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 = 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
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