Electrochemical Reduction of Acetaldehyde, Benzaldehyde & Furfural in Ethanol-Water Mixture

Indian Journal of Chemistry Vol. 17A, June 1979, pp, 564-566

G. A. NOUBI*, M.. F. EL-SHAHED, iF. EL-CHEIKH & H. MANSOUR Chemistry Department, Faculty of SCience, Assiut University, Qena, Egypt

Received 22 July 1978; revised 6 January 1979; accepted 5 February 1979

The electrochemical reduction of acetaldehyde, benzaldehyde, and furfural in ethanol-water

mixture (1: 1) and in the presence of 1·0N H2S04 has been investigated. The reductions have been carried out at metallurgical copper, electrodeposited copper, copper amalgam and electrodeposited cadmium cathodes. The structures of the aldehydes seem to playa definite role in their reducibility. The dipole moments (i.e. polarities) of the aldehydes influence the adsorb- ability and hence the reduction efficiency to a great extent. The cathode potentials at which the reduction takes place are highly dependent on the dipole moments of the aldehydes. It is found that on (i) increasing the aldehyde concentration, (ii) lowering the current density, (iii) raising the temperature of the solution and (iv) addition of some salts, the current efficiency of electrochemical reduction increases. On the other hand on increasing the ethanol content of the solvent a parallel Increase in the reducibility of the aldehydes studied is noticed till a certain limit after which the reducibility decreases.

'" HE mechanism of electrochemical reduction of deposition of a thin layer of metal of the added carbonyl compounds has been studied by many salt on the cJpper spiral cathode. Electrodeposited T workers':", Several theories were proposed to copper and cadmium cathodes were obtained by explain this process. According to these theories, electrodeposition of these metals on the spiral the reduction of carbonyl functions involves the copper cathode for 30 min at 25° from the following formation of the free radical, baths: (i) copper bath - CuS04.5H20 (220 gjlitre), R-C--R cone, H2S04 (60 gjlitre) and current density 1·5 ajdc2; (ii) cadmium bath-CdS0 .8j3H 0 (65 gjlitre), ! 4 2 OR (NH4)2S04 (35 gjlitre). Al2(S04)3.18'H20 (30 gjlitre), glue (0-7 gjlitre) and c.d., 1·0 ajdc2• Amalgamation The activity of carbonyl compounds towards was carried out from a bath containing N'H4C1 electrochemical reduction depends to a great extent (1 g) HgC1 (1 g) per litre for 30 sec. on their structures. The effect of substituents + 2 The amount of hydrogen gas evolved at the on the reducibility and the extent of reduction of cathode was compared with that evolved in a aldehydes and ketones has also been investigated. hydrogen coulometer, connected in a series to the In the present work, an attempt has been made to electrolytic cell containing ethanol-water solvent investigate the role of the simple differences in the and H2S04 (l·ON).- dipole moments of the aldehydes, viz. acetaldehyde, In the case of experiments with added salts, benzaldehyde and furfural on their reducibility. same salts at low concentrations were also added Benzaldehyde and furfural are aromatic but have to the coulometer. The difference between the sufficient :structural differences. Hence the study quantities of hydrogen evolved was considered to be has been extended to differentiate between the equivalent to the quantity of electricity consumed cathodic behaviour of these compounds. in the reduction process of the carbonyl func- Materials and Methods tion. The electrolytic cell used consisted of two com- The temperature during electrolysis was kept at partments, separated by a porous diaphragm of 25° and the cathodic potentials were measured sintered glass. The anode consisted of a platinum against a saturated calomel electrode. sheet of 4·2 em" surface area. The cathode was Results and Discussion made of a copper spiral of 5·0 cm2 surface area. Aqueous ethanol (1 :1) containing 27·8 ml conc. In the present work the current efficiency of reduction (A) was taken as a measure of the H2S04jlitre was used as the electrolyte. The cathode was pretreated chemically before each run and then ease of reduction of the carbonyl function. This is kept in solution under investigation for about given by A = (V - V')jV where V and V' are the 15 min without polarizing current. When small corrected volumes of hydrogen evolved over the amounts of some salts (O·OOlM) w ere added, coulometer and the working cathode respectively. polarization was carried out br about 5 min at Effect of chemical structure of the aldehyde - The 40 majcm2• This cathodic treatment allowed the dipole moments of acetaldehyde, benzaldehyde and

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I NOUBI et at.: ELECTROCHEMICAL REDUCTION OF ALDEHYDES furfural are 2·69, 3·0 and 3·6 respectively". The potentials of reduction of these aldehydes under iden- tical conditions (concentration. temperature, current density and electrode material) are found to become less negative as the dipole moment increases (Fig. 1). Thus, acetaldehyde is reduced at the highest negative A",.50 potential, while furfural at the least negative pot en- tial at the same cathode. This fact confirms the well known rule in polarography on the rela .on 20 between (J. (dipole moment) and El/2 for he 10 different ketones". The higher dipole moment assists OL-L-~ L-__-+ ~~ the adsorption of the organic compound. Thus, the 0~1 ~ depolarization caused by the aldehyde reduction 0·025 0·075 and the reducibility are expected to be functions CONC',M of the dipole moment. Actually, the reducibility Fig_. 2 - Plots of current efficiency (A) versus the concen- (reduction efficiency) is found to be highly dependent tration of furfural for its reduction at Cu-metallurgical cathode at 30° and different current densities [Current on th,: dipole moment. 0.1 all the cathodes em- densities for curves 1-10: 1, 2. 3,4. 5. 6, 8. 10, 20 and 30 ployed in the present study, the reducibility of ma/cm2 respectively] furfural-Is the hijhest , while that of acetaldehyde is the lowest. The electrochemical reduction of furfural in the sorption. At low concentrations the amount of presence of c)p~)er sulphate solution has been aldehyde adsorbed is proportional to its concen- reported to give Iurau and formaldehyde as pro- tratiori' (C). At higher concentrations, the electrode ducts". Benzaldehyde and acetaldehyde are not surface becomes more saturated and further increase capable of such cleavage reactions at the cathode, in C does not affect the adsorption as noticeably This behaviour again may bad to higher efficiencies as at low concentrations. When the degree of of furfural reduction. surface coverage is small, more aldehyde molecules Effect of varying aldehyde concentration -- The can be adsorbed. current efficiency of acetaldehyde, benzaldehyde and . Effec.t of varying current density -- The effect of 'furfural at different cathodes has been investigated. ~ncreasmg the current density on current efficiency The representative curves, shown in Fig. 2 pertain ~s the. same for ~1l the compounds studied. On to the reduction of furfural at metallurgical copper mcreasmf? the applied current, the efficiency decreases cathode. Figure 2 shows that an increase in and at higher current values it is found to approach [furfural] is accompanied by an increase in the a more or less constant value. This behaviour is current efficiency. However, at higher concentra- quite cJmmon. for many organic depolarizers and tions the increase in A becomes less noticeable, may be expl.amed by the competitive reduction especially at low current densities. This behaviour of the orgamc molecules and the hydrogen ions may be explained on the basis of aldehyde ad- at the cathode. Representative curves for the variation of the ~urrent efficiency (A) with ~urrent density. (i) '200r------~~ m the case of furfural reduction at metallurgical 900 copper cathodes are given in Fig. 2. . G~nerally, the reducibility of the aldehydes studied 800 IS hI&her at copper amalgam and electrodeposited 700 cadmium cathodes than at electrodeposited or metallurgical copper at the same values of current 600 density. This phenomenon can be attributed to the a jmetallurg,col copper difference in th~ ?ydrog~n overvoltage of these etectrcdeposued Copper cathodes. In acidic medium, Tafel's constants a UJ for hydrogen ev?lution o~ copper, copper amalgam ~o t-200r > ~======~======~ and electrodeposit ed cadmium are respectively 0·800, >e-. ",00- 1·406 and 1·40 V. The hydrogen evolution at c?pper amalgam and cadmium cathodes is more 1000 difficult than at copper cathode. This must lead to 0-900- higher reduction efficiencies of the organic depolarizer at copper amalgam and cadmium cathodes. Effect of added metal salts -- Representative curves of furfural reduction in the presence of small b \"COPPt" amalgam - amounts of s?~e metal salts (Fig. 3), show etectrcceccsaec cadmium - - - that the addition of metal salts increases 0'500 the r~ducib~lity of the aldehydes as compared to -'·0 -os 0-0 0·5 '·0 1·5 that in their absence. The function of the added LOG i (me cm-2) metal ion~ in solution is mainly the formation of Fig. 1 - Cathodic polarization curves of (1) acetaldehyde a very thm layer of their deposits on the copper (2) benzaldehyde. and (3) furfural at different cathodes cathode. In the case of mercuric sulphate,

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Cadmium sulphate MIlr c uric sui pncte 15 Zinc sutphcte 70 ~5 Cd-electrode posited (,0 Cu-amolgom 55 Copper sulphate A-'.50 __--'5 Cu- eletrodllpOSlted 45 40 ----~& 35 30 50 25 20 10 20 30 40 50 GO 40 TEMP,:C Fig '. 4 - Effect of varying temperature on the current 30 efficIency.(A) of furfural (0'05M) reduction at 30° on copper metallurgical cathode [Current densities for curves 1-6: 2, 4, 6, 10, 20, 30 ma/cm2 respectively) o 2 4 6 a 10 20 30 2 I (ma crn- ) Fig. 3 - Effect of added metal salts (O·OOlM) to furfural Effect of varying t(Jlnperature - The reduction (0'05M) on the current efficiency of reduction of furfural at efficiency of furfural at different temperatures is copper metallurgical cathode at 30° grap~ically shown in Fig. 4. The other aldehydes studied reveal the same characteristics. As can be seen from Fig. 4 raising of temperature leads to a amalgamation of the copper takes place. The formed parallel increase in the reduction efficiency of the deposit is highly dispersed, very fine-grained and of aldehyde. This behaviour is noticed at different spongy film type. The adsorption of' organic matter cathodes stu~i.ed and .over a wide range of applied on such metal films is much greatepthan on compact ,current densities. This phenomenon may be attri- surfaces. Also the real surface' area of spongy buted t.o the different effect of temperature on the metal deposits is much greater' than that of the competmg reactions of hydrogen evolution and compact surfaces. This in t~rn leads to lower aldehyde reduction. current densities on spongy surfaces than on compact smooth cathodes. Solvent effect - The reducibility of aldehydes in- References creases with increase in alcohol content of the 1. ELV"ING,P. 1. & LEONE,!. T., J. Am. chem, Soc. 80 solvent, reaches a maximum and then decreases (1958). 1021. ' with further increase in alcohol content. Such a 2. KORSHOUNOV",1. A., KIRILLOV", A. S. & KOUZNETWOV" behaviour of the current efficiency with the solvent Z. B., Zh.{!z. Khim., 24 (1950), 551. ' 3. ASHowRTH, M., Colln Czech. chem, Commun: 13 (1958) composition may be due to two factors: (i) different 299. ' , solubilities of the reduction products" and. (ii) com- 4. SCHIKATA,M. & TAcHI, L, J. chem, Soc. Japan 53 (1932) petitive adsorption at the cathode surface by alcohol 834. ' , and aldehvde molecules. The decrease in current 5. TOMILOV",A. P., MAIRANOV"SKYS,. G., FIOSHIN, M. YA. & efficiency ·on increasing" alcohol c mt ent may be SMIRNOV,V. A,; Elektrokhim. organivheckikh Seoginenie attributed to the blocking of the cathode surface by Khimiya, Leningrad (1968), 65, 83, 85, 86, 214. ' 6. Chemists handbook-I (Khimiya, Moscow), 1966. alcohol molecules inhibiting the adsorption and 7. REIKHFELD, V. O. & KOUDRNA, S. K., Troudi LT.I. consequently the reduction of aldehydes. Lensooet, Goskhimizetat (1926), 100-136.

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