More About Acids
and Bases
1
2
Molecular Acids
Molecular Acids
Carboxylic acids are a major class
—acetic, citric, malic, acetylsalicyclic, etc.
• Classes of acids—
• Molecular acids, CH3CO2H
Acidic proton
• Cationic acids, NH4+
• Anionic acids,
3
Lactic
H2PO4-
Citric acid
Oxoacids
Note that all have H
attached to an O
atom that is
attached to N, P, S,
etc.
The N, P, S, etc. is also
has a double bond
to another O atom
Carboxylic acid group
4
Acetylsalicylic,
aspirin
Tartaric
5
Cationic Acids
Ammonium ion, NH 4+
NH 4+ + H2O ----> NH3
+ H3 O+
acid
base
base
acid
NH4+ ion is a moderate acid because its
conjugate base is weak.
Ka = 5.6 x 10-10
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6
Ammonium Salts
• Many natural
materials have
the -NR 2 group.
• –NR2 is a base
and forms
–NR2H +
Caffeine
Cadaverine
7
Novocaine
Anionic Acids
8
Molecular Bases
• Examples: H 2PO4-, HPO42-, HSO4-, HCO3 -
—NH3+
9
• Many natural
HPO42-
H2PO4-
materials have
the -NR 2 group.
- H+
• –NR2 is a base
and forms
+ H+
–NR2H +
Nicotine
These anions are all AMPHIPROTICthey are both acids and bases
10
Anions as Bases
11
Anions as Bases
• Carbonate
CO 32-(aq)
aq) + H2O(liq
O(liq)) ¸
HCO3 (aq)
aq) + OH-(aq)
aq)
• Phosphate, PO 43• Sulfate, SO42-
12
Anions as Bases
Lactate
Acetate
All are
conjugate
bases of weak
acids
• Note: All are conjugate bases
of weak acids
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Benzoate
13
Amino Acids
14
Acid Strength: pKa
H3PO4
Internal
proton
transfer
Ka = 7.5 x 10 -3
-
Carboxylic
acid group
Basic
amine
group
Substitutent
on alpha
carbon
CH3CO2H
+
Ka = 1.8 x 10 -5
pKa = -log Ka
pKa = 2.12
pKa = 4.74
Stronger acids have smaller pK a values
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