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.