The effect of substituents at the 5position upon the rate of chlorination and bromination of thiophen at the 2position, in glacial and 15% aqueous acetic acid respectively, has been studied and found to exhibit a Hammett PO relationship, with p values of -6.5 and -1 0. Bromination of deactivated thiophens has been analysed in terms of simultaneous second-and third-order processes and the activation parameters for various substituted thiophens indicate that the rate of reaction is affected more by changes in A M than ASS.THE effect of substituents on the rate of electrophilic substitution has been extensively investigated for substituted benzenes but less so for other ring systems. An increased understanding of structure reactivity relationships depends, to some extent, upon the accumulation of data for ring systems other than benzene. As part of a general investigation to this end, we describe here a study of the effect of substituents upon the rate of chlorination and bromination of thiophen by molecular chlorine and bromine in glacial and 15% aqueous acetic acid respectively . Direct interaction of thiophen and chlorine results in addition rather than substitution2 but this is not so if the reaction takes place in acetic acid solution. Marino 3 reports that, under these conditions, thiophen reacts 1.25 x 10' times faster than benzene. Bromination of thiophen has been investigated more extensively than chlorination and previous work has been summarised in Part I of this series. Marino has studied the effect of substituents upon the rate of reaction and this work will be considered in detail in the discussion. EXPERIMENTALMaterials.-Commercial 5-methyl-, 3-methyl-, 5-ethyl-, 5-chloro-, and 5-bromo-thiophen were distilled before use. Thiophen-5-carboxylic acid was recrystallised from water to constant melting point. 5-Phenylthiophen was prepared by the reaction of 5-lithiothiophen with cyclohexanone and subsequent dehydrogenation of the product with 4 G. Marino, Atti Accad. naz. Lincei, Rend. Classe Sci. fis.

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Protodetritiation of 2and 3-tritiothiophen in aqueous sulphuric and perchloric acids has been studied as a function of acidity and temperature. There is a linear relation between log k and -H, but the activation parameters are independent of acidity. General acid catalysis has been demonstrated in the protodetritiation of 5-methoxy-2-tritiothiophen. The relative reactivity of the 2and 3-positions in thiophen appears to be a function of acidity.The mechanism of hydrogen exchange in thiophen is compared to that in benzene compounds.

A study of salt effects and activation parameters indicates that the mechanism of bromination of thiophen in aqueous acetic acid is the same as that of benzene compounds. There is an isotope effect (k"/kD) of 1-3, which is shown to be secondary and does not represent slow proton loss.

A number of classes of sulphated and sulphonated anionic surfactants have been determined by photometric titration with the cationic surfactant Hyamine 1622. The results obtained are within +5% of the corresponding results obtained by the two-phase mixed indicator titration procedure that is the current IS0 method (IS0 2271). A photometric approach offers considerable advantages over the two-phase titration. These include a single aqueous phase, a non-operator dependent mode of detection and a 50% or more reduction in sample analysis time. The repeatability of the photometric method is similar to that obtained by experienced operators using the two-phase titration.