CHM 501 Lecture



Acids/Bases

Acid-base reactions are atom transfer reactions; these are definition dependent.
We use them in an attempt to classify reactivity and aid us in predicting reactions.

Brønsted/Lowry definition

Acid: H+ donor
Base: H+ acceptor
A/B reaction: H+ transfer

The definition is not solvent or phase dependent; fairly universal.

Acid Strength

In aqueous solutions this is measured by equilibrium constants

HA(aq) + H2O(l) H3O+(aq) + A(aq)

pKa = –logKa

This is a so-so measure of extent of reaction but breaks down at higher concentrations; modified by activity coefficients; tells us nothing about structure or the role of the solvent.

Differentiating solvent: allows relative acid strength to be measured
Leveling solvent: two or more acids give the same acid strength

Water is differentiating for organic acids but leveling for HCl, HBr, HI

A gas phase scale would be simpler:

B(g) + H+(g) HB+(g)    H = –proton affinity = –PA

Periodic trends of PA: increases to the right on the Periodic Table; increases down the Periodic Table

To account for properties in solution, write a Born-Haber cycle:
B(g) + H+(g) HB+(g) –PA
+H2O(l)   +H2O(l)   +H2O(l) solvation energies
B(aq) + H+(aq) HB+(aq) 1/Ka

Use the Born equation to estimate solvation energies.

Structural aspects of Brønsted acids:

1) Aqua acids: water coordinated to a central (usually cationic) species

M(H2O)nm+(aq) + H2O(l) M(H2O)n-1(OH)(m-1)+(aq) + H3O+(aq)

d block metals, some Group 2 metals; not Group 1 metals
ionic models work reasonably well

2) Hydroxy acids: only OH bound to a central atom

H4SiO4 = Si(OH)4 generally p block central atoms

3) Oxoacids: ionizable H from an OH but also has one or more =O groups

HClO4; H2CrO4 both p block and d block examples known

Paulings rules: write the acid structure as OpE(OH)q E = central atom

1. pKa1 ~ 8–5p
2. each subsequent pKa increases by ~ 5 units

HClO4 pKa ~ 8–3(5) = –7
H3PO4 pKa1 ~ 8–5 = 3 (2.1 obs); pKa2 ~ 8 (7.2 obs); pKa3 ~ 13 (11.8 obs)

Oxides: when dissolved in water, can give either an acid or a base

Acidic oxides: covalently bound oxides

CO2(g) + 2H2O(l) HCO3(aq) + H3O+(aq)
SO3(g) + OH(aq) HSO4(aq)

Basic oxides: ionically bound oxides

BaO(s) + H2O(l) Ba2+(aq) + 2OH(aq)
CaO(s) + 2H3O+(aq) Ca2+(aq) + 3H2O(l)

Amphoteric oxides: act as either acid or base; at the line of covalent or ionic bonding

Ga2O3(s) + 6H3O+(aq) + 3H2O(l) 2[Ga(H2O)6]3+(aq)
Ga2O3(s) + 2OH(aq) + 3H2O(l) 2[Ga(OH)4]-(aq)


Lewis Acid/Base Theory

Acid: electron pair acceptor (electrophile)
Base: electron pair donor (nucleophile)
A/B reaction: complex formation where a new covalent bond is formed

Universal: any solvent, any phase, any chemical species

Pearsons Hard/Soft Acid/Base Theory

Hard acids or bases: high charge density, not polarizable
Soft acids or bases: low charge density, polarizable
Operating Principle: hard acids prefer to bind with hard bases and soft acids prefer to bind with soft bases

hard acid/base combinations tend to be more ionic
soft acid/base combinations tend to be more covalent

Nonpolarizable substances (hard species) have a large HOMO/LUMO gap so that the valence orbitals of the species are generally of very different energies; this means that orbital overlap is poor and transfer of electrons is more favorable than sharing; reverse for polarizable substances

HSAB can be used to qualitatively predict reactivity, especially metathesis reactions:
CuF(s)
+
HI(aq)
CuI(s)
+
HF(aq)
S H   H S   S S   H H

CaO(s)
+
2HBr(aq)
Ca2+(aq)
+
2Br(aq)
+
H2O(l)
H H   H S   H   S   H H

Drago-Wayland

A + B A-B

H = EAEB + CACB

EA, EB : "electrostatic" interactions

CA, CB : "covalent" interactions

To get a large –H, need both EA and EB to be large or CA and CB to be large; molecules with similar binding preferences give more exothermic reactivity

 
BF3
+
(CH3)3P
F3B-P(CH3)3
E
20.2
 
0.84
   
C
3.31
 
6.55
(kJ/mol units)

H = (20.2)(0.84) + (3.31)(6.55) = 38.6 kJ/mol)

 
BF3
+
(1,4-C4H8O2
F3B-O(C4H8)O
E
20.2
 
2.23
   
C
3.31
 
4.87
(kJ/mol units)

H = (20.2)(2.23) + (3.31)(4.87) = 61.2 kJ/mol

A third term can be added to account for steric interactions