Chemical Structure and AcidBase Relationship
Proton Affinity on Acid-base force:
Proton affinity (PA) is The proton affinity is a quantitative measure
of the base strength of a chemical species in the gas phase.
-Big PA strong base and its conjugate acid is weak
-Acidity and bacicity of a chemical species are effected than İE, EA,
BE, Inductive effect, rezonance and charge of chemical species.
-The main factor that makes the acid-base strength order in the gas
phase different from that in the solution phase is solvation. (water
for hydration). While the solvation of the acid or base molecule
reduces the acid-base strength, the solvation of the formed ions
increases.
-The acidity of hydrides, which are binary compounds of hydrogen,
increases from left to right in a period and from top to bottom in a
group.
Acid Base Strength
Proton affinity: In gase phase, Enthalpy of reaction of an chemical species
with H+.
IE I<Br<Cl<F
PA HF>HCl>HBr>HI
Acid strenght HF<HCl<HBr<HI
Inductive effect and Rezonans
Inductive effect is property of an atom or atom groups to change
electron density of neighbouring atom due to electronegativity
difference.
Inductive effect cause to chance IE of central atom.
PA increasing Lto R
NF3 NCl3
NH3
NH3
CH3NH2
CF3CH2OH
H2O<OH-<O2-
(CH3)2NH (CH3)3N
CCl3CH2OH
PA decreasing Lto R
NO- NO2- NO3-
CH3CH2OH
Hydration Effect:
In gase phase;
HA(g)  H+(g) + A-(g)
In water solution;
HA(aq)  H+(aq) + A-(aq)
HA(aq)  HA(g) -DHh(HA)
HA(g)  H+(g) + A-(g) –PA (A-)
H+(g) + A-(g)  H+(aq) + A-(aq) ) -DHh(ion)
DH= -DHh(HA)-PA(A-) -+DHh(ion)
H2S and HF in gase phase H2S is more stronger acid but HF is more stronger
in water solution.
Hidration effect on Acid-base force:
DH= -DHh(HA)-PA(A-) -+DHh(ion)
In water solution, hidration of acid molecule
decrease acid forse also for base .
Hidration of ions increase acid forse also for base
Examples: H2S and HF, (CH3)3N and CH3NH2
Acidity resulting from hydration of metal cation
For Mn+ cation in water;
If Mn+ is a transition metal cation or q/r is big, coordinative covalent
bond form.
Other presentation of this reaction
In case of q/r increasing this reaction moves to more left and the
solution becomes more acidic.
As a result;
-In IA and IIA metal cations (they interact water with only ion-dipol
interaction), q/r increase as linear. With increasing q/r ratio,
cation make more positive oxygen and facilitate to leave proton.
-Transition metal cations are more asidic than metal cations with
simmilar q/r ratio. Because these metal cations form more high
kovalent character bond with water and proton easily leave. For
example Co2+, Fe2+ and Cd2+ is mopre acidic than Mg2+.
-Different cations of same element, such as Fe3+, Fe2+’den ve Co3+,
Co2+den more acidic. Q7r bigger cation become more positive to
oxygen.
-Metal cations with +4 and bigger than +^+ oxidation step become
more positive to oxgen thus all of the H leave and Cr+4 and Mn7+
form CrO42- and MnO4-. Their water solutions are quite asidic.
Oxides
Acidity or basicity of an oxide can be explain with Lux-Flood
definition.
The higher the tendency of an oxide to accept oxide ions, the
stronger acid and the higher its tendency to give oxide ions, the
stronger base.
Oxide includes at least one X-O bond (X: metal, semimetal or
ametal), Leaving oxide ion in this structure;
Leaving of oxide ion depending on electronegativity of X atom; increasing of
X atom make leaving of oxide ion diffucult. Thus this inreases the acidity.
Oxy acids are formed from the hydrolysis of acidic oxides and metal
hydroxides are formed from the hydrolysis of basic oxides.
Therefore, acidic oxides are acid anhydrides and basic oxides are
base anhydrides.
Basicity of Nitrogen Compound and Steric Effect
Nitrogen atom in their compounds inclu lone pair and these compounds
bahave as LB.
Steric effect both decrease acidity and basicity of the compounds.
Reacted acid and base don’t close to each other. This is called as
pre-tension
In same molecule, repulsion of substituent each other, molecule
geometry and hybridization type can be change.
Oxyacids
[OpA(OH)q]
p: Oxo number q: number of OH
A: Ametal
An oxyacid includes at least one
X-O-H. Some oxiacids include X-H
Bond.
Pauling Rules
1.
pKa1  8-5p
2.
q> 1 ise
pKa2 = pKa1 + 5
pKa3 = pKa2 + 5
For H2SO4
1. pKa1  8-5x2  -2
2. pKa2 -2 +5  +3
Force of oxyacid depends on leaving of H+ from X-O-H Easy
H
O
H
leaving of proton, more strong acid.
O
S
O
H
O
H
İncreasing of oxygen number increase acidity
HClO4 > HClO3 > HClO2 > HClO
Increasing of electronegativity of ametal
increase. H2SO4 > H2SeO4 > H2TeO4
O
S
O
O
ÖRNEK: H2CO3 asitin denel pKa değeri  6.4 dür.
1. Pauli kuralına göre beklenen pKa değeri nedir?
2. Deneysel veri ile uyumlu mudur? Nedenini izah ediniz.
1. pKa1  8 – 5x1  3
2. CO2 + H2O  H2CO3
Uyumlu değildir. Çözünmüş CO2 nin yaklaşık %1 i H2CO3 halinde bulunur.
O
C
O
O
O
O
+
H
O
OH
C
C
H
-
OH2
OH
ÖRNEK : Aşağıda bazı asitlerin denel PKa değerleri verilmiştir.
H3PO4
2.1
H3PO3
1.8
H3PO2
2.0
Bu asitlerin açık formülünü tahmin ediniz.
pKa  8 – 5 x 1 = 2
Hepsinde 1 okso grubu bulunur.
pKa
8 – 5 x 2 = -3
O
O
O
P
P
P
HO
OH
HO
H
OH
OH
Fosforik asit
Fosforöz asit
HO
H
H
Hipofosforöz asit
Inductive Effect
Electronegative groups
Cl
C
F, Cl, Br, NO2
Electronegative atom remove
electron density from C atom.
Electropositive groups
CH3
C
R, CH3
Alkyl groups direct electron density
to carbon atom.
Lewis acidity BF3>BH3>BMe3
İncreasing
electronegativity increase
acidity
İncreasing methyl number
decrease acidity.
Cl atom move away
decrease
Acidity.
Electronegativity and Size Effect
In hydro acids across a group Acidity increases as
atomic diameter increases.
Large anions have lower charge density and attract
the H atom more weakly.
In hydro acids over a period, as electronegativity increases, acidity
increases.
As the electronegativity of the conjugate base increases H+ ion
dissociates more easily.
.
Hybridyzation Effect
sp3
H
pKa
H C H
50
H
İncreasins s character increase
electronegativity and increase acidity.
H H
sp2
H C C H
35
sp
H C C H
25
H
C
10-45
C
sp2
:
H+ +
C
C
10-26
C
C
H
H+ +
C
C :
sp
Steric and Solvatation
CH 3
COOH
4.75
CH 3CH 2
COOH
4.87
CH 3CH 2CH 2
COOH
4.81
…. Close to each other pKa
CH 3
CH 3
C COOH
CH 3
…. More high pKa
Solvatation decrease ion energy..
H
O H
H O
H
O H
O
C
O H
Branched
H
-
steric effect
H O
O
H
H
H O
C
Not solvatation.
O H
H O
5.02
H
-
H
H O
O H
H O
O H
H
Decrease acidity
Polarization, Hard and Soft Acid-Base
Polarization, , is the distortion that occurs as a result of the migration
of electrons from one region to another under the influence of an
external electric field. As  increases, the electron cloud becomes more
easily polarized.
Polarization
- If the charge of the cation is large and its diameter is smal Fe(III) >
Fe(II)
-If the anion has a large charge and a large diameter S-2 > O-2
-The electron configuration of the cation determines the polarizing
power.* (n-1)dxns0 > (n-1)s2(n-1)p6ns0 Hg2+ (116 pm) > Ca2+ (114
pm)
* Shielding effect of d electrons small and Z* is more
HARD: Small atoms and ions, small polarization, HOMO-LUMO energy
difference is large
SOFT: Larger atoms and ions, larger polarization, smaller HOMO-LUMO
energy difference
Verici atom : N, O, F
Verici atom: P ve S
Pearson Principle
Hard acids prefer hard bases, soft acids prefer soft bases.
Reference Asit
Basicity
HARD
H+, Mg2+, Sc3+
F- > Cl- > Br- > I-
SOFT
Hg2+
F- < Cl- < Br- < I-
Li-I + Ag-F  Li-F + Ag-I
HS
HS
HH
SS
ΔH = -17 kcal/mol
HgF2 + BeI2  HgI2 + BeF2
SH
HS
SS
HH
ΔH = - 95 kcal/mol
The hard-hard interaction is more dominant in terms of energy.
HA
HB
SA
SB
HA
SB
HOMO-LUMO
farkı büyük
Covalent interaction
Ionic interaction
Weak
Strong
Strong
Weak
Weak
Weak
Stable
Stable
Unstable
AgI(k) SA-SB (Covalent)
İnsoluble in water.
AgF(k) SA-HB (Ionic compounds)
Soluble in water.
NaF < NaCl <NaBr<NaI (Ionic compound)
As the cation-anion radius difference increases, the solubility
in water increases.
HA-HB Ionic contribution and water solubility increases.
SUMMARY
• Hard Lewis acids:
• Atomic centres of small ionic radius
• High positive charge
• Species do not contain electron pairs in their valence shells
• Low electron affinity
• Likely to be strongly solvated
• High energy LUMO
• Soft Lewis acids:
• Large radius
• Low or partial (delta+) positive charge
• Electron pairs in their valence shells
• Easy to polarise and oxidise
• Low energy LUMOs, but large magnitude LUMO coefficients
• Hard Lewis bases:
• Small, highly solvated, electronegative atomic centres: 3.0-4.0
• Species are weakly polarisable
• Difficult to oxidise
• High energy HOMO
• Soft Lewis bases:
• Large atoms of intermediate electronegativity: 2.5-3.0
• Easy to polarise and oxidise
• Low energy HOMOs but large magnitude HOMO coefficients.