The science (and some would say art) of building up useful organic molecules from simple, cheap and available starting materials.
For anything but the most simple syntheses, a logical process is used to design possible synthetic routes to a target molecule. For any one target there will be many possible routes, each with a different series of starting materials, chemical transformations and purification steps. Each route will have its own advantages and disadvantages, and a route that is perfectly suited to a laboratory environment (on a gram scale) will be totally useless in an industrial environment (on a ton scale).
Some factors that have to be considered when choosing a synthetic route include:
1. Starting Materials
For an industrial synthesis, where compounds are made on the scale of many tons, the starting materials must be cheap and readily available. Examples of good starting materials are those that are:
A synthetic chemist will sometimes go to great lengths to include a very cheap starting material in an industrial synthesis, even if it means introducing several extra steps.
In the research laboratory, where compounds are only produced on the scale of a few grams or kilograms for testing, these considerations are less important. More complex (and so more expensive) starting materials can be used in order to reduce development time.
2. Number of Steps and Yield
It is desirable for a synthesis to have as few steps as possible. It is rare for an organic reaction to convert all of the starting material to product (i.e. its yield is less than 100%), so at each stage some material is lost.
For this reason, the best syntheses have a small number of high-yielding steps. These are often based on well known, tried and tested chemistry.
This is a very important consideration for large-scale reactions. Reagents that are explosive, highly toxic, carcinogenic or otherwise dangerous are best avoided when planning a synthesis. More dangerous materials require more expensive equipment to ensure safe handling and containment, but more importantly pose more of a danger to researchers and plant workers in the event of an accident.
On a laboratory scale, however, more dangerous reagents are routinely used. As the amount of material involved is small, containment and clean-up are much easier in the event of an accident.
4. Environmental Considerations
Increasing pressure from the public and regulatory authorities has led chemical companies to put a lot more thought into the environmental impact of their activities. Important issues include:
- reducing the need to transport hazardous chemicals
- reducing the amount of waste released into the environment
- recycling as much material as possible (e.g. solvent reclamation)
- containment procedures in the event of an accident (e.g. chemical release or fire)
Several systems exist for the logical design of synthetic routes. One of the most important is the Disconnection Approach (or Synthon Approach).
But, for all the logic and rational thought that is involved in synthesis, it is still considered by many to be an art-form. The design of syntheses has not been automated with much success to date, and many examples of syntheses that can only be described as beautiful exist in the chemical literature.