Stefan Svensson 2004
ACID and BASES - a Summary
Brönsted-Lowry : Acids donate protons
Bases accept protons
Lewis -acid : Electron pair acceptor
Lewis-base: Electron pair donator.
Acetic acid
O
O
CH3 C OH
ättiksyra
Aniline
NH2
+ H2O
CH3 C O
+ H2O
NH3
+
H3O
+
OH
Keq = [H 3 O+] [CH3 COO-]
[CH3 COOH][H2 O]
Acidity constant Ka:
Ka = K eq [H2 O] = [H3 O+] [CH3 COO-]
[CH3 COOH]
Ju starkare syra
-
= 1,76 x 10-5
pKa = -log Ka =4,76
ju svagare korresponderande bas.
Basstyrkan kan relateras till pKa för dess korresponderande syra.
Ju större pKa värde för korresponderande syra- ju starkare är basen.
Relative strength for some acids and their conjugate bases,
Strongest acid
Weakest acid
ACID
HSbF6
HCl
CH3COOH
CH3NH3+
H2O
(CH3)3COH
NH3
CH3CH3
Ex Lewis acids:
HCl, HNO3, HClO4
BF3
pKa
> -12
-7
4.8
10
Conj. BASE
SbF6ClCH3COOCH3NH2
OH-
15,7
18
33
50
AlCl3
Weakest base
(CH3)3CONH2CH3CH2TiCl4
Strongest base
ZnCl2
are complete ionised in water and appears to have the same
strength (the leveling effect of water)
Factors that influence the acidity of an organic compound H-A
A
B
C
D
•
•
•
•
Examples
The strength of the H-A bond
The electronegativity of A
Factors stabilising A- compared with H-A
The nature of the solvent
CH3
H
pKa ≈ 43
CH3 O H
pKa ≈ 16
OH
pKa ≈ 10
RCOOH
pKa ≈ 4-5
A- Bonding strength to the proton
H-F
pKa: 3,2
F-
<
H-Br
-7
>
H2O <
OH - >
B-
H-Cl <
<
-9
Cl -
>
H2S
SH -
Br -
<
>
H-I
-10
>
I-
H2Se
SeH -
Increased acidity ⇔
decreased bonding strength
Increased Basicity
Higher acidity
Increased Basicity
Acidity increase with electronegativity
CH4
<
NH3
<
R-OH <
HF
Electronegativity affect both polarity and the stability of the anion.
Acidity increase with increased s-character in the hybridisation
CH3 -CH3 <
H2 C=CH 2 <
HC≡CH
Increased acidity
sp3
sp2
sp
pKa: ≈
50
44
25
Increased s-character binds the electrons closer to the carbon nucleus
More s-contribution
⇒ lower energy and higher anion stability
Ex.
CH3 C C H
pKa = 25
+
NH 2
NH 3
CH3 C C
+
NH 3
pKa = 33
C
Lower pK a for Carboxylic acids than Phenols due to:
Resonance structures of the anion have identical energy
The anion contain two electronegative oxygen atoms
O
H
C
pKa ≈ 3,8
OH
Phenol:
OH
O
O
pKa
CH3 CH2 OH
15,9
H
O
C
H
O
O
pKa ≈ 10
O
CH3 C OH
4,8
C
O
O
O
O
O
Cl CH2 C OH
2,9
O
Cl CH2 C O
The negative charge is spread by electronwithdrawing and thereby stabilising the anion
D. Polar solvent with high dielectric constant (ε) have better ability to solvate ions
Water is extremely effective as ion solvating medium and is readily polarised, and
can thereby stabilise and solvate both cations and anions
The solvent must act as a base otherwise can not acids dissociate.
Ex. HCl is a strong acid in methanol but not in toluene.
ACIDS
Aliphatic acids
Alkyl groups can inductively decrease the acid strength
Compare: acetic acid and formic acid pKa 4,76 versus 3,77
O
O
Me
H
C
C
O
O
But often depends differences in relative solvation possibilities of ionisation.
∆Go = ∆Ho - Τ∆So
and
∆Go = -2.303 RT log K eq
pKa
∆Go
∆Ho
Τ∆So
Acetic acid
4,76
6,5
-0,13
- 6,6
Kcal
Formic acid
3,77
5,1
-0,07
- 5,17
Kcal
Low enthalpy The energy required for dissociation of the O-H bond is canceled by
the energy evolved in solvating the resultant ions.
Entropy has a greater effect Through solvation of the ions by water molecules is
the orderliness increased. Differential solvation of the acid anions makes the acid
strength to differ. Formiat ion is stronger solvated.
For other short aliphatic acid (C3 -C5 ) are the differences in pKa small,
Minor steric effects may count for the differences (pKa ≈ 4.80 - 5,05)
CH3 CH2 COOH
pKa
4,88
sp3
CH2 CH COOH
4,25
sp2
1,88
sp
Hybridisation
HC
C COOH
Stronger acid
Electrons are drawn closer to unsaturated carbon nucleus - larger s contribution
This change the inductive effect from donating to withdrawing when sp3 → sp.
Similar to acidities in the serie: ethane - ethene- ethyne
Substituted aliphatic acids
CH3
O
C OH
4,76
O
C OH
F CH2
Cl CH2
2,57
Cl CH2
O
C OH
Br CH2
2,86
O
C OH
Cl
Cl
1,25
I CH2
2,90
O
CH C OH
2,86
O
C OH
O
C OH
3,16
O
Cl
Cl C C OH
Cl
0,65
Inductive effects ( electron withdrawing, EW) delocalise the negative charge over
the whole of the anion. The water can be less ordered to solvate the ions.
The changes in pKa and then free energy is largely due to entropy factor also here.
Entalphy differ only little with different substituent.
The anionic charge gets more concentrated as the EW-substituent is situated further
apart ⇒ increased ∆S
Phenols
The Inductive effect falls off with distance from orto > meta > para, but is also
combined with mesomeric effect which affect primary at orto- and para-positions.
C6 H5 OH
o-O2 NC6 H4 OH
m-O2 NC6 H4 OH
p-O2 NC6 H4 OH
2,4-(O2 N)2 C6 H4 OH
2,4,6-(O2 N)3 C6 H4 OH
pKa
9,95
7,23
8,35
7.14
4,01
1,02
O
O
N
O
N
O
O
With more powerful EW groups the negative charge gets delocalised
⇒ decreased ∆S (solvation can be less ordered) ⇒ lower pKa.
Alkyl groups have only marginal effects.
Methyl
pKa
C6 H5 OH
o-MeC6 H4 OH
m-MeC6 H4 OH
p-MeC6 H4 OH
9,95
10,28
10,08
10.19
O
Aromatic carboxylic acid
Benzoic acid pKa 4,20 is stronger than the saturated acid ( 4,87).
Phenyl as double bond is less electron donating than saturated acids.
pKa of X-C6 H4 COOH
H
Me
NO2
Cl
4,20
4,20
4,20
4,24
4,34
2,17
3,45
3,43
2,94
3,83
3,99
O
O
Br
OMe OH
2,85
3,81
4,00
4,09 2,98
4,09 4,08
4,47 4,58
O
O
C
O
O
C
C
X
X
N
O
O
EW-groups increases the acid strength, mesomeric effect may also decrease strength.
HO and MeO- groups may have both inductive (EW) and mesomer effect (ED)
depending on position. The effect can give a weaker acid also.
O
Orto groups may also besides short
inductive distance act through space,
or as few cases with intra-molecular
hydrogen bonding
C
O
H
O
O
- H+
H
C
H
O
O
Again is the ∆S term most important for the pKa value.
Dicarboxylic acids
pKa
1,23
2,83
4,19
HOOC-COOH
HOOC-CH2 -COOH
HOOC-CH2 -CH2 -COOH
The inductive EW- effect of the second
COOH falls off sharply as the COOH-groups
are separated more than one saturated carbon.
Maleic acid has a low pK a1 compared with fumaric acid due to intra molecular Hbonding, which on the other hand also makes pKa2 higher due to stabilisation.
O
H
C
C
O
O
H
C
H
C
O
-H
-
H
C
C
C
H
Maleic acid
C
O
pKa1= 1,92
pKa2= 6,23
O
COOH
C
H
O
H
H
O
C
HOOC
H
Fumaric acid
pKa1= 3,02
pKa2= 4,38
For malic and succinic acid is pKa2 higher as the first COO- group is Elect. Donating.
As the entropy has a major effect on pKa:s also temperature influences the value of
pKa.
BASES
More convenient to use pKa also for bases
Ka =
[B] [ H3 O+ ]
[ BH+ ]
The smaller value of pKa for BH+ the weaker B is as a base
NH4 + +H2 O
NH3
∆Go
12,6
pKa
9,25
∆Ho
12,4
+ H3 O+
Τ∆So
0,2 Kcal
Enthalpy changes are more important than entropy changes.
∆S is low as both sides has the same kind of ions, equally solvated.
Aliphatic bases
pKa:
NH3
Me NH 2
Me
NH
Me
9,25
10,64
10,77
Me
Me N
Me
9,80
Et NH 2
Et
NH
Et
10,67
10,93
Et
Et N
Et
10 88
Alkyl groups on ammonia increases the base strengthThe first one markedly, the
second slightly but the third actually decrease base strength.
Not only electron availability on the nitrogen also solvation of the cation must be
stabilised. Tertiary amines less easily solvated.
The more hydrogen atoms attached on nitrogen the more powerful solvation via Hbonding between these and water.
H2O
H
R
N
H2O
H
H
O2H
>
H2O
H
R
N
H2O
H
R
>
H2O
H
R
N
R
R
Decreasing stabilisation by solvation
Increasing electron-donating inductive effect on basicity
In solvent where ions are not solvated by H-bonding is the order of base strength
the same as the inductive effect of the alkyl groups.
In chlorobensen or gas phase:
But NH2
<
But 2 NH
<
But 3 N
EW inductive groups reduce the base strength: Cl, NO2 CF3
F3 C
F3 C
N
F3 C
O
R C
O
C
N
C
O
O
NH 2
R C
NH 2
H
acidic H
Amide nitrogens are non basis due to mesomeric EW effect (pKa ≈ 0,5).
Pthalimide, with two carbonyls , is acidic and non-basic.
R4 N+ OH- as a ion pair has a base strength alike alkali bases
Guanidine has pKa of ≈ 13,6 and protonation gives three exactly equal resonance
structures.
Anilines
NH 2
NH 2
NH 2
NH 2
NH 2
H NH 2
+
H
pKa ≈ 4,6 (Compare with cyclohexylamine pKa 10,7)
Unshared electrons on N can interact with the delocalised π-electrons in the ring.
In protonated form is this stabilisation not available.
Ph2NH pka ≈ 0,8 and Ph3N is not basic at all.
pKa
Alkyl groups
C6 H5 NH 2
4,62
4,38
o-Me-C6 H4 NH 2
C6 H5 NHMe
4,84
4,67
m-Me-C6 H4 NH 2
C6 H5 NMe2
5,15
5,10
p-Me-C6 H4 NH 2
Alkyl groups do only effect little whatever position, and the main effect on the base
strength is the mesomeric stabilisation of the aniline molecule with respect to the
cation.
Ex Nitro- hydroxy- and methoxy- substituted anilines.
C6 H5 NH 2
4,62
O2 N-C6 H4 NH 2
o- - 0,28
m- 2,45
p- 0,98
NH 2
N
O
O
O
HO-C6 H4 NH 2
MeO-C6 H4 NH 2
o- 4,49
m- 4,20
p- 5,29
o- 4,72
m- 4,17
p- 5,30
NH 2
NH 2
NH 2
N
OMe
OMe
O
Orto position gives greatest effect due to strongest inductive effect but also by
direct interaction by steric and H-bonding.
H
H
O
O
N
O
N
N
O
O
CH3
O
N
N
N
O
CH3
O
N
O
Stronger base
N
O
O
O
2,4,6-trinitro-N,N-dimethyl aniline is much more stronger base than N,N-dimethylaniline or 2,4,6-trinitro-aniline, because the orto groups inhibit resonance
interaction by steric reason.
Hetrocyclic bases
Pyridine (A) aromatic (sp2) pKa 5,2 less basic than ex. triethylamine (sp3)
As nitrogen becomes more multiply bonded its lone pair of electrons is
accommodated in an orbital with more s character., the electron are drawn closer to
the nitrogen nucleus. Compare also MeCN pKa ≈ -4,3 (sp).
Hybridisation
+
N
N
H
N
N
R3 N >
> RC N
N
(A)
N
H
H
N
H
(B)
H
N
H
(C)
Base strength
Pyrrole (B) have aromatic character, the electron pair is incorporated in the aromatic
6 π-system, which gives a weaker base. α-Carbons is more basic.
Pyrrolidine (C) on the other hand have pKa ≈11,3 resembling of diethylamine.