The electrochemistry of alcohols in aqueous phosphate electrolytes under reducing conditions : a thesis presented in partial fulfilment of the requirements for the degree of Master of Philosophy in Chemistry, Massey University, Palmerston North, New Zealand
Few methods are available for the routine reduction of alcohols in synthetic chemistry. These few are dominated by reduction with HI/I2, LiAlH4 or Li/NH3 and typically involve severe conditions for other functionalities and there is little research into less severe synthetic or electrochemical methods. There is also limited mechanistic or kinetic information available for these reduction methods. This leaves an interesting area for development within fundamental knowledge. The development of an effective process for the reduction of alcohols could have many applications in pharmaceutical and chemical industries along with many environmental and economical benefits.
A preliminary study on a range of electrodes established an electrochemical reduction response observed for a number of water-soluble alcohols on rotating disc copper, tin and lead electrodes in 0.1 M phosphate buffers.
A response was observed for ethanol, propanol, propan-2-ol and butanol on copper rotating disc electrodes in the 0.1 M phosphate buffer. Reduction of the alcohols at the copper disc electrodes was observed at pH 8.1 with the production of a limiting current plateau. The reduction was found to be continuous and reproducible. The observed limiting current was found to increase with both increasing concentration and increasing electrode rotation rate. A Koutecky-Levich study suggested the reduction of the alcohol occurred through both mass transport and kinetic processes.
A discrete, reproducible response was observed for ethanol, propanol and propan-2-ol on tin rotating disc electrodes in the 0.1 M phosphate buffer electrolyte at pH 7.3. A reductive peak was observed at −1.1 V vs Ag/AgCl in cyclic voltammetry. This formation of a reductive peak suggests that the reduction becomes progressively hindered, proposed to be due to a passivating layer forming on the surface of the electrode. The charge associated with the peak is relatively invariant with alcohol concentration (in the range 7−20 mM) and with scan rate (over the range 10−500 mV s−1). In the case of ethanol, the peak charge is typically found to be in the range 2.9−3.6 C m−2 suggesting that a passivating layer of reaction products forms with an area of 8.8−10.8 Ǻ2 for each adsorbed molecule (assuming a 2-electron process and a surface roughness factor of one). This suggests formation of
a monolayer with sparsely located binding sites. The peak charge does not change with increasing electrode rotation rate, not inconsistent with the formation of a passivating layer on the surface of the electrode inhibiting any further reduction.
A discrete response was also observed for ethanol, propanol and propan-2-ol on lead rotating disc electrodes in the 0.1 M phosphate buffer electrolyte at pH 8.1. A reduction peak is observed at −0.9 V vs Ag/AgCl in cyclic voltammetry. This suggests that the reduction becomes progressively hindered due to a proposed passivating layer. The passivating layer is not permanent – employing a > 30 second open-circuit rest period or having an anodic limit more positive than −0.6 V will result in the new reduction peak for each subsequent voltammogram. Multiple-cycle voltammograms exhibit only the background response if these conditions are not met.
The charge associated with the peak decreases with scan rate (over the range 10−500 mV s−1) but is relatively invariant with alcohol concentration (in the range 7−20 mM). In the case of ethanol, the peak charge is typically found to be in the range 0.5−4.0 C m−2 suggesting that a passivating layer of reaction products forms with an area of 19−58 Ǻ2 for each adsorbed molecule (assuming a 2−electron process and a surface roughness factor of one). This suggests formation of a monolayer with sparsely located binding sites.
The peak charge decreases with increasing electrode rotation rate. It is proposed that this is due to a surface chemical reaction following the electrochemical process – it is the product of this chemical reaction that results in a transient passivating monolayer. FT−IR analyses of the lead disc systems suggest possible products to be propandiol and butandiol.