Chemical Synthesis

Chemical Synthesis involves the production of complex chemical compounds from simpler ones. Industrial chemical synthesis largely considers 3 factors when evaluating these processes:

The Reaction Rate
The Yield
The impact on the Environment (Green Chemistry)

These factors are considered to ensure the synthesis is as economically and environmentally viable as possible.

Chemists aim to make as much money as possible by both maximising the reaction rate and the reaction yield. They try to do this while also minimising costs. Chemists are also now looking for greener methods of synthesis, while still maintaining the rate and yield of products.

In short, chemists want it all, they want:

An appreciable yield
A fast reaction
A cheap reaction
An environmentally friendly reaction

In reality this is quite hard to achieve. To get a good yield some environmental sacrifices might need to be made. Likewise to perform a ‘green’ reaction it may be more expensive and take longer. Therefore, compromises need to be made.

Key Terms

The Reaction Rate: a measure of the rate at which the products are generated. It is affected by the:

Concentration of reactants
Pressure
Temperature
Presence of catalysts
Surface area

Yield: a measure of the efficiency by which the reactants are converted to the product. It is also affected by the:

Concentration of reactants
Pressure
Temperature

Chemists ideally want a fast reaction with an appreciable yield. A good chemist will also want to do what is best for the environment.

For a general chemical reaction, a chemists can attempt to increase both the rate of reaction as well as the product yield by increasing the:

Concentration
Pressure
Temperature
Surface area
Use of a catalyst

However, whether a change in these factors derives the desired effect upon rate and yield depends upon two general things:

The nature of the reaction as either endothermic, or exothermic.
The number of gaseous particles on both the reactant and product side of the chemical reaction.

This creates a dilemma

For example, in an exothermic reaction a temperature increase would increase the rate of the reaction, but decrease the yield of the reaction as the reverse reaction is favoured. It can help to think of reactions like this:

If a reaction is exothermic heat is released from the system into the surrounding and feels hot to the touch.
If a reaction is endothermic heat is absorbed from the surroundings into the system and feels cool to the touch.
If a forward reaction is exothermic then the reverse reaction will be endothermic and vice versa
- If a reaction is exothermic and the temperature is increased the system will attempt to adjust itself in order to decrease the temperature, which is found by favouring the endothermic reaction (the cooler reaction). This opposes the formation of the products which reduces the yield of the reaction.
- If a reacton is endothermic and the temperature is increased the system will favour the forward reaction as it opposes the change imposed upon the system by having a cooling effect. This works in the favour of increasing the yield of reaction.

Like temperature, pressure can also create a dilemma. Say for example there is a reaction that has two particles on the reactant side and three particles on the product side of the equation:

If the pressure of the system was increased, the reaction would respond in such a way as to reduce the pressure, in this case it would favour the reverse reaction in attempt to reduce the pressure. Again, this would increase the reaction rate and reduce the yield.

As a result, in chemical synthesis a compromise may need to be made regarding the temperature and pressure used in the reaction. This may require a moderate reaction rate and/or a moderate yield.

Another point that needs to be considered is how expensive it is to run a reaction at high temperatures and pressures. The apparatus used needs to be able to sustain these demanding conditions, which can be quite costly.

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