Electrochemistry made easy with continuous flow chemistry techniques

By Andrew Mansfield on October 10th, 2018 in Flow chemistry, The Flow Chemistry Collection

Electrochemistry has a huge potential to offer synthetic organic chemists. The use of electrochemistry in modern laboratories, however, is generally quite low. This is largely due to the absence of suitable equipment that allows non-electrochemists a ‘convenient’ method to carry out this chemistry. For this technique to be more readily accepted as a routine procedure chemists need an easier, more user-friendly way to access it.

Electrochemistry techniques in the past have relied on electrolysis in glass reactors that lead to poor reaction control, low selectivity, and reproducibility and slow reaction rates. These systems can be easily set-up but because of the reasons stated above, there has been a reluctance to adopt them.

Over the past 5 years or so the development of continuous flow electrochemical cells has made selective syntheses with high reactant-to-product conversions possible, more often with a single pass through the device. These devices offer an easy access to electrochemical techniques which is driving its current re-assessment as a viable, attractive synthetic method.

Basic principles of organic electrosynthesis

During an organic electrochemical reaction, organic molecules are activated by the addition or removal of electrons at the surface of an electrode through a heterogeneous process. An electrosynthetic reaction typically requires two electrodes (anode and cathode) in contact with a solution that contains an electrolyte. An electrolyte is a salt that provides ions to improve the conductivity of the solution.

Electrochemistry can be performed in a variety of different methods either Potentiostatic mode (where the voltage across the electrodes is controlled) or Galvanostatic mode (where the current across the electrodes is controlled).

• There are several variables that can be explored in the development of electrosynthetic methods;
  the nature of the electrode
• the voltage/current applied
• whether the chemistry is performed at the anode or cathode
• whether the electrodes are simply either side of the cell or are separated (e.g. by an ion permeable membrane)

Advantages of organic electrosynthesis

• The reaction selectivity can be controlled with the potential applied at the working electrode. Meaning the selection of one electrophore as opposed to another with a similar structure can be achieved. Unlike with the use of redox reagents, the potential can be modified at will
• The reaction rate can be controlled by adjusting the current density or applied potential
• As in classical organic chemistry, the degree of transformation of a molecule (regarding its oxidation state) can be controlled by managing the number of electrons supplied
• The nature of the electrode and the composition of the electrolyte can be used as reaction parameters to control selectivity and reaction rate
• The electrosynthetic experimental conditions and pathway can be predicted
• The reaction conditions are typically mild since in general, electrolysis is performed at room temperature and atmospheric pressure

Electrochemistry as a “green” application

If we consider that our reaction is performed with the addition or removal of electrons it’s easy to consider that our reactions will be REDOX – reductions or oxidations. These reactions often use hazardous and polluting reducing or oxidizing reagents such as OsO4, Pb(OAc)4, NaH etc.; by eliminating the need of these we can consider electrochemistry to be a ‘greener’ method.

Our chemistry is mediated by electrons, so we often perform our chemistry as room temperature reducing energy consumption. We can also consider our electrodes as heterogeneous catalysts allowing less waste in the work-up of our reactions. Furthermore, ionic liquids are often used replacing organic solvents which are readily recovered.

The development of continuous flow electrochemical cells enables us to access this greener method and perform unique transformations enabling selectivity and transformations not possible by other techniques.

Continuous electrochemistry in flow

Traditional methods of electrochemistry involve the use of ‘beaker cells’. Typically, in electrochemical literature, the precise description of this apparatus is not properly described. While electrode material is given, the geometries, positioning, and dimensions are not giving rise to difficulties in reproducing experimentation. The development of continuous flow electrochemical equipment eliminates a lot of the problems with reproducing experimental procedures and confines variables to the electrode material and the synthetic procedure.

Electrochemistry is a surface phenomenon meaning large surface area to volume ratios are required. This is something we know a lot about if we are familiar with flow chemistry in general. Flow reactors have a large surface area to volume ratio compared to equivalent volume batch reactors. It’s not a big leap to design a flow electrochemical reactor that creates a high surface area to volume ratio in respect to the electrode.

One thing to consider with traditional methods of electrochemistry is that the electrodes are separated by a larger distance compared with there flow equivalent. A large distance between electrodes gives rise to loss of control of the charge passing between them resulting in ‘electron gradients’. The diagram below illustrates this with a simple oxidation. A range of electrons in our reaction can give rise to a loss in selectivity.

In a flow electrochemical cell, the distance between the electrodes is drastically reduced. By reducing the distance between the electrodes, we can precisely control the electrons equivalents in our reaction and therefore increase selectivity.