Acidity, Basicity, pH and pKa

Organic Chemistry
8th Edition
Paula Yurkanis Bruice
Acidity,
Basicity,
pH and
pKa
Chapter 2
Acidity, Basicity,
pH and pKa
2
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Brønsted–Lowry Acids and Bases
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An Acid/Base Equilibrium
• Acid donates a proton
• Base accepts a proton
Ka =
[ H 3O + ][ A − ]
[ H 2 O ][ AH ]
LogKa = p Ka
• Strong reacts to give weak
• The weaker the base, the stronger its conjugate acid
• Stable bases are weak bases
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€ Ka: The acid dissociation constant.
The stronger the acid, the larger its Ka
value
€ and the smaller its pKa value.
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The Most Common Organic Acids Are Carboxylic Acids
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Protonated alcohols and protonated carboxylic acids are
very strong acids
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An amine can behave as an acid or as a base
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Strong Acids / Bases React to Form Weak
Acids / Bases
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The Structure of an Acid Affects Its Acidity
• The weaker the base, the stronger is its conjugate
acid
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Electronegatividad
• When atoms are similar in size, the stronger acid will
have its proton attached to the more electronegative
atom
• Stable bases are weak bases
• The more stable the base, the stronger is its conjugate
acid
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Electronegatividad
Polarizabilidad (Tamaño)
The stability of a base is affected by its electronegativity
(and Size)
• When atoms are very different in size, the stronger
acid will have its proton attached to the largest atom
size overrides electronegativity
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Inducción
• Inductive electron withdrawal increases the acidity of a
conjugate acid
Substituents Affect the Strength of an Acid
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Resonancia
Hibridación
Carbon as a very weak acid
Acetic acid is more acidic than ethanol
The delocalized electrons in
acetic acid are shared by more
than two atoms, thereby
stabilizing the conjugated base
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Summary: Factors That Determine Acid Strength
1. Size: As the atom attached to the hydrogen increases
in size, the strength of the acid increases
2. Electronegativity
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3. Hybridization
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5. Electron delocalization (Resonance)
4. Inductive effect
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Lewis Acids and Bases
• Lewis acid: non-proton-donating acid; will accept two
electrons
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Basicity and Drug Design: Methotrexate,
Substituting Nitrogen for Oxygen
• Lewis base: electron pair donors
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An Acid/Base Equilibrium
Acidity and Cosmetics: Skin Peel Agents
• Skin Peels: Remove old skin with an acid to expose new
young-looking skin.
• The stronger the acid, the deeper the peel.
• Examples of skin peel agents:
Mild Peel Agents:
Ka =
Deep Peel Agent:
[ H 3O + ][ A − ]
[ H 2 O ][ AH ]
LogKa = p Ka
€ Ka: The acid dissociation constant.
The stronger the acid, the larger its Ka
value
€ and the smaller its pKa value.
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The Henderson–Hasselbalch Equation
Relación entre pKa y pH (estado de protonación)
Ejemplos: Aspirina (ácido acetyl salicílico)
y aminoácidos
O
pK a = pH + log
[HA]
[A ]
−
OH
O
O
O
O
O
O
CH3
CH3
O
CH3
forma ácida
(protonada)
A pH = 1 (estómago), cuál forma predomina?
forma básica
(deprotonada)
A pH = 9 (antiácido), cuál forma predomina?
ácido carboxílico pKa ~ 5, ión amonio pKa ~ 9
O
pKa
• Compounds exist primarily in their basic form at a pH > its pKa
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O
aspirina
pKa = 3.5
•Compounds exist primarily in their acidic form at a pH < its
• A buffer solution maintains a nearly constant pH upon
addition of small amount of acid or base
O
OH
a) cálculo de pH
b) cálculo de pKa
c) cálculo de razón de especies
The pH indicates the concentration of hydrogen ions (H+)
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O
OH
O
O
O
NH2
NH3
aminoácido Valina
neutral
aminoácido Valina
switterionic form
A pH = 1 (estómago), cuál forma predomina?
O
OH
NH3
forma ácida
(protonada)
NH2
forma básica
(deprotonada)
A pH = 6.8 (fisilógico), cuál forma predomina?
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