`where th(
`Where is
`try, the wate
`when water
`also remov,
`solvent and 1
`be put back i
`formic acid r
`use the Ka ec
`
`This ana
`base as used
`ionization eq
`not used for
`change the fc
`with bases in
`that will be i
`to remembe1
`based on the
`is
`chlorid
`
`ther
`acid?
`
`ABr0nsted-
`18 a proton E
`which is an 1
`tions are rel:
`lie with rem<
`HCl reacts ~
`
`HCOOH(ether) + ~o~ __. Hcoo-(ether) +
`
`and,
`
`3
`
`4
`
`---
`-
`
`~6~
`(ether)
`H
`
`1
`
`5
`
`Why are these observations important? Remember from general chem(cid:173)
`istry that the equilibrium constant, Ka (acidity constant), is given by the follow(cid:173)
`ing equation when the reaction is done in water:
`,-------------------
`'
`
`and for HCI in water
`
`

`

`Base Approach
`
`Why Is an Acid-Base Theme Important?
`
`25
`
`However, the actual equation for the equilibrium constant is
`- '
`----------------
`---
`,--
`: K =
`[conjugate acid]
`[conjugate base]
`:
`[acid]
`[base]
`: so,
`:
`a
`,_ - -- --- - - - - - -- - - -- - - - - - - - - - - -- -- - - - - - - - -'
`
`[Ci-]
`[Hp+]
`[base)
`[HCI]
`
`,
`, K =
`:
`a
`
`where the base in the HCl reaction is actually the water from the solvent.
`Where is the term for water in the first set of equations? In general chemis(cid:173)
`try, the water is removed from the equation to simplify calculations. However,
`when water is removed from the equilibrium constant equation, this act
`also removes the base from the acid-base equation. If water is not the
`solvent and there is a different base in the reaction, the base certainly must
`be put back in the equation. In other words, the equilibrium constants for the
`formic acid reaction with ether as well as the methanol and HCl reaction must
`use the Ka equations:
`
`1sic atom in 3
`re the formate
`5, an oxonium
`n the reaction
`\ition of strong
`It is the same
`es the position
`'.er to the left in
`i water. This is
`e acid depends
`g whether the
`
`re methanol is
`se for reaction
`·m an oxonium
`(the conjugate
`,action to show
`;ion is omitted
`! by the solvent
`r:;try and it will
`ion is the same
`br ionization
`~he acid-base
`icid reacts with
`:vith the solvent
`ie acid and the
`
`,_ 5 is the acid
`
`' Justify your
`
`+ ~ 7 (ether)
`
`H 5
`
`1 general chem(cid:173)
`m by the follow-
`
`:
`:
`i Ka =
`
`,,,.___ + ,,,.___
`/
`-o,
`"-.
`[ H
`[Hcoo-1
`[HCOOHJ [,,/"o~ J :
`
`l
`
`i and
`
`:
`
`I
`
`;-- -- -- - - -- -- - -- - --- - -- - -,
`[CHpH 2J :
`•
`[Ci-J
`i Ka ~ - - - - - '
`[HCI)
`:
`[CH 3OHJ
`
`This analysis relies on the Bn,nsted-Lowry definitions of an acid and a
`base as used in general chemistry, as well as an extension of the fundamental
`ionization equations to solvents other than water. What is the point? Water is
`not used for many reactions found in organic chemistry, and it is important to
`change the focus from acids that ionize in water to the concept that acids react
`with bases in order to ionize. This concept is the basis for many of the reactions
`that will be introduced in succeeding chapters. For the moment, it is important
`to remember that both the acid and the base must be identified in a reaction
`based on their reactivity.
`2,8 What is the base in the reaction that generates hydronium km and
`chloride ion from HCl?
`
`2.9 In the reaction of methanol with HCl, which is the Br~nsted-Lowry
`acid?
`
`A Br0nsted-Lowry acid is defined as a proton donor, and a Br0nsted-Lowry base
`is a proton acceptor. A Lewis acid accepts an electron pair from a Lewis base,
`which is an electron pair donor. There may be confusion about how these defini(cid:173)
`tions are related rather than about just how they differ. Part of the problem may
`lie with removing the base (water) from acid-base reactions done in water. When
`HCl reacts with water, the Br0nsted-Lowry definition states that the proton (H+)
`
`

`

`26
`
`Organic Chemistry: An Acid-Base Approach
`
`is "donated" to water, forming the hydronium ion. In this reaction, the oxygen
`atom of the water "accepts" the proton. How does an oxygen accept a proton?
`
`2.10 If ammonia were to react with HCl rather than water, which atom
`accepts the proton?
`
`What happens when water accepts a proton to form the hydronium ion?
`The answer is that a new a-covalent bond (see Chapter 3) is formed between
`the hydrogen atom and the oxygen atom, represented as O-H. This reaction
`is shown using a Lewis dot formula for water, and one of the unshared elec(cid:173)
`tron pairs on oxygen reacts with H+ to form the new bond to the proton. This
`reaction uses a blue curved arrow, which indicates that oxygen donates two
`electrons to H+ to form a new O-H bond. The color blue will be used to indicate
`an electron-rich species or with curved arrows to indicate electron donation.
`The curved arrow formalism is used in chemical reactions to indicate trans(cid:173)
`fer of electron density in order to form a new chemical bond. A proton has no
`electron density associated with it, so it accepts the electron density from the
`electron-rich oxygen atom. In other words, oxygen is the base and the proton
`is the acid. The resulting hydronium ion has three bonds to oxygen, making it
`electron deficient; it takes on a positive charge as shown (see formal change in
`Chapter 5, Section 5.5).
`
`2.11 Draw the Lewis dot formula for ammonia and then for HCL
`
`H ··o··~
`
`H+
`
`• •• •
`H
`
`H H
`
`:·o·: ..
`
`H +
`
`hydronium ion
`
`Further in this reaction, one electron pair on oxygen is donated to the
`electron-deficient proton to form a new O-H bond. Therefore, the oxygen atom
`"accepts" a proton by donating two electrons to form the new bond. Remember
`that the Lewis definition of a base is an electron donor and a Lewis base
`donates electrons to an atom other than hydrogen. However, in this case, the
`base donated two electrons to a proton. At this point, the definitions merge. A
`Br0nsted-Lowry base accepts a proton by donating two electrons to that pro(cid:173)
`ton. Why two electrons? Chapter 3 will discuss the fact that a covalent bond
`requires two electrons. Therefore, bases may be defined as two-electron donors
`to an electron-deficient center. If the base donates electrons to a proton, it is
`a Br0nsted-Lowry base. If the base donates electrons to an atom other than
`hydrogen, it is formally a Lewis base. The key concept is that a base is a two(cid:173)
`electron donor.
`To donate two electrons, a base must have an excess of electron density.
`Therefore, it makes sense that molecules containing oxygen or nitrogen react
`as bases. The oxygen atom of hydroxide and the nitrogen atom of ammonia
`are examples. The more easily an atom is able to donate electrons, the more
`basic that atom should be, ignoring all other factors. This statement will be
`discussed in more detail in Chapter 6.
`
`Why Is an Ac
`
`tl
`atom?
`
`A Br0nst
`tron pair ace
`the base to t,
`does not dor
`form a new·
`the hydronil
`of electrons,
`sity to a poi,
`is always fri
`color blue is
`sented by the
`a proton. Th
`
`What ha,
`will donate 1
`form the COJ\
`are no free p:
`the hydrogen
`Section 3.3).
`is polarized i
`whereas the
`does not reac
`atom that is
`electrons to t
`What haJ
`bond will bre
`(such as 0-1
`toward the cl
`curved arrov;
`tion is writte
`
`The elect:
`?onates two (
`ing a new 0-
`O-Hbond is
`breaks and t:
`the chloride i
`an acid consi
`
`

`

`Base Approach
`
`Why Is an Acid-Base Theme Important?
`
`27
`
`m, the oxygen
`ta proton?
`
`, which atom
`
`rdronium ion?
`rmed between
`This reaction
`mshared elec(cid:173)
`e proton. This
`:n donates two
`sed to indicate
`tron donation.
`ndicate trans(cid:173)
`proton has no
`nsity from the
`md the proton
`gen, making it
`mal change in
`
`lonated to the
`te oxygen atom
`nd. Remember
`a Lewis base
`7, this case, the
`:tions merge. 'A
`ns to that pro(cid:173)
`. covalent bond
`2lectron donors
`1 a proton, it is
`;om other than
`i base is a two-
`
`ectron density.
`nitrogen react
`,m of ammonia
`;rons, the more
`Ltement will be
`
`2.12 Can the phosphorus atom function as a Lewis base? The sulfur
`atom? The sodium atom in Na+?
`
`A Bnmsted-Lowry acid is a proton donor, whereas a Lewis acid is an elec(cid:173)
`tron pair acceptor. If all bases donate two electrons, the electron flow is from
`the base to the acid, rather than from the acid to the base. Therefore, an acid
`does not donate the proton, but rather the proton is "attacked" by the base to
`form a new bond to the proton, as shown previously for water and H+ giving
`the hydronium ion. As noted, a blue curved arrow is used to indicate the flow
`of electrons, and the electron flow is always from a source of high electron den(cid:173)
`sity to a point of low electron density. In this case, the direction of the arrow
`is always from the base to the proton of the acid, as illustrated. Although the
`color blue is used for the electron-rich base as well as the electron flow repre(cid:173)
`sented by the arrow, the color red is used for the electron-deficient acid-here,
`a proton. This convention is used throughout this book.
`2,13 Why is it incorrect to draw the arrow from H to oxygen in this
`reaction?
`What has been learned from this discussion? A base is electron rich and
`will donate two electrons to an electron-deficient atom, such as a proton, to
`form the conjugate acid. The reaction with water used H+, a free proton. There
`are no free protons. The protons in common acids such as HCl or HCOOH have
`the hydrogen atom attached to another atom by a covalent bond (see Chapter 3,
`Section 3.3). Chapter 3 discusses bond polarization; the hydrogen atom in HCl
`is polarized such that His electron deficient and positive (a proton-like atom),
`whereas the Cl is electron rich and negative (see Section 3.7). Therefore, a base
`does not react with a free proton but rather with an electron-deficient hydrogen
`atom that is attached to another atom, as in HCL When the base donates two
`electrons to the hydrogen atom, it literally pulls the proton away.
`What happens to the two electrons in the bond between H and Cl? That
`bond will break as the two electrons from the base are used to form a new bond
`(such as 0-H), and the two electrons in the former H-Cl bond will migrate
`toward the chlorine atom, forming chloride ion, which is the conjugate base. A
`curved arrow represents the electron flow in this process, and the entire reac(cid:173)
`tion is written as the following:
`
`H••o·•~r.)
`•
`'
`H 'Cl'
`H. •
`•
`•
`
`H H
`;'o':
`H • '+
`
`; Cl:
`
`The electron-rich atom in water is the oxygen atom (the basic atom), and it
`donates two electrons to the electron-deficient hydrogen of HCl (the acid), form(cid:173)
`ing a new 0-H bond in the hydronium ion, which is the conjugate acid. As the
`0-H bond is formed by using the two electrons from the oxygen, the H-Cl bond
`breaks and the two electrons in that bond are transferred to chlorine, forming
`the chloride ion, which has a negative charge. The 8+ hydrogen atom in H-Cl is
`an acid consistent with identification of H as an electron-deficient atom.
`
`

`

`28
`
`Organic Chemistry: An Acid-Base Approach
`
`\/\/hy Is an Acid-!
`
`2.14 In a reaction of HCl and ammonia, identify the electron~dormting
`atom of the base and draw the reaction using the curved arrow
`formalism, giving the products of the reaction.
`If the acid-base reaction is written with the electron pairs and the arrows,
`as shown for water and HCl, the Lewis base definition is quite useful. The
`electron-rich molecule is the base, and the electron-rich atom donates two elec(cid:173)
`trons. The molecule bearing the electron-deficient atom (hydrogen) is the acid.
`For reactions of organic molecules, it is essential to identify electron-rich and
`electron-poor components of molecules, to understand the electron flow, and
`to understand how to predict the products. That process begins with making
`the transition to thinking in terms of Lewis bases/Lewis acids rather than
`Bn;;nsted-Lowry acids and bases.
`
`It is important to identify the acid and the base in the reactions that we have
`discussed. Sometimes an acid-base reaction does not work very well because
`the Ka favors the acid and the base rather than the conjugate acid and the
`conjugate base. If the equilibrium constant Ka for a given acid-base reaction
`is unfavorable, changing the base may make the Ka more favorable. In other
`words, changing the base to one that is stronger for a given acid should shift
`Ka toward the conjugate acid and the conjugate base. To do this, however, it is
`necessary to differentiate one base from another by comparing the electron(cid:173)
`donating capability (the base strength). In other words, one base may be stron(cid:173)
`ger than another, and one must be able to predict relative basicity. Likewise,
`two acids can be differentiated based on their relative strength.
`This analysis can be probed further. Why is one base stronger than another?
`Why is one acid stronger or weaker than ano