CHM 401

 

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

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