Organic
Chemistry
William H. Brown
Christopher S. Foote
Brent L. Iverson
4-1
Acids
and
Bases
Chapter 4
4-2
Arrhenius Acids and Bases
 In
1884, Svante Arrhenius proposed these
definitions
• acid: a substance that produces H3O+ ions aqueous
solution
• base: a substance that produces OH- ions in aqueous
solution
• this definition of an acid is a slight modification of the
original Arrhenius definition, which was that an acid
produces H+ in aqueous solution
• today we know that H+ reacts immediately with a water
molecule to give a hydronium ion
+
H ( aq) + H2 O( l)
+
H3 O ( aq)
Hydronium ion
4-3
Brønsted-Lowry Definitions
 Acid:
a proton donor
 Base: a proton acceptor
H
H O:
:
:O
:
-
: :
:
+
H O H +
+ H O
H
:
H
Proton
donor
Proton
acceptor
H
H
:
:
+
H O H +
H
H
Proton
donor
:N
H
H
Proton
acceptor
H O:
H
+
+ H N H
H
4-4
Conjugate Acids & Bases
• conjugate base: the species formed from an acid when
it donates a proton to a base
• conjugate acid: the species formed from a base when
it accepts a proton from an acid
• acid-base reaction: a proton-transfer reaction
• conjugate acid-base pair: any pair of molecules or ions
that can be interconverted by transfer of a proton
con jugate acid-bas e pair
con jugate acid-bas e pair
HCl(aq)
Hyd rogen
chloride
(acid )
+
H2 O( l)
Water
(base)
+
Cl ( aq)
+ H3 O (aq)
Chlorid e
Hydronium
ion
ion
(conju gate
(conjugate
b ase of HCl) acid of H 2O)
4-5
Conjugate Acids & Bases
• Brønsted-Lowry definitions do not require water as a
reactant
• consider the following reaction between acetic acid
and ammonia
conju gate acid -base p air
conju gate acid -base p air
CH3 COOH + NH3
A cetic acid Ammonia
(acid )
(base)
-
+
CH3 COO
+
NH4
Acetate
Ammonium
ion
ion
(conju gate b ase (conjugate acid
acetic acid )
of ammonia)
4-6
Conjugate Acids & Bases
• we can use curved arrows to show the flow of
electrons in an acid-base reaction
: :
H
+
:N H
H
A cetic acid
Ammonia
(proton d on or) (p roton acceptor)
:O:
CH3 -C-O:-
H
+ H-N-H
H
Acetate ion Ammonium
ion
: :
:O:
CH3 -C-O H
4-7
Conjugate Acids & Bases
 Many
organic molecules have two or more sites
that can act as proton acceptors
 in
this chapter, we limit our discussion to carboxylic
acids, esters, and amides
 in these molecules, the favored site of protonation is
the one in which the charge is more delocalized
 question: which oxygen of a carboxylic acid is
protonated?
+ H
O
CH3 -C-O-H + H2 SO4
O
O
+
CH3 -C-O-H
or CH3 -C-O-H + HSO4
H
A
B
(p roton ation
(p roton ation
on th e
on the
carbonyl oxygen) h yd roxyl oxygen )
4-8
Conjugate Acids & Bases
 for
protonation on the carbonyl oxygen, we can write
three contributing structures
 two place the positive charge on oxygen, one places it
on carbon
+
O
CH3 -C-O-H
H
O
+
CH3 -C-O-H
-C
A-1
(C an d O have
comp lete octets)
A-2
(C h as incomplete
octet)
H
H
O
+
CH3 -C=O-H
A-3
(C an d O have
comp lete octets)
 A-1
and A-3 make the greater contribution because all
atoms have complete octets
 the positive charge is delocalized over three atoms
with the greater share on the two equivalent oxygens
4-9
Conjugate Acids & Bases
 for
protonation on the hydroxyl oxygen, we can write
two contributing structures
O
+
CH 3 -C-O-H
H
B-1
O
+
CH 3 -C-O-H
+
H
B-2
(ch arge sep aration an d
ad jacen t positive charges)
 B-2
makes only a minor contribution because of
charge separation and adjacent positive charges
 therefore, we conclude that protonation of a carboxylic
acid occurs preferentially on the carbonyl oxygen
4-10
Conjugate Acids & Bases
 Problem
4.3 Does proton transfer to an amide
group occur preferentially on the amide oxygen
or the amide nitrogen?
+ H
O
O
CH3 -C-N-H + HCl
H
A cetamide
(an amid e)
CH3 -C-N-H
or
H
A
(p rotonation
on the
amide oxygen)
OH
+
CH3 -C-N-H + Cl
-
H
B
(protonation
on th e
amid e nitrogen)
4-11
Pi Electrons As Basic Sites
 Proton-transfer
reactions occur with compounds
having pi electrons, as for example the pi
electrons of carbon-carbon double and triple
bonds
 the
pi electrons of 2-butene, for example, react with
HBr by proton transfer to form a new C-H bond
CH3 -CH=CH-CH3 + H-Br
2-Butene
H
+
CH3 -C-C-CH3 + Br
-
HH
sec-Butyl cation
(a 2° carb ocation )
 the
result is formation of a carbocation, a species in
which one of its carbons has only six electrons in its
valence shell and carries a charge of +1
4-12
Pi Electrons As Basic Sites
 Problem
4.4 Draw Lewis structures for the two
possible carbocations formed by proton transfer
from HBr to 2-methyl-2-butene
CH3
CH3 -C=CH-CH3 + H-Br
2-Methyl-2-buten e
4-13
Acids & Base Strengths
 The
strength of an acid is expressed by an
equilibrium constant
the acid dissociation of acetic acid is given by the
following equation
O
CH3 COH + H2 O
A cetic acid
Water
O
-
CH3 CO
Acetate
ion
+
+
H3 O
Hyd roniu m
ion
4-14
Weak Acids and Bases
 We
can write an equilibrium expression for the
dissociation of any uncharged acid, HA, as:
-
+
Keq =
+
A + H3 O
HA + H2 O
-
[H3 O ] [A ]
[ HA][ H2 O]
water is a solvent and its concentration is a constant
equal to approximately 55.5 mol/L
we can combine these constants to give a new
constant, Ka, called an acid dissociation constant
+
K a = Keq[ H2 O] =
-
[H3 O ][A ]
[HA]
4-15
Acid
Weaker Ethan e
acid
Ethylene
Ammonia
Hyd rogen
Acetylene
Ethan ol
Water
Formula
CH3 CH3
CH2 =CH2
NH3
H2
HC CH
CH3 CH2 OH
H2 O
Methylammon ium ion CH3 NH3
+
-
Bicarbonate ion
Phen ol
HCO3
C6 H5 OH
Ammoniu m ion
Hyd rogen su lfid e
NH4
H2 S
Carbonic acid
Acetic acid
Benzoic acid
Phosp horic acid
Hyd roniu m ion
Sulfuric acid
Hyd rogen ch loride
Stron ger Hyd rogen bromide
acid
Hyd rogen iod ide
+
H2 CO3
CH3 COOH
C6 H5 COOH
H3 PO4
+
H3 O
H2 SO4
HCl
HBr
HI
pK a
51
44
38
35
25
15.9
15.7
10.64
10.33
9.95
Conju gate Bas e
CH3 CH2
S tronger
- conju gate
CH2 =CH
base
NH2
H
HC CCH3 CH2 O
-
HO
CH3 NH2
2-
CO3
C6 H5 O
9.24
7.04
6.36
4.76
4.19
NH3
HS
HCO3
CH3 COO
C6 H5 COO
2.1
-1.74
-5.2
-7
-8
-9
H2 PO4
H2 O
HSO4
Cl
Br
I
-
Weaker
conju gate
base
4-16
Acid-Base Equilibria
 Equilibrium
favors reaction of the stronger acid
and stronger base to give the weaker acid and
weaker base
CH3 COOH
+
-
NH3
A cetic acid
Ammon ia
(stron ger bas e)
pK a 4.76
(stron ger acid )
CH3 COO
+
NH4
+
Acetate ion A mmonium ion
(w eaker bas e)
pK a 9.24
(w eak er acid)
pK eq = 4.76 - 9.24 = -4.48
Ke q = 3.0 x 104
4-17
Acid-Base Equilibria
 Consider
the reaction between acetic acid and
sodium bicarbonate
 we
can write the equilibrium as a net ionic equation
 we omit Na+ because it does not undergo any
chemical change in the reaction
O
+
HCO3
CH3 COH
A cetic acid
Bicarb on ate ion
pK a 4.76
(stron ger acid )
O
CH3 CO +
H2 CO3
Acetate ion Carbonic acid
pK a 6.36
(w eak er acid)
 equilibrium
lies to the right
 carbonic acid forms, which then decomposes to
carbon dioxide and water
4-18
Molecular Structure and Acidity
 The
overriding principle in determining the
relative acidities of uncharged organic acids is
the stability of the anion, A-, resulting from the
loss of a proton
 the
more stable the anion, the greater the acidity of HA
 Ways
to stabilize anions include having the
negative charge
 on
a more electronegative atom
 on a larger atom
 delocalized through resonance
 delocalized by the inductive effect
 in an orbital with more s character
4-19
Molecular Structure and Acidity
A. Electronegativity of the atom bearing the
negative charge
• within a period, the greater the electronegativity of the atom
bearing the negative charge, the more strongly its electrons are
held, the more stable the anion is, and the stronger the acid
Acid
M ethanol CH3 O H
pKa 16
M ethylamine CH3 N H
pKa 38
H
H
Ethane CH3 C H
pKa 51
H
Conjugate base
CH3 O
–
M ethoxide ion
CH3 N – M ethylamide ion
H
H
CH3 C – Ethyl anion
H
4-20
Molecular Structure and Acidity
B. Size of the atom bearing the negative charge
• within a column of the Periodic Table, acidity is related to the size
of the the atom bearing the negative charge
• atomic size increases from top to bottom of a column
• the larger the atom bearing the charge, the greater its stability
CH3 S H
+
CH3 O
–
Methan ethiol
Meth oxid e
p Ka 7.0
ion
(stronger acid ) (stronger bas e)
CH3 S
–
+
CH3 O H
Methan ethiolate Methanol
ion
p Ka 16
(w eaker base) (w eaker acid)
4-21
Molecular Structure and Acidity
C. Resonance delocalization of charge in A• the more stable the anion, the farther the position of
equilibrium is shifted to the right
• compare the acidity alcohols and carboxylic acids
• ionization of the O-H bond of an alcohol gives an anion
for which there is no resonance stabilization
CH3 CH2 O-H + H2 O
A n alcohol
CH3 CH2 O
-
+
+ H3 O
pK a = 15.9
An alk oxide ion
4-22
Molecular Structure and Acidity
• ionization of a carboxylic acid gives a resonancestabilized anion
• the pKa of acetic acid is 4.76
O
CH3 C
O
+ H2 O
O H
CH3 C
O
+
+ H3 O
CH3 C
O
O
equ ivalen t contrib uting structu res ;
the carb oxylate anion is stab ilized by
delocalization of the n egative ch arge.
• carboxylic acids are stronger acids than alcohols as a
result of the resonance stabilization of the carboxylate
anion
4-23
Molecular Structure and Acidity
D. Electron-withdrawing inductive effect
• the polarization of electron density of a covalent bond
due to the electronegativity of an adjacent covalent
H
F
bond
H C-CH2 O-H
H
F C-CH2 O-H
Ethanol
pK a 15.9
2,2,2-Tri fl uoroethano l
pK a 12.4
F
• stabilization by the inductive effect falls off rapidly
with increasing distance of the electronegative atom
from the site of negative charge
CF3 -CH2 -OH
CF3 -CH2 -CH2 -OH
CF3 -CH2 -CH2 -CH2 -OH
2,2,2-Trifluoroethan ol
(pK a 12.4)
3,3,3-Trifluoro-1prop anol
(p Ka 14.6)
4,4,4-Trifluoro-1butan ol
(pK a 15.4)
4-24
Molecular Structure and Acidity
• we also see the operation of the inductive effect in the
acidity of halogen substituted carboxylic acids
O
O
OH
Bu tanoic
acid
pK a 4.82
Cl
Cl
OH
O
O
OH
OH
Cl
4-Ch lorob utan oic 3-Ch lorobutan oic 2-Chlorob utanoic
acid
acid
acid
p Ka 4.52
pK a 3.98
pK a 2.83
4-25
Molecular Structure and Acidity
E. Hybridization
• for anions differing only in the hybridization of the charged atom,
the greater the % s character to the hybrid orbital of the charged
atom, the more stable the anion
• consider the acidity of alkanes, alkenes, and alkynes (given for
comparison are the acidities of water and ammonia)
Weak
Acid
Conjugate
p Ka
Bas e
Water
HO-H
HO–
Alk yn e
HC C H
HC C
–
25
–
38
H2 N
–
Alken e
CH2 =CH-H
CH2 =CH
Alkan e
CH3 CH2 -H
CH 3 CH2
–
44
51
In
c
re
a
si
n
g
a
ci
d
it
Ammonia H2 N-H
15.7
4-26
Lewis Acids and Bases
 Lewis
acid: any molecule of ion that can form a
new covalent bond by accepting a pair of
electrons
 Lewis base: any molecule of ion that can form a
new covalent bond by donating a pair of
electrons
A
Lewis
acid
+
:B
Lewis
base
-
+
A B
new covalent bond
formed in this Lewis
acid-base reaction
4-27
Lewis Acids and Bases
• examples
:
H : Br :
H
H H
sec-Butyl cation
(a carbocation)
CH3 CH 2
+
: :
+
CH3 -C C- CH3
: Br : Bromide
ion
F
B F
CH3 -C
C- CH3
H H
2-Bromobutane
CH3 CH 2
F
+
:O:
+
:O B- F
CH3 CH 2
F
F
CH3 CH 2
Diethyl ether Boron trifluoride A BF3-ether complex
(a Lewis base) (a Lewis acid)
4-28
Acids and
Bases
End Chapter 4
4-29