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Accounts of Chemical Research tions51 concerning the above interpretation, it provides the most direct explanation of the experimental observations, and is preferred by the writer. Some support can be derived from the fact that related adducts do in fact undergo 6-and 8-bromination.53 It is likely that similar reaction paths, available because of the easy reversibility of attachment of halogen to nitrogen, are to be found with related heterocyclic systems. Reversibility in Aromatic Substitution: Applicability of Linear Free-Energy Relationships Involving Partial Rate Factors for Aromatic SubstitutionThe qualitative electronic theory of organic chemistry, together with attempts to put this theory in quantitative terms by using linear free-energy relationships or by making molecular orbital calculations concerning the relative availability of transition states, involves a number of assumptions which are not always stated explicitly. One of these is that the rate-determining transition states which are being compared have similar compositions and bonding characters. When this is not so, deviations from predicted rates and orientations are expected as possi-bilities, though they are not required. Thus it is by no means clear that two aromatic substitutions, one of which shows a primary H-D isotope effect and one of which does not, should give a reasonable linear free-energy relationship between their relative reaction rates. Quite good correlations are, in fact, found in a number of cases,21 but Berliner and his cowork-er@ have illustrated how reversibility of the first stages of substitution (reactions b and c, Scheme I) can influence the orientation of the products in suitable cases. This provides one example of a number of ways in which the detailed chemistry of the carbocations and other intermediates leading to electrophilic substitution in unsaturated systems can be important in determining the relative rates at which particular products are formed.T h i s Account has surveyed a n u m b e r of examples t o t h e understanding of which t h e writers' couorkers have m a d e contributions; it gives quite inadequate acknowledgment of his personal indebtedness to t h e m , as it does also to t h e m a n y other groups of workers contributing actively to our knowledge of these reactions.

Previous attempts to phosphorylate nucleosides by heating with inorganic phosphate succeeeded only when acid phosphates such as Ca(HPO(4))(2) were used. The addition of urea and ammonium chloride to the reaction mixture permits phosphorylation in high yield with neutral or basic phosphates at temperatures in the range of 65 degrees to 100 degrees C. Since the abundant mineral, hydroxylapatite, is a satisfactory substrate for this reaction, we believe that this procedure plausible model for prebiotic phosphorylation.

Adenosine-5'-monophosphorimidazolide reacts efficiently with adenosine derivatives on a polyuridylic acid template, with the formation of internucleotide bonds.

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