Chem 104A, UC, Berkeley
Molecular Orbitals of Ethene
Chem 104A, UC, Berkeley
1 Chem 104A, UC, Berkeley
Molecular Orbital Analysis of Ethene Dimerisation
the reaction is said to be a "symmetry forbidden" – interestingly, this reaction is rare and very slow !
Chem 104A, UC, Berkeley
Molecular Orbitals of 1,3-Butadiene
2 Chem 104A, UC, Berkeley
Chem 104A, UC, Berkeley
Molecular Orbital Analysis of Diels-Alder reaction
3 Chem 104A, UC, Berkeley
Acid-Base Chemistry
Reading : MT 6
Chem 104A, UC, Berkeley
The Acid Base Theory of Brønsted and Lowry
In 1923, within several months of each other, Johannes Nicolaus Brønsted (Denmark) and Thomas Martin Lowry (England) published essentially the same theory about how acids and bases behave.
•An acid is a "proton donor." •A base is a "proton acceptor."
4 Chem 104A, UC, Berkeley
+ HCl + H2O <===> H3O + Cl¯
HCl - this is an acid, because it has a proton available to be transfered.
H2O - this is a base, since it gets the proton that the acid lost. Now, here comes an interesting idea: + H3O - this is an acid, because it can give a proton. Cl¯ - this is a base, since it has the capacity to receive a proton. + Notice that each pair (HCl and Cl¯ as well as H2O and H3O differ by one proton (symbol = H+). These pairs are called conjugate pairs.
Chem 104A, UC, Berkeley
HCl + NaOH ---> H2O+ NaCl stronger stronger Weaker weaker conjugate conjugate acid base acid base
+ - NH4 +OH---> H2O+ NH3 stronger stronger weaker weaker conjugate conjugate acid base acid base
5 Chem 104A, UC, Berkeley
H n A H 2O H n1 A H3O
[H n1 A ][H3O ] pKa1 log [H n A]
pKa 0 strong
pKa 0 weak
Chem 104A, UC, Berkeley
Amphoteric Compound
H n A H 2O H n1 A H3O
NH 3 H 2O NH 4 OH
6 Chem 104A, UC, Berkeley
Oxyacids AOp(OH)q A=Si, N, P, As, S, Se, Te, Cl, Br, I p=# of nonhydrogenated oxygen atom
H + A ----O
As atom A is rendered more positive, it becomes easier to break the O-H bond Because of enhanced bond polarization.
The greater the formal charge on A. the stronger the acid should be.
Chem 104A, UC, Berkeley
Oxyacids AOp(OH)q
p=# of nonhydrogenated oxygen atom
p=0: PKa 8-9 very weak p=1: PKa 1-2 weak p=2: PKa -2 ~ -3 strong p=3: PKa -7 very strong
A=Si, N, P, As, S, Se, Te, Cl, Br, I
7 Chem 104A, UC, Berkeley
- O
p=3 2 1 0
Empirical Rule (Pauling)
PKa 8 5p
Chem 104A, UC, Berkeley
8 Chem 104A, UC, Berkeley
Chem 104A, UC, Berkeley
As successive protons are removed, the PKa increases by ~4 Or 5 each time.
H3PO4 H 2 PO4 HPO4 (2.15) (7.20) (12.37)
H 2SO4 HSO4 (3.0) (2.0)
9 Chem 104A, UC, Berkeley
HnX
0.1 M HCl, H2S,H2O, H3PO4,H2PO4-,HPO42-,PO43-
Chem 104A, UC, Berkeley
Acidity
H2Se > H2S > H2O
Conjugated bases SeH-, SH-, OH- of larger molecules
lower charge density
weaker attraction for H+
10 Chem 104A, UC, Berkeley
Acidity
NH3 < H2O < HF
- - - Conjugated bases NH2 , OH , F
- NH2 -1/2 on each lone pair HO- -1/3 on each lone pair - F -1/4 on each lone pair Strongest attraction for proton Strongest conjugated base Weakest acid NH3
Chem 104A, UC, Berkeley
Lewis Concept
A base: an electron-pair donor Species with lone pair type orbitals
An acid: an electron-pair acceptor Species with empty non-bonding type orbitals
MO theory
11 Chem 104A, UC, Berkeley * 2 u * 2 g Lewis acid BeL2, MgL2
1 u
b 1 u
1 b Ligand Group Orbitals g LGOs
* Chem * 104A, UC, Berkeley x y ' Lewis acid 2e1 BL3, AlL3 * GaL3 s ' InL3 2a1
2py nb LGO3 z '' LGO2 2p 2p b 1a2 x z b x y LGO1 2s ' 1e1 b s ' B 1a1 3 H
BH3
12 Chem 104A, UC, Berkeley Lewis base NL3, PL3, AsL3, SbL3, BiL3
NH3 (C3v: E, 2C3, 3v)
z N 3 H NH3 2e -13.5 eV H(3) H(1) H(2) 4a1 y e - s2 + s3 x -15.5 eV
The symmetry of 3H’s group orbitals: e (2p 2p ) x, y a1 2s1 - s2 - s3
r = A1 + E
a1 (2pz) 3a 1 s1 + s2 + s3 C E2C 3 3v 3 v -17.0 eV 2 2 2 A1 111zx+y , z 1e a1 (2s) A2 11 -1 -25.6 eV E 2 -1 0 (x,y)
-31.0 eV 2a1
Chem 104A, UC, Berkeley
x
O z H H y Other: OL-, SL-, SeL- 2p z F- 1s Cl- -15.9 eV Br- 2p y H H I-
2px LGO
-32.4 eV Lewis Base 2s OL2, SL2, SeL2, TeL2
O O
13 Chem 104A, UC, Berkeley
Type of non-protic acid-base reactions:
1. Adduct formation: acid-base react to form a bond and produce a single molecule.
2. Displacement: one base (or acid) displace another
3. Double displacement (Metathesis): interchange of acids and bases
Chem 104A, UC, Berkeley
14 * Chem * 104A, UC, Berkeley x y ' 2e1 * s ' 2a1
2py nb LGO3 z LUMO '' LGO2 2p 2p b 1a2 x z b x y LGO1 2s ' 1e1 b s ' B 1a1 3 H
BH3
Chem 104A, UC, Berkeley
NH3 (C3v: E, 2C3, 3v)
z N 3 H NH3 2e -13.5 eV H(3) H(1) H(2) 4a1 y e - s2 + s3 x -15.5 eV
The symmetry of 3H’s group orbitals: e (2p 2p ) x, y a1 HOMO 2s1 - s2 - s3
r = A1 + E
a1 (2pz) 3a 1 s1 + s2 + s3 C E2C 3 3v 3 v -17.0 eV 2 2 2 A1 111zx+y , z 1e a1 (2s) A2 11 -1 -25.6 eV E 2 -1 0 (x,y)
-31.0 eV 2a1
15 Chem 104A, UC, Berkeley Adduct formation
AlCl3 Cl AlCl4
BH3 CO H3B C O
SO3Et2O O3S OEt2
Chem 104A, UC, Berkeley
Displacement: one base (or acid) displace another
A B B' A B'B
F3B SEt2 Et2O F3B OEt2 SEt2
Double displacement (Metathesis): interchange of acids and bases
A B A'B' A B'A'B
Me3Si Cl F3B F Me3Si F F3B Cl
16 Chem 104A, UC, Berkeley
Relative strength of acid/base
Reference acid Base strength
H+,Mg2+, Sc3+ F Cl Br I
Hg2+ F Cl Br I
Chem 104A, UC, Berkeley
Hard and Soft Acids and Bases (HSAB)
Hard (class a): having contracted (tightly held) frontier orbitals Not readily polarized
Soft (Class b): Having diffuse frontier orbitals Readily polarized
17 Chem 104A, UC, Berkeley Have vacant orbitals held in tight
Have vacant orbitals, but larger radius
Chem 104A, UC, Berkeley
Have lone pairs in tightly held orbitals
Have lone pairs in larger orbitals
18 Chem 104A, UC, Berkeley
Table 1 Hard and Soft Acids and Bases
Hard Bases Soft Bases Borderline Bases - - - H2O OH F R2S RSH RS ArNH2 C5H5N - 2- - - - - AcO SO4 Cl I R3P (RO)3PN3 Br 2- - - CO3 NO3 ROH CN RCN CO NO2 - RO R2O NH3 C2H4 C6H6 - - RNH2 H R
Hard Acids Soft Acids Borderline Acids H+ Li+ Na+ Cu+ Ag+ Pd2+ Fe2+ Co2+ Cu2+ + 2+ 2+ 2+ 2+ 2+ 2+ 3+ K Mg Ca Pt Hg BH3 Zn Sn Sb 3+ 2+ 3+ 3+ Al Cr Fe GaCl3 I2 Br2 Bi BMe3 SO2 + + BF3 B(OR)3 AlMe3 CH2 carbenes R3C NO GaH3 + AlCl3 AlH3 SO3 C6H5 + RCO CO2 HX (hydrogen-bonding molecules)
Chem 104A, UC, Berkeley
Hard Bases Donor atoms have high electronegativity (low HOMO) and low (nucleophiles) polarizability and are hard to oxidize. They hold their valence electrons tightly. Soft Bases Donor atoms have low electronegativity (high HOMO) and (nucleophiles) high polarizability and are easy to oxidize. They hold their valence electrons loosely. Hard Acids Possess small acceptor atoms, have high positive charge and (electrophiles) do not contain unshared electron pairs in their valence shells. They have low polarizability and high electronegativity (high LUMO). Soft Acids Possess large acceptor atoms, have low positive charge and (electrophiles) contain unshared pairs of electrons (p or d) in their valence shells. They have high polarizability and low electronegativity (low LUMO)
n.b “The HSAB principle is not a theory but a statement of experimental facts.” Pearson, R.G, Songstad, J.Amer.Chem.Soc., 1967, 89, 1827
19 Chem 104A, UC, Berkeley
Pearson’s Principle:
Hard acids prefer to bind to hard bases And soft acids prefer to bind to soft bases.
Chem 104A, UC, Berkeley
Keq CH3Hg BH CH3HgB H
The simplest hard acid is the proton and methyl mercury cation is the simplest soft acid.
Keq small B hard base
Keq large B soft base
2 2 I Br Cl S2O3 S PPh3 CN SCN SO3 NH3 F OH
20 Chem 104A, UC, Berkeley
Li-I +Ag-F H-F +Ag-I
H=-17kcal/mol
HgF2 +BeI2 HgI2 +BeF2
H=-95kcal/mol
Chem 104A, UC, Berkeley
Absolute Hardness
IE EA (E E ) 2 LUMO HOMO
Hard: large HOMO-LUMO gap
Soft: small HOMO-LUMO gap, ease of mixing ground/excited states, electron Density redistributed (polarized).
21 Chem 104A, UC, Berkeley
Strong polar bond!
Strong covalent bond!
Chem 104A, UC, Berkeley
Empirical approach to determine enthalpy of adduct formation
A + B AB
H EAEB CACB
E: susceptibility to undergo electrostatic interaction C: tendency to form covalent bonds
22 Chem 104A, UC, Berkeley
Chem 104A, UC, Berkeley
23 Chem 104A, UC, Berkeley
SO2 Me32 N Me3 N SO2 H (0.561.211.525.61) 9.2kcal / mol
Experimental value: -9.6 kcal/mol
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