9/5/2014
Acids and Bases
Definitions
1. Arrhenius:
a)
b)
2. Bronsted-Lowry
a)
b)
CH3CO2H
+
Acid
Keq
H2O
CH3CO2–
Conjugate base
Base
H3O+
+
Conjugate Acid
3. Lewis
a)
b)
Cl
+
B
Cl
Cl
H3C
O
C
CH3
In this course/organic chemists:
Acid = proton donor
Lewis Acid = non-protic acids
Chemists use pKa values to discuss relative acidities of protic acids.
We will:
1) reexamine how one experimentally determines a pKa value
2) determine the direction/shift of an acid/base equilibrium
3) examine pKa values for various protic functional groups
9/13/2007
Determination of pKa
Keq
[CH3CO2–][H3O+]
=
Ka
[CH3CO2H][H2O]
=
Keq[H2O]
[CH3CO2–][H3O+]
=
[CH3CO2H][ 1 ]
pKa = –log(Ka)
pH = –log[H3O]+
If you take the log of both sides of equation above, you get:
log Ka = log
[H3O+]+log
[CH3CO2–]
[CH3CO2H]
When the acid is 50% dissociated the acid concentration will equal the
conjugate base concentration. Thus, log 1 = 0 and this leaves
pKa = pH @ 50% dissociation of the acid
Discussing pKa:
The stronger the acid, the smaller the pKa
Strong Acids have weak conjugate bases
Predicting Acid/base equilibria:
1. Find the most acidic proton of each acid
2. Estimate the pKa value based on “known” values
3. The direction of equilibria shifts in the direction of the weaker acid (the one with the
higher pKa value)
Note: the shift will be quantitative (complete) if Δ pKa of acid/conj acid is > 3
9/13/2007
Examples:
+
HBr
H 2O
Br–
H3O+
+
Amines: think organic base
H
H3C N
+
H
O H
H
H 3C N H
H
+
O
Amines can also be acids:
H
H 3C N
+
H
CH3–
H3C N
H
+
Na+ –OCH2CH3
+
CH4
Carboxylic acids…the organic acids:
O
O
H OCH2CH3 +
Na+
O
CH3
H O
CH3
7. (40 pts) For the following reactions:
a) Circle the acids and conjugate acids.
b) Estimate the pKa values for each. The table below may be of some help.
c) Use your pKa values to determine the direction of equilibrium. Indicate whether the reaction
is quantitatively to the left, mostly left, 50:50, mostly right or quantitatively to the right.
Quantitaive means >100:1.
H H
H
H
H
A
–OH
+
H
quant. left
+
CH 3
Circle 1:
mostly left
50:50
O
HO Cl O
O
quant. left
mostly left
+
O
mostly right
H
50:50
O
C
C
NH 4+
+
Circle 1:
quant. left
mostly left
50:50
CH 3
+
D
mostly left
50:50
quant. right
+
mostly right
NH 3
O
O
O Cl O
O
+
O
quant. left
quant. right
H
mostly right
OH
Circle 1:
H 2O
H
Circle 1:
B
+
H
H
O
C
H
mostly right
OH
quant. right
O
H
quant. right
9/13/2007
Common Acids used by Organic Chemists
•
Strong acids:
The mineral acids (HBr, HCl, H2SO4, HNO3, etc)
O
S OH
O
H 3C
TsOH
para-toluenesulfonic acid
•
Weak acids:
Carboxylic acids and phenols
the carboxylic acid
functional group
H 3C
O
C
OH H3C
OH
acetic acid
phenol
OH
4-methylphenol
Common Bases used by Organic Chemists
H 3C
Strong bases
H 3C
H3C
N
H
Li+
CH3
H +
Li
H
Li+
Na+
N
t-butyllithium
n-butyllithium
lithium diisopropylamide
sodium amide
H
K+ –OC(CH3)3
Na+ –OCH2CH3
potassium tert-butoxide (KOtBu)
sodium ethoxide
Na+ –OCH3
sodium methoxide
Na+ –OH
sodium hydroxide
triethylamine (Et3N)
(CH3CH2)3N
Weak bases
N
pyridine
9/13/2007
Acid/Base Trends (pKa)
Conj. Base
HSO4–
Acid
H2SO4
pKa
–10
NO3–
–1.4
CH3CO2H
CH3CO2–
4.76
vs.
HCN
–C
9.1
Electronegative stabilization
H2O
HO–
15.7
CH4
CH3–
55
NH3
NH2–
34
H2O
HO–
15.7
HF
F–
3.2
FCH2CO2H
FCH2CO2–
2.59
ClCH2CO2H
ClCH2CO2–
2.86
CCl3CO2H
CCl3CO2–
0.6
H
H
N
Li
H
H
H
H
H
H
n-butane
H
H
H
H
H
H
H
H H
H
tert-butyllithium
H
H
H
H
H
H
H
H
vs.
H H
H
H
Li
H
n-butyl lithium
H
Inductive effect stabilizes the
conjugate base
H
H
H
Electronegative stabilization
of the conjugate base
Li
H
H
H
Resonance stabilization
HNO3
H
H
Comment
H
H
vs.
H
H
H
H
H
isobutane
Compare the acidity of these
two hydrogens