1
762 Rajamathe et al.
Asian J. Chem.
R2
13
O
and at 15.2 and 65.9 ppm in C NMR. The spectral data of the
X
unreported compound 22 are given below:
Zn
2HCl
ZnCl
2
2
-(4-Nitrobenzylideneamino)benzaldehyde (22): Pale
Et O
2
1
3h , r.t
yellow low melting solid, mp 25 °C; H NMR (400 MHz,
R1
R1 = H; R2 = H
;
R1
1
CDCl
proton), 8.05-6.95 (m, 8H, aromatic protons) ppm; C NMR
(100 MHz, CDCl ) δ 194.0, 188.2, 156.0, 154.6, 134.1, 133.7,
3
) δ 10.35 (s, H, aldehydic proton), 9.10 (s, 1H, alkenic
R1 = H; X = CH
OH
2
13
6;
1
7; R1 = OCH
; X= CH
R1 = H; X= C(CH
)(OH)C(OH)(CH
)C(CH )C
)(OH)C(OH)(C
= H; X = CH(OH)COC H
R1 = OCH ; R = H
2
3
3
3
2
3
4
;
3
8;
9;
)(C
H
)
3
3
6
5
;
;
R1 = H; R2 = CH
R1 = H; R2 = C H
3
131.3, 130.9, 129.6, 129.4, 124.37, 119.7, 124.43, 114.9 ppm.
To compensate the loss of the atomic hydrogen atoms
generated as hydrogen molecules (bubbles) without effecting
reduction, the stoichiometric ratios of the zinc and acid was
scaled up for the optimum yield. The scaled up ratio of the
acid was lesser than that of Zn, to avoid residual acid medium
and loss of acid sensitive products to the aqueous phase. The
acid of the required concentration was prepared by using brine
solution to minimize the loss of water soluble products to the
aqueous phase. By this technique, it was found that the amount
of product in the organic phase was increased by two folds
R1 = H; X = C(CH
R1 = H; X = C(C
; R
2
2
6 5
H
6
5
5;
R1 = H; R2 = COC H5
6
10;
6
H
5
6 5 2
H )
11
1
6
5
Scheme-I: Reduction of aromatic carbonyl compounds 1-5
NO2
X
R2
R2
6HCl
3ZnCl
2
2
2H O
3Zn
Et
2
O
3h,r.t
R1
2; R1 = H; R = H
R1
2
1
1
2
1
8; R = H; R = H; X = NH(OH)
9; R1 = H; R = H; X = NH
; R2 = H; X = NH
21; R1 = CHO; R2 = H; X = N=CH-polymer
1
1
3; R1 = CH
3
; R2 = H
2
1
2
(
mixture NMR evidence) without the regular workup.
The technique of adding dil. HCl in drops by using addition
4; R1 = NH
2
1
2
; R = H
20;
R
= CH
3
2
1
2
1
5; R = H; R = NH2
funnel/burette to organic phase containing zinc and substrate
dispersed in ether, at the rate of 50 mL/h provides a steady
velocity of hydrogen generation and more contact time for
atomic hydrogen to effect reduction on the substrate. The rate
of addition was optimized after several trial runs, by running
TLC with hexanes/ethyl acetate (8:2) as eluent.
1
2
1
6; R1 = CHO; R2 = H
6 4
22; R = H; R =CHO; X= N=CH-C H -NO
2
1
2
1
7; R = H; R =CHO
Scheme-II: Reduction of aromatic nitro compounds 12-17
spectrometers, respectively. The products were characterized
by a comparison with authentic samples (melting or boiling
points) and their H NMR or C NMR or IR spectra. TLC was
applied for the purity determination of substrates, products
and the reaction mixture by using silica gel 60 F254 aluminum
sheet.
As the starting materials for the investigation are known
compounds, the experiments are performed in a higher scale
for the ease of qualitative identification of the products. 0.2
mol of zinc dust was added to a solution 0.05 mol aromatic
carbonyl compound in 150-200 mL of diethyl ether, taken in
a 500 mL 2 necked round bottom flask. To the stirred zinc
suspension of the ethereal solution of the substrate, 150 mL
of very dilute HCl of the concentration 0.3 mol per 150 mL is
added very slowly through the addition funnel at the rate of
1
13
The present investigation is aimed only at finding the
feasibility of the reduction of aromatic carbonyl/nitro group
and the extent of reduction possible, under given the set of
experimental condition, so the technique used to calculate the
yield was to cull the ratio of unreacted substrate and the products
1
formed (percentage of conversion), from the H mixture NMR
of the crude product. It was done on the basis of the peak area
integration of the aromatic protons or the characteristic protons
of the substrate/products.
The principle for spectral assignment for the aromatic
protons is that, when an carbonyl group is converted to alcohol/
alkane, or when a nitro group is converted to hydroxyl amine
or amine, the aromatic protons of the products appear slightly
up field in comparison to the aromatic protons of the substrate
5
0 mL/h which is the optimum rate of addition for the optimum
(
starting material) as most of the product formed are known
velocity of the generation of hydrogen for the maximum yield,
tested by several trial runs and monitored byTLC (8:2) hexanes
and ethyl acetate system as eluant. To compensate the loss of
solvent due to evaporation, 20 mL of ether was added at end
of each hour.
compounds. The products aromatic peaks are identified by
comparing them with the authentic spectra available in NMR
data base [11].
RESULTS AND DISCUSSION
After the completion of the reaction, the zinc was filtered
off and usual work up resulted in the crude. The mixture NMR
of the crude was obtained to determine the product to reactant
ratio (conversion percentage) and in the case of formation new
compound, isolated yield was determined. In the case of the
reduction of nitro compound the amount of zinc and HCl was
scaled up by 3 times to match the stochiometric requirement.
As most of the substrates and products involved in our
study are known compounds, the NMR data of the mixture
are compared with the NMR of the authentic samples available
with SDBS [11] and with the NMR of the starting materials
available. In some mixture NMR, the residual solvent (diethyl
The results of this investigation of reduction of aromatic
carbonyl and nitro compounds by Zn/dil. HCl-Et
are presented in the Table-1.
2
O system
Based on studies of Turro and others [12], we suggest a
plausible free radical mechanism (Schemes III and IV) for
the room temperature reduction of aromatic carbonyl and nitro
com-pounds by Zn/dil. HCl-diethyl ether system.
The mechanism for the carbonyl reduction (Scheme-III)
is suggested to account for the three types of products (normal,
coupled and clemmenson) formed. The first step is the slow
generation atomic hydrogen (free radicals) or the ‘nascent
hydrogen’. The Zn metal suspended in the organic phase,
1
ether) signal occur at 1.16-1.19 and 3.42-3.47 ppm, in H NMR