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44
4-1
Organic Organic Chemistry Chemistry
William H. BrownWilliam H. Brown& Christopher S. Foote& Christopher S. Foote
44
4-2
Acids Acids and and BasesBases
Chapter 4Chapter 4
44
4-3
Brønsted-Lowry DefinitionsBrønsted-Lowry Definitions Acid:Acid: a proton donor Base:Base: a proton acceptor
+
Proton donor
Protonacceptor
-O H
H
OH
H
O HH + +O H H
+
Protonacceptor
Protondonor
+O H
H
OH
H
N H
H
H
H
H
H + +N H H
: ::
: ::
::
: :
::
44
4-4
Conjugate Acids & basesConjugate Acids & bases Conjugate baseConjugate base: the species formed from an
acid when it donates a proton to a base Conjugate acid:Conjugate acid: the species formed from a
base when it accepts a proton from an acid
+ +
Acid Base Conjugate base
Conjugate acid
H3 O+
NH3 H2 O NH4+
44
4-5
Conjugate Acids & BasesConjugate Acids & Bases Which is the favored site of protonation?
CH3-C-O-H
OH+
or CH3-C-O-H
O
H
+
A(protonation on thecarbonyl oxygen)
B(protonation on thehydroxyl oxygen)
CH3-C-O-H
OH+
44
4-6
Conjugate Acids & BasesConjugate Acids & Bases Which is the favored site of protonation?
+ HCl CH3-C-N-H
OH+
H+
or
A(protonation on the
amide oxygen)
H
+
Acetamide(an amide)
CH3-C-N-H
O
H
CH3-C-N-H
O
H
H
B(protonation on
the amide nitrogen)
44
4-7
Strong Acids and BasesStrong Acids and Bases Strong acid:Strong acid: completely ionized in aqueous
solution. Examples are• HCl, HBr, HI, HNO3, HClO4, and H2SO4
Strong base:Strong base: completely ionized in aqueous solution. Examples are• LiOH, NaOH, KOH, Ca(OH)2, and Ba(OH)2
44
4-8
Acids & Base StrengthsAcids & Base Strengths Acetic acid is a weak acid
• it is incompletely ionized in aqueous solution
Base (weaker base)
Acid (weaker acid)
Conjugate base (stronger base)
Conjugate acid (stronger acid)
CH3COH + H2O
CH3CO- + H3O
+
O
O
44
4-9
Weak Acids and BasesWeak Acids and Bases The equation for the ionization of a weak acid,
HA, in water and the acid ionization constant, Ka, for this equilibrium are
=
HA H3O++A
-
[H3O+][A
-]
[HA]Ka
+ H2O
= Keq[H2O]
pKa = - log Ka
44
4-10
Weak Acids and BasesWeak Acids and BasesAcid Formula pKa
Conjugate Base
ethanol
water
bicarbonate ion
ammonium ion
carbonic acid
acetic acid
sulfuric acidhydrogen chloride
10.33
15.7
15.9
4.76
6.36
9.24
-5.2
-7
CH3CH2 OH CH3CH2 O-
H2 O HO-
HCO3-
CO32-
NH4+ NH3
H2 CO3 HCO3-
CH3COOH CH3COO-
H2 SO4 HSO4-
HCl Cl -
44
4-11
Weak Acids and BasesWeak Acids and Bases Equilibrium favors reaction of the stronger
acid and stronger base to give the weaker acid and weaker base
CH3COH NH3 NH4++ +
Acetic acidpKa 4.76
(stronger acid)
Ammonia(stronger base)
Acetate ion(weaker base)
Ammonium ionpKa 9.24
(weaker acid)
CH3CO-
O O
Keq = 3.0 x 104
pKeq = 4.76 - 9.24 = -4.48
44
4-12
Structure and AcidityStructure and AcidityA. 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 A-, and the greater the acidity of the acid HA
pKa 51 38 16
Electronegativity of A in A-H
2.5 3.0 3.5
CH3CH2-H CH3NH-H CH3O-H
Anion CH3CH2- CH3NH- CH3O-
increasing anion stability
Acid
increasing acidity
44
4-13
Structure and AcidityStructure and AcidityB. 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 negative charge, the greater its
stability, and the greater the acidity of HA
CH3O-CH3S-
CH3SH CH3OHMethanethiol
pKa 7.0Methanol
pKa 16
44
4-14
Structure and AcidityStructure and AcidityC. Resonance Delocalization of Charge in A-
• for compounds with the same functional group, the more stable the anion A-, the stronger the acid HA
• compare an alcohol and a carboxylic acid• ionization of the O-H bond of an alcohol gives an
anion for which there is no resonance stabilization
:: + H3O
+
Ethanol Ethoxide ion
+ H2OpKa = 15.9
CH3CH2-O-H CH3CH2-O:-
::
44
4-15
Structure and AcidityStructure and Acidity• ionization of a carboxylic acid gives a resonance-
stabilized anion
• a carboxylic acid is a stronger acid than an alcohol
Delocalization of the negative charge
+CH3 C
O
O
C
O
O
CH3
O H
C
O
CH3
H3O+
+ H2OpKa = 4.76
::
::
: :
:
:
:
:
:
:
::
44
4-16
Structure and AcidityStructure and AcidityD. Electron-withdrawing inductive effect
• the polarization of electron density of a covalent bond due to the electronegativity of an adjacent covalent bond
CH3-C-OH
Acetic acidpKa 4.76
Cl-CH2-C-OH
Chloroacetic acidpKa 2.86
O
O
+C C
Cl O0.5-
O0.5-HH
-
Charge is distributedequally betweenthe two oxygens
The positive chargehelps neutralize the negative charge on the oxygens
44
4-17
Structure and AcidityStructure and Acidity• stabilization of an anion by the inductive effect falls off
rapidly with increasing distance of the electronegative atom from the site of the negative charge
CH3CH2CH2COH
O
CH2CH2CH2COH CH3CHCH2COH
O
Cl
CH3CH2CHCOHO
Cl ClButanoic
acidpKa 4.82
4-Chlorobutanoicacid
pKa 4.52
3-Chlorobutanoicacid
pKa 3.98
2-Chlorobutanoicacid
pKa 2.83
O
44
4-18
Structure and AcidityStructure and AcidityE. Hybridization
• for anions differing only in the hybridization of the charged atom, the greater the % of s character to the hybrid orbital of the charged atom, the more stable the anion, and the greater the acidity of HA
25
44
51
pKa
HC CH
C C
H
H C
HH
C
H
H
Carbon% s
sp 50
sp2 33
sp3 25
Acid Anion
HC C:-
C C
H
H
H
H C C: -
H
H
H H H
H H H
44
4-19
Lewis Acids and BasesLewis Acids and Bases Lewis acid:Lewis acid: any molecule of ion that can form a
new covalent bond by accepting a pair of elections
Lewis base:Lewis base: any molecule of ion that can form a new covalent bond by donating a pair of elections
Lewis base
Lewis acid
+ BA new covalent bondformed in this Lewisacid-base reaction
:+-
A B
44
4-20
Lewis Acids and BasesLewis Acids and Bases
:
: :
Br -CH3-C C-CH3
H H
H
CH3-C C-CH3
H H
H Br
Bromideion
sec-Butyl cation(a carbocation)
2-Bromobutane
++
: :
::
+ -
Diethyl ether(a Lewis base)
Boron trifluoride (a Lewis acid)
O
CH3 CH2
CH3 CH2
F
F
O
CH3 CH2
CH3 CH2
B-F
F
F
+ B
A BF3-ether complex
F:: :
44
4-21
Prob 4.8Prob 4.8 For each conjugate acid-base pair, identify the first
species as an acid or base and the second as its conjugate base or acid.(a) (b)
(c) (d)
(e) (f)
(g)
HCOOH, HCOO- NH4+, NH3
CH3CH2O-, CH3CH2OH
CH3CH3, CH3CH2-H2PO4
-, HPO42-
HCO3-, CO3
2-
CH3S-, CH3SH
44
4-22
Prob 4.9Prob 4.9 Complete a net ionic equation for each proton-transfer
reaction. Label the original acid and its conjugate base.
(b)
(a)
CH3CH2O-
+NH3 HCl
+ HCl
(d)
(c) +HCO3- OH-
+CH3COO- NH4+
44
4-23
Prob 4.10Prob 4.10 Complete a net ionic equation for each proton-transfer
reaction. Label the original acid and its conjugate base.
NH4+
OH-
CH3COO-
CH3NH3++
+(a)
(b)
+ OH-CH3NH3
+
+(c)
(d)
NH4+
CH3CH2O-
44
4-24
Prob 4.11Prob 4.11 Write a structural formula for the conjugate acid
formed by treating each molecule or ion with HCl.
(a) (b)
(c) (d)
OCH3CH2OH HCH
(CH3)2NH HCO3-
44
4-25
Prob 4.13Prob 4.13 Account for the greater stability of the anion derived
from acetone compared with the anion derived from ethane.
CH3CCH2-H
O
CH3CH2-H
AcetonepKa 22
EthanepKa 51
44
4-26
Prob 4.16Prob 4.16 Arrange the compounds in each set in order of
increasing acid strength.
CH3CH2OH HOCO-
Ethanol Bicarbonate ion Benzoic acid
(a)
(b)
C6H5COH
HOCOHCarbonic acid
CH3COHAcetic acid
HClHydrogen chloride
O O
OO
44
4-27
Prob 4.17Prob 4.17 Arrange the compounds in each set in order of
increasing base strength.
NH3 HOCO- CH3CH2O-(a)
(b)
OH-
HOCO- CH3CO-
(c)H2O NH3 CH3CO-
(c) CH3CO-NH2-
OH-
O
OO
O
O
44
4-28
Prob 4.20Prob 4.20 Write an equation for the reaction of acetic acid, pKa
4.76, with each base. Which equilibria lie toward the left? Which lie toward the right?(a)NaHCO3 (b)
(c) (d)
NH3
H2O NaOH
44
4-29
Prob 4.21Prob 4.21 Write an equation for the reaction of ethanol,
CH3CH2OH, pKa 15.9 with each base. Which equilibria lie toward the left? Which lie toward the right?
(a)HCO3- (b)
(c) (d)
OH-
NH2- NH3
44
4-30
Prob 4.22Prob 4.22 Benzoic acid, C6H5COOH, pKa 4.19, is only slightly
soluble in water but its salt, C6H5COO-Na+, is quite soluble in water. In which solutions will benzoic acid dissolve more readily than in water?
(a)
(b)
(c)
Aqueous NaOH
Aqueous NaHCO3
Aqueous Na2CO3
44
4-31
Prob 4.23Prob 4.23 4-Methylphenol, CH3C6H4OH, pKa 10.26, is only slightly
soluble in water, but its sodium salt, CH3C6H4O-Na +, is quite soluble in water. In which solutions will 4-methylphenol dissolve?
(a)
(b)
(c)
Aqueous NaOH
Aqueous NaHCO3
Aqueous Na2CO3
44
4-32
Prob 4.24Prob 4.24 For an acid-base reaction, one way to determine the
predominant species at equilibrium is to say that the reaction arrows points to the acid with the higher value of pKa. Explain why this rule works.
pKa -1.74pKa 9.24
++NH4+ H2O NH3 H3O
+
+ +
pKa 9.24 pKa 15.7NH4
+ OH- NH3 H2O
44
4-33
Prob 4.25Prob 4.25 Will acetylene react with sodium hydride according to
the following equation to form a salt and hydrogen?
HC CH + Na+H-
Acetylene Sodiumhydride
HC C-Na+ + H2
Sodiumacetylide
Hydrogen
44
4-34
Prob 4.26Prob 4.26 Using the values of pKa given in Table 4.1, calculate
the equilibrium constant, Keq, for this acid-base reaction.
+ +H3 PO4 CH3CH2 OH H2 PO4- CH3CH2 OH2
+
44
4-35
Prob 4.28Prob 4.28 Complete the equation for each Lewis acid-base
reaction.
(b)
(a) +
+
Cl
ClCH3-C-Cl
CH3-C+
Al-Cl
H-O-H
CH3
CH3
CH3
CH3
(c)+
+ Br -
+(d) CH3-O-H
CH3-CH-CH3
+CH3-CH-CH3
44
4-36
Prob 4.29Prob 4.29 For each reaction, label the Lewis acid and the Lewis
base, and use curved arrows to show the flow of electrons in each reaction.
(a) +CH3-CH=CH2 H-Cl CH3-CH-CH2+
+ Cl -
(b) +CH3-C=CH2 Br-Br CH3-C-CH2-Br + Br -+
H
CH3 CH3
44
4-37
Prob 4.30Prob 4.30 2,4-Pentanedione is a considerably stronger acid than
acetone. Write a structural formula for each conjugate base, and account for the greater stability of the conjugate base from 2,4-pentanedione.
CH3CCH2-H
O
CH3CCHCCH3
O
HAcetonepKa 22
2,4-PentanedionepKa 9
O
44
4-38
Prob 4.32Prob 4.32 In which equation does the sec-butyl cation react as a
Lewis acid? In which equation does it react as a Brønsted-Lowry acid? Write Lewis structures for the reactants and products, and show by the use of curved arrows how each reaction occurs.
CH3-CH-CH2-CH3 H2O
CH3-CH-CH2-CH3 H2O
CH3-CH-CH2-CH3
OH H
CH3-CH=CH-CH3 H3O+
+
sec-Butyl cation
+
+
sec-Butyl cation
+
++
(1)
(2)
44
4-39
Prob 4.34Prob 4.34 Write equations for the reaction of each compound
with H2SO4, a strong protic acid.
(a) (b)
(c) (d)
(e) (f)
CH3OCH3 CH3CH2SCH2CH3
CH3CCH3
CH3NCH3CH3CH2NHCH2CH3
CH3COCH3
O
CH3
O
44
4-40
Prob 4.36Prob 4.36 Write a structural formula for the conjugate base
formed when each compound is treated with one mole of a base stronger than the compound’s conjugate base.
(a) (b)
(c) (d)
(e) (f)
HSCH2CH2NH2
CH3CHCOH
HOCCH2CH2SHHOCH2CH2C
H3NCH2CH2COH
(g) (h)HSCH2CH2OH
HOCH2CH2NH2
CHO
HO O O+
H3NCH2CH2CO-O+
44
4-41
Prob 4.39Prob 4.39 For each pair of molecules or ions, select the stronger
base and write its Lewis structure.
(a) (b)
(c) (d)
(e) (f)
CH3NH- or CH3O-
NH3 or OH-
CH3CH2O- or H-
NH3 or H2O
(g) (h)HSO4- or OH-CH3COO- or HCO3
-
CH3S- or CH3O-
(i)
CH3COO- or OH-
OH- or Br-
44
4-42
Prob 4.43Prob 4.43 When imidazole is dissolved in water, proton transfer
to it gives a cation. Is the cation better represented by A or B?
N
N
H
+ H2ON
N
HN
N
H
+ OH-
H
H
or
A BImidazole
+
+
:
:
:
:
44
4-43
Acids Acids and and
BasesBasesEnd Chapter 4End Chapter 4