Solvent free facile room temperature reduction of aromatic carbonyl and nitro compounds by zn/Conc. HCl system - An experimental and DFT study

Article abstract of DOI:10.14233/ajchem.2018.21071

This experimental and DFT studies involve a novel technique for the reduction of aromatic carbonyl/nitro compounds to the corresponding alcohols/amines, in high yield under laboratory conditions. The reducing species is the nascent hydrogen generated by the Zn/HCl system. The novelty of this work is that the comparison of yield with and without solvent. The yield is increased by many folds in the solvent free method compared to the solvent method reported earlier. The technique followed is to make a 'slurry' of the substrate with zinc dust (zinc slurry) and to add (in small portion of the dry slurry) to the optimized amount of conc. HCl, over a period of 3 to 4 h at room temperature. In this technique the substrate, adsorbed on zinc dust being very proximal to the site of generation nascent hydrogen, the reduction is very effective and the yield is high. The novelty is that zinc dust acts as catalyst (adsorbent role) and reactant (hydrogen generation role). The DFT study with B3LYP/6.311g ++ (d,p) basis set revealed that the stability of first formed free radical (energy factor) and the homo nuclear nature of carbonyl and nitro group (charge factor) decide the yield. The electrostatic potential calculated by DFT studies correlates well with Mullikan charges in deciding the charge factor. The free radical mechanism was confirmed by the formation of pinacol coupled product in one instance.

Full text of DOI:10.14233/ajchem.2018.21071

Asian Journal of Chemistry; Vol. 30, No. 3 (2018), 639-644
A
SIAN
J
OURNAL OF HEMISTRY
C
https://doi.org/10.14233/ajchem.2018.21071
Solvent Free Facile Room Temperature Reduction of Aromatic Carbonyl and
Nitro Compounds by Zn/Conc. HCl System-An Experimental and DFT Study
2
S. RAJAMATHE1,* and M. BHUVANESWARI
Department of Chemistry, Kanchi Mamunivar Centre for Post Graduate Studies, Puducherry-605 008, India
*Corresponding author: E-mail: rajamathi@gmail.com
Received: 29 September 2017;
Accepted: 28 November 2017;
Published online: 31 January 2018;
AJC-18758
This experimental and DFT studies involve a novel technique for the reduction of aromatic carbonyl/nitro compounds to the corresponding
alcohols/amines, in high yield under laboratory conditions. The reducing species is the nascent hydrogen generated by the Zn/HCl
system. The novelty of this work is that the comparison of yield with and without solvent. The yield is increased by many folds in the
solvent free method compared to the solvent method reported earlier. The technique followed is to make a 'slurry' of the substrate with
zinc dust (zinc slurry) and to add (in small portion of the dry slurry) to the optimized amount of conc. HCl, over a period of 3 to 4 h at
room temperature. In this technique the substrate, adsorbed on zinc dust being very proximal to the site of generation nascent hydrogen,
the reduction is very effective and the yield is high. The novelty is that zinc dust acts as catalyst (adsorbent role) and reactant (hydrogen
generation role). The DFT study with B3LYP/6.311g ++ (d,p) basis set revealed that the stability of first formed free radical (energy
factor) and the homo nuclear nature of carbonyl and nitro group (charge factor) decide the yield. The electrostatic potential calculated by
DFT studies correlates well with Mullikan charges in deciding the charge factor. The free radical mechanism was confirmed by the formation
of pinacol coupled product in one instance.
Keywords: Solvent free, Zinc slurry, Atomic hydrogen reduction, Carbonyl compounds, Nitro compounds, Mullikan charge.
temperature by the nascent hydrogen generated from Zn/HCl
system was only reported in earlier investigation [22] but the
yield was poor.
INTRODUCTION
The easiest functional group transformation which one can
think of is the conversion of aromatic nitro group to amines and
aromatic carbonyl group to alcohol/alkane and can be effected
by reduction. Reduction is one of the frequently used reaction in
organic synthesis and a vast variety of reducing agents have been
introduced for this achievement [1,2]. The synthesis and biolo-
gical evaluation of aromatic amines by derivative formation [3,4]
constitutes the most important study in chemistry. Likewise, the
conversion of carbonyl compound to alcohol or alkane is important
in pharmaceutical industries as the transformation changes the
hydrophilicity of the compound.
A plethora of reducing agents is available for the reduction
of aromatic carbonyl/nitro compounds claiming to be chemo/
regio/stereo-selective agents [5-11]. Inspite of the presence of
mushroom of reducing agents for the reduction of aromatic
carbonyl/nitro group, even though a few researchers [12-15]
used zinc for reduction, including the recently reported reduction
of graphene oxide to graphene by metal mediated direct electron
transfer reduction [16-19] and metal mediated atomic hydrogen
reduction [20,21], the feasibility of using a commonly available
non-toxic reagent which can act as the reducing agent at room
The aims of the present investigation are to devise a method,
which can enhance the yield of earlier investigation [22] and
also confirm the principles derived from the DFT studies of the
substrates used. This method is called ‘solvent free slurry’
(SFS) method. In this method, the substrates (carbonyl/nitro
compounds) are not dissolved in any solvent (unlike earlier
report [22] wherein the solvent used was Et2O) but an intimate
dry slurry of the substrate and zinc dust was prepared and
used as substrate for the reduction in the presence of conc. HCl.
In this SFS method, zinc acts as a reactant and also a catalyst
(adsorbent).
EXPERIMENTAL
All the reagents and substrates were purchased from comm-
ercial sources with A.R. quality and used without further
purification. SiO2 was used in the form of silica gel 60 (70-230
meshASTM) and was purchased from Merck company. 1H and
13C NMR spectra were recorded on Bruker Avance DPX-400
MHzspectrometer. Theproductswere characterizedbycomparison

640 Rajamathe et al.
Asian J. Chem.
with authentic samples and their 1H and 13C NMR spectra. TLC
was applied for the purity determination of substrates, products
and the reaction monitoring was carried out using silica gel 60
F254 aluminum sheet.
O
R2
X
ZnCl2
+
2 HCl
Zn
+
+
rt , 4-5 h
R1
General procedure for the reduction of aromatic carbonyl/
nitro compounds by solvent free slurry (SFS) method.
R1
1. R1 = H; R2 = H
1
6.
R = H; X = CH(OH)CH(OH)C6H5
Preparation of slurry: As the starting materials for the
investigation are known compounds, the experiments are perfor-
med in a higher scale for the ease of qualitative identification of
the products. Zinc dust (0.45 mol) was added to 0.05 mol of
aromaticcarbonylcompound/0.025molofaromaticnitrocompound
to make slurry. To reduce the viscosity, an excess Zn dust (about
10 g) was added to make slurry for the aromatic carbonyl group/
nitro compound reduction. In case of solid subs-trate, a minimum
amount of diethyl ether (about 15 mL) was added and then
treated with Zn dust for the slurry preparation. Slurry was
warmed in water bath kept at 45 ºC for few minutes in order to
evaporate diethyl ether.
7. R1 = OCH3; X= CH3
1
2
2.
R = OCH3; R = H
1
8.
R = H; X= C(CH3)(C6H5)COCH3
3. R1 = H; R2 = CH3
4. R1 = H; R2 = C6H5
5. R1 = H; R2 = COC6H5
9. R1 = H; X = C(OH)(C6H5)C(OH)(C6H5)2
10. R1 = H; X = CH2CH2C6H5
11. R1 = H; X = COCH2C6H5
Scheme-I: Reduction of aromatic carbonyl compounds (1-5)
NO2
X
R2
+
R2
3 Zn
+ 6 HCl
3 ZnCl2
+
+ 2 H2O
rt , 4-5 h
Experimental procedure: In a typical experiment, 150
mL of conc. HCl of 1.8 mol was taken in 250 mL two-necked
round bottom flask, which was kept in a water bath with stirring
in order to maintain room temperature. To the stirred conc. HCl
in a water bath, 0.45 mol of Zn dust in 0.05 mol of carbonyl
substrate (slurry)/0.025 mol of nitro substrate (slurry) was added
very slowly through the spatula at a rate of 0.5g/h, which was
found to be the optimum rate of addition of product formation
for maximum yield. As the generation of hydrogen from acid
by zinc exothermic the water bath was kept in room temperature
by monitoring the temperature by thermometer and maintaining
the temperature by intermittently addition of ice cubes/ice water.
The reaction was tested by several trial runs for optimum reaction
time, whichwas monitored byTLC (8:2) hexane and ethyl acetate
system as eluent. The reaction mixture was vigorously stirred
at room temperature for the optimum reaction time of 4-5 h.
After the completion of reaction, the mixture was filtered
using a Whatmann filter paper No. 42 and extracted with diethyl
ether. To the aqueous layer, 100 mL of 20 % aqueous NaCl was
added (salting out effect) and extracted with diethyl ether again.
On repeating the procedure 3-4 times, the multiple extracted
fractions were combined and dried over anhydrous Na2SO4 and
then the solvent was evaporated under reduced pressure (rotor
evaporator) to obtain the crude product. The mixture NMR of
the crude was obtained to determine the product to reactant ratio,
character-ization and to find the conversion percentage.
The general scheme for the reduction by Zn/conc. HCl using
solvent free slurry (SFS) method of aromatic carbonyl and nitro
groups is given in Scheme I and II, respectively.
R1
R1
14. R1 = H; R2 = H; X = NH2
12. R1 = H; R2 = H
13. R1 = CH3; R2 = H
15. R1 = CH3; R2 = H; X = NH2
Scheme-II: Reduction of aromatic nitro compounds (12-13)
base [23]. The ratio of aromatic peak area of the substrate and
that of products is a measure of yield and percentage of conversion.
The absence of the substrate's characteristic peak in the NMR
spectra of the products is an indication of improvement in the
yield and that will serve present objective of enhancing the yield
under the experimental conditions.
Spectral data
1
1,2-Diphenylethane-1,2-diol (6): H NMR: (400 MHz,
CDCl3) δ 4.739 (s, 1H), 5.36 (s, 1H, hydroxyl protons), 6.93-
7.11 (m, 10H, aromatic protons) ppm; 13C NMR: (100 MHz,
CDCl3) δ 77.89, 126.80, 127.92, 128.07, 139.38 ppm.
1
1-Methoxy-4-methylbenzene (7): H NMR: (400 MHz,
CDCl3) δ 2.33 (s, 3H), 3.82 (s, 3H), 6.84-6.87and 7.12-7.14
(m, 5H, aromatic protons); 13C NMR: (100 MHz, CDCl3) 20.55,
55.31, 113.70, 129.91, 129.99, 157.55 ppm.
1
3,3-Diphenylbutan-2-one (8): H NMR: (400 MHz,
CDCl3) δ 1.76 (s, 3H), 1.99 (s, 3H), 7.08-7.24 (m, 10H aromatic
protons); 13C NMR: (100 MHz, CDCl3) δ 26.49, 27.66, 62.37,
126.97, 128.36, 128.41,143.64, 209.19 ppm.
1
1,1,2,2-Tetraphenylethane-1,2-diol (9): H NMR: (400
MHz, CDCl3) δ 2.96 (s, 1H, hydroxyl protons), 7.08-7.23 (m,
20H, aromatic protons) ppm; 13C NMR: (100 MHz, CCl4-
CDCl3) δ 83.15, 127.08, 127.43, 128.74, 144.29 ppm.
Diphenylethane (10): 1H NMR (400 MHz, CDCl3) δ 3.07
(s, 2H), 7.06-7.49 (m, 10H, aromatic protons) ppm; 13C NMR:
(100 MHz, CDCl3) δ 38.01, 125.99, 128.70, 130.97, 141.83
ppm.
After adopting the experimental procedure described in the
experimental section, 1H NMR and 13C NMR of crude products
are obtained. The 1H NMR was used for fixing the ratios of the
products formed and the nature of the products formed. The
principle for fixing the yield ratio is that when a carbonyl group
is converted to alcohol/alkane, or when a nitro group is converted
to hydroxylamine or amine, the aromatic protons of products
appear slightly up field in comparison to the aromatic protons
of the substrate (starting material).As most of the products formed
are known compounds the products peaks are identified by comp-
aring them with the authentic spectra available in NMR data
1,2-Diphenylethanone (11): 1H NMR: (400 MHz, CDCl3)
δ 4.40 (s, 2H), 7.34-7.49 and 8.10-8.20 (m, 10H, aromatic
protons) ppm; 13C NMR: (100 MHz, CDCl3) δ 45.40, 126.80,
128.53, 129.56, 133.29, 136.76, 197.50 ppm.
Aniline (14): H NMR (400 MHz, CDCl3) δ 3.65 (NH2
broad signal), 6.69-6.71 (d, 1H, aromatic proton) ppm, 6.67-
6.81 (m, 2H, aromatic protons) 7.16-7.20 (m, 2H, aromatic
1

Vol. 30, No. 3 (2018)
Solvent Free Reduction of Aromatic Carbonyl and Nitro Compounds by Zn/Conc. HCl System 641
TABLE-1
protons) ppm; 13C NMR: (100 MHz, CDCl3) δ 115.11, 118.50,
129.30, 146.47 ppm.
DETAILS OF THE REDUCED PRODUCTS OF THE AROMATIC
CARBONYL (6-11) AND NITRO COMPOUNDS (14-15) BY SFSM
p-Toluidine (15): 1H NMR: (400 MHz, CDCl3) δ 2.29 (s,
3H), 3.40 (NH2 protons), 6.63-6.6m (m, 3H, aromatic protons),
7.00-7.02 (m, 2H, aromatic protons) ppm; 13C NMR: (100 MHz,
CDCl3) δ 20.47, 115.32, 127.77, 129.78, 143.88 ppm.
Theoretical studies: The yield percentage of the reduced
products can only be reasoned out by theoretical studies, so as
reported in earlier study [22], DFT calculations were performed
in order to calculate the energy difference between the energy
of optimized geometry of the substrates and that of first formed
free radicals. The DFT calculations were performed at B3LYB/
6.311g ++ (d,p) level of theory by using the quantum mechanical
calculation Software (Gaussian 03) program. From the geometry
optimized substrate Mullikan charges were also obtained.
In addition to those calculations, now we have performed
electrostatic potential (ESP) calculation of the geometry
optimized substrates to study the correlation between
electrostatic potential and Mullikan charges (calculated earlier
[22]), in deciding the homonuclear nature of bond (π) present
in the carbonyl and nitro compounds.
The electronegativity difference of carbonyl and nitro group
in as substrate can be calculated from the residual partial charges
on the atoms of geometry optimized substrate molecules and is
called Mullikan charges. The Mullikan charges for carbon and
oxygen atoms of the carbonyl group and nitrogen, and oxygen
of nitro group, are obtained from the geometry optimized substrates.
The electronegativity difference or homonuclear nature is
denoted as charge factor. It is arrived by adding the magnitudes
of 'unlike' Mullikan charges and by subtracting the magnitude
of 'like' Mullikan charges on C and O atoms in case of carbonyl
group and on N and O atoms in the case of nitro group of the
substrates. The charge factor is an approximate measure of the
electronegativity difference of the atoms involved, lesser the
charge factor, more is the homonuclear nature of (π) bond present
between the atoms (C/O or N/O) and hence more likely is the
homolytic fission of (π) bond in second step of the suggested
mechanisms [22], resulting in more yield.
Compound
number
Entry
R1
R2
Yield* (%)
1
2
3
4
5
6
7
H
OCH3
H
H
H
H
H
80
50
60
90
70**
75
6
7
8
CH3
C6H5
COC6H5
H
9
10
14
15
H
CH3
H
60
*isolated crude with negligible minor products; **with minor amount
of compound 11
TABLE-2
COMPARISON OF THE YIELDS OF REDUCED PRODUCTS
OF THE AROMATIC CARBONYL (6-10) AND NITRO
COMPOUNDS (14-15) BY SFSM AND SM
Yield (%)
(solvent
method)*
Yield (%)
(solvent free
slurry method)**
Entry
R1
R2
1
2
3
4
5
6
7
H
OCH3
H
H
H
H
H
42
4
80
50
60
90
70
75
60
CH3
C6H5
COC6H5
H
13
42
25
25
5
H
CH3
H
*culled from 1H mixture NMR spectra containing starting material;
**isolated crude with negligible minor products.
two schools thought, One group [29] emphasizes the non-exis-
tence of species called 'nascent hydrogen' (atomic hydrogen),
and the reduction involves only direct transfer electron by the
metal (zinc) to substrate, not to H+ ions [16-19]. There are two
mechanistic pathways suggested by this group [28,30].
The first mechanism (zinc-carbenoid) [26] suggested the
involvement of the transfer of an electron from zinc metal to
carbonyl group of ketone, leading to a radical species which
is presumed to react further to yield zinc-carbenoid species.
Upon subsequent addition of protons, methylene product is
formed. Zinc-carbenoid mechanism does not involve alcohol
intermediate.The second mechanism(carbanion)[24]assuggested
involves ionic mecha-nism, in which after the protonation of
oxygen of the carbonyl group, metal transfers electrons to the
carbocation of ketone, formed after the protonation of oxygen
of carbonyl group. The electron transfer results in the formation
of metal-carbon bond leading to the formation of α-hydroxy
alkyl zinc chloride intermediate. The intermediate leads to
carbanion intermediate by the elimination of H2O and ZnCl2.
The carbanion again eliminates ZnCl2 and forms the product
on protonation.
We firmly believed that the above mentioned mechanisms
do not operate in earlier studies [22] and present investigations
due to the following reasons: (a) Some of the products obtained
earlier/present studies are pinocol-coupled products (compound
6, 9), rearranged pinacol-coupled product (compound 8) and
alcoholic product (compound 16).The formation of these products
cannot be explained by zinc-carbenoid/carbanion mechanisms.
(b) The experimental conditions in present investigation is highly
acidic. Under these conditions, zinc metal cannot transfer electron
RESULTS AND DISCUSSION
The results of present investigation are given in Table-1.
Apart from the usual reduction products, an unexpected pinacol
coupled and rearranged products 6, 8 and 9 were also obtained.
The formation of pinacol coupled products 6, 9 is in accordance
with report of Ranu et al. [23]. The formation of pinacol rearranged
product 8 is in accordance with the report of Salama et al. [24].
The formation of bibenzyl 10 is in accordance with report of
Mahajan et al. [25]. All the reduced products are obtained at
room temperature, in solvent free condition (SFSM) within 4-5 h.
The yield percentages of the reduced products are very high
in solvent free slurry (SFS) method, in comparison with solvent
method. The conversion rate for all the reduction reactions
are 100 %, as we found the absence of characteristic peaks of
the starting materials in 1H and 13C NMR spectra of the crude. For
ease of comparison, the yields of two methods are presented
in Table-2.
The mechanism of zinc/conc. HCl reduction has been
obscure right from the era of Clemmenson [26-28]. There are

642 Rajamathe et al.
Asian J. Chem.
to carbonyl carbon or oxygen, but it can only transfer electron
to H+ ions. Based on its high redox potential, H+ ions receive
electrons from metal to form nascent hydrogen (atomic hydrogen),
which reduces carbonyl/nitro group. The reduction by atomic
hydrogen has also been recently reported in the study reduction
of graphene oxide to graphene [20,21]. (c) The increase in the
yield from solvent method to solvent free slurry (SFS) method
is an evidence for the reduction by atomic hydrogen. The enhanced
yields are due to the decrease in the distance 'b' between the
substrate and nascent hydrogen in solvent method to 'a' in solvent
free slurry method (Figs. 1 and 2). (d) As shown Figs 1 and 2,
there are two competitive reactions in the system, one is reduc-
tion of the substrate by atomic hydrogen (path A) and other is
recombination of atomic hydrogen atoms to form hydrogen
molecules (path B), of which the reduction is favoured in solvent
free slurry method (SFSM) due to the proximity (distance 'a')
of the substrate and atomic hydrogen in SFSM. The adsorption
of substrate on zinc metal (during slurry preparation) is physical
adsorption only and is not chemical adsorption, as zinc does
not react with substrate but react with H+ ions to produce the
atomic hydrogen. This is evident from the fact that more of pinacol
coupled products are formed in solvent free slurry method
(SFSM), as the carbon free radicals formed in the process of
atomic hydrogen reduction are proximal due to the physical
adsorption and they easily undergo coupling and rearrangement.
Present finding is a path breaking and contrary to the general
belief that there is no such thing called 'nascent hydrogen'.
The above mentioned facts ruled out the metal mediated
direct electron transfer mechanism and hence present investig-
ations should follow metal mediated atomic hydrogen reduction
(free radical) mechanism suggested in earlier investigation [22]
which was also based on the studies of Martin et al. [31].According
to the suggested mechanisms, the success of carbonyl/nitro
group reduction depends on three factors: (i) Homonuclear
nature of carbonyl group and nitro group. It is decided by the
electronegatively difference of carbonyl carbon and oxygen
atoms present in the carbonyl compound and in the case of
nitro compound, it depends on the electronegativitiy difference
of nitrogen and oxygen atoms of the given nitro compound.
(ii) Stability of the free radical intermediates formed in the
second step of both mechanisms, and (iii) Distance between
the generated nascent hydrogen and substrate molecule.
The third factor is taken care by preparing a intimate slurry
of the substrate and zinc in solvent free slurry (SFS) method.
Present aim was to quantify the first two factors and correlate
them with the experimental yields and a positive correlation
serves as a support for the suggested mechanism. With this
aim, the DFT calculations were performed at B3LYB/6.311g
++ (d,p) level of theory by using the quantum mechanical calcul-
ation software (Gaussian 03) program in earlier study [22] to
find 'energy factor' and 'charge factor'. Now in the present study,
electrostatic potential (ESP) calculation of geometry optimized
substrates were performed to study the correlation between
electrostatic potential and Mullikan charges (calculated earlier
[22]), in deciding the homo nuclear nature of π-bond present
in carbonyl and nitro compounds.
Sub
Path A
H2
b
Path B
H•
Solvent-Et2O
In present study, we found that charge factor can also be
correlated to calculated electrostatic potential of substrate.
More, the negative electrostatic potential on oxygen atom of
carbonyl or nitro group, more is the charge factor for substrate
and less is the yield as illustrated in Figs. 3 and 4. For substrate
4, the electrostatic potential correlates well with the prediction
made from Mullikan charges for all substrates but for substrate
5 it is not, which is due the fact that the positive potential on
carbonyl carbon is more (indicated by intense blue color in
electrostatic potential structure).
H•
2H+
2e–
2Cl–
Zn dust
particle
Zn2+
The energy difference between the substrate and first formed
free radical is called 'energy factor' and its value decides the further
course of action in both mechanisms. So, from the DFT calcul-
Fig. 1. Solvent method (SM)
H•
H•
–6.430e–2
6.430e–2
Sub
a
2H+
2e–
2Cl–
Zn dust
particle
Zn2+
Fig. 2. Solvent free slurry method (SFSM)
Fig. 3. Electrostatic potential (ESP) structure of benzophenone (4)

Vol. 30, No. 3 (2018)
Solvent Free Reduction of Aromatic Carbonyl and Nitro Compounds by Zn/Conc. HCl System 643
the energy factor. So from the results obtained, one can conclude
both factors are important for high yield, with the charge factor
having an edge over energy factor.
Conclusions
The investigations of solvent free facile room temperature
reduction of aromatic carbonyl and nitro group by Zn/conc.
HCl highlighted the following factors:
• In complement to earlier investigation [21] in same
theme, the present study also confirms that reduction of aromatic
carbonyl and nitro group by Zn/conc. HCl, is only by generated
hydrogen free radical (nascent hydrogen) and not by metal
mediated direct electron transfer. It also confirms that the yield
of product depends both the stability of free radical generated
(in second step of suggested mechanisms), denoted as energy
factor and the charge factor, which makes π-bond of both nitro
and carbonyl group amenable for homolytic fission, which is
a precondition for the free radical reactions.
• It is also found that apart from Mullikan charges, the
electrostatic potential calculated by DFT method, can also be
useful in quantifying the charge factor.
• The yield of reductions products are high, due to the proxi-
mity of the substrate and liberated nascent hydrogen under
the experimental conditions (SFS method).
• Due to the adsorption of substrate on zinc surface which
acts as a catalyst and reactant, the radical intermediates formed
are also proximate and undergo coupling to yield unexpected
coupled and rearranged products.
• The reaction is facile as it involves cheap chemicals, less
reaction time and more yields and hence, this method can be
included in the undergraduate curriculum as a preferred alternate
method for the synthesis of the reported reduced product
(aniline, toluidine, etc.).
• In the adopted experimental method (SFS method), the
most elusive nascent hydrogen was tamed and used effectively
for the efficient reduction in a situation where competitive reaction
is possible. So in future investigations, the reduction efficiency
of this method for reducing aromatic alkenes, esters and amides
would be investigated.
Fig. 4. Mullikan charges for geometry optimized structure of benzophenone
(4)
ations Esubstrate − EFree radical were obtained to quantify the energy
factor. Higher value the energy factor, more stable is the free
radical because energy factor indicates how much lower is the
energy of free radical when compared to the substrate from
which it formed. The results obtained showed the following
trend in charge factor and energy factor for carbonyl and nitro
compounds.
Charge factor: Smaller the charge factor, more is the
probability of free radical production by homolytic fission
(yield).
(i) Carbonyl compounds: 0.005 (benzophenone) < 0.018
(acetophenone) < 0.094 (benzaldehyde) < 0.191 (4-methoxy-
benzaldehyde) < 0.302 (benzil).
(ii) Nitro compounds: 0.118 (nitrobenzene) < 0.138 (2-nitro-
benzaldehyde)
Energy factor: Higher energy factor more is the stability
of free radical and yield.
(i) Carbonyl compounds: 1532.338 (benzil) > 1527.651 (benzo-
phenone) > 1487.420 (benzaldehyde) > 1480.798 (acetophen-
one) > 1478.341 ( 4-methoxybenzaldehyde).
(ii) Nitro compounds: 1537.578 ( 4-nitrobenzene) > 1534.744
4-nitrotoulene.
The results are presented in Table-3 along with yields
obtained earlier [32] and present study. For positive correlation
1/charge factor is also presented in Table-3. It is heartening to
observe that the yield prediction from the values of charge
factor and energy factor (which are based on the suggested
mechanisms) agrees very well with the experimental yield and
thereby supported the metal mediated atomic hydrogen
reduction (free radical) mechanism. It is also observed that in
case of substrate 5, the charge factor takes an upper hand over
ACKNOWLEDGEMENTS
The authors are thankful to Prof. Dr. H. Surya Prakash Rao,
Department of Chemistry, School of Physical, Chemical &
Applied Sciences, Pondicherry University, Puducherry, India
and his research scholars for their encouragement, support
TABLE-3
ENERGY DIFFERENCE OF THE GEOMETRY OPTIMIZED SUBSTRATES (1-5) AND (12-13) AND THEIR FREE
RADICALS (B3LYB/6.311g ++ (d,p) ) WITH ESP/CHARGE/ENERGY FACTORS AND YIELD OF SM/SFSM
Energy difference
(kJ/mol) (Energy
factor)
Charge
difference
(Charge factor)
Yield (%)
(SM)
Yield (%)
(SFSM)
Compound
name
ESP of oxygen
atom (V)
1/Charge
difference
S. No.
1
2
3
4
5
6
7
Benzaldehyde
Anisole
Acetophenone
Benzophenone
Benzil
1487
1478
1481
1528
1532
1538
1535
-7.526 × 10–2*
-8.242 × 10–2
-7.848 × 10–2
-6.430 × 10–2
-5.118 × 10–2*
-6.223 × 10–2
-6.667 × 10–2
0.094
0.191
0.018
0.005
0.302
0.118
0.140
10.6
5.2
55.6
200.0
3.3
42
4
80
50
60
90
70
75
60
13
42
25
25
5
Nitrobenzene
4-Nitrotoluene
8.5
7.1
*The potential on carbonyl carbon is more positive.

644 Rajamathe et al.
Asian J. Chem.
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rendered in recording the NMR spectra. Thanks are also due to
the Director, Kanchi Mamunivar Centre for Post Graduate
Studies, Puducherry and The Government of Puducherry, for
providing the infrastructure in the Department of Chemistry
of the College to carry out this research work.
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