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Introduction to Organic Chemistry

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pair
donor
electron-
pair
acceptor
Lewis
acid
Lewis
base
Note that, although we speak of a Lewis base as “donating” a pair of electrons, the term 
is not fully accurate. “Donating” in this case does not imply that the electron pair under 
consideration is removed completely from the valence shell of the base. Rather, “donating” 
means that the electron pair is shared with another atom to form a covalent bond.
As we will see in the chapters that follow, a great many organic reactions can be 
interpreted as Lewis acid–base reactions. Perhaps the most important (but not the only) 
Lewis acid is the proton. Isolated protons, of course, do not exist in solution; rather, a 
proton attaches itself to the strongest available Lewis base. When HCl is dissolved in wa-
ter, for example, the strongest available Lewis base is an H2O molecule, and the following 
proton-transfer reaction takes place:
H¬O H¬Cl Cl+ ¡
ƒ
H
H¬O¬H +
ƒ
H
Hydronium
ion
+ –
When HCl is dissolved in methanol, the strongest available Lewis base is a CH3OH mol-
ecule, and the following proton-transfer reaction takes place. An oxonium ion is an ion that 
contains an oxygen atom with three bonds and bears a positive charge.
CH3¬O H¬Cl Cl+ ¡
ƒ
H
CH3¬O¬H +
ƒ
H
An oxonium
ion
Methanol
(a Lewis base)
+ –
Table 2.3 gives examples of the most important types of Lewis bases we will en-
counter in this text arranged in order of their increasing strength in proton-transfer 
T a b l e 2 . 3 Some organic Lewis Bases and Their Relative Strengths in 
Proton-Transfer Reactions
Halide Ions
Water, Alcohols, 
and Ethers
Ammonia 
and Amines
Hydroxide Ion and 
Alkoxide Ions Amide Ions
CCla C- HiOa iH Hi NAiH
 ƒ
 H
HiOa C- HiNAC-
 ƒ
H
CBra C- CH3iOa iH CH3i NAiH
 ƒ
 H
CH3iOa C- CH3iNAC
-
 ƒ
H
CIaC- CH3iOa iCH3 CH3iNA iH
 ƒ
 CH3
CH3i NAC
-
 ƒ
CH3
 CH3iNA iCH3
 ƒ
 CH3
Very weak Weak Strong Stronger Very strong
C H A P T E R 2 Acids and Bases56
 reactions. Note that each of the Lewis bases has at least one atom with an unshared pair 
of electrons. It is this atom that functions as the Lewis base. Ethers are organic derivatives 
of water in which both hydrogens of water are replaced by carbon groups. We study the 
properties of ethers along with those of alcohols in Chapter 8. We study the properties of 
amines in Chapter 10.
E x a m p l E 2.5
Complete this acid–base reaction. Use curved arrows to show the redistribution of electrons in the reaction. In addition, 
predict whether the position of this equilibrium lies toward the left or the right.
 CH3iO+iH + CH3iNiH m
 ƒ ƒ
 H H
 
S T R AT E g y
First, add unshared pairs of electrons on the reacting atoms to give each a complete octet. Then identify the Lewis 
base (the electron-pair donor) and the Lewis acid (the electron-pair acceptor). The position of equilibrium lies on the 
side of the weaker acid and weaker base.
S o L U T i o n
Proton transfer takes place to form an alcohol and an ammonium ion. We know from Table 2.3 that amines are stron-
ger bases than alcohols. We also know that the weaker the base, the stronger its conjugate acid, and vice versa. From 
this analysis, we conclude that the position of this equilibrium lies to the right, on the side of the weaker acid and the 
weaker base.
CH3¬N¬H CH3¬N¬HCH3¬O+
ƒ
H
ƒ
H
ƒ
H
H
ƒ
+
Weaker
acid
Weaker
base
Stronger
base
Stronger
acid
CH3¬O¬H
ƒ
H
+ +
See problems 2.7, 2.8, 2.11, 2.26–2.32
p r o b l E m 2.5
Complete this acid–base reaction. First add unshared pairs of electrons on the reacting atoms to give each atom a complete 
octet. Use curved arrows to show the redistribution of electrons in the reaction. In addition, predict whether the position of 
the equilibrium lies toward the left or the right.
CH3iO- +
 H
 ƒ
CH3 iN + iCH3 
 ƒ
 CH3 
m
Another type of Lewis acid we will encounter in later chapters is an organic cation in 
which a carbon is bonded to only three atoms and bears a positive formal charge. Such car-
bon cations are called carbocations. Consider the reaction that occurs when the following 
organic cation reacts with a bromide ion:
57Summary of Key Questions
E x a m p l E 2.6
Complete the following Lewis acid–base reaction. Show all 
electron pairs on the reacting atoms and use curved arrows 
to show the flow of electrons in the reaction:
CH3i C 
+
HiCH3 + H2O ¡
S T R AT E g y
Determine which compound will be the electron-pair 
donor and which will be the electron-pair acceptor. Hint: 
Compounds with empty orbitals in their valence shell usu-
ally act as Lewis acids.
S o L U T i o n
The trivalent carbon atom in the organic cation has an 
empty orbital in its valence shell and, therefore, is the Lewis 
acid. Water is the Lewis base.
CH3¬C¬CH3 CH3¬C¬CH3H¬O¬H ¡+
ƒ
H
ƒ
H
O
ƒ
≈ √
HH
+
An oxonium
ion
Lewis
base
Lewis
acid
+
p r o b l E m 2.6
Write an equation for the reaction between each Lewis 
acid–base pair, showing electron flow by means of curved 
arrows. (Hint: Aluminum is in Group 3A of the Periodic Table, 
just under boron. Aluminum in AlCl3 has only six electrons 
in its valence shell and thus has an incomplete octet.)
(a) Cl- + AlCl3 ¡ (b) CH3Cl + AlCl3 ¡
See problems 2.30–2.32
Summary of Key QueSt ionS
•	 An Arrhenius acid is a substance that dissolves in aqueous 
solution to produce H3O
+ ions.
•	 An Arrhenius base is a substance that dissolves in aque-
ous solution to produce OH- ions. 
2.1 What Are Arrhenius Acids and Bases?
CH3¬CH¬CH3 CH3¬CH¬CH3Br ¡+
+
 Br
ƒ
2-BromopropaneBromide ion
(a Lewis base)
An organic
cation
(a Lewis acid)
bromine uses a lone
pair of electrons to form
a new bond to carbon
In this reaction, the organic cation is the electron-pair acceptor (the Lewis acid), and bro-
mide ion is the electron-pair donor (the Lewis base).
•	 Neutralization of an acid by a base is a proton-transfer 
reaction in which the acid is transformed into its conjugate 
base, and the base is transformed into its conjugate acid.
•	 A Brønsted–Lowry acid is a proton donor.
•	 A Brønsted–Lowry base is a proton acceptor. 
2.2 What Are Brønsted–Lowry Acids and Bases?
C H A P T E R 2 Acids and Bases58
•	 The strength of a weak acid is expressed by its ionization 
constant, Ka.
•	 The larger the value of Ka, the stronger the acid, pKa = 
- log Ka.
•	 A strong acid or strong base is one that completely ion-
izes in water. 
•	 A weak acid or weak base is one that only partially ionizes 
in water.
2.3 How Do We Measure the Strength of an Acid or Base?
•	 A Lewis base is a species that forms a new covalent bond 
by donating a pair of electrons (an electron-pair donor).
•	 A Lewis acid is a species that forms a new covalent bond 
by accepting a pair of electrons (an electron-pair acceptor).
2.6 What Are Lewis Acids and Bases?
•	 the electron-withdrawing inductive effect, which also sta-
bilizes the conjugate base.
•	 the size of the atom with the negative charge on the con-
jugate base.
The relative acidities of the organic acids, HA, are deter-
mined by 
•	 the electronegativity of A.
•	 the resonance stabilization of the conjugate base, A-. 
2.5 What Are the Relationships between Acidity and Molecular Structure?
•	 In an acid–base reaction, the position of equilibrium 
favors the reaction of the stronger acid and the stronger 
base to form the weaker acid and the weaker base.
2.4 How Do We Determine the Position of Equilibrium in an Acid–Base Reaction?
 1. If NH3 were to behave as an acid, its conjugate base 
would be NH2
2. (2.2)
 2. Delocalization of electron density is a stabilizing factor. 
(2.5)
 3. Amide ion,