Acids and Bases

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Acids and Bases
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Different Definitions of Acids and Bases

• Arrhenius definitions for aqueous solutions.
• acid a substance that produces H (H3O) ions
  aqueous solution
• base a substance that produces OH- ions in
  aqueous solution

Bronsted-Lowry definitions for aqueous and
non-aqueous solutions. Conjugate acid base
pair molecules or ions interconverted by
transfer of a proton. acid transfers the
proton. base receives the proton.
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Lewis Acids and Bases
Focuses on the electrons not the H. An acid
receives electrons from the base making a new
bond. Acid electron receptor. Base electron donor.
Types of electrons
Energy
lone pairs pi bonding electrons
sigma bonding electrons
Basicity
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Acid Base Eqilibria
The position of the equilibrium is obtained by
comparing the pKa values of the two acids.
Equivalently, compare the pKb values of the two
bases.
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Acid Base Eqilibria
Same equilibrium with electron pushing (curved
arrows).
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Lone Pair acting as Base.
Note the change in formal charges. As reactant
oxygen had complete ownership of lone pair. In
product it is shared. Oxygen more positive by
1. Similarly, B has gained half of a bonding
pair more negative by 1.
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An example pi electrons as bases
Bronsted Lowry Base
Bronsted Lowry Acid
For the moment, just note that there are two
possible carbocations formed.
The carbocations are conjugate acids of the
alkenes.
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Sigma bonding electrons as bases. Much more
unusual!!
A very, very electronegative F!! A very
positive S!! The OH becomes very acidic because
that would put a negative charge adjacent to the
S.
Super acid
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Trends for Relative Acid Strengths
Totally ionized in aqueous solution.
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Example
pKa 15.9 Weaker acid
pKa 9.95 Stronger acid
H2O PhOH H3O PhO-
H2O EtOH H3O EtO- Ka
H3OEtO-/EtOH 10-15.9
Ka H3OPhO-/PhOH 10-9.95
Ethanol, EtOH, is a weaker acid than phenol,
PhOH. It follows that ethoxide, EtO-, is a
stronger base than phenolate, PhO-. For reaction
PhOH EtO- PhO- EtOH where does
equilibrium lie?
Weaker base.
Stronger base
K 10-9.95 / 10-15.9 106.0
Query What makes for strong (or weak) acids?
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What affects acidity?
1. Electronegativity of the atom holding the
negative charge.
Increasing electronegativity of atom bearing
negative charge. Increasing stability of anion.
Increasing acidity.
2. Size of the atom bearing the negative charge
in the anion.
Increasing size of atom holding negative charge.
Increasing stability of anion.
Increasing acidity.
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What affects acidity? - 2
3. Resonance stabilization, usually of the anion.
Increasing resonance stabilization. Increased
anion stability.
Increasing basicity of the anion.
Acidity
No resonance structures!!
Note that phenol itself enjoys resonance but
charges are generated, costing energy, making the
resonance less important. The more important
resonance in the anion shifts the equilibrium to
the right making phenol more acidic.
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An example competitive Bases Resonance

• Two different bases or two sites in the same
  molecule may compete to be protonated (be the
  base).

Acetic acid can be protonated at two sites.
Pi bonding electrons converted to non-bonding.
Which conjugate acid is favored? The more stable
one! Which is that? Recall resonance provides
additional stability by moving pi or non-bonding
electrons.
No valid resonance structures for this cation.
Non-bonding electrons converted to pi bonding.
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An example competitive Bases Resonance
Comments on the importance of the resonance
structures.
All atoms obey octet rule!
The carbon is electron deficient 6 electrons,
not 8. Lesser importance
All atoms obey octet rule!
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What affects acidity? - 3
4. Inductive and Electrostatic Stabilization.
Increasing anion stability.
Increasing anion basicity.
Acidity.
d
d
Due to electronegativity of F small positive
charges build up on C resulting in stabilization
of the anion.
Effect drops off with distance. EtOH pKa 15.9
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What affects acidity? - 4
Note. The NH2- is more basic than the RCC- ion.
5. Hybridization of the atom bearing the charge.
H-A ? H A-.
sp3 sp2 sp
More s character, more stability, more
electronegative, H-A more acidic, A- less
basic.
Increasing Acidity of HA
Increasing Basicity of A-
Know this order.
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Example of hybridization Effect.
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What affects acidity? - 5
6. Stabilization of ions by solvents (solvation).
Solvation provides stabilization.
Comparison of alcohol acidities.
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pKa 15.9
Crowding inhibiting solvation
(CH3)3CO -, crowded
Solvation, stability of anion, acidity
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Example
Para nitrophenol is more acidic than phenol.
Offer an explanation
The lower lies further to the right.
Why? Could be due to destabilization of the
unionized form, A, or stabilization of the
ionized form, B.
B
A
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Examine the equilibrium for p-nitrophenol. How
does the nitro group increase the acidity?
Examine both sides of equilibrium. What does the
nitro group do? First the unionized acid.
Note carefully that in these resonance structures
charge is created on the O and in the ring
or on an oxygen. This decreases the importance of
the resonance.
Structure D occurs only due to the nitro group.
The stability it provides will slightly decrease
acidity.
Resonance structures A, B and C are comparable to
those in the phenol itself and thus would not be
expected to affect acidity. But note the to
attraction here
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Now look at the anion. What does the nitro group
do? Remember we are interested to compare with
the phenol phenolate equilibrium.
In these resonance structures charge is not
created. Thus these structures are important and
increase acidity. They account for the acidity
of all phenols.
Structure D occurs only due to the nitro group.
It increases acidity. The greater amount of
significant resonance in the anion accounts for
the nitro increasing the acidity.
Resonance structures A, B and C are comparable to
those in the phenolate anion itself and thus
would not be expected to affect acidity. But
note the to attraction here
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Sample Problem