Acid-Base Chemistry HA+:B A - + HB + K a is the equilibrium constant for the dissociation of an acid into H + and its conjugate base. Ka values are typically measure in water. aci d bas e conjuga te base conjuga te acid We define pK a = -logK a just as pH = -log[H 3 O + ] Weak acids have small K a and large pK a values Strong acids have large K a and small pK a values Remember – when comparing acids, the weaker acid will have…the stronger conjugate base. HA H + + A - HA H + + A - HA A H a a a a K HA and A - are a conjugate acid- base pair. HB + and B: are also a conjugate acid-base pair. 1
Transcript
Acid-Base Chemistry
HA + :B A- + HB+
Ka is the equilibrium constant for the dissociation of an acid into H+ and its conjugate base. Ka values are typically measure in water.
acid base conjugate base
conjugate acid
We define pKa = -logKa just as pH = -log[H3O+]
Weak acids have small Ka and large pKa valuesStrong acids have large Ka and small pKa values
Remember – when comparing acids, the weaker acid will have…the stronger conjugate base.
HA H+ + A-HA H+ + A-
HA
AHa a
aaK
HA and A- are a conjugate acid-base pair.HB+ and B: are also a conjugate acid-base pair.
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N C H S
H
N C H S
H
N C H O
H
HN C H
H
O
H
H O NCH NCH
OH
A Brønsted acid is simply an electrophile that loses H+ to a base. The acidic site is the hydrogen atom. A Brønsted base is simply a nucleophile that makes a bond to the hydrogen atom of an acid. The basic site is the atom/bond donating the electrons used to make this bond.
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Acid Ka pKa
HClO4 1010 -10
HF 7.2 x 10-4 3.1
NH4+ 6.3 x 10-10 9.2
H2O 2.0 x 10-16 15.7
H2 10-35 35
some typical pKa values
Acid-Base Chemistry
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So, for an equilibrium between two conjugate acid/base pairs, the equilibrium will always favour the side which has…the weakest acid.
What does this mean for ‘real life chemistry’?
OH
+ CH3CO2
O
+ CH3CO2H
KapKa
1.0 x 10-10
101.8 x 10-5
4.72
So, if you wanted this equilibrium to be product favoured, you would use a base…whose conjugate acid was weaker than phenol. 4
pKa Compound types
-10 mineral acids: H2SO4, HI, HBr, HCl, sulfonic acids RSO3H
Resonance eff ects greatly enhance the acidity of "activated" C—H bonds...
N C CH2
H 25
O O
CH3
CH4
36
48
919
H
H9
Inductive and Field effects
H2CX C
O
O H
pKa4.874.763.172.902.862.592.461.68
XCH3HIBrClFCNNO2
+ H2OH2CX C
O
O
+ H3O+
10
ClH2C C
O
OH
Cl2HC C
O
OH
Cl3C C
O
OH
2.85 1.48 0.70
These effects are…additive.
COH
O
COH
O
COH
O
COH
O
Cl
Cl
Cl
2.86 4.05 4.52
4.82
These effects…drop off rapidly with increasing number of bonds.
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pKa + pKb for a conjugate acid/base pair = 14
Bases
Although potentially confusing, when evaluating base strength we often look at the pKa of its conjugate acid. The higher the pKa of the conjugate acid, the stronger the base.
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N:
R3N:
H2N
N
H O
R O
Conjugate acid pKa of conjugate acid
N H
R3NH
H2O
ROH
H2NH
NH
5
11
15
16-18
38
40
Base
13
The same things that affect acidity, also affect basicity...
Across the periodic table...
H3N > H2O > HF
Down a column...
H2N: > H2P: HO > HS
H3N > H3P H2O > H2S
Hybridization...
NH N:
Conj. pKa: 12.20 5.3
O
H
O
-2 -4 to -10
14
NHH
NHH
NOOpKa of conjugate is 4.62
pKa of conjugate is 1.00
Resonance…
NH NH2
Et2NH Et2NH2
base CA pKa
11.0
12.2
Sterics…
15
Inductive effects...
NH3 CH3NH2 (CH3)2NH (CH3)3N
pKa of conj. 9.25 10.65 10.73 9.78
Amphiprotic species
CH3OH + H2SO4
CH3OH + NH2
CH3OH2+ + HSO4
-
CH3O + NH3
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Acids, bases, electrophiles, and nucleophiles.
For the following molecule, identify which nitrogen atom is more basic. Why?
O
CCH2N
NH2
H H
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Acids, bases, electrophiles, and nucleophiles.
For the following molecule, identify the acidic and basic sites and rank them in terms of reactivity (by category; don’t compare acid to base). Rationalize your rankings.
O
CO
O
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Acids, bases, electrophiles, and nucleophiles.
For the following molecule, identify the acidic, basic, electrophilic, and nucleophilic sites and rank them in terms of reactivity (by category). Rationalize your rankings.
O
CCH3C
C
H H
CH
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Acids, bases, electrophiles, and nucleophiles.
When solving a mechanism, we can use the different kinds of sites (A, B, E, Nu) to help us.e.g. How can we rationalize the following reaction?
O
C CH
H
C
H
Cl
D D
H3C OO
C CC
H
D
DOH3C
HH
+ + Cl
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Acids, bases, electrophiles, and nucleophiles.
•It is incorrect to mix-and-match the terms acid and electrophile, or base and nucleophile. They are not interchangeable.
e.g. I – is a good nucleophile but not a good base
•It is, however, entirely possible for a molecule to be both an acid and an electrophile – or both a base and a nucleophile. It is even possible for the same molecule to be an acid, a base, a nucleophile and an electrophile all at the same time. In that case, how it reacts will depend on the other species in the reaction flask (since usually the most nucleophilic site reacts with the most electrophilic site – assuming it can reach).
