Table 2. The ordered stabilities of surface carboxylate intermediates, the gas phase acidity of their parent acid, the reactions used to test their relative stabilities, and the equilibrium constants relative to acetate.

	Conjugate base	Gas phase acidity a (KJ mol–1)	Probe reaction	Exp. K b
	Propanoate	1454 ± 12	CH3COO(a) + CH3CH2COOH → CH3COOH + CH3CH2COO(a)	4
	Trifluoroacetate	1351 ± 17	CF3COO(a) + CH3CH2COOH → CF3COOH + CH3CH2COO(a)	2
			CH3COO(a) + CF3COOH → CH3COOH + CF3COO(a)	2
	Acetate	1459 ± 9	CH3COO(a) + CH3COOH ↔ CH3COOH + CH3COO(a)	1
	Formate	1444 ± 12	CH3COO(a) + HCOOH ← CH3COOH + HCOO(a)	0.9

^aGas phase acidity (taken from the NIST database)50 is defined as ΔH for BH_(g) → B_(g)^– + H_(g)⁺ (KJ mol^–1).

^bEquilibrium constant is determined at 260 K for formate/acetate and 300 K for other pairs.