Is This Reaction a Substitution, Oxidation-Reduction, or Transfer? Naum S. lmyanitov VNIINeftekhirn,Zheleznodorozhny Pr., 40,193148 St. Petersburg, Russia Because the number of chemical reactions is tremendous, chemists have been able to study and describe them only by classifying them into gmups of the same type. An example of such successful classification is given in the organic chemistry text by Hendrickson, Cram, and Hammond ( I ). However, when we consider the various areas of chemistry individually-inorganic, coordination, organic, orgauometallic, and orgauometalloidal-we fmd that they all deal with many reactions of the same type. Further, many reactions that are put in different p u p s actually have much in common. Unfortunately, their separation is based on a convention that does not always follow objective and consistent rules. "Nucleophilic Substitution" at Various Atoms The following well-investigated reaction is usually classified as a nucleophilic substitution at the carbon (1,2).
In the table, such reactions are generalized as reaction type a. (For example, for eq 1,Y-is HO-; Z is C; R is H; n = 3; and X is Br). However, when nucleophilic substitution occurs at a hydrogen rather than at a carbon, the reaction is generally regarded as a proton transfer.'
However, this approach to classification fails to point out the chemical similarity between nucleophilic substitution at hydrogen and nucleophilic substitution at carbon. The same is true for nucleophilic substitution at other non-carbon atoms (4, 5). For example, at a halogen we get the following similar reaction.
'
Reactions of type 2 were classifiedonly by Hammen in 1940 as nucleophilic substitution at hydrogen (3). Equivalent Notions In Different Classifications Sign
Reaction
-
Substitution
Transfer
Z is the reaction
RnZ is the group
center' nucleoohilic \T + RnZ-X + [Y.. . Z(R,). . . XI-
b
d
-t
[Y...Z(R,)...X]'+ Y-ZRn+Xa 6
+ RnZ-X + [Y. . . Z(Rn) . . . XIt + Y-ZRn + Xt
'In cased, Z and R.Z are an electmn.
Journal of Chemical Education
electrophilic electrophiiic
6
V + X + [ ~ . . . e. - -X I +
14
free radical
&
Y+R,Z-X c
+ Y-ZRn + K
+
Y+X+
transferred^ transfer oi R Z + (cationic) transfer of RZ' (freeradical) transfer of RnZ(anionic) transfer of e(electronic)
0xidatiol~-Reduction Z is a bridge' oxidation of '. . C reduction of X
reduction of Y+ oxidation of X reduction of Y+ oxidation of X
Viewing the Reaction as a Cation Transfer
On the other hand, all the preceeding transformations (reactions 1-3) could be classified,with equal justification, as cation transfers involving
or, for reaction type a in general,
This approach can be very useful. For instance, using the Marms theory, Albery ( 6 ) regarded nucleophilic substitution at C H 3 as a methyl transfer, similar to an electron transfer or proton transfer. Then he was able to calculate the free activation energy of the following reaction
and where E.is the standard electrode potential of the nucleophile in the following reaction. 2T-2e-+2Y+Y2 In other words, nucleophilicity is quantitatively related to the abilitv of the nucleoohile to undenro oxidation. Thus. reactions 1-6 may also beregarded as oGdation-reduction reactions (see reaction type a in the table). In a similar way, electrophilic substitution may be regarded as a transfer or as an oxidation-reduction (twe c). In free-radical substitution, redox reactions do not r&e place (type b). In the limiting case, when the anion being transferred (reaction center) is assumed to decrease to the size of an electron, we have an electron transfer. which can be viewed as an el&trophilic "substitution" at the electron (type d).
The One-sided Approach to Classifying Reactions
by starting with the free activation energies of degenerate reactions 5 and 6.
Albery has also separated the SwainScott nucleophilicitv parameter into its kinetic and thermodvnamic contributions. Then the change seen in the reaction rate upon deuteration (CH3 + CD3) characterizes the value of the positive charge on carbon in the transition state (6). Viewing the Reaction as an Oxidation-Reduction
These nucleophilic substitution reactions (reaction 1 and reactions of type a) can also be regarded as oxidation-reduction reactions. In reaction 1the charge on the bromine atom in CH3Br is only 4% that of an e l e c t r ~ nClearly .~ the bromine is reduced
whereas the hydroxyl group is oxidized.
Ingold (2) suggested using this approach with such reactions. It is not formally "eccentrice to think of reaction 1as an oxidation-reduction reaction rather than as a nucleophilic substitution. In Edwards equation, which is shown below, the reactivity of the nucleophile in a substitution reaction is measured at the saturated carbon atom.
All three of the classifications considered here are quite eauallv valid: substitution transfer. and oxidation-reduction. However, the classification &at is preferred differs among the various areas of chemistry. For instance, in organic chemistry, all the reactions listed in the table are usually regarded as substitutions, whereas in coordination chemistry all three classifications are applied, depending on the reaction groups. In all areas of chemistry, acid-base transformations are usually described as transfer react-. in ..n..a...
For chemical classification or description as a whole, this situation seems unsatifactorv: Instead of four tvDes of reactions (a-d), there are nine-types. Moreover, even within one branch of chemistry, similar reactions are ofien regarded as belonging to different classes. For example, in organic chemistry, reaction 1 and reaction 2 are classified separately In coordination chemistry, all three are classified separately: substitution, transfer, and redox reactions. Obviously it would be helpful to have a system that classified reactions uniformly across the various areas of chemistry: inorganic, coordination, organic, organometallie, and organometalloidal. Since free-radical substitution cannot be described in terms of oxidation-reduction, this variant can not be used for universal classification. The most widespread approach classifies all reactions of types a-c as substitution reactions. Unfortunately, it seems artificial to describe reaction type d as electrophilic substitution at the electron. Since "substitution at hydrogen" has not become a popular term for reaction 2, "s6bstitution at the electron" is less likely to be widely used to describe reaction type d. I believe that the best and most universal appproach would classify all these transformations, as seen in types a-d, as transfer reactions. In other words. each reaction would be seen as a eationic, free-radical, anionic, or electronic t r a n ~ f e rAlso, . ~ this approach does not have contradictions.
where
The Simultaneous Approach to Understandina Reactions
This value is taken from a table in ref 7as a function of the d Rerence oetween the mo ecular elenroneaativlt es of CH, and Br reported in the same monograph. Even f& flourine, whic6 is the most electronegativeatom, the charge in CH3F is-0.36 (7) or -0.22 (8). Naturallv, it cannot be assumed that the R.2 is transferred ,, arouo ~~,~ -as a free on or radlcal in lnese reactions. On y a partial cnarge (an unpa red electron) in the trans uon state is meant. however, tne -doOat ng' gr0t.p X ts anually transformeo nto an on or raa ca
Unfortunately, classification always fosters formalization. We must remember that the full concept of light was developed only by simultaneously considering both its wave-like and particle-like properties. Similarly, full understanding of each reaction can be reached only by simultaneously consideri~xall three of its asoects: substitution. transfer,-and oxidati&reduction. As shown above. when we broadened our conceot * oftransfer, we strengthened our approach to nucleophilic substitution reactions. Similarly, an extended view of oxi-
-
En = E D+ 2.60
-
~~~
~
~
Volume 70 Number 1 January 1993
15
dation-reduction processes led to the quantitative characterization for the reactivity of nucleophiles using Edwards equation. Using an earlier undescribed example, I will illustrate the usefulness of the reverse approach: regarding oxidation-reduction as nuoleophilic substitution. Typical redox reactions 7 and 8 proceed by the inner-sphere mechanism with the intermediate formation of halogen atom bridges (9-11 ).
The Benefit of the Flexible Approach
The present paper shows the fruitfulness of regarding reaction type a from three aspects: nucleophilic substitution cationic transfer oxidation-reduction It also seems promising to interpret the classical electrophilic substitution (reaction type c) as both an anionic transfer and an oxidation-reduction. Other nontraditional variations are also of interest, including the unusual concept that reactions of type d be viewed as substitutions at the electron. In this work I have restricted the discussion to bimolecular reactions (reactions 1-8 and the types listed in the table) because monomolecular mechanisms are usually formulated when using an unjustifiably simple approach to bimolecular reactions (12). Examples from organic and coordination chemistry show that any reaction with the followmg form where Y is a nucleophile, a free radical, or an electrophile may be regarded as every one of the three types of reactions listed below. .Substitution reaction involving a nucleophilic, free-radical. or electroohilic substitution .!lkansfer reaction involving the transfer of an atom, a group (Z', Zo,Z 1, or an electron ( Z = e-) Ozidation-reduction reaction involving the oxidation of Y and the reduction of X, or vice versa
Careful comparison shows that reactions 7 and 8 are similar to reaction 3, which is a nucleophilic substitution at the halogen. Then reactions 7 and 8 can be adequately described by reation type a, which is common for all nucleophilic substitution reactions. (For reaction 7, Y-is Co" in Con(CN)g-;n = 0; Z is C1, and X is Co"' in *Con'(NH3)P.For reaction 8, Y is Fen in FeE(Hz0)8';n = 0; Z is Cl; X is Con' in Con1(NH3)P.) Thus, there is sufficient reason to classify them as nucleophilic substitutions at the chlorine atom (5). A Conceptual Approach to the Effect of Leaving Group Structure
In this case, the C O ( N H ~group ) ~ in reaction 8 is the leaving group. We can study the effect of the leaving group's structure on the reaction rate by investigating the complexes in which ammonia is substituted by different ligands. Previous detailed investigations (9) show that the action of the leaving group's ligand depends on its position with respect to the Co-C1 bond being b ~ o k e nSurprisingly, .~ the strength of the trans cobalMigand bond proved to be an important factor in this reaction. Although this bond is not broken in the reaction, it becomes longer in the transition state (9). These details on the effect of the leaving group's structure in nucleo~hilicsubstitution are new. Moreover. they were obtained not by experimentation but by develop: ing a nontraditional wncept of reaction 8. in this caae, reaction 8 has naturally been regarded as an oxidation-teduction reaction.
16
Journal of Chemical Education
A table showing the relationships among these approaches is presented on the first page of this article. AU three classifications are quite equally correct, but in different areas of chemistry one of them is generally preferred. As a result, these areas are artificially separated. When considering chemistry as a whole, classification becomes unnecesarilv comolicated. For classikcatik purposes, it is desirable to restrict ourThe w n c e ~ of t transfer is oreselves to a sinde - aa~roach. -. posed for this purpose because the approach it involv& is the most universal, vet the least contradictom. Besides aiding cl&sification, simultaneouhy cousidering the same reaction 'om several different aoomaches is very fruitful, as shown by considering an oxid&on-reduction as a nucleophilic substitution. Other examples include interpreting an electrophilic substitution as an anionic transfer and an oxidation-reduction, and interpreting an electron transfer as a substitution at the electron. Literature Cited
1. Hend"cksan,
J. B.: Cram, D. J.: Hammond, G. S. O r g ~ i Chemishy, c 4th ed.; McGraw-Hill: NevYork. 1980. 2. Ingold, C. K Sfrmfunond Mechanismin h.goniChamishy, 2nd ed.: CameUUniversity: Ithaw 1969: pp 236246. 3. Hsmmett, L. PPhyslml Organic C h i s b y ; Maraw-Hill: New York, 1940; p 144. Cham.Rau. Im2.82.615-624. 4. Zefimv,N.S.:Ma*honXov,D.I. 5. 1myanitw.N.S.Zh. ObshchKhim. lOBl,61.&10. 6. Albery, W. J.Ann. Re".Phys. Cham. 1980,31,227-263. 7. Balsano", S. S. Eksporimedol'nynya osnovy Sfrukturnoi mimii; ledatel'stvo Standartov: Moskva, 1986:pp 198-200. 8 . Ekcfmnmya Sfruktum Ftomganlcheskikh Saodimnfi; Zemskov, S. V., Ed.: Nauka: Novmibirak. 1988;pp 4 6 4 8 9. Basdo, F.; Pearson, R. G. Mechanisms of Inogonlc Rp~pfions:Why: New York, 1967:pp 466473,479485.51M15. 10. lbbe, M. L. InorganiRezLlan Meehonisms; Nelaon: landon, 1912:pp 13L137. 11. Haim,A Prprgr Inorg. Chem. 1883,30,273357. 12. 1myanitav.N. S.Zh. Obsheh. Khim. 1990,60,481-484.
