Transcript Introduction to Organic Chemistry 2 ed William H. Brown

2
Introduction to
Organic
Chemistry
2 ed
William H. Brown
2-1
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
2
Acids
and
Bases
2-2
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
2 Brønsted-Lowry Definitions
• Acid: a proton donor
• Base: a proton acceptor
H
+
O H
+
H
Proton
donor
-
O
H
H
O
+ H
H
Proton
acceptor
H
Proton
donor
+
N
H
H
H
Proton
acceptor
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
H
H
H
+
O H
O
H
O
H
+ H
+
N H
H
2-3
2 Conjugate Acids & Bases
• When an acid donates a proton, it is converted to
its conjugate base
• When a base accepts a proton, it is converted to
its conjugate acid
+
H3 O
Acid
+
NH3
Base
H2 O
C onjugate
base
+
NH4
+
C onjugate
acid
2-4
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
2 Strong Acids and Bases
• Strong acid - completely ionized in aqueous
solution. Examples are:
• HCl, HBr, HI, HNO3, HClO4, and H2SO4
• Strong base - completely ionized in aqueous
solution. Examples are:
• LiOH, NaOH, KOH, Ca(OH)2, and Ba(OH)2
2-5
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
2 Weak Acids and Bases
• Acetic acid is a weak acid
• it is incompletely ionized in aqueous solution
O
CH 3 COH
+
H2 O
Acid
Base
(weaker acid) (weaker base)
O
CH3 CO
-
+
+
H3 O
Conjugate base Conjugate acid
of CH3CO2 H
of H2 O
(stronger base) (stronger acid)
2-6
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
2 Weak 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 +
H2 O
A
-
+
+
Ka =
Keq [H 2 O] =
+
H3 O
-
[H 3 O ][A ]
[HA]
pKa = - log Ka
2-7
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
2 Weak Acids and Bases
Acid
ethanol
water
bicarbonate ion
ammonium ion
carbonic acid
acetic acid
pKa
Formula
CH3 CH2 OH 15.9
H2 O
15.7
10.33
HCO 3
+
NH 4
H2 CO 3
9.24
6.36
CH3 CO2 H
4.76
sulfuric acid
H2 SO 4
hydrogen chloride HCl
Conjugate
Base
CH3 CH2 O
HO
2-
CO 3
NH 3
HCO 3
CH3 CO2
-5.2
HSO 4
-7
Cl
-
-
-
2-8
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
2 Structure & Acidity
• Electronegativity
• the relative acidity of HA within a period is determined
by the stability of A• the greater the electronegativity, the greater the
stability of A-
pKa
Electronegativity
of A inA-H
H3 C-H
51
H2 N -H
38
HO- H
15.7
F-H
3.5
2.5
3.0
3.5
4.0
Increasing acid strength
2-9
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
2 Structure & Acidity
• Resonance - delocalization of charge in A• carboxylic acids and alcohols both contain an -OH
group
• a carboxylic acid is a stronger acid than an alcohol
O
CH3 COH + H2 O
A carboxylic
acid
O
+
CH3 CO + H3 O
A carboxylate
ion
CH3 CH2 OH + H2 O
An alcohol
pKa = 4.76
+
CH3 CH2 O- + H3 O
pKa = 15.9
An alkoxide
ion
2-10
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
2 Structure & Acidity
• Resonance - delocalization of charge in A-
••
• delocalization of charge stabilizes the carboxylate ion
relative to the alkoxide ion
••
O
CH 3
C
+
H2 O
••
O
H
••
••
••
••
C
C
O
CH3
••
••
O
CH 3
O
••
These contributing structures are equivalent;
the carboxylate anion is stabilized
by delocalization of the negative charge
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
+
H3 O
••
••
••
O
••
+
2-11
2 Structure & Acidity
• Inductive effect
• the electron-withdrawing inductive effect of the
adjacent carbonyl group weakens the O-H bond of the
carboxylic acid
O
+
CH 3 C
O H
• Carboxylic acids are more acidic than alcohols
• delocalization of charge in the carboxylate ion
• electron-withdrawing inductive effect of adjacent C=O
2-12
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
2 Position of equilibrium
• Favors reaction of the stronger acid and stronger
base to give the weaker acid and weaker base
HA
Stronger
acid
+
B
-
Stronger
base
CH3 CO2 H + NH 3
Acetic acid
pKa 4.76
(stronger acid)
A
-
+
Weaker
base
CH3 CO2
-
HB
Weaker
acid
NH 4 +
Ammonium ion
pKa 9.24
(weaker acid)
+
2-13
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
2 Position of equilibrium
• Stronger acid and stronger base react to give
weaker acid and weaker base
-
CH3 CO2 H + HCO3
Acetic acid Bicarbonate
ion
pKa 4.76
-
C6 H5 OH + HCO3
Phenol
Bicarbonate
ion
pKa 9.95
CH3 CO2
Acetate
ion
-
+
H2 CO3
Carbonic acid
pKa 6.36
-
C6 H5 O + H2 CO3
Phenoxide Carbonic acid
ion
pKa 6.36
2-14
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
2 Position of equilibrium
• Stronger acid and stronger base react to give
weaker acid and weaker base
-
C6 H5 OH + HCO3
Phenol
Bicarbonate
ion
pKa 9.95
-
C6 H5 O + H2 CO3
Phenoxide Carbonic acid
ion
pKa 6.36
2-15
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
2 Lewis Acids and Bases
• Lewis acid: accepts a pair of electrons to form a
new covalent bond
• Lewis base: donates a pair of electrons to form a
new covalent bond
A + B
Lewis Lewis
acid
base
A
B
new covalent bond
formed in this Lewis
acid-base reaction
2-16
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
2 Lewis Acids and Bases
H O H
H
Lewis acid
(accepts an
electron pair)
- O-H
+
H- O
Lewis base
(donates an
electron pair)
H
CH3 -C+
+ H- O-H
H
H
+
Cl
-
CH3
A carbocation Chloride ion
(a Lewis acid) (a Lewis base)
CH3 -C- Cl
CH3
2-17
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
2
Acids
and
Bases
End Chapter 2
2-18
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.