Principles of Green Chemistry

We have had a look at how we can manipulate reaction conditions to favour the formation of our products, however in this page we will look at how we can make these industrial processes align with green chemistry principles.

Green Chemistry is an approach to chemical synthesis based on twelve principles. It aims to minimise the use and creation of hazardous substances and wastes in chemical synthesis.

The 12 principles are:

They key principle to take away from this table is principle number 9, the ‘use of catalysts where possible’, more specifically the use of a specical type of biological catalyst called enzymes.

Enzymes

Enzymes are biological catalysts. Their surface shape is specifc to certain reactant molecules. This means that only specific molecules can interact with its surface. This interaction causes a reduction in the activation energy of reaction and consequently increases the rate of reaction.

The lowered activation energy pathway of a reaction provided by enzymes allows for a sufficient reaction rate at lower pressures and temperatures, making the process cheaper.

Another characteristic is the recyclability of catalysts, as they are not consumed in the reaction and can therefore be reused. However, due to their specificity they can only catalyse specific reactions. A great way to understand this characteristic of catalysts is to think of the enzyme – molecule interaction as a lock and key:

• Enzymes have an active site, this is the place where catalysis occurs. This active site has a distinct size and shape, much like locks:

• Due to the active site’s specificity, only reactant molecules of the correct shape, can successfully bind, through weak intermolecular forces. The interacting molecule is much like a key:

• Once the reactant molecule is contained within the enzyme, the reaction can proceed more easily at a lower activation energy.

• When there is a lower activation energy of a reaction, it means that there is a greater proportion of successful collisions between the reacting particles, therefore increasing the rate of reaction.

• Once the reaction has finished, the product particles are released, and the enzyme repeats this cycle with new reactants.

Here are some of the advantages and disadvantages of enzymes:

Enzymes - Temperature and pH

Enzymes are proteins and are therefore sensitive to temperature and pH, as exposure can alter the shape and size of their active site. Therefore, there is an optimal temperature and pH for enzymes.

For enzymes, optimal conditions are always at a relatively low temperature. As you slowly increase the temperature, the reaction rate will steadily increase from an increase in enzyme activity (from particles binding more frequently and with more kinetic energy). However, if you continue past the optimum temperature, the bonds, and intermolecular forces within the secondary and tertiary structure of the enzyme, will break down and the active site will lose its shape.

At this point, the enzyme becomes denatured (ineffective) as the reactants no longer have the same shape as the active site and cannot bind.

Much like temperature, enzymes can also denature due to pH changes. When enzymes are outside of their optimum pH range, the amino and carboxyl groups contained within the side chains of the enzyme will donate/accept protons (hydrogen ions). This alters the types of bonds and intermolecular forces within the enzyme and therefore alters its overall structure.

Therefore, at any pH outside of an enzyme’s small optimal pH range, the enzyme will be denatured, and the reactants can no longer bind to the active site.

You might be wondering, why enzymes?

Enzymes can be obtained from renewable sources and have the ability to catalyse various chemical reactions, for this reason enzymatic catalysis is intrinsically linked to green chemistry. They offer a promising, green alternative to classical chemical catalysis.