Case 2 .sx - One part of the addendum stable as an anion and the other as a cation , the cationic component being the more reactive ( e.g. ) .sx Structural influences ( a ) favour and ( b ) inhibit anionotropy .sx Case 3 .sx - One part of the addendum stable as an anion and the other as a cation , the anionic component being the more reactive ( e.g. , .sx Case 4 .sx - Both parts of the addendum stable as cations ( e.g. ) .sx Examples of all four cases have been investigated by Ingold and his collaborators and the results have established in broad outlines a comprehensive theory of orientation in additive reactions of conjugated systems .sx Detailed discussion of the results lies somewhat outside the scope of this monograph , but the subject is so closely bound up with the nature of tautomeric systems that this account would be incomplete unless the main conclusions were briefly summarized .sx It will be convenient to deal with each of the four cases in turn .sx Case 1 ( a) .sx In considering the addition of halogens to ethylenic linkages it must be borne in mind that it has been shown by F. G. Soper and G. F. Smith that the attack is by the molecule and not by either of its ions , even in an ionizing solvent .sx Combined with this , cognizance must be taken of the conclusion based on the work of E. M. Terry and L. Eichelberger and of A. W. Francis , that , in halogen additions , one ion enters before the other , the second ion only being liberated during the addition of the first .sx Thus the second stage of the presumed mechanism , .sx may be modified by the presence of a competing anion such as hydroxide or nitrate ions .sx The addition of such .sx to the cation initially formed results in the formation of the compounds or , respectively , in place of the dibromide .sx Now with such a scheme of addition it will be noticed that the first stage involves the formation of a cation and an anion , and , in additions to conjugated systems , the cation so formed will contain a potentially anionotropic system .sx It will not necessarily follow , therefore , that the anion will co-ordinate at the original seat of the positive charge on the electromeric cation , but will be determined by the conditions governing the position of equilibrium in the anionotropic system so formed .sx The second bromine atom may co-ordinate at either the A- or the d-positions ( or both ) according to the distribution of the positive charge on the electromeric cation first formed .sx Thus the whole process of addition of halogens to conjugated systems may be summarized by the scheme :sx In applying such a scheme , it must be remembered that a free halogen ion would only be liberated under conditions favouring ionization .sx Thus trans-hexatriene , which gives the with molecular bromine , affords mainly the when bromination is effected with quinoline dibromide hydrobromide in glacial acetic acid .sx The question therefore resolves itself into two parts , ( a ) what is the initial point of attack of the reagent , and ( b ) what is the final stable product .sx The former will depend on the direction of the initial polarization of the conjugated system , which in turn will be governed by the polar char- .sx acter of the attached groups .sx The latter is determined by the conditions governing the position of equilibrium in the anionotropic system first formed , and can often be predicted on lines already discussed .sx Experimental evidence regarding the point of incidence of reagent attack may be obtained by either of two methods .sx The first depends on the use of a competing anion in completing the second stage of the addition , in which case the position of the foreign ion in the final product will indicate the point of completion of the reaction and will thus yield indirect evidence as to the point of reaction incidence .sx The second involves the use of iodine monochloride as the addendum .sx Since the first stage of the addition involves the co-ordination of the positively polarized portion of the reagent molecule to the negatively polarized end of the ethylene bond , it follows , from the known polar characters of iodine and chlorine , that the location of the iodine atom will indicate the position of initiation of addition , a conception the validity of which has been proved by additions to systems in which the direction of the initial polarization is beyond doubt .sx From the standpoint of this monograph , this aspect of the problem is somewhat irrelevant and it is sufficient to notice that both methods have been applied and have con-firmed the predictions based on the known electronic characters of the groups attached to the conjugated systems .sx The success which has attended the prediction of the final stable product on the basis of the known conditions governing anionotropic systems may be gauged by the typical examples tabulated on p. 254 .sx The major direction of polarization in the original unsaturated compound , the anionotropic system formed by the addition of the first bromine atom , and the final stable product are tabulated in succeeding columns , whilst the last column recordsreferences to the actual experimental results obtained .sx The groups terminating the anionotropic systems resulting from the initial bromine addition are printed in heavy type , that which would be expected the more effectively to absorb the cationic charge on the adjacent carbon atom being underlined .sx The most favoured position of co-ordination for the second bromine ( anion ) will thus be the carbon atom in the y-position to this underlined group .sx The predictions based on the view already outlined are seen to be consistent with available data and this , despite the fact that , in some cases , at the time the predictions were made , contradictory experimental evidence , subsequently proved to be erroneous , existed in the literature .sx Amplifying a particular example it will be noticed that the much-discussed 1 :sx 2-addition of bromine to -diphenylbutadiene and the 3 :sx 4-addition of the same halogen to -phenylbutadiene is readily explained on the grounds of the much stronger facilitation of anionotropic change by -aryl than by -alphyl groups .sx In the former example , .sx if , or its ions , were first formed ( the bracket indicating the anionotropic system ) , the relatively slow and incomplete change ( promoted by , i.e. by alphyl ) , to , would be at once succeeded by the much more rapid and complete change ( promoted by , i.e. by aryl ) to .sx Thus the is not isolated and the main product is the .sx Case 2 ( a) .sx The addition of the hydrogen halide will be initiated by the more reactive cationic component of the addendum , .sx and the point of incidence of the reaction will be determined by the polarization of the unsaturated compound , deduced from the polarity of the substituents in the manner already illustrated .sx In the case of unsaturated conjugated acids or esters the potentially mobile cation , hydrogen , may be assumed to enter via the carbonyl oxygen ; its final position will normally be the -position unless the conditions are suitable to --prototropic change .sx The stable position ( or positions ) of the halogen will , however , again depend on the equilibrium in the anionotropic systems so formed .sx For the systems already considered in the case of bromine addition , hydrogen bromide should add on in the same positions as does bromine , the position of the hydrogen ( cation ) being that occupied by the initially entering bromine cation in the former case .sx Thus the monohydrobromide of -dimethylbutadiene has the structure :sx the main constituent of isoprene monohydrobromide is , whilst the orientation of the addition to both and butadiene should be , the actual products being those expected , namely , where R = Ph or Br , respectively .sx Similarly , cinnamylideneacetic and -malonic acid or ester should , and do , give rise only to -addition :sx whereas sorbic acid and its esters should give both - and -addition :sx Case 3 .sx Strictly speaking , this and the following case should not be considered under the heading of anionotropy , for , as we shall see , such systems do not enter into additive reactions .sx of these two types .sx It will be convenient , however , to include a brief survey of such cases in order that this summary of the theory of orientation of additive reactions may be moderately complete .sx This case differs from Case 2 in that the reaction is initiated by anion attack , whereas in the latter it is initiated by the attack of the more reactive cation .sx One consequence of this , as J. Bloom and C. K. Ingold point out , .sx " is that the place of anionotropic phenomena in the orientation theory of Case 2 is taken by reversibility effects in Case 3 .sx Whilst , in Case 2 , the electromeric displacements which determine the distribution of the anion take place in the first-formed cation , which , after co-ordination , yields the molecular products , in Case 3 they occur in the original unsaturated molecule , which , after co-ordination , gives the anions of the products ; in short , the distribution of the anion is determined in the final stage of addition in Case 2 , but in the initial stage of addition in Case 3 .sx " For additions of this type , the presence of an electron-sink in the unsaturated molecule is essential , for , whereas in Case 2 the anion-affinity required for the addition of the anionic component of the addendum is furnished by the positive charge introduced in the first stage of the addition , in Case 3 the mechanism does not thus automatically furnish the necessary form of reactivity , which renders necessary the presence of the electron-sink .sx The mechanism of addition in Case 3 may be typified by the addition of malonic ester , which has lost its hydrion and is in the condition represented by ( II ) , to acrylic ester ( I ) , in which the initiating minute positive charge on the b-carbon atom arises from the pull of the carbethoxyl group on the C = C electrons :sx These initial electron displacements proceeding to completion give rise to the anion ( III ) of the additive product , the hydrion being introduced during the subsequent acidification .sx In additions of this type to a conjugated system .sx the positive charge produced by the primary polarization will be distributed between Cs and C. The relative pro-portions of addition to the unsaturated units and will be determined by their respective conditions of substitution which determine , in turn , the respective degrees of reversibility of the corresponding addition reactions .sx Thus the place of anionotropic phenomena in the orientation theory in Case 2 is taken by reversibility effects in Case 3 .sx The structural conditions governing the latter .sx have been considered by K. E. Cooper , C. K. Ingold and E. H. Ingold on the basis of the known polar effects of groups and have enabled the successful prediction of orientation in Michael addition reactions on the basis of the theory outlined above .sx Case 4 .sx This case , in which both portions of the addendum are stable as cations , includes the addition of hydrogen to conjugated systems .sx According to H. Burton and C. K. Ingold , reduction of unsaturated compounds by means of a metal dissolving in aqueous ( or alcoholic ) medium is regarded as a three-stage process .sx In the first stage , the unsaturated molecule , polarized in the electric field at the surface of the metal , extracts a proton from the aqueous or alcoholic solution , this proton becoming attached to that atom ( Ca ) which most readily tolerates the negative polarization charge developed prior to co-ordination .sx The resulting cation possesses an electron-sextet at Cb and this immediately takes up two electrons from the metal surface , thus converting it from a cation into an anion .sx This anion then combines with a second proton , giving the final reduction product .sx It will be seen that at the conclusion of the second stage negative hydrogen ( ) has been added to the double linking and thus we perceive a close analogy with the halogen additions discussed under Case 1 :sx for just as in the latter unstable positive halogen first unites , leaving the .sx ordinary halide ion to combine later , so according to this reduction hypothesis , unstable negative hydrogen is first added on leaving the ordinary positive hydrogen ion to unite later .sx Moreover , just as in halogen additions to conjugated systems the first stage results in the production of an electromeric cation , so in the addition of hydrogen to such systems , the initial addition of negative hydrogen results in the formation of an electromeric anion :sx