What is the Difference Between Linear and Convergent Synthesis

Organic synthesis involves building complex carbon-based molecules, like those found in living things and plastics. Linear and convergent synthesis are two main approaches used in organic chemistry to create complex molecules.

What is the difference between linear and convergent synthesis? Linear synthesis is longer and less efficient whereas convergent synthesis is shorter and more efficient.

Key Areas Covered

1. What is Linear Synthesis
      – Definition, Features
2. What is Convergent Synthesis
      – Definition, Features 
3. Similarities Between Linear and Convergent Synthesis 
      – Outline of Common Features
4. Difference Between Linear and Convergent Synthesis
      – Comparison of Key Differences
5. FAQ: Linear and Convergent Synthesis
      – Answers to Frequently Asked Questions

Key Terms

Linear Synthesis, Convergent Synthesis

What is Linear Synthesis

Linear synthesis is a strategy for building molecules in a step-by-step manner. Linear synthesis starts with simpler molecules. The effectiveness of linear synthesis decreases as the complexity of the molecules increase. At each stage, the product from the previous reaction becomes the starting material for the next, with the molecule undergoing a transformation. This method is most efficient for creating molecules that don’t require a large number of transformations.

However, as the number of steps in linear synthesis increases, the overall product yield decreases. Linear synthesis involves different types of reactions such as substitution reactions (where atoms or groups are being replaced) and elimination reactions (where atoms or groups are removed). Here, the functional group is changed by these types of reactions, creating a new molecule. The choice of reaction depends on the specific functional groups present and the desired outcome.

There are many uses of linear synthesis. It can be used to produce simple, natural products and fine chemicals with different functions such as the chemicals used in dyes, flavors, and fragrances.

What is Convergent Synthesis

In convergent synthesis, many simpler molecules (synthons) are independently synthesized and then strategically combined to form the final product. Unlike linear synthesis, convergent synthesis suits much more complex molecules.

The mechanism of convergent synthesis includes the following steps.

1. Fragmentation – First the target molecule is divided into more simpler subunits called synthons. These synthons have functional groups.

1. Independent synthesis – Each synthon is independently synthesized through its own optimized pathway, which could be linear or even another convergent synthesis for particularly complex synthons.
2. Coupling – This is the final step of the convergent synthesis. Here synthons are strategically combined together to form the final complex molecule.

Improved yield, flexibility and access to complex molecules are some benefits of convergent synthesis.

Similarities Between Linear and Convergent Synthesis

1. Both methods rely on the concept of functional group transformations as the building blocks for constructing a molecule.
2. Both methods require careful planning.

Difference Between Linear and Convergent Synthesis

Definition

Linear synthesis is a step-by-step chemical assembly of a molecule starting from simpler compounds to build complexity sequentially. On the other hand, convergent synthesis is a strategy in organic chemistry where smaller fragments or intermediates are synthesized separately and then combined to form the final desired molecule in fewer steps.

Ideal for

Linear synthesis is ideal for simpler molecules with fewer transformations, while convergent synthesis is ideal for complex molecules with multiple functional groups.

Yield

In linear synthesis, yield often decreases significantly with each step, limiting the final amount of product. However, convergent synthesis minimizes yield loss by creating synthons independently, resulting in higher overall yield.

Planning

Linear synthesis involves relatively straightforward planning, focusing on the order of reactions. Meanwhile, convergent synthesis requires more complex planning, including fragmentation of the target molecule, synthon synthesis strategies, and coupling reactions.

Flexibility

While linear synthesis has limited flexibility, convergent synthesis has greater flexibility.

Conclusion

Linear synthesis and convergent synthesis are two strategies that make new molecules in organic chemistry. Linear synthesis is a step-by-step approach whereas convergent synthesis is a much more complex approach. This is the basic difference between linear and convergent synthesis.

FAQ:

1. What is the convergent reaction strategy?

The convergent reaction strategy involves synthesizing several simple fragments independently at first. In the next step, these fragments are strategically connected to form the target molecule. This allows for efficient assembly of complex molecules by focusing on separate, simpler components

2. What is an example of a divergent synthesis?

An example of divergent synthesis is the creation of dendrimers.  First, the process starts from a single molecule such as a hydrocarbon chain. By reactions, new monomers are attached to functional groups on the initial molecule. Hence, branches are formed. The same reaction used in step 2 can now be applied to each of the newly added functional groups. This creates another layer of branching on every branch from the previous generation. This process could be repeated to create highly complex molecules.

3. Why convergent synthesis is advantageous over linear approach?

The convergent synthesis is advantageous over the linear approach because of improved overall yield, increased efficiency, enhanced flexibility and due to purification advantages.

4. What is the longest linear sequence in synthesis?

The longest linear sequence (LLS) in synthesis refers to the highest number of consecutive reaction steps required to transform a starting material into the target molecule. The length will vary depending on the specific molecule being synthesized and the chosen reaction pathway.

5. How to calculate overall yield for convergent synthesis?

Look at all the individual pathways that come together in the convergent synthesis and identify the one with the most reaction steps (identify the longest linear sequence). Each reaction step in the LLS will have its own yield, typically expressed as a percentage. Convert each percentage yield into a decimal (by dividing by 100) and multiply all the decimal yields together. The final product of multiplying all the decimal yields will be a value between 0 and 1. Multiply this value by 100 to express the overall yield as a percentage.

Reference:

1. “Convergent synthesis.” Wikipedia. Wikipedia Foundation.

Image Courtesy:

1. “BiyouyanaginATotalSynthesis” By en:User:V8rik – en:Image:BiyouyanaginATotalSynthesis.png (CC BY-SA 3.0) via Commons Wikimedia