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Evrokhim-SPb-Trading, VNIINeftekhim, 193148 St.Petersburg, Russia

At the modern stage of development of the chemical science the unimolecular heterolytic reaction mechanisms become an unjustified simplification for it is definitely established that a dissociation into ions of a covalent bond can occur only forcibly, by the action of electrophiles or nucleophiles.

In organic chemistry reactions R₃C–Hlg that currently are classed as unimolecular nucleophilic substitution at a carbon atom proceeding by the dissociative mechanism S_N1 are really substitutions at Hlg occurring by a synchronous mechanism ^HlgS_E. Similarly in coordination chemistry reactions [L₅M – Hlg]⁺⁺ classified now as substitution at a metal atom by a dissociative mechanism are actually electrophilic substitutions at Hlg by a synchronous mechanism.

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It is suggested here to classify these and analogous reactions according to their true mechanism. This classification provides a better understanding of the reaction regularities and makes possible adequate conclusions and permits the consideration of a large number of versatile substitution reactions from a unique viewpoint.

The suggested approach to considering reactions based on the true mechanisms provides easier and clearer interpretations and can significantly facilitate teaching chemistry and its understanding and mastering by students.

In the nineteen thirties at the early stage of investigating reaction mechanisms in organic chemistry all reactions were represented as occurring at carbon reaction center, like the antique astronomy that regarded the Earth as the center 1) of the Universe (Ptolemaic system). For instance, any replacement of one nucleophilic group by another was regarded as nucleophilic substitution at a carbon atom (3). This was quite natural since the data for more adequate classification were lacking. In keeping with the fundamental works of the school of C.K. Ingold two generally acknowledged mechanisms of nucleophilic substitution were formulated and found a prominent place in modern textbooks:

1. S_N2, bimolecular, or synchronous, or direct

   

                                                                          I

    

2. S_N1, unimolecular, or pre-dissociative

Therewith the mechanism (2) + (3) = (4) based on the nucleophilic character of the HO^- reagent in the overall reaction (4) is classified as a nucleophilic substitution at the carbon atom although the stage (2) of this reaction comprises dissociation, and stage (3) is reaction of ions, and neither of these elementary stages represents a nucleophilic substitution at a carbon.

The nucleophilic or electrophilic attack is performed by a reagent, a catalyst, or a solvent providing the latter possesses these nucleophilic or electrophilic properties. For the pre-dissociative mechanism (2) + (3) = (4) the dissociation along equation (2) is effected by nucleophilic attack of water that may be described as follows 3)

There is a substantial literature on the importance of nucleophilic solvent participation in the solvolysis of tert-butyl chloride (8-10). Therewith finally was found a reason for classing the pre-dissociative mechanism (2) + (3) = (4) as nucleophilic substitution at carbon.

The dissociation along equation (2) in the pre-dissociative mechanism (2) + (3) = (4) is effected by the electrophilic influence of acetic acid and may be written as

Already Hammett (11) concluded that specific solvation of the halogen in the transition state by hydrogen bond formation was the driving force of solvolytic reactions of alkyl halides. The requirement of electrophilic assistance to stabilize the “departing” anion has been repeatedly emphasized (5-7). Mixed, push-pull (3,5-7,12) mechanisms with simultaneous involvement of nucleophilic and electrophilic reagents were also suggested. An impression was created that all the above versions were possible.

The situation is paradoxical: The reaction described in the modern textbooks as nucleophilic substitution at carbon by the S_N1 mechanism actually occurs as electrophilic attack on the adjacent heteroatom! And this atom which really is the reaction center is called a “departing”, “leaving” group. This strange picture is a result of the common practice of fitting disagreeing facts into a generally adopted pattern by means of additional assumptions. Therewith the majority of chemists in discussions agree that terms and concepts are contrary to fact but the tradition is conserved.

Now a need is generated of essential changes in description of these reaction mechanisms (13,14). Similarly to transition made by astronomy in XVI century from Ptolemaic system where the Earth has been the center of the Universe to Copernicus system where the Earth is only one among planets it seems appropriate in XXI century for organic chemistry to take into consideration in classification of reaction mechanisms not only the substitution at carbon but also substitution at heteroatoms on the other end of the bond. In other words, the case in point is replacing the formal classification of reaction mechanisms according the reagents in the overall reaction [for instance, in reaction (4)] by the classification in keeping with actual mechanisms of the elementary stages [for reaction (4) by stage (6)].

The most efficient way to improving the concepts on reaction mechanisms is the introduction of the corresponding changes into the teaching the reaction mechanisms to the students.

Not S_N1 at carbon but S_E at chlorine

It is readily seen (13,14) that stage (6) is a synchronous reaction similar to (1). The likeness becomes especially clear considering the modification of the reaction (2) + (3) = (4) catalyzed by electrophilic reagents (3,6,7)

The fundamental difference between reaction (1) on the one hand and stages (6), (7) on the other hand is the site of attack and the character of the acting reagent. In the first case the attack is directed at the carbon, in the second case to the heteroatom at the other end of the bond; in the former case the attacking species is a nucleophile, in the latter it is electrophile. Therefore the first reaction (1) should reasonably be called nucleophilic substitution at carbon and designated as ^CS_N (instead of S_N2). The reasonable symbol for reaction (2) + (3) = (4) in keeping with the limiting stage (6) or (7) is electrophilic substitution at chlorine with the corresponding designation ^ClS_E (instead of S_N1). The reactions of type (5) should be classified as ^CS_N together with reaction (1).

Therewith the molecularity of a reaction need not be defined, especially since fundamental alternative versions exist: Ingold insisted (3) on his initial definition of molecularity as “the number of molecules necessarily undergoing covalency change” 4) , many authors define the molecularity as the kinetic order of the reaction. Besides, “Also, the unimolecularity has never been demonstrated, but is an assumption only.”(1).

If formerly the molecularity has been almost the main characteristic (S_N1 or S_N2 ?), now it is not so important compared to the site of attack and character of the attacking reagent (^Cl S_E or ^CS_N ?).

The study of reaction mechanisms in inorganic chemistry started later than in organics, and it was based on concepts developed in the organic chemistry (15). In inorganic chemistry the reaction center is as a rule placed on the centrally located element, usually a metal.

In reactions classified as following a dissociative mechanism many of relations are valid as described above for the S_N1 reactions. The majority of the above-considered conflicts between the classification and real mechanisms also exists. For instance, in aquation of octahedral complexes (16)

(M = Co³⁺, Rh³⁺, Cr³⁺; X = Hal^-, NCS^-, CN^-) reactions (10) are classified as following the dissociative mechanism (8) + (9). They are catalyzed with ions Ag⁺, Hg⁺⁺, Cd⁺⁺ and Tl³⁺ along equations of the type

Even at the first glance the great resemblance between reactions (2) and (8), (7) and (11) is obvious (13,17).

Similarly to the above discussed reactions of synchronous substitution at the heteroatom (not at carbon) in organic chemistry, reactions (8) + (9) = (10) in inorganic chemistry proceed as synchronous substitution at the ligand (not at the metal). Therefore the correct classification (13,17) should be a synchronous substitution at the ligand, and not a formal one, as dissociative substitution at the metal.

It is very important to make students understand that the considered versions of mechanisms for reaction (1) involving a transition state I and for reaction (6) with a transition state II are the limiting cases.

In the first limiting case, in the “pure” nucleophilic substitution at carbon the nucleophile HO^- in the transition state I (Table 1) is maximally close to the carbon whereas the electrophile HOC₂H₅ is outside of the transition state. In going downstairs along Table 1 the electrophile (HOC₂H₅, Å, HOOCCH₃) approaches the chlorine whereas the nucleophile (HO^-, Nu, H₂O) moves away from the carbon. In the limiting case II the electrophile is maximally close to the chlorine, and the nucleophile is outside of the transition state: we has come to the “pure” electrophilic substitution at chlorine.

The frontier between nucleophilic substitution at carbon and electrophilic substitution at chlorine is set by line III (Table 1) where both interactions are equal. It is shown in Table 1 that every version of the transition state gradually and regularly transforms into the neighboring ones similarly to changes of colors in the spectrum of sunlight or in a rainbow. Therewith the majority of the reaction mechanisms are situated between the limits I and II (Table 1), in these intermediate cases are revealed both the features characteristic for the main and partially for the assisting interactions.

Table 1. Substitution mechanisms range (spectrum) from nucleophilic to electrophilic.

“Push-pull” transition states of III type were described long ago (12) and are widely believed (3,7). They were also suggested as generalized transition state S_N for S_N2 and “S_N1” mechanisms (1,3,7). A novel concept in Table 1 consists in a statement of the fact that below the frontier line III occurs an electrophilic substitution at halogen.

It should be specifically emphasized that the designation S_N1 (^XS_E) applies to all the mechanisms occurring below the line III and not only the “pure” electrophilic substitution at halogen characterized by the transition state II. To the latter case corresponds formation of a free carbocation and consequently racemization (inversion by 50%). Whereas in electrophilic substitution at chlorine with nucleophilic assistance at carbon with the transition state IV the attack by nucleophile Nu contributes to the reaction and the resulting inversion is over 50%.

Strong arguments were advanced (1) against the possibility of carbocation formation in reactions (2), (6), and (7), i.e. against the limiting case II in Table 1. Presumably carbocation formation occurs not so frequently as it is assumed nowadays, but a complete denial of its existence (even in a solvated form) is hardly justified. It would have signified complete rejection of transition state II in favor of IV, but it has already been indicated that there is no evidence for significant nucleophilic participation of the solvent in the S_N1 reaction of any tert-alkyl chloride in any solvent (2).

In a more general form the limiting electrophilic synchronous stages (6), (7), and (11) can be expressed by an equation

In the case illustrated in (6) the electrophilic attack on X forms a hydrogen bond, and in cases (7) and (11) it is a covalent bond. This is no reason for contrasting these mechanisms: The modern quantum-chemical concepts (18) regard the hydrogen bond as a kind of ordinary chemical bond for it forms with reconstruction of electron orbitals, collectivization, and transfer of electrons. At the same time such characteristics as bond length and its energy do not define the bond type (18). Besides the small energy gain at hydrogen bond formation is supplemented with energy solvation of ions which in this case were generated from an uncharged species.

Actually the thorough investigation of stereochemistry observed in spontaneous (12, caused by solvent E = H₂O, hydrogen bond) and induced (12, caused by reagent E = Hg⁺⁺, covalent bond) reactions of a series of complexes A – X of a general formula (+)-cis-[CoEn₂LX]ⁿ⁺ established that no fundamental difference exist between these processes (19). In other words, the solvent in the spontaneous reaction (H₂O) acts in the same way as the reagent (Hg⁺⁺) in the induced one. Thus the assumption that the hydrogen bond formation (12, E = H₂O) is not a chemical reaction but a “simple solvation” is outdated. By the way, just this assumption is the reason of survivability of the error concerning the nucleophilic unimolecular substitution at carbon or at metal in reactions of (4) and (10) type, for manipulations with solvation make possible overlooking of the electrophilic character inherent to the hydrogen bond formation with the halogen.

Apart from the already discussed cases, a lot of existing reactions possess limiting electrophilic synchronous stages (12). Some well studied examples are listed in Table 2 (13, 14, 17).

Table 2 illustrates the wide opportunities provided by the approach under consideration. Within the framework of a classical S_N1 mechanism it is hardly expected to see many common points in substitution reactions at carbon [Table 2, ¹ 1–3, A = (C₆H₅)₃C] and at transition metals [Table 2, ¹ 4, 5, A = (NH₃)₅M³⁺]. In contrast, mechanism (12) treats both reactions as occurring at the X atom; providing it is the same in both cases, the rules governing these reactions should be similar. Actually, when X is a halogen the reactivity of both R₃C - Hlg, and [(NH₃)₅M - Hlg]⁺⁺ for all E’s considered here, excluding H⁺, changes in the same order: F < Cl < Br < I. For E = H⁺ also in both case the series is reversed (14).

Table 2. Examples of limiting synchronous stages (12)

Finally, the catalytic effect of cations similar to types (7) and (11) is not limited by solvolysis of C – Hlg and M – Hlg bonds but is also characteristic of B – F, P - F, As – F and even Xe – F (20). In particular, the hydrolysis of the difluoride of the noble gas, xenon, is accelerated with Al³⁺, Be⁺⁺ and Ti⁴⁺ similar to the acceleration observed with the same cations in the hydrolysis of (CH₃)₃CF (organic compound), Co(NH₃)₅F⁺⁺ (coordination compound), and BF₄^- (inorganic anion).

Thus the suggested approach provides a possibility to treat large series of reactions belonging to organic, organometallic, and coordination chemistry from a single viewpoint as a substitution at the same common atoms for all these series X.

A number of essential regular trends characteristic of the reactions in question may be explained more convincingly based on the suggested concept of the decisive role of the electrophilic attack on X in (12) stages:

· It is known (3) that a modification of X strongly affects the rates of these reactions [compared with synchronous processes of type (1)]; now it looks natural in reactions for type (12) with just X as the reaction center;

· The acceleration of reactions (12) when A contains electron-donor groups (a strange phenomenon for a true nucleophilic substitution) is readily understood because of growing electron density on X facilitating the electrophilic attack thereto;

· Bulky substituents at A do not hamper reactions as the attack is directed to X;

· Abnormally strong (considering the increase in the dielectric constant of the medium) reaction acceleration on addition of small amounts of polar compounds is easily explained by the fact that these substances play the role of reagents or catalysts E;

· Applied directly a very strong sensitivity of the reactions to the solvent follows from the fact that it can really function as a reagent (E).

The concepts described here on the decisive role of the electrophilic impact on the heteroatom (ligand) in reactions (4) and (10) provides a possibility to formulate some regularities.

Upon initial consideration of the mechanism, it is useful to remember that the attack treated here on the peripheral atoms is more probable than the attack on the carbon atom shielded by four substituents or on the metal surrounded by six ligands:

Simple calculations revealed that the carbon in the first structure and the metal in the second are nearly completely “closed” and chlorine is “opened” to 60-80% (14).

The stereochemistry of electrophilic synchronous substitution may essentially differ from that of nucleophilic synchronous substitution. Since the Pauli principle excludes the presence of more than two electrons in the same orbital a nucleophile carrying its electron pair in reaction (1) can approach only from the side opposite to the group to be substituted and along the straight line extending the bond to be broken. As a result the synchronous nucleophilic substitution occurs with reverse of configuration, and the size of substituents at the reaction center strongly affects the reaction rate.

In the electrophilic attack no additional electrons are involved, and the electrophile may also approach sideways (3,17). Therefore if instead of chlorine in reactions (6), (7) or of X in reactions (12) operate relatively complex groups, the reversal of configuration in these groups (14) would not be obligatory (15), and steric deceleration would not always be great (15).

It should be noted that the approach suggested in this article does not formally regard terminology and does not simply propose to replace one name by another. Here we intend

· to find the classification of a true mechanism of reaction

· to move the principal attention to the true reaction center (heteroatom, ligand) and not to regard it as “departing”, “leaving” group

· to attract attention to the key importance of bond formation between X and E along equation (12): The energy of the new bond and that of ion solvation making up for the energy loss at the rupture of the A – X bond, and that the reaction is impossible without it

· to make evident that “pure dissociative” reactions of nucleophilic substitution of the type (2) + (3) = (4) or (8) + (9) = (10) under common conditions as a rule do not exist: they proceed through synchronous mechanisms (6), (7) or (11)

· to emphasize that the practice of dividing reactions of substitution into nucleophilic synchronous (S_N2) and nucleophilic dissociative (S_N1) in organic and inorganic chemistry creates erroneous notions