Activation parameters for the competing electron transfer and SN2 pathways of the reaction of anthraceneradical anion with cyclopropylmethyl bromide

Abstract

The reaction between the electrogenerated radical anion of anthracene and alkyl halides has been studied in the temperature interval of −50 to 40 °C in N,N-dimethylformamide in order to describe in detail the competition between the electron transfer (ET) and S_N2 pathways. For cyclopropylmethyl bromide the competition can be quantified directly on the basis of an analysis of the distribution of ring-opened versus ring-closed products. In general, the ET pathway is found to prevail although the S_N2 pathway becomes of greater importance as the temperature is lowered. When the product analysis is combined with the measurement of the overall rate constant for the reaction the pertinent activation parameters can be extracted for both mechanisms. As expected ΔH^‡_SN2 is smaller than ΔH^‡_ET (by 7 kJ mol⁻¹) because of the stronger bonding interaction present in the transition state of S_N2. On the other hand, this effect is more than counterbalanced by the fact that ET is entropically favoured because of smaller geometrical restrictions (ΔS^‡_ET − ΔS^‡_SN2 = 37 J mol⁻¹ K⁻¹). The S_N2 mechanism leads to substitution in the 9 position of anthracene while for the ET mechanism it takes place in all three positions of 1, 2 and 9. The overall distribution of products in the three positions is largely independent of temperature. This is due to the fact that the increased amount of S_N2 products formed in the 9 position as the temperature is lowered is accompanied by an equally large decrease in the amount of ET products obtained in the very same position. Exactly the same constancy in the product distribution is seen for the reaction of anthracene radical anion with bromoethane, where both mechanisms also exist. For the reaction involving the sterically hindered 1-iodoadamantane, on the other hand, the S_N2 pathway is precluded and in this case there is a profound decrease in the amount of 9-substitution as the temperature is lowered.