An expeditious and efficient method for the oxidation of benzyl alcohols by homogeneous electrolysis

Article abstract of DOI:10.1080/00397911.2021.1960377

A greener and inexpensive electrochemical method has been developed for the oxidation of benzyl alcohols by homogeneous electrolysis. The electrochemical reaction was carried out in an undivided cell equipped with carbon and stainless steel electrodes at room temperature. The homogeneous solution made up of acetonitrile/water containing substrate and ammonium bromide with a catalytic amount of H₂SO₄ as supporting electrolyte. The reaction condition was optimized with various electrochemical experimental parameters and evaluated with various substituted benzyl alcohols to result in excellent yield of aldehydes (>83%).

Full text of DOI:10.1080/00397911.2021.1960377

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
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An expeditious and efficient method for the
oxidation of benzyl alcohols by homogeneous
electrolysis
Rathinavel Jagatheesan, Poovan Shanmugavelan, Subramaniyan
Sambathkumar & Pugalenthi Ramesh
To cite this article: Rathinavel Jagatheesan, Poovan Shanmugavelan, Subramaniyan
Sambathkumar & Pugalenthi Ramesh (2021): An expeditious and efficient method for the
oxidation of benzyl alcohols by homogeneous electrolysis, Synthetic Communications, DOI:
10.1080/00397911.2021.1960377
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https://doi.org/10.1080/00397911.2021.1960377
An expeditious and efficient method for the oxidation of
benzyl alcohols by homogeneous electrolysis
Rathinavel Jagatheesana, Poovan Shanmugavelanb, Subramaniyan
Sambathkumara, and Pugalenthi Ramesha
aDepartment of Chemistry, Vivekanandha College of Arts and Sciences for Women (Autonomous),
b
Namakkal, Tamil Nadu, India; Department of Chemistry, School of Chemistry, Tamil Nadu Open
University, Chennai, India
ABSTRACT
ARTICLE HISTORY
Received 11 June 2021
A greener and inexpensive electrochemical method has been devel-
oped for the oxidation of benzyl alcohols by homogeneous electroly-
sis. The electrochemical reaction was carried out in an undivided cell
equipped with carbon and stainless steel electrodes at room tem-
perature. The homogeneous solution made up of acetonitrile/water
containing substrate and ammonium bromide with a catalytic
amount of H2SO4 as supporting electrolyte. The reaction condition
was optimized with various electrochemical experimental parameters
and evaluated with various substituted benzyl alcohols to result in
excellent yield of aldehydes (>83%).
KEYWORDS
Ammonium bromide;
benzaldehyde; benzyl
alcohol; electrochemical
oxidation; homogeneous
electrolysis; undivided cell
GRAPHICAL ABSTRACT
O
HO
SS
C
2F, 1OOmA cm-2
H2O + CH3CN
Electrochemical oxidation of benzyl alcohol to benzaldehyde
Introduction
Since the corresponding carbonyl compounds can be used as important and versatile
precursors for the synthesis of fine chemicals such as fragrances, pharmaceutical inter-
mediates and beverage or food additives, the selective oxidation of alkyl aromatics to
corresponding carbonyl compounds is one of the most significant reactions in labora-
tory and industrial synthetic organic chemistry.[1–7] Oxidation of organic substrate in
the absence of oxidizing agent is an important transformation in synthetic organic
CONTACT Rathinavel Jagatheesan
jagan26@yahoo.com
Department of Chemistry, Vivekanandha College of Arts
and Sciences for Women (Autonomous), Namakkal, Tamil Nadu, India.
Supplemental data for this article can be accessed on the publisher’s website.
ß 2021 Taylor & Francis Group, LLC

2
R. JAGATHEESAN ET AL.
CHO
CH2OH
C/SS, NH4Br/CH3CN
r.t, 100 mA/cm2
R
R
Scheme 1. Homogeneous electrolysis of benzyl alcohol to benzaldehydes.
chemistry. This subject is intensively investigated and achieved by electrochemical
method in the present study.
Generally electrochemical oxidation is carried out using transition metal compounds
such as ruthenium (VIII) oxide, chromium (VI) oxide, permanganates and dichromates
to bring about these transformations. Most of these reagents are toxic, expensive and
also generates environmental wastes.[8–17] Moreover, electrochemical method has
numerous remarkable advantages like an ecofriendly process, short reaction time, simple
electrochemical cell set up, easy-control condition and economically cheap reagents
compared with the conventional reactions.[18,19] Further, the existing electrochemical
methods for the conversion of alcohol to aldehydes is carried out by biphasic system
which demands mediators[20,21] and harmful acid like HBr.[22] In continuation of our
previous work,[23] herein we examined and report a simple method for the preparation
of arylaldehydes from its respective benzyl alcohols by homogeneous electrolysis at
ambient temperature as shown in Scheme 1.
In this electrolysis reaction, benzyl alcohols as substrates, aqueous ammonium brom-
ide salt, supporting electrolyte and minimum amount of organic solvent are the
required substances. No additional substances such as catalyst, mediators, oxidants, or
ligands are necessary to achieve effective electrochemical oxidation.
Results and discussion
Eletrochemical oxidation of benzyl alcohol
In two-phase electrolysis, mediator is required to bring the product conversion.
Similarly, in emulsion electrolysis, electrode passivation occurs. Conversely, homogenous
electrolysis has a discrete advantage over the above two methods by providing adequate
contact for generated transient hypobromous acid with reactant. Hypobromous acid is
an unstable substance and is therefore prepared in situ by electrolysis. In the selective
oxidation of benzyl alcohol by homogenous electrolysis, the following optimization
studies were carried out to afford the excellent yield and the results are reported.
Further, the optimized homogeneous constant current electrolysis system is applied for
the oxidation of substituted benzylic alcohols to the corresponding aldehydes at room
temperature in an undivided cell.
Effect of current density
The reactions were carried out at different current densities from 30 to 100 mA cmꢀ2
by passing 2 F of electricity. The maximum yield and current efficiency was achieved at

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SYNTHETIC COMMUNICATIONSV
3
100 mA cmꢀ2 (Table 1). Furthermore, it takes very short time to carry out the electroly-
sis. Hence, this higher current density (100 mA cmꢀ2) was elected as an optimum cur-
rent density for further optimization studies.
Effect of charge passed
To afford good yield in short reaction time, 100 mA cmꢀ2 was chosen and the electroly-
sis was carried out by applying various charges from 2.0 to 4.0F. The maximum yield is
reached with high current efficiency at a charge of 2.0F/mole. There are no remarkable
changes in the product yield even after passing upto 4.0F/mole as shown in Table 2.
Effect of temperature
Table 3 shows the effect of temperature on the electrochemical conversion of benzyl alco-
hol. There is an increase in benzaldehyde yield with decrease of temperature from 50 to
30ꢁC. It clearly indicates about the good stability of hypobromous acid at room tempera-
ture. Hence, 30 ꢁC was preferred as optimum temperature for this electrolysis reaction.
Effect of bromide salt
In order to achieve the electrochemical oxidation of benzyl alcohol by using simple and
cheaper salt, the effect of various bromide salts as KBr, NaBr and NH4Br was investi-
gated (Table 4).
ꢂ
Table 1. Simple salt mediated electrochemical oxidation of benzyl alcohol .
S. No.
Current density (mA cmꢀ2
)
Benzaldehyde yield (%)
Current efficiency (%)
1
2
3
4
*
30
50
70
81
81
82
83
81
81
82
83
100
Reaction mixture and conditions: 60% NH4Br þ 0.3 M H2SO4 þ 10 mmol benzyl alcohol þ water 80% – acetonitrile
20%. Charge passed, 2 F; Electrodes, C/SS, Temperature, 30ꢁC, undivided cell.
Table 2. Effect of charge passed on the electrochemical oxidation of benzyl alcohol by homoge-
ꢂ
neous electrolysis .
S. No.
Charge passed (F)
Benzaldehyde yield (%)
Current efficiency (%)
1
2
3
*
2
3
4
83
84
85
83
56
43
Reaction mixture and conditions: 60% NH4Br þ 0.3 M H2SO4 þ 10 mmol benzyl alcohol þ water 80% – acetonitrile
20%. Current density, 100 mA cmꢀ2; electrodes, C/SS, temperature, 30ꢁC, undivided cell.
ꢂ
Table 3. Effect of temperature on selective oxidation of benzyl alcohol .
S. No.
Temperature (ꢁC)
Benzaldehyde yield (%)
Current efficiency (%)
1
2
3
4
5
*
50
40
30
20
10
45
56
83
60
10
45
56
83
60
10
Reaction mixture and conditions: 60% NH4Br þ 0.3 M H2SO4 þ 10 mmol benzyl alcohol þ water 80% – acetonitrile
20%. Current density, 100 mA cmꢀ2; charge passed, 2 F; electrodes, C/SS, undivided cell.

4
R. JAGATHEESAN ET AL.
ꢂ
Table 4. Effect of halide salt variation on oxidation of benzyl alcohol .
S. No.
Bromide salt
Benzaldehyde yield (%)
Current efficiency (%)
1
2
3
*
KBr
NaBr
NH4Br
78
80
83
78
80
83
Reaction mixture and conditions: 60% Bromide salts þ 0.3 M H2SO4 þ 10mmol benzyl alcoholþ water 80% – acetonitrile
20%. Current density, 100mA cmꢀ2; charge passed, 2F; electrodes, C/SS, temperature, 30˚C, undivided cell.
Table 5. Effect of solvent on electroselective oxidation of benzyl alcohol.
S. No.
Solvent
Benzaldehyde yield (%)
Current efficiency (%)
1
2
3
4
*
DMSO
76
81
80
83
76
81
80
83
CH3OH
ꢂꢂ
CHCl3
CH3CN
Reaction mixture and conditions: 60% NH4Br þ 0.3 M H2SO4 þ 10 mmol benzyl alcohol þ water 80% – solvent 20%.
Current density, 100 mA cmꢀ2; charge passed, 2 F; electrodes, C/SS, temperature, 30˚C, undivided cell. Biphasic
ꢂꢂ
electrolysis.
From the table it was observed that the electrochemical oxidation of benzyl alcohol
was effectively achieved when using NH4Br salt. Further, minimum quantity of NH4Br
is required to prepare saturated solution. Thereby this salt was suggested for this reac-
tion condition.
Effect of acid
To scrutinize the best supporting electrolyte for the present study, the electrochemical
conversion of benzyl alcohol to benzaldehyde was carried out in aqueous solution of
saturated NH4Br containing catalytic amount of various supporting electrolytes like
HNO3, HCl, HClO4 and H2SO4. All the acids are performed equally well, but H2SO4
was preferred as good supporting electrolyte due to its low cost, availability and higher
conductivity (<2 V).
Effect of solvent
Concerning the influence of solvent, the electrolysis was attempted with different sol-
vents like DMSO, CH3OH, CHCl3 and CH3CN at room temperature (Table 5). Among
these solvents CH3CN performed well at 2 F/mole and was suggested as a suitable solv-
ent for the present optimization study.
Reusability of spent aqueous bromide salt solution
After the completion of homogeneous electrolysis, benzaldehyde and unreacted benzyl
alcohol was separated using appropriate solvents from the reaction mixture containing
aqueous salt solution. Thereby the remaining aqueous salt solution was available for fur-
ther electrolysis reaction. The yield was obtained as 65, 52, and 38% in first, second and
third reuse, respectively. From the result, it states that the amount of yield was
decreased in every further reuse of the bromide salt solution. It indicates the availability
of bromide source was less in every successive reuse.

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SYNTHETIC COMMUNICATIONSV
5
ꢂ
Table 6. Electrochemical oxidation of alcohols by homogeneous electrolysis system .
Entry
1.
Substrate
Product
Yield (%)
83
Current efficiency (%)
83
CH2OH
CHO
CH2OH
2.
CHO
80
80
3.
79
79
CH2OH
CHO
4.
79
79
CH2OH
CHO
(continued)

6
R. JAGATHEESAN ET AL.
Table 6. Continued.
Entry
5.
Substrate
Product
Yield (%)
83
Current efficiency (%)
83
CH2OH
CHO
6.
80
80
CH2OH
CHO
OMe
CH2OH
OMe
CHO
7.
68
68
Cl
Cl
8.
74
74
CH2OH
CHO
Cl
Cl
(continued)

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SYNTHETIC COMMUNICATIONSV
7
Table 6. Continued.
Entry
9.
Substrate
Product
Yield (%)
78
Current efficiency (%)
78
CH2OH
CHO
Cl
Cl
ꢂꢂ
10.
0
0
CH2OH
CHO
NO2
NO2
*
Reaction mixture and conditions: 60% NH4Br þ 0.3 M H2SO4 þ 10 mmol benzyl alcohol þ water 80% – acetonitrile
20%. Current density, 100 mA cmꢀ2; charge passed, 2F; electrodes, C/SS, Temperature, 30˚C, undivided cell. Stirring
ꢂꢂ
extended up to 5 hours after electrolysis.
Evaluation of optimized electrolysis condition with substituted benzyl alcohols
To evaluate the scope of this optimized condition and to test the generality of this
method, a variety of benzyl alcohols were subjected to the homogeneous electrolysis
reaction at room temperature in a single compartment and its corresponding results are
listed in Table 6. From the results we observed that, effective electrochemical oxidation
occurred at benzyl alcohols substituted with electron donating groups (Table 6, entries
2–6) in excellent yields, whereas benzyl alcohols containing electron withdrawing groups
(Table 6, entries 7–9) gave moderate yields at lowest charge of 2 F. Surprisingly, benzyl
alcohol substituted with highly deactivating group (Table 6, entry 10) did not follow the
electrochemical oxidation reaction.
Based on a literature report, a plausible mechanism for the electrochemical oxidation
is given in Scheme 2.[24] The electrochemically generated bromine undergoes hydrolysis,
giving hypobromous acid (HOBr) and HBr. The hypobromous acid is unstable thus
forms transient Brþ which can subsequently oxidizes the benzyl alcohols to the corre-
sponding aldehydes as shown in Scheme 2.

8
R. JAGATHEESAN ET AL.
-2e
Br2
2Br
Br2 + H2O
HOBr
HOBr + HBr
H
Br + H2O
Br
H
O
C
O
C
CH2OH
H
Br
-H
H
-Br
-HBr
R
Br
R
R
Scheme 2. Plausible mechanism for electrochemically generated transient bromonium ion mediated
oxidation of benzyl alcohols.
Experimental
General experimental procedure
In an undivided cell (beaker type), 10 mmol of benzyl alcohols, 20 ml of acetonitrile,
80 ml of aqueous ammonium bromide solution (60%) containing catalytic amount of
H2SO4 (0.3 M) was taken. Two electrodes such as carbon (anode) and stainless steel
(cathode) of 15 cm2 area were introduced at a distance of 2 cm2 into the undivided cell
containing homogeneous solution. The electrolysis was conducted by passing 2 F of elec-
tricity galvanostatically at a current density of 100 mA cmꢀ2. The temperature of the
electrochemical cell contents was maintained between 30–33 ꢁC during the electrolysis
to reduce the solvent evaporation. The whole homogeneous solution was stirred con-
stantly throughout the reaction. After passing required charge per mole (2 F), the cur-
rent supply was stopped and the reaction mixture alone is allowed to stir another
1 hour to complete the reaction. At the end, the reaction mixture was extracted with
ethyl acetate (3 ꢃ 25 mL) and the combined organic layer was washed with water
(20 mL) and dried over anhydrous sodium sulfate. The solvent was removed by
distillation.
The electrochemical reaction was monitored by TLC and Shimadzu HPLC with LC-
8A column (250 mm ꢃ4.6 mm) as stationary phase. The eluent consisted of acetonitrile/
water (80: 20) at a flow rate of 1 mL minꢀ1. Samples were analyzed at a wavelength of
254 nm with a UV detector coupled to a printer. Authentic samples of benzaldehyde are
used to calculate the peak area of the corresponding experimental product for yield
calculation.
Conclusion
On electrolysis aqueous ammonium bromide provides bromonium ion for the oxida-
tions of benzyl alcohols to its corresponding aldehydes. The advantages of the present
method include simple experimental set up, low cost bromide salt, cheaper electrodes,
room temperature, in-situ generated transient hypobromous acid, short reaction time,
green solvent water used as maximum and no by-product. Further, the reuse of the

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SYNTHETIC COMMUNICATIONSV
9
spent electrolyte solution is the remarkable advantage in this method. The above fea-
tures also lead to electrochemical regeneration with high current efficiencies at high cur-
rent densities and rapid oxidation without oxidizing agent.
Supplementary information (experimental procedures, 1H and 13 C NMR spectra,
HPLC traces) associated with this article can be found via the “Supplementary Content”
section of this article’s webpage.
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