Resolved isomers explained

Encyclopedia Article

Several herbicides on the market are referred to as resolved isomers.  The most recent examples are Dual II Magnum (metolachlor - Dual) and Outlook (dimethenamid - Frontier).   The concept of resolved isomers can be difficult to comprehend, but hopefully this article will clear up some of the confusion.   Isomers are compounds that have the same chemical formula, but the elements of the molecules are arranged slightly differently.  In the illustration below, each compund has a black center ball with side branches of blue, green and orange balls.  The balls represent different elements (hydrogen, carbon, nitrogen, etc.)  in a molecule.  At casual glance the molecules look identical, but the branches are arranged differently around the center of the molecule.  This can be seen by 'visually' rotating the molecule on the right so that the red ball and black center ball are in line with those of the molecule on the left.  When this is done, the blue ball of the molecule on the right projects backward whereas on the left molecule the blue ball projects forward.  Thus, while the 'chemical' formula of these two molecules are identical, they are different in structure.  A pair of compounds with this arrangement is referred to as a stereo isomer.  Some people like to make the analogy of a pair of gloves and isomers.  Both gloves in a pair have four fingers and a thumb, but a right-hand glove will only fit on the right hand.

So what does this have to do with herbicides?   Certain herbicides may be produced as two or more isomers during the manufacturing process.  Depending on the location of the chemical bonds resulting in the isomers, different isomers may bind more easily to the target site.  If one isomer does not bind as easily to the herbicide target site as the others, this isomer will not contribute as much to controlling weeds as the better binding isomer.  In some cases one isomer may not have any herbicidal activity.  In the latter case, if the two isomers were present at equal quantities in the herbicide product, only 50% of the active ingredient would actually kill weeds. 

Chemists have found ways to change the manufacturing process so that only the active isomer of the herbicide is produced.  By marketing a product that contains only the active isomer, a company can reduce the amount of product required for weed control.  The two latest examples of resolved isomer products are Dual II Magnum and Outlook.  The following table compares the amount of product in the old 'unresolved' product and the new 'resolved' isomer product.  Table 1 provides the active ingredient provided by the old and new products at equivalent rates for the two products on similar soil types (e.g. 1.5 pt Dual is recommended on coarse textured soils with less than 3% OM, whereas 1 pt of Dual II Magnum is recommended for this soil type). 

 

Table 1.  Relative rates of the resolved isomer products of dimethenamid and metolachlor.

Frontier rate (oz/acre)

dimethenamid 
(lb/acre)

Outlook rate 
(oz/A)

dimethenamid-P 
(lb/acre)

20 0.94 10 0.47
24 1.13 14 0.66
28 1.31 16 0.75
32 1.50 21 0.98
       

Dual II rate 
(pt/A)

metolachlor 
(lb/A)

Dual II Magnum 
(pt/A)

S-metolachlor
(lb/A)

1.5 1.46 1.00 0.96
2.0 1.95 1.33 1.27
2.5 2.44 1.67 1.59
3.0 2.92 2.00 1.91
       

In summary, the resolved isomer product is a refined formulation of the herbicide active ingredient.  In some situations the inactive isomer may not contribute to the performance of the herbicide, whereas in others in may simply have a lower level of activity than the active isomer.  By marketing the active isomer, less chemical is needed (an envirnomental benefit) and this may allow the manufacturer to reduce the cost of the product or allow increased performance of the product.

 

Prepared by Bob Hartzler, extension weed management specialist

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Origin: 
Iowa State Weed Science Online
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