CHM 401

 

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

compound

DN

benzene

0.1

water

18

pyridine

33



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

compound

AN

hexane

0

benzene

2.3

water

82

SbCl5

100


Drago-Wayland

A + B → A-B

–ΔH = EAEB + CACB

EA, EB: "electrostatic" interactions

CA, CB: "covalent" interactions

To get a large –ΔH, 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)

–ΔH = (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)

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