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
Three categories of Lewis acid/base reactions:
1. Complex Formation: a Lewis acid reacts with a Lewis base to form a single product.
I–(aq)
+
I2(s)
→←
I3–(aq)
base
acid
BF3(g)
+
CH3CH2OCH2CH3(g)
→
F3B:O(CH2CH3)2(s)
acid
base
Al(OH)3(s)
+
OH–(aq)
→←
Al(OH)4–(aq)
acid
base
2. Substitution or Displacement reactions
F3B:O(CH2CH3)2(s) + NH3(g) → F3B:NH3(s) + (CH3CH2)2O(l)
XeF4(s) + Pt(s) → Xe(g) + PtF4(s) (this is more typically thought of as a redox reaction)
3. Metathesis reactions
4 HF(aq) + SiO2(s) → 2 H2O(l) + SiF4(g)
2 XeF6(s) + 3 SiO2(s) → 2 XeO3(s) + 3 SiF4(g)
Pearson's Hard/Soft Acid/Base Theory
This can be used to predict reactivity.
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
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) + 2 HBr(aq) → Ca2+(aq) + 2 Br–(aq) + H2O(l)
H-H
H-S
H
S
H-H