1 W. B. Jensen, The Lewis Acid-Base Concepts (1980)
Most of this chapter is concerned with the Lewis definition, its more recent explanation in terms of molecular orbitals, & its application to inorganic chemistry.
2 6-1-1 History
3 6-2 Major Acid-Base Concepts 6-2-1 Arrhenius Concept Arrhenius received 1903 Nobel Prize in chemistry for this theory, Arrhenius acids form hydrogen ions (now frequently called hydronium or oxonium + ions, H3O ) in aqueous solution, Arrhenius bases form hydroxide ions in aqueous solution.
4 This explanation works well in aqueous solution, but it is inadequate for nonaqueous solutions & for gas & solid phase reactions in which H+ & OH- may not exist.
Definition by Br∅nsted & Lewis are more appro- priate for general rule.
5 6-2-2 Br∅nsted-Lowry concept
In 1923, Br∅nsted-Lowry defined an acid as a species with a tendency to lose a hydrogen ion & a base as a species with a tendency to gain a hydrogen ion. This definition expanded the Arrhenius list of acids & bases to include the gaseous HCl & NH3.
6 This definition also introduced the concept of conjugate acids & bases, differing only in the presence or absence of a proton, & described all reactions as occurring between a stronger acid & base to form a weaker acid & base.
7 Na + NH3
8 6-2-3 Solvent System Concept The solvent system definition applies to any solvent that can dissociate into a cation & an anion (autodissociation), where the cation resulting from autodissociation is the acid & the anion is the base. Solutes that increase the conc of the cation of the solvent are considered acids & solutes that increase the conc of the anion are considered bases.
+ - 2H2O ' H3O + OH Solvent: H2O + - H2SO4 + H2O ' H3O + HSO4 Solute: H2SO4 (acid) + - 2BrF3 ' BrF2 + BrF4 Solvent: BrF3 + - SbF5 + BrF3 ' BrF2 + SbF6 Solute: SbF5 (acid) - - - F + BrF3 ' BrF4 Solute: F (base) 9 Properties of Solvents
10 6-2-4 Lewis concept A base as an electron-pair donor and an acid as an electron-pair acceptor. Adduct is a combination of a Lewis acid & base (metal-containing adducts are called coordination compounds).
acid base adduct
11 Because fluorine is the most electronegative element, the boron atom in BF3 is quite positive, & the boron is frequently described as electron-deficient. The lone pair in the HOMO of NH3 combine with the empty LUMO of BF3, which has very large, empty orbital lobes on boron, to form the adduct.
LUMO HOMO
A + :B → A-B Acid adduct Base 12 LUMO HOMO empty orbital lone pair lobes on boron on nitrogen
Note the change of the geometry of BF3.
13 14 b.p. : BF3 (-99.9 °C); diethyl ether : 34.5 °C adduct : 125 - 126 °C. The formation of the adduct raises the boiling point enormously, a common result of such reactions.
15 6-2-5 Frontier orbitals and acid-base reactions Frontier orbitals : those at the occupied-unoccupied frontier (HOMO - LUMO)
+ + NH3 + H NH4 acid-base reaction
The combination of the HOMO of the base (NH3) & the LUMO of the acid (H+).
16 In most acid-base reactions, a HOMO-LUMO combination forms new HOMO & LUMO orbitals of the product.
17 Td C3v 18 (I) A-B dismatch in energy (A < < B)
2+ - 2H2O + Ca Ca + 2OH + H2 (water as oxidant) (II) A-C match in energy (A < C)
- - nH2O + Cl [Cl(H2O)n] (water as acid- solvation of anion)
19 - H2OCaCl (III) A-D match in energy (A > D) 2+ 2+ 6H2O + Mg [Mg(H2O)6] (water as base – solvation of cation) (IV) A-E dismatch in energy (A > > E) - + 2H2O + 2F2 4F + 4H + O2 (water as reductant)
20 Lewis definition of acid & base in terms of frontier orbitals : A base has an electron pair in a HOMO of suitable symmetry to interact with the LUMO of the acid. The better the energy match between the base’s HOMO & the acid’s LUMO, the stronger the interaction.
21 6-2-6 Hydrogen bonding
Symmetrical H-bonding: FHF-
Note that the LUMO of HF is mainly located on H-atom side.
F- FHF- HF 22 unsymmetrical H-bonding: BHA
There are three possible cases, as shown in Figure 6-8.
23 Figure 6.8 (a) Poor match of HOMO and LUMO. LUMO
No energy gain with adduct formation. HOMO Little or no H-bonding.
For example: H2O H2O···H-CH3 CH4 24 Figure 6-8 (b) Good HOMO (base) and LUMO (acid) match Both occupied orbitals are lowered in energy, with net energy gain. If the B HOMO is higher than the A LUMO, as in this figure, the H-A portion is stronger. HOMO If the B HOMO is lower LUMO than the A LUMO, the H-B portion is stronger.
It is closer so that the HOMO of the adduct B…H-A contains more A. 25 Figure 6-8 (c) The HOMO-LUMO energy match is so poor that H+ transfer occurs.
More energy gain than loss, so that proton prefers transferring to B (base).
Energy loss Energy gain
26 487 507 (6 eq) Orange brown 558
Yellow Violet (1) 475 (MLCT) 13.08 13.09 UV-vis spectra of 1 in MeCN solution and the 13.10 addition of 1 equiv of anions and 6 equiv TBAF. 13.10 13.45 H-bond 13.88 vanished
1 H-NMR spectra of 1 in DMSO-d6 in the Ye et al, Inorg. Chem. 2007, 46, 6427. absence and presence of 1 equiv of anions. 6-2-7 Electronic spectra (including Charge Transfer) blue-shift
28 CT bands : 230-400 nm 520 nm D-A bands : ~500 nm
520 nm
500 nm
450 nm
I -;360 nm 3 29 Charge-transfer spectra Charge-transfer absorption a strong interaction between a donor solvent and a halogen molecule, X2, leads to the formation of a complex in which an excited state
(primarily of X2 character, LUMO) can accept electrons from a HOMO (primarily of solvent character) on absorption of light of suitable energy : 30 . + - X2 donor → [donor ][X2 ] The absorption band, known as a charge-transfer band, can be very intense; it is responsible for the vivid colors of some of the halogens in donor solvents.
31 e- CoIII−X- CoII···X CT Charge-transfer bands Blue-shifted from Br- to F-
Hight of X- HOMO: Br- > Cl- > F- Ease of oxidation: Br- > Cl- > F-
d-d transition of Co3+(d6)
1kK = 1000 cm-1 CT usually has larger absorption coefficient than d-d transition.32 6-2-8 Receptor-Guest Interactions
Also called inclusion complex (C60 + C60H24) A shortest C…C distance of 3.128 Å
(rvw = 1.70 Å)
33 Schematic illustration of the attractive electrostatic interaction between the σ framework & the π electron density in an offset π-stacked & in a T-shaped geometry because of decreased π⋅⋅⋅π repulsion 6-3 Hard and Soft Acids and Bases
1. Relative solubility of halides
Mercury(I) halides have a similar trend
(Ksp : Hg2F2 > Hg2I2).
35 2. Coordination of thiocyanate (SCN-) to metals : 2- 2- [Hg(SCN)4] vs [Zn(NCS)4] 3. Equilibrium constants of exchange reactions.
+ + [CH3Hg(H2O)] + HCl ⇔ CH3HgCl + H3O K = 1.8 x 1012 + + [CH3Hg(H2O)] + HF ⇔ CH3HgF + H3O K = 4.5 x 10-2
36 37 Linkage Isomerism
2- 2- 2- [Co(NCS)4] , [Hg(SCN)4] , [Pt(SCN)4]
2- [Pd(SCN)4] at RT (solid) 2- [Pd(NCS)4] at HT (solid) or in solution
[Pd(bpy)(SCN)2] : two isomers obtained. Factors influencing Solubility :
1. Solvation of the ions (exothermic) 2. Ag-X interactions [hard and soft acids and bases (HSAB)] (endothermic)
39 • Hard : small and nonpolarizable (polarizability : charge separation) Soft : larger and more polarizable Soft-soft > hard-hard > hard-soft
• Color : depends on the difference in energy between occupied and unoccupied orbitals. AgI yellow; AgBr slightly yellow; AgCl, AgF white. (LMCT)
40 Rules by Fajan in 1923 (Third edition) 1. For a given cation (smaller EN), covalent character increases with increase in size of the anion (smaller EN). 2. For a given anion (larger EN), covalent character increases with decrease in size of the cation (larger EN). 3. Covalent character increases with increasing charge on either ion. 4. Covalent character is greater for cations with nonnoble gas electronic configurations.
41 (S-S)
(H-H)
42 Solubility:
MgCO3 > CaCO3 > SrCO3 > BaCO3 Fajan‘s Rule 2 predict the reverse of this order. Mg2+ (small, with higher charge density) – strongly solvated with water (HSAB) Ba2+ (large, with smaller charge density) The difference appears to lie in the aquation of the metal ions. Mg2+ attracts water molecules much more strongly than the others, with Ba2+ the least strongly solvated.
43 Ahrland, Chatt, and Davies divided the metal ions into two classes :
The solubility of class (b) metal halides : F- > Cl- > Br- > I- The order is reverse for class (a) metal halides.
Class (b) metal ions have a larger enthalpy of reaction with P-donors than with N-donors. This is again reverse for class (a) metal ions.
44 Class (a)
Class (b) Borderline elements, whose behavior depends on their O.S. & the donor.
45 Class (b) ions have d electrons available for π bonding (back bonding). Donor molecules or ions that have the most favorable enthalpies of reactions with class (b) metals are those that are readily polarizable and have vacant d or π* orbitals available for π bonding.
46 6-3-1 Theory of Hard and Soft Acids and Bases
Pearson designated the class (a) ions hard acids and class (b) ions soft acids.
Reactions are more favorable for hard-hard and soft-soft interactions than for a mix of hard and soft in the reactants.
47 48 Much of the hard-soft distinction depends on polarizability.
Hard acids : 1. Cations with large positive charge (3+ or larger). 2. Those with d electrons are relatively unavailable for π bonding.
49 Soft acids : 1. Those with d electrons or orbitals are readily available for π bonding (+1 cations, heavier +2 cations).
2. The larger and more massive the atom, the softer it is likely to be, because the large numbers of inner electrons shield the outer ones and make the atom more polarizable.
50 Soft and hard bases : More electrons and larger size lead to soft behavior
• I- is softer than F- • S2- is softer than O2- • S2- is softer than Cl-
51 6-4
S, P
O, N
52 6-5
53 HSAB (2) vs inherent acid-base strength (1)
ZnO + 2 LiC4H9 ' Zn(C4H9)2 + Li2O soft-hard hard-soft soft-soft hard-hard In this case, the HSAB parameters are more important than acid-base strength, because Zn2+ is considerably softer than Li+. As a general rule, hard-hard combinations are more favorable energetically than soft-soft combinations.
Acid strength : Zn2+ > Li+ Acid-base strength : 2- - Base strength : O > C4H9 Ka and Kb values
54 6-6
Increase [S-2]
55 Hard-Hard Soft-Soft
Hard-hard interactions considered as simple electrostatic interactions, with large HOMO and LUMO separation.
56 As shown from previous page: A somewhat oversimplified way to look at the hard-soft question considers the hard-hard interactions as simple electrostatic interactions, with the LUMO of the acid far above the HOMO of the base & relatively little change in orbital energies on adduct formation. A soft-soft interaction involves HOMO & LUMO energies that are much closer & give a large change in orbital energies on adduct formation.
57 Many comparisons of hard-hard & soft-soft interactions indicate that the hard-hard combination is stronger & is the primary driving force in reactions.
58 6-3-2 Quantitative measures :
There are two major approaches to quantitative measures of acid-base reactions.
Pearson’s & Drago’s approaches.
59 I. Pearson’s approach : It uses the hard-soft terminology, and defines the absolute hardness, η, as one-half the difference between the ionization (I) & electron affinity (A) (both in eV) : η = (I-A) / 2
60 This definition of hardness is related to Mulliken’s definition of electronegativity (χ), called absolute electronegativity by Pearson.
χ = (I + A) / 2
61 This approach describes a hard acid or base as a species that has a large difference between its ionization energy and its electron affinity. Ionization energy is assumed to measure the energy of the HOMO and electron affinity is assumed to measure the LUMO for a given molecule :
EHOMO = -I ELUMO = -A
Softness σ = 1/η η = (I-A) / 2
62 χ = (I + A) / 2
η = (I-A) / 2
63 6-7
64 II. Drago’s approach : It proposed a quantitative system of acid-base parameters to account more fully for reactivity by including electrostatic and covalent factors.
-ΔH = EAEB + CACB where ΔH is the enthalpy of the reaction (A + B ' AB) in the gas phase or in an inert solvent, and E & C are parameters calculated from experimental data.
65 E is a measure of the capacity for electrostatic (ionic) interactions and C is a measure of the tendency to form covalent bonds. The subscripts refer to values assigned to the acid and base, with I2 chosen as the reference acid and N,N’-dimethylacetamide and diethyl sulfide as reference bases.
66 6-8
67 6-9
68 This experimental value of ΔH is -1.3 kcal/mol, or -5.5 kJ/mol, 9% larger. This is a weak adduct
(other bases combining with I2 have enthalpies 10 times as large), and the calculation does not agree with experiments as well as many. Drago developed statistical methods for averaging experimental data from many different combinations (within 5% deviation).
69 Drago’s system emphasized the two factors involved in acid-base strength (electrostatic and covalent) in the two terms of his equation for enthalpy of reaction. Pearson proposed the equation
log K = SASB + σAσB A + B ' AB with inherent strength S modified by a softness factor σ. Larger values of strength & softness then lead to larger equilibrium constants or rate constants)
70 Both Drago’s E and C parameters and Pearson’s HSAB are useful, but neither covers every case, and it is usually necessary to make judgments about reactions for which information is incomplete. With E & C numbers available, quantitative comparisons can be made. When they are not, the qualitative HSAB approach can provide a rough guide for predicting reactions.
71 Solvation : under most conditions, reactions will be influenced solvent interactions (neither of the two quantitative theories takes this factor into account), & they can promote or hinder reactions, depending on the details of these interactions.
72 6-4 Acid and Base Strength 6-4-1 Measurement of Acid-Base Interactions
A + B ' AB
73 74 6-4-2 Thermodynamic measurement
For example: enthalpy (ΔH3) & entropy (ΔS3) of the ionization of a weak acid, HA, if ΔH1, ΔH2 and Ka can be measured.
Another way: Plot ln Ka vs 1/T 75 6-11
76 66--44--33 ProtonProton AffinityAffinity
+ + BH (g) Æ B(g) + H (g) Proton Affinity = ΔH Purest acid-base strength, with no solvation effect, is gas phase proton affinity. Mass spectroscopy & ion cyclotron resonance spectroscopy (離子迴旋共振光譜, yields information on many aspects of ion-molecule chemistry) can be used to measure the reaction indirectly. Pyridine & aniline are found to have larger gas phase proton affinity than NH3, but they are less basic in aqueous solution because of the better interaction of ammonium ion with water. 77 6-4-4 Acidity and basicity of binary hydrogen compounds
proton affinity
In one period, more electronegative Æ more acidic In one group, heavier atom Æ more acidic (electronegativity less important)
78 66--44--55 InductiveInductive EffectsEffects
less basic From MO viewpoint, HOMO is lowered or stabilized. more electronegative
Alkyl groups are electron-donating:
Base strength in gas phase : NMe3 > NHMe2 > NH2Me > NH3
But the acid strength : BF3 < BCl3 ≦ BBr3 Because of the ease of π-bonding : F > Cl > Br to increase the electron density on B. 79 66--44--66 StrengthStrength ofof OxyacidsOxyacids
Acid strength in aqueous solution:
Pauling’s equation: pKa ≈ 9-7n Because Oxygen is Another equation: pKa ≈ 8-5n highly electronegative.
80 6-4-7 Acidity of Cations in Aqueous Solution
In general, metal ions with larger charges and smaller radii are stronger acids. 6-12
81 Solubility of the metal hydroxides is also a measure of cation acidity. The stronger the cation acidity, the less soluble the hydroxides. 6-13
-3 Ka=6.7x10 -4 Ka=1.6x10 -5 Ka=1.1x10 better hydroxide affinity
At the highly charged extreme, free metal cation is no longer a detectable species. (too oxophilic!) For example: - - + + 2+ MnO4 , CrO4 , UO2 , VO2 and VO etc 82 6-4-8 Steric Effect
83 Combination of Steric & Inductive Effects, plus Solvation (may be in opposite directions)
6-14
proton affinity little steric effect
84 6-4-9 Solvation and Acid-Base Strength
Solvent molecules may stabilize species in solution.
+ + RnH4-nN (g) + H2O Æ RnH4-nN (aq) Solvation energies are in the order: + + + RNH3 > R2NH2 > R3NH
Solvation depends on the number of H atoms
available for hydrogen bonding to H2O. Competition of inductive, steric effects and solvation scramble (混亂) the order of solution basicity.
85 66--44--1010 NonaquesouNonaquesou SolventsSolvents andand AcidAcid--basebase StrengthStrength Acids and bases in water:
In glacial acetic acid as solvents, only very strong acid can protonate acetic acid. This differentiates the acid strengths of strong acids.
Any base stronger than acetate reacts with acetic acid to form acetate.
86 Leveling (拉平) Effect
The term leveling effect refers to a solvent's ability to level the effect of a strong acid or base dissolved in it.
Therefore, nitric acid, sulfuric acid, perchloric acid and hydrochloric acid are all equally strong acid in dilute aqueous solution, reacting with H2O to form hydronium, + H3O , the strongest acid possible in water. However, in acetic acid as solvent, their acid strength is in the order:
HClO4 > HCl > H2SO4 > HNO3
87 88 George Andrew Olah 66--44--1111 SuperacidsSuperacids 1994 Nobel price Acid solutions more acidic than sulfuric acid are called superacids. Hammet acidity function: 6-15
The stronger the acid, the more negative its H0 value.
89 Examples of Superacids:
These acids are very strong Friedel-Crafts catalysts. For this purpose, the term superacid applies to any acid stronger than AlCl3, the most common Friedel-Crafts catalyst.
90