Pesticide Chemistry in the 20th Century - ACS Publications - American

6 The Environmental Chemistry of Herbicides D O N A L D G. C R O S B Y

Downloaded by CORNELL UNIV on June 6, 2017 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0037.ch006

Department of Environmental Toxicology, University of California, Davis, Calif. 95616

One of the p r i n c i p a l problems i n discussing the environmental chemistry of herbicides lies i n deciding where to start and where to stop. As an initial o v e r s i m p l i f i c a t i o n , one could write Herbicides-->Nontoxic Inorganic Products and be close to the truth. However, the recent history of public and scientific concern over herbicide efficacy, t o x i c i t y , side -effects, and s i m i l a r issues requiresthat we consider at least some of the intermediate steps of that process. This consideration i s overdue, not only among herbicide chemists but p a r t i c u l a r l y among other s c i e n t i s t s and scienti fically-aware attorneys, public officials, managers, and even professors. Therefore, t h i s Chapter i s not so much directed toward "experts" as it is toward a more diverse and perhaps more critical audience. By "environment", I refer to the physical and chemical world which surrounds us. We usually tend to think of it in terms of "compartments"--atmosphere, s o i l (lithosphere), water (hydro sphere) and living plants and animals (biosphere)--although a moment's r e f l e c t i o n on soil microorganisms, airborne dust, or the clouds in the sky should tell us that t h i s categorization, too, i s oversimplified. However, the compartment concept does form a framework of chemistry by which our all-encompassing surroundings can be assigned some chemical c h a r a c t e r i s t i c s - - c h a r a c t e r i s t i c s which existed before, and exclusive of, man-made chemicals. For example, from the composition of the atmosphere, we may surmise that oxidations will represent an important group of reactions i n that compartment, ionic reactions such as nucleophilic displace ments should be especially important i n the hydrosphere, and so on. Unlike other pesticide groups such as the insecticides or fungicides, herbicides now encompass a very wide range of struc tural types ( F i g . 1). A l i p h a t i c , aromatic, and heterocyclic systems; a variety of common and less common functional groups 93

Plimmer et al.; Pesticide Chemistry in the 20th Century ACS Symposium Series; American Chemical Society: Washington, DC, 1977.


PESTICIDE C H E M I S T R Y IN T H E 2 0 T H

VIII Figure 1.

C!

Chemical structures of some important herbicides

CI 0CH C00C H 2

4

9

H^O

0CH C00H 2

CI

CI CI

C=CH Cf ^ ^ C O N H - C - C H , V)

S CH NHC-SH 3

CI

I CH,

~ CH NCS + 3

H S 2

XI Figure 2.

Typical dark reactions of 2,4-D butyl ester (III), pronamid (VI), and metham (XI)


CENTURY

6.

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Environmental

Chemistry

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Herbicides

95

i n c l u d i n g e s t e r s , a c i d s , a m i n e s , n i t r o compounds, a n d t h i o - a c i d s ; a continuum o f p o l a r i t i e s from w a t e r - s o l u b l e s a l t s t o hydrophobic h y d r o c a r b o n s — a l l seem t o s h a r e a common p r o p e r t y : reactivity. I t i s w i t h the c h e m i c a l consequences o f the i n t e n t i o n a l o r i n a d v e r t e n t i n t r o d u c t i o n o f the t w o — r e a c t i v e h e r b i c i d e s and t h e c h e m i c a l compartments o f t h e e n v i r o n m e n t — t h a t t h i s p a p e r w i l l deal.


Interactions D e s p i t e the c h e m i c a l d i v e r s i t y o f the s e v e r a l hundred s t r u c t u r e s r e p r e s e n t i n g h e r b i c i d a l a c t i v i t y , most r e a c t i o n s o f h e r b i c i d e s f a l l w i t h i n o n l y a l i m i t e d number o f m e c h a n i s t i c t y p e s : o x i d a t i o n , r e d u c t i o n , n u c l e o p h i l i c displacements (such as h y d r o l y s i s ) , e l i m i n a t i o n s , and a d d i t i o n s . "Herbicides", after a l l , are m o r e - o r - l e s s o r d i n a r y c h e m i c a l s , and t h e i r p r i n c i p a l t r a n s f o r m a t i o n s i n t h e e n v i r o n m e n t a r e f u n d a m e n t a l l y no d i f f e r e n t f r o m those i n l a b o r a t o r y glassware. Figure 2 i l l u s t r a t e s three t y p i c a l examples w h i c h have r e c e i v e d t h e i r s h a r e o f c l a s s i c a l l a b o r a t o r y study—the a l k a l i n e hydrolysis of a carboxylic ester (in this case, an e s t e r o f 2,4-dichlorophenoxyacetic a c i d , I X ) , the c y c l o a d d i t i o n o f a n a l c o h o l t o a n o l e f i n (as i n t h e a c e t y l e n e , V I ) , a n d t h e β-éliminâtion o f a d i t h i o c a r b a m a t e w h i c h p r o v i d e s t h e u s u a l s y n t h e t i c r o u t e t o a n i s o t h i o c y a n a t e ( c o n v e r s i o n o f a n N.Ndimethylcarbamic a c i d s a l t , XI, t o methyl isothiocyanate). Allow the s t a r t i n g m a t e r i a l s h e r b i c i d a l a c t i o n (which they have), g i v e them names s u c h a s "2,4-D e s t e r " o r " p r o n a m i d e " o r "Vapam", a n d l e t s o i l form the w a l l s o f an outdoor r e a c t i o n k e t t l e ; the r e a c t i o n s a n d p r o d u c t s r e m a i n t h e same. Generally these environmental reactions i n s o i l o r water p r o c e e d r a t h e r s l o w l y compared t o what we m i g h t be u s e d t o u n d e r the f o r c i n g c o n d i t i o n s o f the l a b o r a t o r y . For example, t h e h y d r o l y s i s o f h a l f t h e 2,4-D e s t e r i n n a t u r a l w a t e r r e q u i r e s 220 d a y s a t pH 6 ( 1 ) , a n d a p p r e c i a b l e c y c l i z a t i o n o f V I t a k e s 40 d a y s i n s o i l ( 2 ) . However, r e a c t t h e y do. As s e e n f r o m T a b l e I , c o m p a r i s o n o f t h e t r a n s f o r m a t i o n r a t e s o f a number o f common h e r b i c i d e s i n s t e r i l e a n d n o n s t e r i l e s o i l c l e a r l y show t h a t s u c h n o n b i o l o g i c a l r e a c t i o n s must be a t l e a s t a s i m p o r t a n t a s metabo l i s m i n b r i n g i n g a b o u t f u n d a m e n t a l e n v i r o n m e n t a l changes among h e r b i c i d e s when p r o v i d e d enough t i m e . Many o f t h e s e same r e a c t i o n s a r e m a r k e d l y a c c e l e r a t e d b y t h e e n e r g y o f s u n l i g h t ( 3 ) , a n d a number a r e u n e x p e c t e d l y r a p i d . F o r e x a m p l e , a f t e r t h e 2,4-D e s t e r s a r e h y d r o l y z e d b y w a t e r a n d l i g h t (1), the r e s u l t i n g a c i d undergoes o x i d a t i o n , r e d u c t i o n , and n u c l e o p h i l i c d i s p l a c e m e n t o f r i n g - c h l o r i n e s , a t ambient tempera t u r e s , w h i c h w o u l d be v e r y d i f f i c u l t t o p e r f o r m u n d e r o r d i n a r y (dark) l a b o r a t o r y c o n d i t i o n s ( 4 ) . B e s i d e s l i g h t , the d e g r a d a t i o n o f t h e s e phenoxy a c i d h e r b i c i d e s r e q u i r e s a t m o s p h e r i c o x y g e n , t h e h y d r o x i d e i o n n o r m a l l y p r e s e n t i n w a t e r (10 M a t n e u t r a l i t y ) , w a t e r , a n d some e x t r a c t a b l e s o u r c e o f h y d r o g e n ( F i g . 3) ( 5 ) . The 7


96


PESTICIDE C H E M I S T R Y IN T H E 2 0 T H C E N T U R Y

CI Figure 4.

CI

Metabolism and photodecomposition products of monuron (XHI)


6.

Environmental

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Table I.

Chemistry

97

D e g r a d a t i o n o f H e r b i c i d e s i n S t e r i l e and N o n - s t e r i l e Soil.

Sterilization Method

Herbicide


of Herbicides

Amiben ( e s t e r ) Amitrole Atrazine Bromoxynil Pronamid Dalapon Dichlobenil Diphenamid Diuron

Steam

R e l a t i v e Rate (sterile/non-sterile) 1/1 1/1

N a; N e t h y l e n e o x i d e KN3 Autoclave Steam Autoclave Autoclave Radiation Chloropicrin 3

1/1

1/10 1/1

Pesticide Chemistry in the 20th Century - ACS Publications - American

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