Table 1.

Comparison of Properties for Phosphate and Sulfate Monoesters.^a

	Phosphate Monoesters	Sulfate Monoesters
P(S)-OR Bond Length (Å)	1.56 ± 0.02	1.60 ± 0.01
P(S)-O Bond Length (Å)	1.52 ± 0.02	1.46 ± 0.01
kw (M-1s-1)b	3 × 10-11	9 × 10-12
Transition Statec	Loose	Loose
Charge	−2	−1

^aBond lengths are for ethyl phosphate and ethyl sulfate.⁶⁹-⁷¹ A comparison of bond lengths for the p-nitrophenyl esters was not possible because no structure is available for pNPS.

^bValues of k_w, the nonenzymatic hydrolysis rate constants for pNPP and pNPS hydrolysis at 25 °C, are from literature sources.²¹, ²⁴

^cIn a loose transition state, there is advanced bond cleavage to the leaving group and little bond formation to the nucleophile.²⁵-³²