No4 (2016.5.26)

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R.G.Pearson,J.Am.Chem.Soc.,1963,85,3533R.G.Pearson,J.Songtad,J.Am.Chem.Soc.,1967, 89, 1827

the small, charged and electronegative fluoride ioncan be seen to be hard, while the large, uncharged andnot strongly electron negative hydrogen sulfide is soft.

F- hardH2S soft

the small, charged proton or the lithium cation can beseen to be hard, while the large silver cation and theuncharged sulfur dioxide are soft.

H+, Li+ hardAg+, SO2 soft

Q: What’s the product?

Q: Which way does the reaction proceed?

3.2 The Principle of Hard and Soft Acids and Bases (HSAB)

“hard-likes-hard and soft-likes-soft”

χ:absoluteelectronegativity

η:hardness

I:ionizationpotential

A:electronaffinity

R.G.Pearson(1968)

H+ I- Ag+ OH- H+ I-Ag+OH-+ +

Hard Acid: small, high positive charge, High-energy LUMOHard Base: small, high negative charge, Low-energy HOMO

Koopman’s theoremIonizationpotential=- HOMOElectronaffinity=- LUMO

“Principle of maximum hardness”

η =9.82– 3.40=6.42=> η =18.7

η =9.82– 3.06=6.76=> η =9.30

R.G.Pearson,Acc.Chem.Res.1993,26,250

Reactions take place in the direction that increase hardness.

R. G. Paar and R. G. Pearson, J. Am. Chem. Soc., 1983, 105, 7512; R. G. Pearson, J. Am. Chem. Soc., 1985, 107, 6801; R. G. Pearson� J. Org. Chem., 1989, 54, 1423; P. K. Chattaraj and P. von R. Schleyer�J. Am. Chem. Soc., 1994, 116, 1067

Scale of Local Hardness

Hard-with-hard, soft-with-soft

(bonddissociation energy)

(acid)

(Base) 3.3 Transition Structure

rate-equilibriumrelationship

reactivityselectivityprinciple?

Thetransitionstructureforanexothermicreactioniscloserinenergytotheenergyofthestartingmaterials,andsoithasmoreofthecharacterofthestartingmaterials (A<B).

Thetransitionstructureforanendothermicreactionisproduct-like(B<A).

Natureoftheproductswillbeinfluential inaffectingtheratesofendothermicreactions(Fig.3.4b),butthatorbitalinteractionswillbeinfluential inexothermicreactions(Fig.3.4a).

Hammond postulate 3.4 Perturbation Theory of Reactivity

ActivationenergySterichindrance

Nostabilizationrepulsion

Frontier orbitals (HOMO, LUMO) interaction is important

2EA >2EB

The Salem-Klopman Equation

1st term:Filled-filledorbitalinteraction(antibondingeffect)à Ignore

2nd term:Coulombic interactionà Importantforcharged/polarmolecules

3rd term:Filled-unfilledorbitalinteractionsà HOMOandLUMO

Example

shouldbehighenergy

Example

3.6 Hard and S0ft Nucleophiles and Electrophile

HSAB=>HSNE

Simplified Klopman-Salem Equation

ΔE = - QnucQelec + 2(cnuccelec β)2

ε R EHOMO - ELUMO

2nd term 3rd term

HARD nucleophile/HARD electrophile

SOFT nucleophile/SOFT electrophile

NUMERATOR DENOMINATOR

2nd TERM 3rd TERM

STEPWISE CONCERTED

4 Ionic Reactions − Reactivity

Curved Arrows show the flow of electrons

Addition

Why would a pair of electrons act together to move from one bond into another?

SN2

AN

AE

4.1 Single Electron Transfer (SET) in Ionic Reactions

R.A.Rossi,Chem.Rev.2003,103,71Charge-Transfer Complex

slow

fast

SRN1 reaction (Radical Nucleophilic Substitution) Nucleophilic and electrophilic attack on a double bond via CT complex

4.2 Nucleophilicity

4.2.1 Heteroatom Nucleophiles

kNu: rate constant (or equilibrium constant) for a reaction K0: rate constant (or equilibrium constant) for water as the nucleophile EN: rate of the reaction with methyl bromide H: (pKa + 1.74) ( = pKa of H3O+)

softnesshardness

J.O.Edwards,JACS 1954,76,1540 (Scale of nucleophilicity)

α/β ~large=> Soft electrophile

Softness and Hardness of Inorganic Nucleophiles and Electrophiles

G.Klopman,JACS1968,90,���

Solvationcorrection

(i)ELUMO =-7eV (HS- >I- >CN- >Br->Cl->HO- >F- )

(ii)ELUMO =-5eV (HS->CN- >I- >HO->Br->Cl->F- )

(iii)ELUMO =+1eV (HO->CN- >HS->F- >Cl- >Br-> I- )

G.Klopman JACS 1968,90,2234.2.2 Solvent Effects

SN2 Reaction in the Gas Phase

J.Mikosch,Science 2008,319,183

J.I.Brauman,Science 2008,319,168

Solvent changes the reaction pathways

HOMOsolvent/LUMOreagent andLUMOsolvent/HOMOreagent Using the concept of HSAB theory, predict which way, LEFT or RIGHT, that the following reactions will tend to proceed:

a. CaS + H2O ↔ CaO + H2S

b. AlI3 + 3NaF ↔ AlF3 + 3NaI

c. Mg(OH)2 + Cu(SCN)2 ↔ Mg(SCN)2 + Cu(OH)2the S end of SCN is coordinated

d. CdI2(s) + CaF2(s) ↔ CdF2(s) + CaI2(s)

e. [CuI4]2–(aq) + [CuCl4]3–(aq) ↔ [CuCl4]2–(aq) + [CuI4]3–(aq)

f. NH2–(aq) + H2O(l) ↔ NH3(aq) + OH–(aq)

g. 2 LiI + CoCl2 ↔ 2 LiCl + CoI2

Report No4 (5/27)