Difference Between Synthon and Synthetic Equivalent

The key difference between synthon and synthetic equivalent is that synthon is a moiety of a chemical compound that can be formed by a known synthetic process, whereas synthetic equivalent is a reagent that carries out the function of a synthon.

The terms synthon and synthetic equivalent come under the branch of retrosynthetic analysis. It is a technique that is useful for solving the problems that arise during the planning of an organic synthesis process. In this analysis technique, we need to transform a target molecule into a simple structure without the effect of the interaction of a reagent. Sometimes, we use the terms synthon and synthetic equivalent interchangeably, but they are two different components.

CONTENTS

1. Overview and Key Difference
2. What is Synthon 
3. What is Synthetic Equivalent
4. Side by Side Comparison – Synthon vs Synthetic Equivalent in Tabular Form
5. Summary

What is Synthon?

Synthon is a moiety of a chemical compound which can be formed by a known synthetic process. It is a hypothetical unit within a target chemical compound (organic compound). A synthon represents the potential starting reagent for the retroactive synthesis of that target molecule. The concept of synthon was developed by E.J.Corey in 1967. At that time, he used the term synthon to name a retrosynthetic fragmentation structure, but now we mostly use it to name synthetic building blocks.

Synthons are charged chemical moieties. But, in the synthesis process, we mainly use neutral forms because charged species can be potentially volatile synthons. If we consider an example, for the synthesis of phenylacetic acid, we can find two synthons when planning this synthesis process. The two synthons present in the phenylacetic acid molecule are carboxylic group or –COOH and electrophilic benzyl group or –PhCH₂⁺ group.

During this planning, we need to identify suitable synthetic equivalents as well. For this example of the synthesis of phenylacetic acid, the suitable synthetic equivalent for the carboxyl group is cyanide anion. For the –PhCH₂⁺ group, benzyl bromide is the appropriate synthon. Then the reaction steps for the two synthons are as follows:

PhCH₂Br + NaCN → PhCH₂CN + NaBr

PhCH₂CN + 2 H₂O → PhCH₂COOH + NH₃

We can categorize synthons as carbanionic synthons and carbocationic synthons. In the retrosynthesis technique, we usually break down bonds heterolytically (not homolytically), which forms carbanions and carbocations. These two forms are then available for the chemist to build up complicated organic structures.

What is Synthetic Equivalent?

Synthetic equivalent is a reagent that carries out the function of a synthon. Synthons are reacted with the corresponding synthetic equivalent to obtain the desired target molecule. For example, the synthetic equivalent for carboxylic acid group in the synthesis of phenylacetic acid is cyanide anion.

What is the Difference Between Synthon and Synthetic Equivalent?

The terms synthon and synthetic equivalent come under the branch of retrosynthetic analysis. The key difference between synthon and synthetic equivalent is that synthon is a moiety of a chemical compound that can be formed by a known synthetic process, whereas synthetic equivalent is a reagent that carries out the function of a synthon. That means; synthon is a part of a substrate molecule which we are going to change its structure in order to obtain the desired structure, while synthetic equivalent is the molecule that we need to react with the synthon in order to get the desired compound.

Below is summary of the difference between synthon and synthetic equivalent.

Summary – Synthon vs Synthetic Equivalent

The terms synthon and synthetic equivalent come under the branch of retrosynthetic analysis. The key difference between synthon and synthetic equivalent is that synthon is a moiety of a chemical compound that can be formed by a known synthetic process, whereas synthetic equivalent is a reagent that carries out the function of a synthon.

Reference:

1. “Synthon.” Wikipedia, Wikimedia Foundation, 27 Nov. 2019, Available here.
2. “Retrosynthetic Analysis.” Wikipedia, Wikimedia Foundation, 14 July 2019, Available here.

Image Courtesy:

1. “Retrosynthetic analysis of phenylacetic acid” (Public Domain) via Commons Wikimedia