12 Photolysis of Alkyl Nitrites and Benzyl Nitrite at
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Low Concentrations—An Infrared Study BRUCE W. GAY,JR.,RICHARD C. NOONAN, PHILIP L. HANST, and JOSEPH J. BUFALINI Chemistry and Physics Laboratory, Environmental Protection Agency, Research Triangle Park, N.C. 27711 Abstract M e t h y l , e t h y l , p r o p y l , and benzyl nitrite were photolyzed at low c o n c e n t r a t i o n in air or oxygen with a n a l y s i s o f products by long path i n f r a r e d techniques. In all but the methyl nitrite p h o t o l y s i s the major product was a peroxyacyl nitrate type compound. The p h o t o l y s i s o f methyl nitrite gave nitric a c i d as the major n i t r o g e n c o n t a i n i n g product, with formaldehyde, carbon monoxide, and formic a c i d as carbon c o n t a i n i n g p r o d u c t s . The carbon balance in all systems was good. The n i t r o g e n balance became poorer as percentage of PAN-type product decreased and nitric a c i d i n c r e a s e d . The poor n i t r o g e n balance i s a s c r i b e d to nitric a c i d absorption on the r e a c t o r s u r f a c e .
Introduction Many of the interesting compounds that make up the urban smog are formed by photochemical reactions of compounds emitted into the atmosphere. The photooxidation of certain hydrocarbons in the presence of oxides of nitrogen forms a class of compounds that are strong oxidizing agents. The most abundant in this class of compounds found in the atmosphere is peroxyacetyl n i trate (PAN). Recent studies suggest that a well-reacted atmosphere, i . e . , stagnant atmosphere in late afternoon, contains PAN as the major nitrogen containing product QJ . Peroxy propionyl nitrate (PPN) a homolog of the PAN series has been observed in the atmosphere along with PAN but at lower concentrations (2J, (3). The presence of PAN type compounds is important. They are responsible for severe plant damage (2), (4) and concentrations in the parts per billion level range can cause eye irritation (5). Peroxybenzoyl nitrate (PBzN) has been shown to be a much stronger eye irritant than PAN by two orders of magnitude (5J. Severe eye irritation during low or moderate smog conditions may be a result of low concentrations of PBzN. A l though PBzN has not been detected in the urban atmosphere, i t has been observed as a product in laboratory studies of surrogate 132 Deitz; Removal of Trace Contaminants from the Air ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
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atmospheric mixes. I t i s p o s s i b l e t h a t an i n c r e a s e i n a r o m a t i c c o n t e n t i n a u t o f u e l c o u l d r e s u l t i n PBzN f o r m a t i o n and d e t e c tion. The s i m p l e s t homolog o f PAN t y p e compounds t h a t c o n t a i n s o n l y one c a r b o n p e r m o l e c u l e i s p e r o x y f o r m y l n i t r a t e ( P F N ) . U n t i l r e c e n t l y PFN h a d n e v e r been o b s e r v e d a l t h o u g h many h a d a t t e m p t e d t o s y n t h e s i z e t h i s compound. I t h a s been p o s t u l a t e d t h a t PFN does f o r m b u t q u i c k l y d i s s o c i a t e s t o n i t r i c a c i d and c a r b o n d i o x i d e v i a a s i x membered r i n g c o n f i g u r a t i o n ( 6 ) . Peroxyformyl n i t r a t e i s p r e s e n t l y under study i n t h i s l a b oratory. P r e l i m i n a r y r e s u l t s i n d i c a t e t h a t i t i s c r e a t e d when f o r m y l r a d i c a l s a r e p r o d u c e d i n one a t m o s p h e r e o f o x y g e n c o n t a i n i n g 30 p a r t s p e r m i l l i o n o f N 0 - The compound a p p e a r s t o decompose a t room t e m p e r a t u r e w i t h a h a l f l i f e o f a few m i n u t e s . PAN t y p e compounds f o r l a b o r a t o r y s t u d i e s h a v e been p r e p a r e d i n t h e p a s t b y v a r i o u s methods. S t e p h e n s (7) p h o t o l y z e d o l e f i n s i n t h e p r e s e n c e o f o x i d e s o f n i t r o g e n and o b t a i n e d PAN. The p h o t o l y s i s o f d i a c e t y l and n i t r o g e n d i o x i d e i n a i r p r o d u c e s PAN ( 8 ) . T u e s d a y (9) s y n t h e s i z e d PAN i n t h e d a r k r e a c t i o n o f a c e t a l d e h y d e w i t h N 0 and o z o n e . S t e p h e n s (10) p h o t o l y z e d a l k y l n i t r i t e s i n o x y g e n t o o b t a i n t h e compound. More r e c e n t l y Gay (11) p h o t o l y z e d c h l o r i n e i n t h e p r e s e n c e o f a c e t a l d e h y d e , NO^, and o x y g e n t o p r o d u c e PAN. T h i s s t u d y was u n d e r t a k e n t o i n v e s t i g a t e t h e p r o d u c t s o f p h o t o l y s i s o f a l k y l and b e n z y l n i t r i t e a t l o w c o n c e n t r a t i o n i n a photochemical r e a c t i o n c e l l with f o u r i e r transform i n f r a r e d e q u i p m e n t . From p r o d u c t f o r m a t i o n and r e a c t a n t disappearance, t h e mechanism and k i n e t i c s o f t h e r e a c t i o n were d e t e r m i n e d . 2
2
Experimental The i r r a d i a t i o n s o f t h e compounds were c a r r i e d o u t i n a l o n g c y l i n d r i c a l g l a s s chamber o f 6.18 m e t e r s l e n g t h a n d 0.31 meter diameter. The chamber was c o n s t r u c t e d b y c o n n e c t i n g f o u r b o r o s i l i c a t e C o r n i n g QVF (PS 12-60) p i p e s . Aluminum s p a c e r s 1.91 cm w i d e w i t h 0.32 cm t e f l o n g a s k e t s on e i t h e r s i d e were s a n d w i c h e d between t h e p i p e s e c t i o n s . T h r e e s u c h s p a c e r s h a v i n g i n j e c t i o n p o r t s t o i n t r o d u c e s a m p l e s i n t o t h e c e l l were u s e d . The chamber ends were c a p p e d w i t h 3.18 cm f l a t p l e x i g l a s e n d p l a t e s and 0.32 cm t e f l o n g a s k e t s . The i n t e r n a l volume o f t h e chamber was 460 l i t e r s . A l a r g e d i s p l a c e m e n t vacuum pump was c o n n e c t e d t o one end p l a t e w i t h a b a l l vacuum v a l v e t h r o u g h w h i c h t h e chamber c o u l d b e e v a l u a t e d t o a p r e s s u r e l e s s t h a n one t o r r . Leak r a t e o f t h e chamber was l e s s t h a n 50 t o r r p e r day. Samples were m i x e d and i n t r o d u c e d i n t o t h e c e l l t h r o u g h a g l a s s m a n i f o l d f r o m a g l a s s gas sample h a n d l i n g s y s t e m i n c l u d i n g m i x i n g b u l b s o f known v o l u m e , p r e s s u r e gauges o f 0-50 t o r r and 0 - 800 t o r r . M e a s u r e d amounts o f g a s e o u s m a t e r i a l were t r a n s f e r r e d f r o m t h e known volume b u l b s o f t h e gas h a n d l i n g s y s t e m i n t o t h e chamber v i a t h e m a n i f o l d and p o r t s i n t h e aluminum s p a c e r s . Surrounding
Deitz; Removal of Trace Contaminants from the Air ACS Symposium Series; American Chemical Society: Washington, DC, 1975.
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R E M O V A L OF
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e a c h o f t h e f o u r chamber s e c t i o n s were c y l i n d r i c a l aluminum l i g h t b a n k s i n two s e c t i o n s . Each s e c t i o n c o n t a i n e d e i g h t GE-F40 BLB b l a c k l i g h t lamps w i t h e n e r g y maxima o f 3660 X. R u n n i n g t h e l e n g t h o f e a c h chamber s e c t i o n were s i x t e e n lamps a r r a n g e d and c o n t r o l l e d t o g i v e u n i f o r m l i g h t i n t e n s i t y when a l l , o n e - h a l f , o r one q u a r t e r o f t h e lamps w e r e ^ t u r n e d on ( w i t h one q u a r t e r o f t h e lamps on k j ^ Q ) 0.17 m i n " ) . Lamp b a l l a s t s =
(GE-6G 1020 B) were mounted on a h e a t s i n k away f r o m t h e chamber. The l i g h t i n t e n s i t y , k ^ ^ was m e a s u r e d by t h e p h o t o l y s i s o f NO^ i n o x y g e n f r e e n i t r o g e n ( 9 ) . The chamber u s e d f o r t h e p h o t o c h e m i c a l r e a c t i o n s c o n t a i n e d w i t h i n i t an e i g h t m i r r o r o p t i c a l s y s t e m (12) . T h u s , t h e chamber s e r v e d a l s o as a l o n g p a t h m u l t i p l e r e f l e c t i o n i n f r a r e d a b s o r p t i o n c e l l c a p a b l e o f l e n g t h s o f 43n m e t e r s , where η i s an i n t e g e r between 1 and 15. In these experiments path l e n g t h s o f 172 and 344 m e t e r s were u s e d . P o t a s s i u m b r o m i d e e n t r a n c e and e x i t windows were u s e d on t h e c e l l . The i n f r a r e d i n s t r u m e n t was a D i g i l a b F o u r i e r T r a n s f o r m S p e c t r o m e t e r t h a t u s e s a s c a n n i n g M i c h e l s o n i n t e r f e r o m e t e r w i t h a KBr beam s p l i t t e r c o a t e d w i t h germanium. L i q u i d n i t r o g e n c o o l e d d e t e c t o r s were u s e d t o c o v e r t h e s p e c t r a l r e g i o n f r o m 700 t o 3400 cm . A more d e t a i l e d d i s c u s s i o n o f t h e o p t i c a l s y s t e m , i n t e r f e r o m e t e r and s p e c t r a l d a t a t e c h n i q u e s a r e g i v e n e l s e w h e r e ( 1 3 ) . The ab s o r p t i o n c o e f f i c i e n t s o f r e a c t a n t s and p r o d u c t s were m e a s u r e d i n most c a s e s u n d e r c o n d i t i o n s s i m i l a r t o t h o s e u s e d i n t h e ex p e r i m e n t , i . e . 750-760 t o r r t o t a l p r e s s u r e o f t a n k oxygen o r a i r , 30 C, low r e l a t i v e h u m i d i t y (
