A Level Chemistry Revision: Organic Chemistry – Organic Synthesis

If you’re preparing for your A Level organic chemistry exam, you’ll need to have a good understanding of organic synthesis. This includes learning about the different types of synthesis and the methodology involved. Read on to find out more.

Organic synthesis: an introduction

The artificial synthesis of organic compounds is a specialised field of chemistry that focuses on the design and assembly of organic compounds. Synthetically-produced organic compounds are either derived from the reactions of existing organic compounds or the decomposition of a complex organic or biological compound.

Historically, chemists believed organic compounds could only be synthesised by living organisms rather than in a laboratory setting. However, in 1828 Friedrich Wöhler discovered that an organic compound, urea, could be synthesised from inorganic materials. This is widely considered to be the beginning of organic synthesis as a specialised field.

Beaker of urea pellets

Today, organic synthesis plays a key role in biological, medical, agricultural, and industrial applications. The process is used to synthesise products such as medicines, fertilisers and petroleum derivatives like plastics.

Research in organic synthesis tends to focus on several key areas total synthesis, semisynthesis, and methodology of synthesis. All of these fields of study have direct practical applications, such as in the synthesis of petrochemical derivatives.

Key takeaways

  • Organic synthesis is a field of chemistry that focuses on the design and creation of new organic products. It also looks at how existing organic compounds can be made from other compounds.
  • The process may involve multiple stages or just one step and can be linear or convergent.
  • Organic synthesis may also involve a retrosynthesis process. Akin to reverse engineering, scientists start with complex compounds and then work backwards to determine the initial compounds necessary to produce complex products.

    Scientist using a pipette for testing organic synthesis
    Organic synthesis focuses on the design and creation of new organic products

Total synthesis

Total synthesis can be divided into two main categories pure academic research and industrial applications. The former is primarily concerned with synthesising various types of organic compounds solely to study them. Meanwhile, industrial total synthesis typically focuses only on products that have practical or commercial applications and can be cost-effectively mass-produced.

Total synthesis applies to various types of naturally occurring organic products, which are either mimicked or converted into other products like terpenes, plant alkaloids, polyketides, and polyethers. Organic products are typically derived from living organisms such as plants, fungus, and marine organisms.

There are usually multiple steps involved in total synthesis, which may either be purely linear or convergent. Each reaction step modifies the previous compound(s) to bring it closer to the intended final compounds. Precursors, reagents, and catalysts may also be used along the way.

A convergent synthetic approach is often needed to synthesise more complex organic compounds or biomolecules. This shortens the time required and reduces the unnecessary intermediate products. It’s akin to putting different puzzle pieces together from two or more pre-assembled pieces. This approach also has a comparably higher product yield than the linear process.

Examples of total synthesis

The synthesis of the peptide hormones oxytocin and vasopressin in 1954 was one of the earliest examples of total synthesis. Chemical structure and formula for strychnine

In the same year, the father of modern organic synthesis, Robert Burns Woodward, worked on the synthesis of strychnine, which is an alkaloid used as a pesticide. He later won the 1965 Nobel Prize for Chemistry for his work on total synthesis. 

Other examples of total synthesis include the synthesis of polypeptides and polynucleotides.

Semisynthesis

As the name implies, semisynthesis or partial synthesis is not a complete set of steps. Nor is it about synthesising organic products from simpler precursors. Instead, the process starts with naturally occurring organic compounds that are isolated from living organisms such as bacteria and plants.

These compounds serve as the starting materials from which more complex compounds are produced. The substances created via semisynthesis are usually intended for medicinal applications. 

 The compounds created through semisynthesis have a higher molecular weight and/or more complex molecular structures than those produced from total synthesis. As the process involves fewer steps, the medicinal products derived from semisynthesis tend to be cheaper. Here’s an example of the semisynthesis of the anticancer drug, paclitaxel.

The structure of semisynthesis of paclitaxel

It starts from 10-deacetylbaccatin III, which is extracted from the evergreen tree commonly known as Japanese yew (Taxus cuspidata). The necessary sidechain is added through a short series of steps.

Methodology research

Just like many other processes in chemistry, organic synthesis should ideally be efficient and cost-effective. It’s therefore essential to research the best methodology.

Methodology research involves three main aspects discovery, optimisation, and the study of the scope and limitations.

  • Discovery finding the right chemical path, whether through total synthesis or semisynthesis, requires vast amounts of knowledge and expertise in chemical reactions and reagents. Precise measurements, calculations, and experimentations are necessary.
  • Optimisation it’s important to find the most efficient conditions when it comes to the type of starting compounds, temperature, reagents, reaction time, solvent, etc. It takes a lot of experimenting to find the most economic way to produce the desired product at high yield and purity.
  • Scope and limitations once the method is finalised, it can be tested with a broader range of starting materials. The scope and limitations of the method can then be calibrated.

Some organic synthesis methods and results are only appropriate for academic purposes, and might not be practical or economically viable for use in industry. Methods that are designed for industrial applications tend to focus on synthesising useful products such as plastic polymers.

For more organic chemistry revision guides, visit our A Level chemistry resources hub.

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