Chapter 6. Acid-Base and Donor- Acceptor...

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Chapter 6. Acid-Base and Donor-Acceptor Chemistry

Definitions

HOMO (fairly high lying)

LUMO (relatively low energy)

Modern

e- pair donore- pair acceptorLewis

H+ acceptorH+ donorBrrnsted-Lowry

OH- producerH+ producerArrheniusBaseAcid

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Acid-Base ReactionsDefinitions

Acid Base

Arrhenius H+ producer OH- producer

Brønsted-Lowry H+ donor H+ acceptor

Lewis e- pair acceptor e- pair donor

Developmentof concept

Defined only in aqueous solutions

acid base

HCl(aq) + NaOH(aq) NaCl(aq) + H2O

salt water


Acid Base





Defined in both aqueous and non-aqueous solutions

acidbase baseacid

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Acid Base






acid base acid and base

NH4+ + NH2

- 2NH3

A Brønsted acid must contain at least one ionizable proton!


Acid Base






acid base acid and base

NH4+ + NH2

- 2NH3

How about?

SbF5 + BrF3 BrF2+ + SbF6

-

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Acid Base

Solvent System cation anion

(from solvent dissociation)

or solute increaing the cation conc. decreasing anion conc.

How about?


-


Acid Base




2BrF3 BrF2+ + BrF4

-

solvent acid base


-

acid

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Acid Base




2BrF3 BrF2+ + BrF4

-

solvent acid base

F- + BrF3 BrF4-

base


Acid Base



or solute increasing the cation conc. increasing anion conc.

2H2O H3O+ + OH-

solvent acid base

H2SO4 + H2O H3O+ + HSO4

-

acid Acid-base reactions in the solvent system concept are the

reverse of autodissociation.

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Acid Base




How about?

NH3+ BF3 NH3BF3


Acid Base




acidbase baseacid

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Acid-Base Reactions

How about?

NH3+ BF3 NH3BF3

Definitions

Acid Base




base acid adduct

A2' E'

MOs of Polyatomic MoleculesBF3 D3h

A1' E'

A1' E'

A2'' E''

3F(2px)

3F(2py)

3F(2pxz

3F(2s)

SALCs

B

2pz A2''

2s A1'

E'

2py

2px

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MOs of Polyatomic MoleculesBF3 D3h

bonding

non-bondingalmost non-bonding

antibonding

Lewis ?

VBT?

? σ and ? π

same for SO3, NO3-, CO3

2-

MOs of Polyatomic MoleculesNH3 C3v

pz characternon-bonding

2s

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How about?

NH3+ BF3 NH3BF3

acid base adduct

Lewis Acid-Base

coordination compoundwhen involving metal

stabilized

How about?

NH3+ BF3 NH3BF3

acid base adduct

Frontier Orbitals and Acid-Base Reactions

coordination compoundwhen involving metal

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Frontier Orbitals and Acid-Base ReactionsH+ + NH3 NH4

+

2s

NH3

Td

stabilized


+

2s

NH3

Td

a HOMO-LUMO combination new HOMO-LUMO of the product

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+

2s

NH3

Td

Need shape (symmetry) and energy match !!


E

A + B No adductInstead, an electron transfer can occur (Redox).

H2O + Ca 2H2O- + Ca2+ (?)H2O + Ca 2OH- + H2 + Ca2+

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E

A + C AdductnH2O + Cl- [Cl(H2O)n]-

A + D Adduct6H2O + Mg2+ [Mg(H2O)6]-


E

A + E No Adduct

2H2O + 4F2 4H+ + O2 + 4F-

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A + E No Adduct

2H2O + 4F2 4H+ + O2 + 4F-

A + C AdductnH2O + Cl- [Cl(H2O)n]-

A + D Adduct6H2O + Mg2+ [Mg(H2O)6]-

A + B No adductH2O + Ca 2OH- + H2 + Ca2+

H2O

Oxiant

Acid

Base

Reductant

Hydrogen Bonding

Dipole-dipole attraction in which hydrogen is bound to a highly electronegative atom. (F, O, N)

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Hydrogen Bonding

Dipole-dipole attraction in which hydrogen is bound to a highly electronegative atom. (F, O, N)

Why is the difference so big for H2O?

Hydrogen Bonding

Ice Water

b.p ↑m.p ↑ d↓

forming clathrates

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Hydrogen Bonding

α-helix

β-sheet

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C

147152155120van der Waals radius (pm)

71737532Covalent radius (pm)

FONHelement

~2.7 Å

What happens?

Remind....FHF- D∞h D2h

F(2px)+F(2px)2 -2 0 0 0 0 2 -2

F(2py)+F(2py)2 -2 0 0 0 0 -2 2

F(2pz)+F(2pz)2 2 0 0 0 0 2 2

F(2s)+F(2s)2 2 0 0 0 0 2 -2

B3u + B2g

B2u + B3g

Ag + B1u

Ag + B1u

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Remind....FHF-

F---F: SALCs H: 1s orbital

Agcan combine to form MOs

Remind....FHF-

(-18.7 eV)

(-40.2 eV)

(-13.6 eV) H 1s

H 1s will strongly interact with F2Pzs (Ag).

Don't forget if F2s contributes, 3 MOs are formed.

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Remind....FHF-

bonding

antibonding

non-bonding* there are slight long-range interactions.

Remind....FHF-

F H F

Lewis structure

MO3-center 2-electron bond

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Other way to think ...FHF-

2pz

H1S ± F2pz

MOs for H-bonding in FHF-

Hydrogen Bonding

FHF-

MOs for H-bonding in FHF-

2pz

MOs for asymmetricH-bonding

H-bonding can be describedby HOMO(Base)-LUMO(Acid) interaction !!not just dipole-dipole interaction

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When Hydrogen Bonding ?

poor matchno reaction

ex) CH4 in H2O

good matchforming H-bonding

ex)acetic acid, water,acetic acid in water

very poor matchH+ transfer

ex) HCl in H2O

HCl + H2OH3O

+ + Cl-

Evidences ofAcid-Base Adduct Formation (I2)

π* σ*

σ σ*

purple

purple

red-violet

brown

actually, hasfine structures

CT

hv I2

.Donor [I2]-.Donor+

yellow-brown (water)colorless(I3

-)

adduct formation with water (minor) and I- (major)

weak donor

strongdonor

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Hard-Soft Acid-Base (HSAB)

AgF(s) + H2O Ag+(aq) + F-(aq) Ksp= 205

AgCl(s) + H2O Ag+(aq) + Cl-(aq) Ksp= 1.8 x 10-10

AgBr(s) + H2O Ag+(aq) + Br-(aq) Ksp= 5.2 x 10-13

AgI(s) + H2O Ag+(aq) + I-(aq) Ksp= 8.3 x 10-17


Acids and bases can be divided up into groups based on their hardness or softness.

“Hard” means the acid or base has a high charge density:

- it has a small radius and a large charge (less polarizable)

“Soft” means the acid or base has a low charge density:

- it has a large radius and a small charge (more polarizable)

The important predictive ability of HSAB theory is that hard

acids prefer to react with hard bases, and soft acids prefer to

form adducts with soft bases.

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HSAB theory is based on polarizability.

Polarizability is “The ease of distortion of the electron cloud

of a [molecule] by an electric field (such as that due to the

proximity of a charged reagent).”

By this definition, hard acids and bases have tightly bound

electron clouds that are difficult to polarize. Soft acids and

bases have looser electron clouds that are easier to distort.


Polarizabilityincreases down the groupincreases with increasing negative charge (base)increases with decreasing positive charge (acids)

hardnessreverse

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AgF(s) + H2O Ag+(aq) + F-(aq) Ksp= 205

AgCl(s) + H2O Ag+(aq) + Cl-(aq) Ksp= 1.8 x 10-10

AgBr(s) + H2O Ag+(aq) + Br-(aq) Ksp= 5.2 x 10-13

AgI(s) + H2O Ag+(aq) + I-(aq) Ksp= 8.3 x 10-17

Hardness

F- > Cl- > Br- > I-Ag+ : soft cation


O2- O22- O2

- O3-

hardness ??

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O2- O22- O2

- O3-

hardness ??

LiNaKRbCs

+ O2


염기

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Hard-Soft Acid-Base (HSAB)Quiz

Some of the products of the following reactions will be insoluble and some form soluble adducts. Consider only the HSAB characteristics in your answer.

a. Will Cu2+ react more strongly OH- or NH3? With O2- or S2-?b. Will Fe3+ react more strongly OH- or NH3? With O2- or S2-?c. Will Ag+ react more strongly with NH3 or PH3?d. Will Fe, Fe2+, or Fe3+ react more strongly with CO?

Hard-Soft Acid-Base (HSAB)Quiz

Some of the products of the following reactions will be insoluble and some form soluble adducts. Consider only the HSAB characteristics in your answer.


AnswerSome of the products of the following reactions will be insoluble and some form soluble adducts. Consider only the HSAB characteristics in your answer.


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HSAB is in general working but not always.

Hard-Soft Acid-Base (HSAB)Ambidentate ligand : ex) SCN-

soft borderline

hard soft

borderline

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Quantitative HSABAbsolute hardness (η), Absolute softness (σ)

(EHOMO = -I, ELUMO = -A)

Mulliken’s electronegativity (χ)

A for anion is approximated fromthe corresponding atom.

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Quantitative HSABAbsolute hardness (η)

* Absolute hardness is working in gas phase.

Quantitative ABDrago-Wayland Equation

-∆H = EAEB + CACB for A + B AB( in gas phase or inert solvent)

E: capacity for electrostatic (ionic) interactionC: measure of the tendency to form covalent bonds

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Quantitative AB

most acids

CA < 1EA > 1

most bases

CB > 7.40EA < 1.32

Drago-Wayland Equation

Drago's AB and Peasrson's HSAB

Drago Pearson

electrostatic and covalent covalent

Both are useful but neither covers all the cases.When E and C are available, use it !!When not, predict with HSAB.

Don't forget solvation.

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How to measure?• bp/mp: predict the presence of adduct• calorimetric (thermodynamic)• formation of protonated species (gas phase)• IR: ex) Ni(CO)4 ν(C-O)• NMR• UV/VIS

Acid-Base Interaction

Acid-Base Strengths

Acidity constant (Acid dissociation constant): KAcidity constant (Acid dissociation constant): Kaa

HA(aq) + H2O(l) ⇌ H3O+(aq) + A-(aq)

Ka = [H3O+][A-] = [H+][A-]

[HA] [HA]

pKa = -log[Ka]

ln Ka = -∆H/RT + ∆S/R

* pKa ranges between –10 (very strong acids) to 50 (very weak acids).

*The stronger an acid is, the lower (more negative) its pKa is.

Calorimetric

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Acid-Base StrengthsHA(aq) + H2O(l) ⇌ H3O

+(aq) + A-(aq)

Ka = [H3O+][A-] = [H+][A-]

[HA] [HA]

pKa = -log[Ka]

Acid-Base Strengths

BasicityBasicity constant : Kconstant : Kbb

NH3 (aq) + H2O(l) ⇌ NH4+(aq) + OH-(aq)

Kb = [NH4+][OH-]

[NH3]

•The higher Kb is, the stronger basicity is.

• Strong base is virtually fully protonated.

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Acid-Base StrengthsProton Affinity (gas phase)

BH+(g) B(g) + H+(g) proton affinity = ∆H

Mass spec., Ion cyclotron resonance spec.

∆H ↑ : Basicity of B ↑, acidity of BH+ ↓

Basicity: LiOH < NaOH < KOH < CsOH in gas phaseLiOH ~ NaOH ~ KOH ~ CsOH in aqueous soln

Basicity: > NH3 in gas phase

< NH3 in aqeous soln

Useful for sorting out the factors of acid-base behavior

AH(g) A-(g) + H+(g) ∆H: enthalpy of dissociation:

acidity

EN

why?

Acid-Base StrengthsBinary Hydrogen Compounds

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Acid-Base Strengths

AH(g) A-(g) + H+(g) ∆H: enthalpy of dissociation:

acidity

ENwhy? Do your homework.

acidity

EN

Binary Hydrogen Compounds

Acid-Base StrengthsInductive Effect

P

H HH

P

F FF

Basicity? >

NMe3 NHMe2 NH2Me NH3> > >

BF3 BCl3 BBr3Acidity? > > ?

BX

XX

π-bonding strength

electron density on B

BF3 BCl3 BBr3< <

acidity

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Acid-Base StrengthsOxoacids

pKa (exp)

pKa (Pauling)= 9-7npKa (modified)= 8-5nn (# of non-hydrated oxygen)

0 1 2 3

7.2 2 -1 (-10)

9 2 -5 -12

8 3 -2 -7

Acid-Base StrengthsOxoacids

n (# of nonhydrated oxygen) ↑positive charge on central atom (A)↑pull hydrated oxygen toward A weaken O-H bond

or n (# of nonhydrated oxygen) ↑

spread e- density of the conjugate base in wider rangestability ↑ basicity of the conjugate base ↓

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Acid-Base StrengthsAqua acids

Fe(H2O)63+ + H2O(l) ⇌ H3O

+(aq) + Fe(H2O)5 (OH)2+

pKa ∝ (Electrostatic parameter)-1

Working well for s- and p-blocksDeviation: due to covalency

Q/r

Positive cations in aqueous soln

Acid-Base StrengthsSteric Effect

N N N N

Basicity toward hydrogen ion

NN NN

Basicity toward BF3 or BMe3

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Acid-Base StrengthsSteric Effect

Acid-Base StrengthsLeveling Effect

2H2O H3O+ + OH-

Strongest acid in aq soln Strongest base in aq soln

Kw = 10-14

CH3COOH + H2O H3O+ + CH3COO-

HCl + H2O H3O+ + Cl- 100%

Any stronger acid forms hydronium ion immediately.

than H3O+

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2H2O H3O+ + OH-

Strongest acid in aq soln Strongest base in aq soln

Kw = 10-14

NH3 + H2O NH4+ + OH-

Na2O + H2O 2Na+ + 2OH- 100%

Any stronger base forms hydroxide ion immediately.

than OH-


H2SO4 + CH3COOH CH3COOH2+ + HSO4

-

Strongest base in pure acetic acid

NH3 + CH3COOH NH4+ + CH3COO- 100%

Any stronger base forms acetate ion immediately.

than acetate ion

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Superacids

A medium having a high acidity, generally greater than that of 100 wt.% sulfuric acid.”…“By analogy, a compound having a very high basicity, such as lithium diisopropylamide, is called a ‘superbase’

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