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Substrate A is first reduced by one electron, and then the reduced species undergoes a follow-up chemical reaction. The follow-up reaction can be a simple first order reaction, or something more complicated such as dimerization or a catalytic process. With direct electrochemical techniques, electron transfer to substrate occurs at the electrode surface, where the concentration ratio of oxidized and reduced couple is governed by the Nernst equation (Eq. 1) if electron transfer is a faster step relative to the follow-up reaction.
Linear sweep voltammetry (LSV) and cyclic voltammetry (CV) measure the current versus the potential applied to the electrode; the applied potential is a linear function. The most useful information obtained from voltammetry are the peak current (ip), peak potential (Ep), half peak potential (Ep/2, the potential when current is half of the peak current), and peak width |Ep- Ep/2|. Fig. 2. Characteristic of linear sweep voltammetry (LSV) and cyclic voltammetry (CV)
Three different cases need to be analyzed separately by considering the extent of the two steps:
Totally reversible (or Nernstian)
If all of B that was formed from the forward step is readily available to the reverse sweep, then the system is known as a totally reversible or a Nerstian system.
The chemical step is not very slow, some amount of B is converted to C but not all, and in this case a partial reverse wave can be seen.
If the follow-up chemical reaction is very fast, such that all of B is converted to C, no reverse wave observed from the reverse scan.
Indirect electrochemistry technique
When a direct electrochemical method does not give enough information or there are other factors to limit its application, such as the required potential of the substrate is outside the stability window of the solvent. Indirect method can be applied to achieve more information. One of the most important applications is homogenous redox catalysis. The general mechanism is as follows1:
Scheme 2. Homogeneous redox catalysis
The following condition must pertain for homogeneous redox catalysis to be useful:
The mediator must be totally reversible, in other words when no substrate is in presence, P/Q shows a totally reversible CV.
P is more easily reduced than A; P+B are thermodynamically unfavorable. The mediator P must be chosen such that its reduction potential is more positive than that of substrate A.
3. B→C must be irreversible. The chemical reaction is irreversible and facile, providing the driving force to draw the catalysis to the forward direction.
Review of Kolbe electrolysis
Since it was discovered more than one and a half centuries ago, the Kolbe reaction has been widely used as an electrochemical synthetic method. The Kolbe reaction can be regarded as a chemical process involving one electron oxidation of carboxylate followed by a coupling reaction.
Scheme 3. The Kolbe reaction
If reaction (8) does not occur, instead the alkyl radical is oxidized to a carbocation; in that case it is known as Non-Kolbe reaction.
Scheme 4. The Non-Kolbe reaction
The competition between these two pathways depends on different circumstances2:
Nature of electrode
Nature of solvent
Structure of carboxylate
In the following sections, it will deal with several application and advances of Kolbe electrolysis.
Electrochemical oxidation of tetrabutylammonium carboxylate in acetonitrile
Oxidations of a series of aliphatic tetrabutylammonium carboxylate have been studied in acetonitrile3, where the oxidation was carried out at a potential less anodic than the oxidation potential of solvent, so that the oxidation of solvent does not interfere with the experiment. The results from cyclic voltammetry indicate a stepwise mechanism: electron transfer step followed by decarboxylation. The alkyl radical generated by the decarboxylation step may react via different pathways either undergoing a dimerization or being further oxidized to a carbocation. Analysis of the reaction mixture by GC/MS reveals the only formation of N-acylamides. However no dimer product has been detected such as butane, hexane, which indicates that no Kolbe coupling occurs.
The absence of coupling product suggests that the oxidation of alkyl radical is more favored with respect to dimerization pathway. The carbocation formed subsequently reacts with the solvent molecule to form N-acylamides as the principal product. 4The proposed mechanism is shown below as Scheme 5.
The decarboxylation is typically a very facile step with the rate constant at about 109-1011 s-1.5 The formation of a carbocation requires that the oxidation potential of alkyl radical is less positive than the carboxylate anion substrate. To confirm this, an independent experiment has been carried out. From cyclic voltammetry, the oxidation potential of trimethyl methyl radical is 0.09 V/SCE, which is less than that of trimethylacetate anion at 1.05 V/SCE. This shows that the alkyl radical is easier to oxidize with respect to carboxylate anion.6 In agreement with the fast decarboxylation step, the alkyl radical is formed very rapidly at the electrode surface and it has instantly been oxidized to carbocation before it diffuses to the bulk solution. This is consistent with the fact the only carbocation-derived products are achieved.
The carbocation can react with the solvent to form an alkylnitrilium ion as shown in Rxn. (15). Then the alkylnitrilium ion combine with the substrate acetate anion to form an intermediate (16) which will undergoes O, N-acyl migration subsequently to get the N-acylamides (17).
Then the total net reaction is concluded as Rxn. (18)
Both Rxns. (12) and (14) show one electron oxidation, so the overall reaction would seem to be two electrons oxidation. However half of the carboxylate has been consumed in reaction (16), so that the overall reaction is actually single electron oxidation. This is consistent to the total reaction (18), results from coulometric analysis also agree with one electron oxidation.
Furthermore, the product studies indicate that for acetate and propionate only one peak is observed in GC/MS7, however for carboxylates has longer chain such as butyrate and hexanoate, two isomer of N-acylamides are observed. This is probably due to rearrangement of primary carbocation to secondary carbocation. From earlier reference8, the rearrangement of carbocation is more likely to happen with regard to the rearrangement of carboxyl radical.
Reaction between carboxylate and Tris(4-bromophenyl)-ammoniumyl
For a while, a controversy existed about the reaction between carboxylate and TBPA, whether it is an electron transfer step or a polar reaction. Compton and Laing first investigated this reaction as electron transfer process based on rotating disc voltammetry. 9 The cation radical () is generated electrochemically from tris(4-bromophenyl)-ammoniumyl (TBPA) the structure is shown on the right (1).
They came up with the plot of the transport-limited current as a function of the square root of the disc rotation speed (ω/Hz) for various ratios of acetate. More than two electrons per mole of TBPA were consumed when Bu4NOAc was employed as the acetate anion source. Upon adding acetic acid, a stage was eventually reached when the TBPA /TBPA behave as one electron couple. This was taken as evidence that naked acetate ion undergoes ET step with TBPA. The proposed mechanism
However, Eberson strongly deputed this rationale.10 He suggest that the actual acetate source is in the form (MeCO2)2H-, there is an equilibrium reaction:
They revisited the reaction with cyclic voltammetry and found that no alkyl radical derived product is formed; this is a clear evidence to rule out the electron transfer mechanism.
Electron transfer mechanism and rate constant
The Kolbe reaction involves the formation of coupling product. However if the alkyl radical is more ready oxidized rather than forming a dimer, products will be derived from the carbocation formed by the oxidation of alkyl radical; this latter process is called non-Kolbe reaction. Electron transfer and bond breaking occur in Rxn. (23), whether or not these two events occur stepwise or concerted is a debated question.
One remarkable work to investigate the mechanism was reported by Saveant. He had developed an extensive analysis of a series of arylmethyl compounds by varying the R group and then studied their cyclic voltammetry response. He discovered that in most cases removal of the first electron and cleavage of the bond which results in the formation of CO2 are stepwise rather than concerted processes.11
Scheme 8. Possible pathways to form alkyl radical and CO212
Before his work, it was known that the decarboxylation of acyloxyl radicals is a favored reaction. From photochemical studies acyloxy radicals have a finite lifetime range from 10-10 to 10-9 s this suggests a stepwise pathway is possible but it does not rule out the possibility of concerted mechanism.
Concerning arylmethyl carboxylate oxidation at carbon electrodes, previous work founds that sometimes a modified electrode surface accompany with the oxidation.15 This monolayer coating will may lead to inconclusive results. However, by raising the scan rate this can be avoided to some extent. 18
The arylmethyl compounds investigated are listed below as Scheme 9.
An obvious phenomenon is that all the CVs are irreversible. In the case where the electron transfer is step is rate limiting, the variation of peak potential with the scan rate and peak widthwith the scan rate depend on a transfer coefficient α, as depicted by Eqs. (2) and (3)
When the electron transfer step is not rate limiting, there is a follow-up reaction or mixed kinetics are involved compound 4b for instance, equations (2) and (3 still apply. But is no longer a true transfer coefficient, it becomes an apparent transfer coefficient. Table 1 shows the calculated values.
Table 1. Average values of the transfer coefficient (or apparent transfer coefficient )
From another perspective, useful information can be obtained from homogeneous redox catalysis, by adding an excess of carboxylate, the peak current will increase. In this case determining the number of electron exchanged per molecule is very valuable. This data is an indication of the fate of the arylmethyl radical generated by the oxidative decarboxylation of the carboxylate ion, and also it may give additional information about the mechanism of the type of bond cleavage. Fig. 3 illustrates the increasing of peak current in homogeneous redox catalysis.
Fig. 3 (copied form ref. 11) cyclic voltammetry of 1b in CH3CN + 0.1M n-Bu4PF6 in the absence (dotted line) and presence of a three-time excess of tetramethyl ammonium methyl carbonate. Scan rate: 0.1 V/s, Temperature: 298 K
By summarizing the electron stoichiometry derived from the peak currents as a function of excess factor which is the ratio of base relative to acid. One explanation of the increment of the electron stoichiometry upon adding of a base comes from preparative experiment. After consumption of the one electron per mole, almost all the initial carboxylate ion disappears. Neutralization of the electrolysis solution by a strong base regenerate the carboxylic acid produced during electrolysis.
For all the carboxylates whose electrolysis consumes 2e- per molecule, these compounds are believed to undergo a non-Kolbe reaction leading to product base via a carbocation intermediate. In case of 4b, the number of electrons required per molecule is 1.5, which lies between a Kolbe and non-Kolbe pathways. This value is consistent with the fact that dimer product is found in accompany with non-Kolbe product.
Among most of the investigated compounds in scheme 9, the unpaired electron is located at one of the carboxyl oxygen of the acyloxyl radical which undergo a fast hemolytic cleavage. Thus reaction is controlled by the electron transfer step. The alkyl radical is generated so quickly and close to the electrode that it has no time to diffuse to the bulk solution and form a dimer, instead it is oxidized at the electrode and follows a non-Kolbe reaction pathway. So there is not any coupling product formed but carbocation derived product exclusively.
In other cases like compound 4b whereas the aromatic portion of the carboxylate is easier to oxidize, it follows a heterolytic cleavage which is slower than homogeneous bond cleavage. 19In this case the cleavage step becomes the rate control step. The slower cleavage step is also an explanation to the observation that both dimer and carbocation-derived products are formed.
To summarize, in no case of the investigated series of compounds was a value of significantly below 0.5 is observed. For concerted pathway is much lower than 0.5, typically around 0.3. For stepwise pathway is almost equal to 0.5. The value result is a strong signal that these compounds undergo stepwise pathway. The unpaired electrons is mostly located at one of carboxyl oxygen as the result get from density functional calculation of carboxyl radicals , the acyloxyl radical follow a rapid hemolytic bond cleavage with a rate constant in the order of 1010 s-1.22 Such a distribution of unpaired electrons indicates that the standard potential of the carboxylate will not change significantly with the different substituent group. Because the cleavage is fast, the radical is formed very close to the electrode surface; it then oxidized to carbocation before it has time to diffuse to the bulk solution and forming dimer product.
However a different oxidation mechanism is involved in the case of 4b 4-dimethylaminobenzyl carboxylate, whose electron transfer coefficient is 0.82. In this case the electron transfer is no longer rate limiting but the bond cleavage step. This indicates a different location of the unpaired electron in the acyloxyl radical.
Scheme 10. Heterolytic bond cleavage and homolytic bond cleavage
Owing to the electron delocalization, the unpaired electron is located at the aromatic portion of the acyloxyl radical, with a negative charge on the oxygen and a positive charge on the nitrogen. As a consequence of the electron distribution, the cleavage should be heterolytic rather than homolytic which involve intramolecular dissociative electron transfer reaction. 25Accordingly the rate of bond cleavage is slower than homolytic pathway and the rate limiting step passes from the first electron transfer step to the cleavage step. Also this slowness is relative to the fact that to some extent dimer product is formed along with the no-Kolbe product.
Review of Marcus Theory
Marcus Theory proposes a relationship between activation energy and standard free energy.
λ is the reorganization energy, which is the sum of two terms, the inner reorganization energy and the solvent reorganization energy. Derive from conformation changes of electron donor and acceptor. is the energy required to change of arrangement of solvent molecule and the electrostatic effect between it with the reactive compounds.26
For dissociative electron transfer (DET), is dominated by bonding breaking energy (BDE). The theory relates to two important equations (6) and (7), D is the bond dissociation energy (BDE).
The shape in some voltammetry is related to. When, is. The reorganization energy is much larger than a normal electron transfer, DET is usually a kinetically slow step. In the CV, it will require more negative potential than standard reduction potential to drive the reaction, in other word it requires a large over potential.
Review oxidation of ferrocenes
Reaction of ferrocene with carbon-centered free radical
Ferrocenes are interesting substrate which can be utilized to investigate electrochemistry reaction and mechanism. Previous study suggested that ferrocene by itself is not reactive toward free radicals29, but an oxidation step of ferrocene to ferricenium ion is a prerequisite. The oxidation of ferrocene is very easy as its oxidation potential is 0.72 V vs. NHE in CH3CN.30 With regard to the reaction between free radical and ferricenium ion31, it is suggested that the free radical first attack the iron atom, and then rearrange to a positive charge σ complex, after that a proton transfer step leading to the substituted ferrocene.
The alkyl radical is generated by the reaction of methyl radical with the corresponding alkyl iodide in DMSO with H2O2. The mechanism involving forming a hydroxyl radical as precursor
The efficiency of the substitution with nucleophilic radical is relatively low. Radical with more electron withdrawing substituent reach high yield, this may due to better delocalization effect to stablize the σ complex intermediate.
Electron transfer reaction of PINO with ferroecene
A kinetic study of the one electron oxidation a series of ferroecene by phthalimide-N-oxyl radical (PINO) has been investigated. 32
The PINO radical is generated by a previous reaction of N-hydroxyphthalimide (NHPI) with the cumyloxyl radical which is generated by 355 nm laser flash photolysis of dicumyl peroxide.
The rate of reaction is determined by monitoring the decay rate of PINO radical as a function of the concentration of ferrocene. The significance of this work is that, because the intrinsic barrier for the ferrocene/ferrocenium couple is known, from Marcus equation it allow us to determine a self-exchange ET intrinsic barrier for PINO/PINO-.
Research Objective: carboxylate oxidation in electrochemistry
The earliest work is oxidation of acetate by ferrocene as homogeneous redox catalysis. The proposed mechanism is as follows:
0.5 M t butyl-ammonium perchlorate as supporting electrolyte
5mM ferrocene in acetonitrile
Excess factor, g = 5 ([AcË‰]/[Fe(Cp)2] = 5)
At this condition cyclic voltammetry shows catalysis happens as at low scan rate as there is an obvious increase of the peak current in cyclic voltammetry.
Next step is to investigate how these experimental conditions affect the extent of catalysis. Step (3) is typical dissociative electron transfer step which has been covered by reviewing electron transfer mechanism. Of course solvent is one of the most important factor affect the reaction, so the same reaction will run in a parallel way by comparing absent of water and presence of water. To extend the scan rate range could reduce possible disturb at low scan rate, the excess factor will change to a higher value but also with good compliance not to arouse much noise in the cyclic voltammetry.
By varying the concentration of ferrocene and plot these experimental data in a series, we can determine which step is rating limiting. The rate determine step (RDS) can be determined by the effect of mediator concentration on catalysis. For rate limiting electron transfer (step 2), the rate of reaction shows a concentration dependence on the mediator. For rate limiting chemical step, the electron transfer is a pre-equilibrium, concentration of the mediator is not a factor in the observed rate of reaction.
More importantly, experiment will be carried out with a series of different mediator, in this way it actually change the driving force of reaction, thus I can obtain a Marcus plot to get a deeper understanding to the mechanisms of dissociative electron transfer, whether it is concerted or stepwise. And get some information about the intrinsic property of carboxylate.
From this review, several other techniques need to be incorporated to current work.
Coulometric analysis gives important information about reaction stoichiometry, which could unravel valuable kinetic and thermodynamic data for the investigated system. Laser flash photolysis is another powerful tool to study electrochemistry, not only it can be applied to generate radicals of interest, but also it can be utilized to monitoring rate constant of reaction. Theoretical calculation is also worthwhile to consolidate experimental result and give fundamental explanation to the intrinsic mechanism.