Arrhenius parameters in the solvolysis of alkyl chlorides and bromides

Abstract

The reactivities of alkyl halides, BX, in nucleophilic substitution reactions increase in the order RF < RCl < RBr < RI. It has been suggested that this reactivity sequence arises almost entirely from a decrease In activation energy, E, along this series, consistent with the decreasing ionisation energy of the C-halogen bonds, i.e., C-F > C-C1 > C-Br > C-I. On the other hand some authors have concluded that a change in the entropy of activation, ΔS(^+) , plays the most important part in controlling reaction rate in this series. In many cases, however, the activation parameters of the different halides referred to different temperatures. Such comparisons may be misleading since recent work has clearly shown that E and ΔS(^+) can vary with temperature; any valid comparison of these parameters must, therefore, involve quantities which all refer to the same temperature.' A study of the reactions of several pairs of alkyl chlorides and bromides with aqueous acetone is now reported. Reaction rates, activation parameters and the temperature coefficients of these parameters have been determined and the results show that, for hydrolysis at the same temperature, the change in rate caused by replacing an alkyl chloride by the corresponding bromide arises almost entirely from a change in the activation energy; this applies to both S(_N)1 and S(_N)2 reactions. It has recently been suggested that the value of ΔC(^+)/ΔS(^+), where ΔC(^+) is the heat capacity of activation, should be independent of the nature of the substrate in SNl solvolysis and that this ratio will have a lower value for solvolysis by mechanism S(_N)2 under the same experimental conditions. This suggestion was based on results observed with alkyl chlorides. All the alkyl chlorides and bromides now studied behave in accordance with the requirements of this hypothesis. During this work the solvolysis of benzyl bromide was studied and the results indicated that this substance reacted by mechanism S(_N)2. This is of interest, for although the hydrolysis of benzyl chloride occurs near the point which marks the transition from reaction by mechanism S(_N)2 to reaction by mechanism S(_N)1, the replacement of the chlorine atom by a bromine atom does not appear to cause a major mechanistic change.