Acidity and Basicity
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Transcript of Acidity and Basicity
1/22/2012
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Acidity and
Basicity
Ms. Anjelyn del Rosario
UNIVERSITY OF THE PHILIPPINES MANILA Padre Faura, Ermita, Manila
A.Y. 2009 – 2010, Second Semester
CHEMISTRY ORGANIC
Chemistry 31 • Second Sem 2009-2010
+ H2O acid
+ base
solute which produces H3O+
ion when dissolved in H2O
solute which produces OH-
ion when dissolved in H2O
example:
HCl H3O+ + Cl-
example:
NH3 H2O NH4+ + OH-
H2SO4 (aq) + H2O(l) HSO4- (aq) + H3O
+(aq)
HSO4- (aq) + H2O(l) SO4
2- (aq) + H3O
+(aq)
1. Arrhenius Concept classifies solutes according to their behavior in H2O
KOH(aq) OH- (aq) + K+
(aq)
CO32-
(aq) + H2O(l) OH- (aq) + HCO3
- (aq)
Acid- Base Theories
HA (aq) + H2O(l) A- (aq) + H3O
+(aq) MOH (aq) M+
(aq) + OH-(aq)
a proton donor
binary acids <HnX>, oxyacids
<(HO)nXOm>, aquocomplexes
of metal ions,…
a proton acceptor
chemical species that has an
atom with an available electron
for pairing
2. Brønsted-Lowry Concept based on the behavior of a substance during proton (H+) transfer; H+
(aq) + H2O(l) H3O+
(aq)
Acid- Base Theories
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Acid- Base Theories
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• Proton donors and acceptors.
• H+ or H3O+ = a proton
2. Brønsted-Lowry Concept
Conjugate acid-base pair a pair of substances that are related through the loss or gain of a proton; in any acid-base reaction we can identify two sets of conjugate acid-base pairs.
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Acid- Base Theories
2. Brønsted-Lowry Concept
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Acid- Base Theories
Explanation of Acid Strength
1. Positivity of H: The more positive H is, the more acidic it is, the higher the acid strength. RCOOH > H-O-H > R-O-H
2. Stability of the conjugate base: the more stable the resulting conjugate base, the more willing the acid is to donate its proton, the higher the acid strength RCOOH > phenol > HOH > ROH
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Structural Effects on Acidity and Basicity
In the Bronsted-Lowry Concept:
ACID strength – proton-donating ability BASE strength – proton-accepting ability Factors Affecting Acidity and Basicity
1. Direction of polarity of H-X bond 2. Strength of the H-X bond 3. Stability of the conjugate base 4. Type of solvent
ACID and BASE strength
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Factors Affecting Acidity and Basicity
1. Direction of polarity of H-X bond
The H in an H-X bond should be partially positive. The more polar the bond, the stronger the acid.
2. Strength of the H-X bond
Stronger bonds are less easily dissociated than
weaker bonds.
3. Stability of the conjugate base
In general, the more stable the conjugate base, the
stronger the acid.
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ACID strength
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• Anything that stabilizes a conjugate base A:¯ makes the
starting acid H—A more acidic.
• Four other factors affect the acidity of H—A. These are:
Element effects (trends in periodic table)
Inductive effects (electronegativity)
Resonance effects (multiple resonance structures)
Hybridization effects (sp, sp2, sp3)
3. Stability of the conjugate base
ACID strength
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• No matter which of these factors is discussed, to compare
the acidity of any two acids:
o Always look at the conjugate bases.
o Determine which conjugate base is more stable.
o The more stable the conjugate base, the more acidic the acid.
• The strengths of a conjugate acid and its conjugate base are inversely related.
• A strong conjugate base has a weak conjugate acid.
• A weak conjugate base has a strong conjugate acid.
3. Stability of the conjugate base
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Element Effects—Trends in the Periodic Table.
Across a row of the periodic table, the acidity of H—A
increases as the electronegativity of A increases.
Factors that determine ACID strength
Down a column of the periodic table, the acidity of H—A increases
as the size of A increases.
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• Size, and not electronegativity, determines acidity down a column.
• The acidity of H—A increases both left-to-right across a row and down a column of the periodic table.
• Although four factors determine the overall acidity of a particular
hydrogen atom, element effects—the identity of A—is the single most important factor in determining the acidity of the H—A bond.
Factors that determine ACID strength
Element Effects—Trends in the Periodic Table.
Positive or negative charge is stabilized when it is spread over
a larger volume.
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Inductive Effects
• An inductive effect is the pull of electron density through
bonds caused by electronegativity differences between atoms.
• In the example below, when we compare the acidities of ethanol and 2,2,2-trifluoroethanol, we note that the latter
is more acidic than the former.
Factors that determine ACID strength
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• The reason for the increased acidity of 2,2,2-
trifluoroethanol is that the three electronegative fluorine atoms stabilize the negatively charged conjugate base.
• This effect is limited to a three bond distance.
Inductive Effects
Factors that determine ACID strength
Resonance Effects
• Electron delocalization
• Resonance structures – Individual Lewis structures in
cases where two or more Lewis structures are equally good descriptions of a single molecule.
• ethanol vs. acetic acid
Factors that determine ACID strength
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• When the conjugate bases of the two species are compared, it is evident
that the conjugate base of acetic acid enjoys resonance stabilization, whereas that of ethanol does not.
Resonance Effects
Factors that determine ACID strength
• Resonance delocalization makes CH3COO¯ more stable than CH3CH2O¯, so
CH3COOH is a stronger acid than CH3CH2OH.
• The final factor affecting the acidity of H—A is the hybridization.
• The higher the percent of s-character of the hybrid orbital, the
closer the lone pair is held to the nucleus, and the more stable
the conjugate base.
Let us consider the relative acidities of three different compounds
containing C—H bonds.
Hybridization Effects
Factors that determine ACID strength
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Figure. Electrostatic Potential plots
Hybridization Effects
Factors that determine ACID strength
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Summary of the factors that determine acidity
Factors that determine ACID strength Factors that determine ACID strength
Factors that determine ACID strength
Exercise:
For each of the following compounds, indicate which is the stronger acid:
For each of the following compounds, indicate which is the stronger base:
Factors that determine ACID strength
Exercise:
“Hard” acids and bases have low polarizability. Their orbitals do not change their shapes, so the interaction is mostly driven by electrostatic forces. “Soft” acids and bases have high polarizability. Their orbitals change their shapes and drive the interaction.
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Aromaticity and acidity Compound pKa1 pKa2 Struct. Effects
A
Benzoic Acid 4.20
o-methylbenzoic Acid 3.91
o-chlorobenzoic acid 2.94
o-tertbutylbenzoic acid 3.46 STERIC and POLAR o-nitrobenenzoic acid 2.17
B
O-hydroxybenzoic acid 2.98 Intramolecular H-Bond 2,6-dihydroxybenzoic acid 2.30
C
Pthalic Acid 2.98 5.28
H-bond and Polar Isophthalic Acid 3.46 4.46
Terephthalic Acid 3.51 4.82
acidity
Compound pKa1 pKa2 Structural Effects
D
p-methoxybenzoic acid 4.47
p-cyanobenzoic acid 3.55
p-methylbenzoic acid 4.34 Electron delocalization p-nitrobenzoic acid 3.44
acidity
Compound Ka Ethanol 1 .0 x 10-18
Phenol 1.2 x 10-10
o-nitrophenol 6.8 x 10-8
m-nitrophenol 5.0 x 10-9
p-nitrophenol 7.0 x 10-8
2,6-dinitrophenol 1.0 x 10-4
2,4-dintrophenol 5.6 x 10-4
2-nitroresorcinol 1.59 x 10-6
4-nitroresorcinol 1.04 x 10-6
acidity
Compound pKb Compound pKb
Ammonia 4.75 o-methylaniline 9.62
Methylamine 3.36 m-methylaniline 9.33
Dimethylamine 3.23 p-methylaniline 9.00
Trimethylamine 4.20
o-nitroaniline 14.28
Ethylamine 3.33 m-nitroaniline 11.55
Diethylamine 3.07 p-nitroaniline 13.02
Aniline 9.38
N-methyl aniline 9.60 p-methoxyaniline 8.71
p-hydroxyaniline 8.50
triethylamine 3.42
Basicity
Strength of Organic Bases when Acid is a Proton