Using Structure to Predict Acidity/Basicity Effect of Structure on Effect ...
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Using Structure to Predict Acidity/Basicity Effect of Structure on Effect ...
Using Structure to Predict Acidity/Basicity Strong acid
Weak acid
ÿ relatively unstable acid
ÿ relatively stable acid
ÿ relatively stable conjugate base
ÿ relatively unstable conjugate base
Strong base
Weak base
ÿ relatively unstable base
ÿ relatively stable base
ÿ relatively stable conjugate acid
ÿ relatively unstable conjugate acid
Effect of Structure on pKa I 1. Charged vs. noncharged species a charged molecule is more acidic than a neutral molecule NH4+ pKa = 9.4
NH3 + H+
NH3 pKa = 36
NH2- + H+
2. Electronegativity of the atom attached to H the more EN the attached atom, the more acidic the molecule C F
I- > Br- > Cl- > F-
HI > HBr > HCl > HF
relative size
relative stability of conjugate bases
relative acidity
4. Inductive electron withdrawal a. Magnitude of electronegativity (the more EN, the more acidic) b. Distance away from (-) charged conjugate base (closer = more acidic) O
O I
H3C
OH
pKa = 4.76
C OH H2 pKa = 3.15
O
O
Br
C OH H2 pKa = 2.86
O
Cl
F
C OH H2 pKa = 2.81
C OH H2 pKa = 2.66
Example: N vs O vs S H H3C S
H H3C O
H H3C N H
H
H
pKa = -6.8
pKa = -2.4
H3CH2C S H
H3CH2C O H
pKa = 10.6
pKa = 15.9
H
pKa = 9.8 H H3CH2C N
pKa = 35
H
Nitrogen is most effective at stabilizing a positive charge and least effective at stabilizing a negative charge Sulfur is most effective at stabilizing a negative charge and least effective at stabilizing a positive charge
2
Effect of Structure on pKa III 5. Hybridization (sp, sp2, sp3)
Relative Acidity sp sp2 sp3 > > 50% s character 33% s character 25% s character H
H HC C H
>
>
H2C
H3C C H
H
pKa = 25
H
pKa = 44
pKa = 50
Conjugate base has negative charge, which is stabilized by orbitals with greater s character
Effect of Structure on pKa IV 6.
Resonance resonance stabilizes the negative charge of the conjugate base the more resonance contributors, the more stable the molecule
