Chemistry 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)

baseacid

 

BF₃(g)+ CH₃CH₂OCH₂CH₃(g) → ← F₃B:O(CH₂CH₃)₂(s)

acidbase

 

Al(OH)₃(s) + OH^–(aq) → ← Al(OH)₄^–(aq)

acidbase

 

2. Substitution or Displacement reactions

F₃B:O(CH₂CH₃)₂(s) + NH₃(g) → F₃B:NH₃(s) + (CH₃CH₂)₂O(l)

acid                           base

XeF₄(s) + Pt(s) → PtF₄(s) + Xe(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)

red = soft species; black = hard species

CaO(s) + 2 HBr(aq) → Ca²⁺(aq) + 2 Br^–(aq) + H₂O(l)

Thermodynamic measures of Lewis A/B interactions

Donor and Acceptor Numbers:

Donor Number = DN = –ΔH for the reaction:

SbCl₅(l) + :B → Cl₅Sb-B

compares base strength to the very strong Lewis acid SbCl₅

compoundDN

 

benzene0.1

 

water18

 

pyridine33

 

Acceptor Number = AN is a value scaled to the ³¹P NMR chemical shift of the product in the reaction:

(CH₃CH₂)₃P=O + A → (CH₃CH₂)₃P=O:A

compoundAN

 

hexane0

 

benzene2.3

 

water82

 

SbCl₅100

 

Drago-Wayland

A + B → A-B

–ΔH = E_AE_B + C_AC_B

E_A, E_B: 'electrostatic' interactions

C_A, C_B: 'covalent' interactions

To get a large –ΔH, need both E_A and E_B to be large or C_A and C_B to be large; molecules with similar binding preferences give more exothermic reactivity

BF₃+(CH₃)₃P → F₃B–P(CH₃)₃

 

20.20.84E

 

3.316.55C

 

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

BF₃+1,4-C₄H₈O₂ → F₃B-O(C₄H₈)O

 

20.22.23E

 

3.314.87C

 

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