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Π Molecular Orbitals of Ethene 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 Me23 N Me3 N SO2 H (0.561.211.525.61) 9.2kcal / mol Experimental value: -9.6 kcal/mol 24.
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