24 Aromatic Anion Radicals as Bronsted Bases
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Correlation of Protonation Rates with Singlet Energies of the Precursor Aromatic Hydrocarbon* H A I M L E V A N O N , P. N E T A , and A. M . T R O Z Z O L O R a d i a t i o n L a b o r a t o r y a n d D e p a r t m e n t of C h e m i s t r y , U n i v e r s i t y of N o t r e D a m e , N o t r e D a m e , IN 46556
One o f the aims o f p h y s i c a l o r g a n i c c h e m i s t r y is to d e r i v e g e n e r a l i z a t i o n s regarding structure-reactivity r e l a t i o n s h i p s which can be r e l a t e d t o the f u n d a mental electronic p r o p e r t i e s of a molecule. In t h i s paper a r e d e s c r i b e d experiments and i n t e r p r e t a t i o n s of t h e s e experiments which seem t o i n d i c a t e t h a t t h e r e is a fundamental correlation between the singlet energy o f an a r o m a t i c h y d r o c a r b o n and the r a t e o f p r o t o n a t i o n o f its a n i o n radical. A n i o n radicals o f a r o m a t i c hydrocarbons a r e known t o undergo p r o t o n a t i o n in protic media (1-5). The spectrophotometric pulse radiolysis t e c h n i q u e has been used t o demonstrate this p r o c e s s in alcoholic s o l u t i o n s o f s e v e r a l a r o m a t i c compounds (2). In such experiments, the a n i o n radicals a r e produced by the r e a c t i o n of the hydrocarbon w i t h the s o l v a t e d e l e c t r o n
P r o t o n a t i o n can t a k e p l a c e by r e a c t i o n o f the a n i o n radical w i t h the s o l v e n t m o l e c u l e
The a d d i t i o n a l p r o t o n a t i o n p r o c e s s is the r e a c t i o n o f År w i t h ROH+2 produced i n the p u l s e -
which is negligible a t low dose r a t e s . Dorfman and coworkers (2) found t h a t the r a t e s of r e a c t i o n 2 were s t r o n g l y dependent on the acidity of the s o l v e n t , e.g., i-PrOH10 M " s " , and i n most c a s e s i t i s d i f f u s i o n c o n t r o l l e d . Under the e x p e r i m e n t a l c o n d i t i o n s used, t h i s p r o c e s s was complete l o n g b e f o r e the p r o t o n a t i o n took p l a c e . The a n i o n r a d i c a l s can p r o t o n a t e by r e a c t i o n w i t h the s o l v e n t m o l e c u l e ( r e a c t i o n 2) and by the s m a l l amount o f a c i d produced by the r a d i a t i o n p u l s e ( r e a c t i o n 3 ) . The l a t t e r r e a c t i o n was m i n i m i z e d by use o f v e r y low doses, p r o d u c i n g o n l y 1 - 2 χ 1 0 ~ M o f ROH2. However, when the r a t e o f r e a c t i o n 2 becomes lower than ^ 1 0 s the c o n t r i b u t i o n o f r e a c t i o n 3 cannot be n e g l e c t e d . I t was, t h e r e f o r e , n e c e s s a r y t o overcome t h i s c o m p l i c a t i o n by a d d i n g a s m a l l amount o f base. F o r t h i s purpose, sodium m e t a l was d i s s o l v e d i n 2-propanol p r i o r t o the experiment t o produce a s o l u t i o n o f ( C H 3 ) C H O " N a i n the r e q u i r e d c o n c e n t r a t i o n . S i n c e the c o n c e n t r a t i o n o f t h i s base was o n l y i n the mM range i t was n o t e x p e c t e d t o a f f e c t the r a t e o f r e a c t i o n 2. Experiments have shown t h a t the a d d i t i o n #
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Pryor; Organic Free Radicals ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
LEVANON ET
24.
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Anion
405
Radicals
o f base has no e f f e c t on the r a t e s o f p r o t o n a t i o n when they a r e h i g h e r than 1 0 s " . A small decrease i n r a t e was n o t i c e d upon a d d i t i o n o f base when ^2 s i n t h e range o f 1 0 - 1 0 s ~ as e x p e c t e d . The use o f base has another e f f e c t on t h i s system. The r a d i c a l from 2-propanol, (CH3) ÔOH, d i s s o c i a t e s i n t o ( C H ) 6 o " w i t h a m i d - p o i n t a t 7xlO" M of ( C H ) C H O - a ( 8 ) . 4
w
a
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N
(CH ) CHCT
(CH ) COH + 3
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J
(CH^CO" +
(CH^CHOH
(4)
The b a s i c form o f t h i s r a d i c a l i s known t o be a s t r o n g e r r e d u c i n g agent than the n e u t r a l form (9_) . Thus, i t was found t o reduce the b e t t e r e l e c t r o n a c c e p t o r s among the compounds used i n the p r e s e n t s t u d y . In such c a s e s , the i n i t i a l r a p i d f o r m a t i o n o f A r " by r e a c t i o n 1 i s f o l l o w e d by a slower i n c r e a s e i n a b s o r p t i o n due t o r e a c t i o n 5 Ar +
(CH ) CO~ + Âr" + 3
2
(CH ) CO 3
2
(5)
Rate c o n s t a n t s f o r t h i s e l e c t r o n t r a n s f e r p r o c e s s were determined from measurements o f the r a t e o f f o r m a t i o n a t v a r i o u s c o n c e n t r a t i o n s o f A r and the r e s u l t s a r e g i v e n i n T a b l e I a l o n g w i t h the r a t e s o f p r o t o n a t i o n . Discussion The r e a c t i v i t i e s o f a r o m a t i c compounds have been c o r r e l a t e d w i t h v a r i o u s p h y s i c a l parameters, such as i o n i z a t i o n potentials, t r i p l e t energies, free valence, and energy l e v e l s o f l o w e s t u n o c c u p i e d o r b i t a l s (10,11). I t was noted t h a t s i m p l i f i e d HMO c a l c u l a t i o n s were s u f f i c i e n t t o account f o r many e x p e r i m e n t a l o b s e r v a tions. However, the r e a c t i v i t i e s o f a r o m a t i c a n i o n r a d i c a l s toward p r o t o n donors have not been s u c c e s s f u l l y c o r r e l a t e d w i t h any p h y s i c a l parameter. Such a c o r r e l a t i o n may shed l i g h t on the mechanism and a l l o w p r e d i c t i o n o f unknown r a t e s . In the e a r l y work on a r o m a t i c a n i o n r a d i c a l s , P a u l , L i p k i n , and Weissman (1) c o r r e l a t e d the r e a c t i v i t i e s of these r a d i c a l s with e l e c t r o n a f f i n i t i e s d e r i v e d from t h e i r s p e c t r a . Bank and B o c k r a t h (£) compared the r a t e s o f p r o t o n a t i o n o f naphthalene and a n t h r a c e n e a n i o n r a d i c a l s and c o n c l u d e d t h a t the h i g h e r r a t e f o r naphthalene was i n c o n t r a d i c t i o n t o t h e i r p r e d i c t i o n from m o l e c u l a r o r b i t a l l o c a l i z a t i o n energy c a l c u l a t i o n s . F r y and S c h u e t t e n b e r g (5) c o r r e l a t e d the r a t e s o f p r o t o n a t i o n o f v a r i o u s a r o m a t i c
Pryor; Organic Free Radicals ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
ORGANIC
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RADICALS
anion r a d i c a l s with the c a l c u l a t e d e l e c t r o n d e n s i t i e s a t t h e p o s i t i o n o f h i g h e s t e l e c t r o n d e n s i t y i n each o f these r a d i c a l s . T h i s c o r r e l a t i o n a l s o appears un satisfactory. I t seems t h a t parameters d e r i v e d from s i m p l e MO c a l c u l a t i o n s a r e n o t adequate t o e x p l a i n t h e r e a c t i v i t y of anion r a d i c a l s . In even a l t e r n a n t a r o m a t i c hydrocarbons, t h e d i f f e r e n c e i n energy between t h e l o w e s t u n o c c u p i e d m o l e c u l a r o r b i t a l (LUMO) and t h e h i g h e s t o c c u p i e d one (HOMO) i s e s s e n t i a l l y e q u a l t o t h e s i n g l e t energy d i f f e r e n c e Δ Ε . . The u n p a i r e d e l e c t r o n i n t h e a n i o n r a d i c a l o c c f i p i e s t h e LUMO. Upon p r o t o n a t i o n o f t h i s r a d i c a l an odd a l t e r n a n t s t r u c t u r e i s formed i n which t h e u n p a i r e d e l e c t r o n o c c u p i e s t h e non-bonding orbital. The e x o t h e r m i c i t y upon p r o t o n a t i o n i s , t h e r e f o r e , ^ Δ Ε . . I f t h i s energy change a f f e c t s the r a t e o f protonaÇion one would e x p e c t a c o r r e l a t i o n between t h e s e r a t e s and Δ Ε . F i g . 1 shows t h a t a 1 ο p l o t of log k vs A E ^ gives a reasonable l i n e a r dependence. D e v i a t i o n s ?rom l i n e a r i t y appear t o be r e l a t i v e l y l a r g e r when t h e r a t e s o f p r o t o n a t i o n a r e low, p r o b a b l y because t h e lower r a t e s a r e more s u s c e p t i b l e t o experimental complications, p a r t i c u l a r l y impurity e f f e c t s . I t s h o u l d be noted t h a t t h e p r o t o n a t i o n r a t e s determined by F r y and S c h u e t t e n b e r g (5) a l s o g i v e a r e a s o n a b l e s t r a i g h t l i n e when we p l o t them a g a i n s t Δ Ε ^ . As e x p e c t e d f o r a l t e r n a n t hydro c a r b o n s , linearièy i s a l s o o b t a i n e d when l o g k i s p l o t t e d versus the i o n i z a t i o n p o t e n t i a l I (plot not shown, see v a l u e s i n T a b l e I ) . I n p r i n c i p l e , k c a n be c o r r e l a t e d a l s o w i t h e l e c t r o n a f f i n i t y (12) o r w i t h t h e p o l a r o g r a p h i c r e d u c t i o n half-wave p o t e n t i a l , b u t t h e l i t e r a t u r e v a l u e s f o r t h e s e parameters a r e w i d e l y s c a t t e r e d . The m o l e c u l a r dimensions seem t o have l i t t l e e f f e c t on k . N o n - a l t e r n a n t hydrocarbons do n o t appear t o f i t e a s i l y i n t o t h e l i n e a r r e l a t i o n w i t h t h e s i n g l e t sepa r a t i o n ( F i g . 1 ) . A l s o , they do n o t f i t on t h e l i n e of l o g k versus i o n i z a t i o n p o t e n t i a l . F o r example, the i o n i z a t i o n p o t e n t i a l o f a z u l e n e i s s i m i l a r t o t h a t o f anthracene b u t t h e r a t e o f p r o t o n a t i o n o f t h e forme r i s f o u r o r d e r s o'f magnitude slower (see T a b l e I) , which i s i n l i n e w i t h t h e l a r g e d i f f e r e n c e between their respective Δ Ε _ v a l u e s . The same c o n s i d e r a r*" o t i o n s h o l d a l s o f o r f l u o r a n t h e n e and a c e n a p h t h y l e n e . Because o f s p a r s e e x p e r i m e n t a l d a t a t h e a p p a r e n t f i t o f t h e s e l a t t e r compounds w i t h t h e l i n e i n F i g . 1 s h o u l d be t r e a t e d c a u t i o u s l y . These f i n d i n g s i n d i c a t e 3
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Pryor; Organic Free Radicals ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
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Figure 1. Correlation singlet separation ( Δ Ε
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of protonation rate constants (k ) with singlet«_ J. The compounds are identified by the num bers as given in Table I. 2
Λ1
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Pryor; Organic Free Radicals ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
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the r e l a t i o n between i o n i z a t i o n p o t e n t i a l and s i n g l e t s i n g l e t s e p a r a t i o n does n o t h o l d f o r n o n - a l t e r n a n t hydrocarbons as d i s c u s s e d i n d e t a i l by M i c h l and T h u l s t r u p (L3) f o r t h e case o f a z u l e n e . The p r e s e n t study shows t h a t t h e r a t e o f p r o t o n a t i o n o f an a r o m a t i c a n i o n r a d i c a l c a n be c o r r e l a t e d w i t h t h e energy l e v e l (LUMO) o f t h e u n p a i r e d electron. T h i s f i n d i n g s u g g e s t s , on t h e b a s i s o f Hammond's p o s t u l a t e (14), t h a t t h e t r a n s i t i o n s t a t e i n t h e p r o t o n a t i o n p r o c e s s resembles t h e a n i o n r a d i c a l more than i t resembles t h e p r o d u c t ( n e u t r a l r a d i c a l ) . The c o r r e l a t i o n d e s c r i b e d by F i g . 1 does n o t h o l d f o r b i p h e n y l and t h e t h r e e t e r p h e n y l s whose r a t e s o f p r o t o n a t i o n (2) d e v i a t e from t h e l i n e , most p r o b a b l y because o f n o n - p l a n a r i t y . The r a t e s o f r e d u c t i o n o f t h e a r o m a t i c compounds (Table I) , appear t o i n c r e a s e a s A E ^ and I d e c r e a s e , i n c o n t r a s t w i t h t h e b e h a v i o r o? k . T h i s t r e n d i s t o be e x p e c t e d s i n c e k5 r e f l e c t s t h e e l e c t r o n a f f i n i t y which d e c r e a s e s when I and Δ Ε increase. The v a l u e s o f k a l s o can be r e l a t e d ° t o t h e p o l a r o g r a p h i c h a l f - w a v e p o t e n t i a l s , E^, o f t h e a r o m a t i c h y d r o c a r b o n s and t h e g e n e r a l t r e n d i s , o f c o u r s e , a d e c r e a s e i n k when E ^ i s more n e g a t i v e . S
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*The r e s e a r c h d e s c r i b e d h e r e i n was supported by t h e D i v i s i o n o f B a s i c Energy S c i e n c e s o f t h e Department o f Energy. T h i s i s Document No. NDRL- 1839 from t h e N o t r e Dame R a d i a t i o n L a b o r a t o r y .
Literature 1. 2. 3. 4. 5. 6. 7. 8.
Cited
Paul, D.E., Lipkin, D. and Weissman, S.I., J. Am. Chem. S o c . (1956) 78, 116. See r e v i e w by Dorfman, L.M., A c c o u n t s Chem. R e s . (1970) 3, 224, and r e f e r e n c e s t h e r e i n . Levanon, H . and N e t a , P., Chem. P h y s . L e t t . (1977) 48, 345. Bank, S. and B o c k r a t h , B., J. Am. Chem. S o c . (1972) 94, 6076. F r y , A.J. and S c h u e t t e n b e r g , Α., J. O r g . Chem. (1974) 39, 2452. S h i d a , T . and Iwata, S . , J. Am. Chem. S o c . (1973) 95, 3473. S i m i c , Μ . , N e t a , P . and Hayon, Ε., J. P h y s . Chem. (1969) 73, 3794. N e t a , P . and Levanon, H., J. P h y s . Chem. (1977) 81, 0000.
Pryor; Organic Free Radicals ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
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see e . g . , Neta, P . , Adv. Phys. Org. Chem. (1976) 12, 223. Streitwieser, Α., "Molecular Orbital Theory for O r g a n i c C h e m i s t s , " W i l e y , New York (1961), Chapters 11 and 13. Salem, L., "The M o l e c u l a r Orbital Theory o f Conjugated Systems," Benjamin, New York (1966), Chapter 6. It was shown by Hush, N.S. and Pople, J.A., T r a n s .
13.
a r o m a t i c hydrocarbons t h e sum o f e l e c t r o n affinity and ionization potential i s constant. Michl J . and T h u l s t r u p , Ε.W., T e t r a h e d r o n (1976)
10. 11.
Faraday Soc. (1955) 51, 600, t h a t f o r a l t e r n a n t
32, 205. 14. Hammond, G . S . , J . Am. Chem. Soc. (1955) 77, 334. Received December 23, 1977.
Pryor; Organic Free Radicals ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
