Linear free-energy relationships between the rate constants for hydrolysis and the pK, of the leaving group are obtained for both mono-and dianions of phosphate esters, ROPO(OH)2. The reactivity of the monoanions depends only weakly on the leaving group, but that of the dianions is nine times more sensitive to the basicity of RO~. Both ionic species are considered to be hydrolyzed by unimolecular mechanisms, involving the elimination of the leaving group from ROPCVor RO+HPOs2-, and the formation of monomeric metaphosphate, although nonselective phosphorylation of mixed alcohol-water solvents is not in general observed. The hydrolysis of the dianion of 2,4-dinitrophenyl phosphate is accelerated many hundredfold in the presence of dipolar aprotic solvents.

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The reactions of amine nucleophiles at the phosphorus centre of monoand di-anions of phosphate monoesters involve concerted displacement processes. The reactivity of the dianions depends strongly on the leaving group, but only weakly on the basicity of the nucleophile, and for a sufficiently good leaving group the Bronsted coefficient is zero. Thus, substituted pyridines differing in basicity by more than eight powers of ten attack the dianion of 2.4-dinitrophenyl phosphate at the same rate. These reactions must lie close to the borderline between unimolecular and bimolecular processes. The reactions with the monoanions are sensitive to the basicity of the nucelophile, and also depend strongly on the leaving group. The reactivity of phosphate monoester monoanions is similar to that of the anions of the diesters, and both react readily with fluoride anion.CONTISCISG our study of the simple reactions of esters of phosphoric acid, we have examined in more detail the process of nucleophilic substitution at the phosphorus centre of phosphate monoesters. We showed previously that the reactivity of a series of amine nucleophiles towards p-nitrophenyl phosphate dianion can be described by a linear free-energy relationship between the logarithms of the second-order rate constants for the displacement reaction and the pK, values of the conjugate acids of the amines. Similar relationships between rate and the pK, of the leaving group are observed for the hydrolysis reactions of the mono-and dianions of phosphate monoesters.2 We now deal with the effect on reactivity of variation in both nucleophile and leaving group, and the markedly different behaviour of mono-and di-anions. ESPERIMEXTAL i~~ateviaZs.-Inorganic salts were of analytical grade, and were used without further purification, except that solutions were filtered before use. Distilled water was further twice glass-distilled. Solid compounds, and the hydrochlorides of volatile or thermally unstable amines, were recrystallised at least twice. Liquids were distilled twice under nitrogen and stored in the dark. Aniline was purified by way of the hydrochloride, by treatment with stannous chloride.3 Commercial 2,6-lutidine contained significant quantities of lower homologues, and was purified by treatment with boron t r i f l ~o r i d e . ~ NN-Dimethylglycine hydrochloride was prepared from glycine and f~rmaldehyde,~ and 4-methoxypyridine by treatment of the 4-cliloro-compound with sodium rnethoxide.'j 3-Nitropyridine was prepared from 2-chloro-3-nitropyridine by dechlorination with copper in molten benzoic acid,' using the conditions described by Brown.* 3-Aminopyridine was prepared inadvertently in an attempted 1

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