Have you ever wondered what is the difference between Metalaxyl and Metalaxyl-M?

Nov 28, 2022

Many customers were curious about the relationship and difference between Metalaxyl and Metalaxyl-M (also known as Mefenoxam). This article will reveal the mystery of these two fungicides based on their chemical structures and physical/chemical properties.

Due to their particular chemical structures, Metalaxyl and Metalaxyl-M are widely used as systemic fungicides to control plant diseases such as, Oomycetes Pythium, Phytophthora, and several of the downy mildews.

Metalaxyl and Metalaxyl-M are chemical compounds with a pair of different structures and same formulates co-existing together (Called chiral compounds, see example in Figure 1).

The two chiral compounds are S-Metalaxyl and R-Metalaxyl enantiomers based on their 3-D atom arrangement (see Figure 2). Metalaxyl contains S-Metalaxyl and R-Metalaxyl with nearly 1:1 ratio. Metalaxyl-M contains nearly pure R-Metalaxyl.

Figure 1. Example of Chiral Object

Figure 2. Structures of the two Metalaxyl enantiomers.

The chemical structures of the two compounds (enantiomers) are the same unless they have different space (spatial) arrangements. For S-Metalaxyl, the substituents are arranged clockwisely from the one with the highest priority (with the highest atomic number) to the one with the lowest priority (with lowest atomic number).

For the R-enantiomer, the arrangement order is counterclockwise. The spatial difference makes R-Metalaxyl more active in plant diseases control than either the S-Metalaxyl or the combination of the enantiomers. This is the reason that Metalaxyl-M is also known as high-efficiency Metalaxyl, because it retains primarily the R-Metalaxyl by using special synthetic methods.

Metalaxyl-M contributes to risk reduction for Metalaxyl. Comparing to Metalaxyl, Metalaxyl-M provides the same efficacy but at a lower application rate. This brings to farmers lower exposure risk and labor cost. Several studies indicate that different enantiomers of Metalaxyl had different degradation rates in different soils.

In the German soil, the degradation of the R-enantiomer was much faster than the S-enantiomer. In the Cameroonian soil, the degradation of the R-enantiomer was lower than the S-enantiomer.

These studies mark that different soil ecosystem may cause different degradation preferences for different structures. Lower application rate and lower degradation rate in some soils than Metalaxyl are the two main reasons that Metalaxyl-M is replacing Metalaxyl in some parts of the world.

Even though Metalaxyl and Metalaxyl-M’s chemical structures look the same, some of their properties have striking difference. The most obvious difference is their appearance. Metalaxyl exhibits as white powder, but Metalaxyl-M exhibits as yellow to brown liquid (See Figure 3).

Metalaxyl Tech - Heben
Metalaxyl-M (Mefenoxam) Tech - Heben Fungicide

Figure 3. Metalaxyl and   Metalaxyl-M Technical Appearance

Such distinct phases are caused by their disparate melting points. The melting points for Metalaxyl and Metalaxyl-M are 72.0℃, -38.7℃, respectively. How come their melting points have such a big difference as they are constructed by the same atoms and same chemical groups?

This is caused by the different arrangement of atoms in space for these two products. The Metalaxyl’s atom arrangement provides its stronger attraction (intermolecular forces) between atoms, which leads to a higher melting point and solid phase at room temperature.

The Metalaxyl-M’s atom arrangement provides its weaker attraction (intermolecular forces) between atoms, which leads to a lower melting point and liquid phase at room temperature.

Metalaxyl and Metalaxyl-M also have big difference in solubility, as shown in Chart 1. Metalaxyl-M is more soluble than Metalaxyl in water, but Metalaxyl is more soluble than Metalaxyl-M in organic solvent. This difference is caused by their different polarities due to different spatial arrangements. The solubility difference leads to different product application.

Solvent Metalaxyl Metalaxyl-M
Water 8.4 26
Benzene 550 59
Hexane 9.1 Miscible
Methanol 650 Miscible
Acetone Miscible Miscible

Chart 1. Solubility of Metalaxyl and Metalaxyl-M at 20℃

Overall, Metalaxyl and Metalaxyl-M are like identical twins share DNA code (chemical formula) to each other, but have different personalities (properties). Agronomists utilize each product’s advantages to apply them in different ways in order to better plant disease control and risk control.

Heben is one of the top Metalaxyl and Metalaxyl-M manufacturers. In Heben, here are many chemists and agronomists who have in-depth studies on these two products. If you have any question, feel free to contact