Preparation of Pinacol by the electrochemical reduction of carbonyl compounds at stainless steel cathode.
Electrochemical reduction has since been known as simple and cheep and ecofriendly process for the preparation of organic compounds. The study of the electrochemical reduction of carbonyl compounds was very limited. Therefore aim of the work was to investigate and develop a new synthetic method for the preparation and evaluation of Pinacol on easily available conditions (reagent, apparatus, temperature and pH).
The present work deals with the electrochemical reduction of various carbonyl compounds such as (1) Acetone (2) Acetophenone (3) 4-Choloro benzaldehyde was carried out by using the weak acid (Phenol) in aprotic solvent for the synthesis of the Pinacol. The Electrochemical reduction has been carried out at Constant Potential at Stainless steel electrode. Stainless Steel electrode have been used in Constant Potential Elecrtolysis technique [1-3] in our laboratory for the electrochemical reduction of carbonyl compounds [1-10].
Potentiostat cum Galvanostat was used for the carrying out control potential electrolysis to control the potential and measuring the current during the electrolysis. Multi opening round shaped (flat bottomed) flask was used as electrolysis cell to carry out constant potential electrolysis. The solution was stirred by a Remi 2LH hot plate stirrer throughout the electrolysis. Rectangular plates of Stainless Steel (type 316) of size 4cm x 4cm were used as cathode while platinum foil as auxiliary electrode. The saturated calomal electrode(SCE) used as reference electrode.
Solutions were prepared from distilled and purified non-aqueous solvent (DMSO and DMF) and double distilled water. Reagents such as Acetone, Acetophenone, 4-Choloro benzaldehyde,KC1 were of A.R. grade. The purity of reactant was checked by single spot T.L.C. I.R. Spectra of reactant and the product obtained, were recorded and studied to confirm the structure.
Reaction mixture was prepared by taking 50 ml of non-aqueous solution of 0.26 N carbonyl compounds (Acetone, Acetophenone, 4-Choloro benzaldehyde) and 50 ml. of aqueous solution of KCl, then 5 ml. of phenol was added to the solution. A thermometer (0-100[degrees]C) was kept dipped under the surface of the catholyte and temperature was recorded periodically. pH meter was used for pH measurements. Pinacol is prepared in neutral conditions and chromium (III) salts are added to the solution.
All the electrolysis was carried out at controlled anode potential. Reduction potential of the carbonyl compounds was recorded by polarographic technique. During the constant potential electrolysis, current-potential data was recorded at an interval 15 minutes and carried out for 3 hours each. After 3 hours no reduction product was found to diffuse in the bulk.
Result and Discussion
Constant Potential electrolysis as a new eco-friendly synthetic method has been developed for the preparation of Pinacol based on easily available conditions. The carbonyl group is an electronic group offering interesting synthetic possibilities. The ease of the electro-reduction is influenced by the nature of alkyl or aryl group.
The respective current in the time interval of 15 Minutes at applied constant potential are depicted in the Table 1. It is evident from the Table 1 that at constant potential electrolysis the respective current is also almost constant.
Results of the % Yield with the different reactant are depicted in Table 2. In all these cases (Table 2) yield is above 75%. The percentage yield of Pinacol synthesized by acetone, Acetophenone and 4-chlorobenzaldehyde were 75%, 85%, 90% respectively.
In general aromatic carbonyl compounds are easier to reduce than aliphatic carbonyl compounds and Aldehydes are easier to reduce than Ketones. When acetone and Acetophenone were compared for the formation of the Pinacol. Acetophenone yields more Pinacol indicating that aromatic ketones are more prone to the formation of the Pinacol. When we compare Acetophenone and 4-chlorobenzaldehyde, 4-chlorobenzaldehyde yields more of Pinacol indicating that aldehydes are easily reducible than ketones.
Behavior of the carbonyl compounds in aqueous or non-aqueous system is determined by the availability of protons. In water and the alcohal system, this can be measured by pH and controlled by buffer, whereas in aprotic solvents, it can be controlled by addition of proton donors.
The electrochemical reduction of functional groups containing multiple bonds such as carbonyl or nitro groups tends to be more complex than the system having single bonds . The principal which governs whether a pinacol or alcohol is formed when the carbonyl compounds is electrolyzed may be classified on the basis of conjugation . Pinacol has been prepared in good yield under neutral condition if chromium (III) salt added to the solution [12,13]. Given and Peover  have examined in carbonyl reduction that phenol serves as proton donor without being itself reduced in the aprotic solvent.
Mechanism of carbonyl reduction involves a reversible one electron reduction to form an anion radical, which in absence of proton donors, can diffuse into the solution to dimerise. With sufficient proton donors, the rate of protonation can be more rapid than the rate of diffusion. For Acetone, Acetophenone and 4-chlorobenzaldehyde the reaction mechanism is as follows.
Band appeared in FTIR of products at 1400-1310 [cm.sup.-1] and C-O stretching near 1100 [cm.sup.-1] for secondary alcohol in the product. A Peak at 1710 [cm.sup.-1] due to C=O stretching of carbonyl compounds was observed in the starting material which disappeared in the products. A broad band at 3392 [cm.sup.-1] for Acetophenone, 3336 [cm.sup.-1] for 4-chlorobenzaldehyde indicates that products are pinacols.
Aqueous solution of ceric ammonium nitrate is mixed with the product, red colour is appeared, which indicate the presence of diol. This test gives negative results with the reagents.
(1) The production of Pinacol from Electro-organic synthesis method is better than the routine method as far as the cost of the Pinacol is concerned, when we examine our technique at laboratory scale.
(2) Stainless Steel electrode is used as a cathode in the present work. The peculiarity of this Stainless Steel electrode is that it is economical viable, readily available and long lasting, particularly in solution having pH above 6.
(3) Reaction takes place at room temperature and reaction does not requires hazardous chemicals and reagents.
(4) Reaction is carried out in presence of very weak acid.
Authors wish to thanks the Head, Deptt. of Chemistry, University of Raj., Jaipur for providing the necessary facilities. This work is carried out under the WOS-B project sanctioned to Dr. Sangita Gupta by DST,GOI, New Delhi.
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* Sangita Gupta (a), Manish Guptab, Nidhi Jain (a) and P S Verma (a)
(a) Department of Chemistry, University of Rajasthan, Jaipur (Rajasthan)-302004 INDIA
(b) Dy Director (IT), Rajasthan Council for Elementary Education, Dr. R.K.Shiksha Sankul, Jaipur(Rajasthan)-302017 INDIA
* Corresponding author E-mail: sangita. uor@gmail. com
Table 1 Sr.No. Measured Current Measured Current Measured Current when when reactant is when reactant is reactant is 4- Acetone at Acetophenon at Chlorobenzaldehyde constant constant at constant potential potential 0.95V potential (mA) 1.73V 1.80V (mA) (mA) 1 0.42 0.32 0.08 2 0.42 0.32 0.08 3 0.42 0.31 0.07 4 0.43 0.30 0.08 5 0.42 0.29 0.07 6 0.41 0.29 0.06 7 0.41 0.30 0.05 8 0.42 0.29 0.04 9 0.41 0.31 0.05 10 0.42 0.30 0.06 11 0.43 0.31 0.07 12 0.43 0.31 0.07 Table 2 Sr.No. Reactant % Yield 1 Acetone 75 2 Acetophenone 85 3 4-Chlorobenzaldehyde 90
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|Author:||Gupta, Sangita; Gupta, Manish; Jain, Nidhi; Verma, P.S.|
|Publication:||International Journal of Applied Chemistry|
|Date:||Sep 1, 2009|
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