Halogenation by molecular halogens

The reactions described below refer to halogenation in aqueous solution under conditions where the halogenating agent is the molecular halogen with a possible contribution from the trihalide ion. Almost all the work comes from the research groups of Bell and Dubois. The majority of the studies refer to bromination, but comparisons are made with the other halogenation reactions where the results are available. 

Another approach to this problem is to compare the rate coefficients for reaction with the different molecular halogens. The latest results, in Table 8 (Dubois and Toullec, 1973), indicate that the reactivity of a given enol to the three halogens is remarkably similar, suggesting that all three reactions occur on encounter. With less reactive compounds, the reactivities of the three 

16 The lowest value is based on the assumption that the halogenation of acetone occurs on encounter with the molecular halogens. 

Rate coefficients (k, eqn 35) for the halogenation by molecular halogens (XJ of ketones in aqueous acids ([H+] 1 mol dirr ) at 25° ° 

The problems arising from the uncertainty in the values of Kmol can be avoided by changing to compounds for which the enol is the bulk component of the keto-enol equilibrium. This is true for the compounds [ 1 ]-[4] for which the enol content in aqueous solution and the p7sTa-value is also given (Bell and Davis, 1965). The corresponding rate coefficients for bromination by molecular bromine are given in Table 9. The values of k (eqn 36) show little variation with the reactivity of the enol and are similar (106—107 mol-1 s-1 dm3) to that reported above for acetone. These values appear curiously low for reaction on encounter. 

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In many solvents iV-halogenation by molecular halogen seems to be unimportant compared with C-halogenation not because in principle it cannot occur, but because under the conditions of the reaction the A -halogen compound is converted back to halogen and amine, whereas C-halogenation is non-reversible. 

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