The definition is in its name: organic synthesis simply means making organic compounds from scratch in laboratories or industries.
Having learned all of the theories behind Organic Chemistry, you may wonder, how do you actually use them in real life? Well, organic synthesis is an area where the principles of organic chemistry are applied. It is a topic that requires you to use what you’ve learned in organic chemistry and work out a solution. By the end of this series you should be able to connect the dots between the different principles you have covered in the previous explanations on organic chemistry, and truly understand how the different functional groups interrelate with each other.
Importance of organic synthesis
As chemistry students, you may no doubt ponder the importance of organic synthesis. You might ask yourselves, why is it that scientists are working so hard in figuring out how organic compounds can be made?
You should already know that most organic compounds come from living things. For example, ethanol comes from the fermentation of biomass. Ethanol is an example of a simple and abundant organic compound, but there are more complex organic compounds that are important but less common in nature.
One good example is the drug aspirin. Aspirin originates from willow bark. However, extracting aspirin from it is too time-consuming and wasteful, as willow bark only contains very small amounts of aspirin.
As such, scientists have developed steps to synthesise aspirin from laboratory compounds such as salicylic acid. That way aspirin can be produced with a high yield and low cost.
Therefore, one can say that the key importance of organic synthesis is to produce organic compounds (for example drugs and pesticides) efficiently.
Fig. 1 - Aspirin synthesis
Organic synthesis flow chart
Before we discuss the organic synthesis flowchart, let’s cover a few key terms relating to organic synthesis.
The molecule to be synthesised in organic synthesis is called the target compound. The target compound is usually derived from a ‘scaffold’, known as the starting material. As you can see from the aspirin example above, aspirin is the target compound whereas salicylic acid is the starting material.
Organic synthesis of the target compound, therefore, involves figuring out steps to convert the starting molecule to the target compound. To achieve this, functional groups of both the target group and starting molecule are identified. Once that is done, you then figure out the steps to convert the functional group of the starting molecule to that of the target molecule. You can use the flowchart detailed below to help you identify the relevant steps.
Fig. 2 - Organic synthesis routes
The organic synthesis route can be either in one step or it might require multiple steps. A step corresponds to a single reaction.
Of course, you may identify multiple possible steps to convert from the functional group of the starting molecule to that of the target molecule. In such instances, a key rule is to keep the number of steps as small as possible to maximise the product yield.
Furthermore, when deciding on the most appropriate reaction step(s), do not forget to consider the reagents and conditions relating to the step(s) you have chosen. Whether the reagent is oxidising, reducing, or dehydrating, or whether heat and or a catalyst is required - these are the questions you should ask yourselves when deciding on the most appropriate reaction steps. Scientists would also consider which molecules or steps are cheaper, safer, and produce fewer waste products and a higher yield, to keep the costs and the risks as low as possible.
Another option to decide how to produce a certain compound is to start from the final compound and retrace the steps needed to produce it from other (more common, cheaper, safer) molecules, as the starting material. This is called retrosynthesis.
Retrosynthesis is the process of coming up with a synthesis pathway starting from the end molecule (target molecule) instead of the initial one.
Organic synthesis examples
The two organic synthesis examples covered in the AQA syllabus include the synthesis of propanoic acid from 1-bromopropane and propylamine from ethene. Please refer to Synthetic Routes for a detailed breakdown and explanation of each synthetic step.
Mapping the synthesis of propanoic acid from 1-bromopropane
Below is a diagram showing how the synthesis of propanoic acid from 1-bromopropane is mapped.
Fig. 3 - The synthesis of propanoic acid from 1-bromopropane
How do you map a synthetic route?
These are the steps to follow when mapping a synthetic route:
From the functional group interconversion flowchart above, list out the possible molecules that can be made from the starting molecule and the molecules that can be converted into the target molecule.
Identify any common intermediates between the starting material and the target. In this case, propan-1-ol is the intermediate.
List out the reaction steps. In the case of the synthesis of propanoic acid from 1-bromopropane, it entails a two-step reaction that is as follows:
1-bromopropane → propan-1-ol → propanoic acid
Mapping the synthesis of propylamine from ethene
Likewise, below is a diagram showing how the synthesis of propylamine from ethene is mapped.
Fig. 4 - Synthesis of propylamine from ethene
The synthesis of propylamine from ethene is slightly more complex. The steps relating to its mapping are listed as follows:
From the functional group interconversion flowchart above, list out the possible molecules that can be made from the starting molecule and the molecules that can be converted into the target molecule.
Identify any common intermediates between the starting material and the target.
In this case, there is no common intermediate. As such, you need to determine whether any molecules that can be made from the starting molecule can be converted into one of the molecules that is derived from the target molecule. For the above case, a haloalkane can be converted into propanenitrile.
List out the reaction steps. In the case of the synthesis of propylamine from ethene, it entails a three-step reaction that is as follows:
ethene → haloalkane → propanenitrile → propylamine
The development of the olefin metathesis method was an innovation in the field of organic synthesis. This method comprises a diverse set of reactions in forming and rearranging double bonds so that the side groups linked by them can be exchanged between two molecules. This method involves metal catalysts. The researchers who discovered it; Dr. Yves Chauvin, Professor Robert H. Grubbs, and Professor Richard R. Schrock, were granted the Nobel Prize for chemistry in 2005.
Fig. 5 - Metathesis method
Organic Synthesis - Key takeaways
- Organic synthesis is the process of making organic compounds from scratch in laboratories or industries. The key importance of organic synthesis is to produce organic compounds efficiently.
- The key players in organic synthesis include the starting material and the target compound. Steps (reactions) to reach the target compound need to be planned out taking into account the interconvertability of the groups of the molecules involved.
- To map the synthesis of a starting material into a target compound, the reaction intermediates are identified, then the reaction steps are written out in consideration of the necessary reagents and conditions.
- The synthesis map can be planned either from the starting material or from the target compound (retrosynthesis).
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