Post on 16-Dec-2015
Acids and Bases
Different Definitions of Acids and Bases
• Arrhenius definitions for aqueous solutions.
acidacid:: a substance that produces H+ (H3O+) ions aqueous solution
basebase:: a substance that produces OH- ions in aqueous solutionH+(aq) + H2O(l) H3O
+(aq)Hydronium ion
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.
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.
H
O
H
H+
H
O
H
H
base
acid
lone pairs pi bonding electrons sigma bonding electrons
Energy
Basicity
Types of electrons:
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.
Acid – Base Eqilibria
Same equilibrium with electron pushing (curved arrows).
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.
An example: pi electrons as bases
Bronsted Lowry Acid
Bronsted Lowry Base
The carbocations are conjugate acids of the alkenes.
For the moment, just note that there are two possible carbocations formed.
Sigma bonding electrons as bases. Much more unusual!!
Super acid
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.
Trends for Relative Acid Strengths
Totally ionized in aqueous solution.
Aqueous Solution
Totally unionized in aqueous solution
Example
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?
pKa = 9.95
Stronger acid
H2O + PhOH H3O+ + PhO-
Ka = [H3O+][PhO-]/[PhOH] = 10-9.95
OH
phenol, PhOH
CH3CH2OH
ethanol, EtOH
Recall
H2O + EtOH H3O+ + EtO-
Ka = [H3O+][EtO-]/[EtOH] = 10-15.9
pKa = 15.9
Weaker acid
Stronger base
Weaker base.
Query: What makes for strong (or weak) acids?
K = 10-9.95 / 10-15.9 = 106.0
What affects acidity?1. Electronegativity of the atom holding the negative charge.
CH3OH CH3O - + H+
CH3NH2 CH3NH - + H+
CH3CH3 CH3CH2- + H+
Increasing electronegativity of atom bearing negative charge. Increasing stability of anion.
Increasing acidity.
Increasing basicity of anion.
2. Size of the atom bearing the negative charge in the anion.
CH3OH CH3O - + H+; pKa = 16
CH3SH CH3S - + H+; pKa = 7.0
Increasing size of atom holding negative charge. Increasing stability of anion.
Increasing acidity.
Increasing basicity of anion.
OO
What affects acidity? - 23. Resonance stabilization, usually of the anion.
OH
phenol, PhOH
OO
ethanol, EtOHCH3CH2OH CH3CH2O
- + H+
Increasing resonance stabilization. Increased anion stability.
Aci
dit
y
Increasing basicity of the anion.
No resonance structures!!
OH OH
etc.
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.
An example: competitive Bases & Resonance
• Two different bases or two sites in the same molecule may compete to be protonated (be the base).
O
O H
acetic acid
H+O
O H
HH+
O
O H
H
Acetic acid can be protonated at two sites.
Which conjugate acid is favored?
The more stable one! Which is that?
Recall resonance provides additional stability by moving pi or non-bonding electrons.
Pi bonding electrons converted to non-bonding.
O
O H
H
O
O H
H
Non-bonding electrons converted to pi bonding.
No valid resonance structures for this cation.
An example: competitive Bases & Resonance
H+O
O H
H
O
O H
H
O
O H
H
O
O H
acetic acid
All atoms obey octet rule!
All atoms obey octet rule!
The carbon is electron deficient – 6 electrons, not 8.
Lesser importance
Comments on the importance of the resonance structures.
What affects acidity? - 34. Inductive and Electrostatic Stabilization.
F3CCH2O - + H+
H3CCH2O - + H+H3CCH2OH
F3CCH2OH
Due to electronegativity of F small positive charges build up on C resulting in stabilization of the anion.
Increasing anion stability.Acidity.Increasing anion basicity.
Effect drops off with distance. EtOH pKa = 15.9
What affects acidity? - 45. 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.
Incr
easi
ng
Aci
dit
y o
f H
A
Incr
easi
ng
B
asic
ity
of
A-
Note. The NH2-
is more basic than the RCC-
ion.
Know this order.
Example of hybridization Effect.
RCCH + AgNO3 AgCCR (ppt)
acid base
terminal alkyne
non-terminal alkyne
RCCR + LiCH2CH2CH2CH3 No Reaction
RCCH + LiCH2CH2CH2CH3 HCH2CH2CH2CH3 + RCCLi
RCCR + AgNO3 NR
What affects acidity? - 5
6. Stabilization of ions by solvents (solvation).
H
O RO R + H
H
O
H
H
O
HH
OH
Solvation provides stabilization.
OH
ethanol
OH
propan-2-ol
OH
2-methylpropan-2-olCrowding inhibiting solvation
Solvation, stability of anion, acidity
pKa = 15.9 17 18
(CH3)3CO -, crowded
Comparison of alcohol acidities.
Example
Para nitrophenol is more acidic than phenol. Offer an explanation
OH
OH
N
O O
O
O
N
O O
+ H
+ H 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.
A B
OH
N
O O
Examine the equilibrium for p-nitrophenol. How does the nitro group increase the acidity?
O
N
O O
+ H
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
OH
N
O O
OH
N
O O
OH
N
O O
OH
N
O O
A B C D
Structure D occurs only due to the nitro group. The stability it provides will slightly decrease 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.
OH
N
O O
O
N
O O
+ H
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
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.
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.
O
N
O O
O
N
O O
O
N
O O
O
N
O O
A B C D
3. (3 pts) Which is the stronger base and why?
HNvs
HN O
Sample Problem
H2N H2N O H2N O