In vinylic halides which can exist in geometrical isomeric forms, the steric course of substitution is of special interest since information about the geometry of the transition state can, in principle, be obtained. We have examined the products and kinetics of substitution in the ethyl-p-chlorocroton-ates, the configurations of which are known6. Both forms readily underwent bimolecular reaction, in ethanol solvent, with thioethoxide, thiophenoxide, ethoxide and phenoxide ions. An examination of products showed that with ethoxide ions or phenoxide ions the same product was obtained from both chloroesters. With thioethoxide ions, however, two products were obtained, their relative amounts depending on which chloroester was used. These two products, which we will call the higher boiling-point (and lower boiling-point) isomers, had very similar ultra-violet spectra and both gave aceto-acetic ester on mild treatment with acid. All the evidence indicated that they were the isomeric ethyl p-thioethoxycrotonates. Essentially similar results were obtained with thiophenoxide ions as nucleophilic reagent. It was found that with appropriate precautions and under conditions in which isomerization of factors and products was shown not to occur, the products of reaction could be isolated in about 90 per cent yield. The results are illustrated in Table 1; kinetic data refer to solutions of concentrations approximately M/20.A comparison of the infra-red spectra of the thioethoxy-esters with those of the chloro-esters showed that, although no unequivocal assignment of the bands could be made, the spectra in the range 600 1,700 cm.-1 were best interpreted on the assumption that the lower boiling-point and higher boiling-point thioethoxy-esters had the cis and trans configurations respectively. Examination of the products obtained by treatment of the thioethoxy-esters with Raney nickel led to the same conclusion. From the lower boiling-point isomer we isolated, after cautious hydrolysis of the products of the reaction, c^s-crotonic acid in about 30 per cent yield; the higher boiling-point isomer, under the same conditions, gave Jrans-crotonic acid in similar yield. On the basis of this assignment, it is seen that the reactions proceed largely, though not completely, with retention of geometrical configuration. The transition state in substitution at a vinylic centre has been discussed by Bunnett2 and Gold3. The most acceptable model for the present case is that in which the valency bonds at the carbon centre undergoing substitution become, or tend to become, tetrahedrally arranged. Since an extra 39-orbital is required to form the sphybrids, the double bond is lost in the transition state and the carbon atom adjacent to the centre of substitution acquires a negative charge. On this view, and assuming that no rotation occurs in the transition state, substitution should proceed, in a synchronous process, that is, in the absence of a definite intermediate adduct, with retention of geometrical configuration and should be enormously facilitated in those molecules containing a group capable of delocalizing the negative charge formed in the transition state. It is clearly possible to extend this formulation to include the existence of an intermediate of considerable stability. The longer the life of such an intermediate the greater will be the degree of isomerization produced' by free rotation about the single bond which in the intermediate replaces the double bond of the original compound. The reactions of the ethyl p-chlorocroton-ates with thioethoxide ions result in predominant re-tqntion of geometrical configuration and are best considered, therefore, as proceeding through a shortlived intermediate: OH3 ClI SC2H5H CO2C2H5 Ethyl jS-chloro-Zraws-crotonateCH8 Cl C SC2H5! o~ o~ C O /VOC2Hft CH3 SC2H5 CH C02C2H5 (92%) From studies on the corresponding unsubstituted compounds, the carbethoxy group was found to increase reactivity by a factor of about 106. This large effect can be understood on the formulation given, since, as Bunnett2 has pointed out, one of the reasons for the inertness of simple vinylic halides may well be that the carbon atom adjacent to the centre of substitution does not readily accept a negative charge.Table 1 Compound Nucleophilic reagent T C. kz E Iog10 B ProductsEthyl /3-chloro- OCHaCH8 0 0-055 19-2 14-2 Ethyl /3-ethoxycrotonaterans-crotonate(m.p. 30 -2 C) it SCH2CH3 0 0-336 14-5 11-2 8 per cent lower b.p. isomerEthyl /8-chloro- OCH2CH, 0 0-26 17-0 13-0 92 per cent higher b.p. isomer Ethyl /3-ethoxycrotonate cts-crotonate (m.p. 30 -2 C.)SCHaCH8 0 0-148 14-1 10-5 81 per cent lower b.p. isomer 19 per cent higher b.p. isomer" SCH2CH8 78 87 per cent lower b.p. isomer 13 per cent higher b.p. tsomer Ara and E are expressed in gm.mole."1 min."1 and kcal.mole"1.As in the parent compounds, cis and trans refer to the configuration of the methyl and carbethoxy groups. The reactions with ethoxide ions in which only one product was isolated whichever chloro-ester was used may involve an intermediate of longer life; or alternatively, the isomer wliich was not found may be unstable under the experimental conditions. Since all attempts to prepare this isomer failed, we are, at the moment, unable to distinguish between these possibilities.We thank Profs. E. D. Hughes and C. K. Ingold for their help and encouragement, and Dr. P. B. D. de la Mare, Dr. E. S. Graham and Mr. T. M. Dunn for much valuable discussion. Thanks are also due to Dr. D. J. Millen and Mr. D. Watson for measurements of the infra-red spectra and for valuable discussion, and to Mr. D. V. Banthorpe for carrying out the experiments with Kaney nickel.