Chapter 2
Specific Phototransformation of Xenobiotic Compounds: Chlorobenzenes and Halophenols 1
2
1
1
Pierre Boule , Claude Guyon , Annie Tissot , and Jacques Lemaire
Downloaded by UNIV OF MICHIGAN ANN ARBOR on March 8, 2013 | http://pubs.acs.org Publication Date: December 8, 1987 | doi: 10.1021/bk-1987-0327.ch002
1
Laboratoire de Photochimie Moléculaire et Macromoléculaire, U.A. C.N.R.S. 433, University of Clermont II, B.P. 45, 63170 Aubière, France Rhône Poulenc Santé, Centre de Recherches de Vitry, 13, Quai J. Guesde, 94400 Vitry-sur-Seine, France 2
An original aspect of aqueous photochemistry rela ted to the anticipated fact that water would favour pho tochemical processes implying polarized or ionic excited states, is emphasized in the class of chlorobenzenes and halogenophenols. Such photoprocesses would be more spe cific than homolytic photodissociation leading to radi cals. In chlorobenzenes, a photohydrolysis mechanism is observed. The monochlorobenzene is quantitatively trans formed into phenol in aqueous solution, even in acidic media (1 CH -CO-CH -CO-CH
(acetylacetone)
> CH -CO-CO-CH
(biacetyl)
3
2 CH -CO" -
3
3
2
3
3
Acetylacetone i s one of the major products of the photolysis of ace tone (20). In the presence of chlorobenzene, the self-quenching r e action competes with the energy transfer and the r a d i c a l s induce the formation of by-products. This self-quenching reaction i s much redu ced with hexadeuterated acetone. Up to 0.1 m o l . l " , the major photoprod-uct from CD ~CO-CD i n water i s CD -CO-CO-CD . In the presence of chlorobenzene, the formation of biacetyl-D6 i s i n h i b i t e d through an energy transfer and the photohydrolysis i s s e n s i t i z e d . The active v i b r a t i o n mode associated with CD group favors the non-radiative t r a n s i t i o n T ^ S and shortens the i n t r i n s i c l i f e t i m e of the t r i p l e t state. This accounts f o r the lack of self-quenching reaction. Thus hexadeuterated acetone appears to be a more s p e c i f i c photosensitizer than acetone i n water (19). The s p e c i f i c i t y of the photohydrolysis allows to rule out a mechanism involving an homolytic s c i s s i o n of the C-Cl bond with form ation of r a d i c a l s . A d i r e c t photohydrolysis probably occurs from a polarized excited state. I t i s suggested t h i s excited state would be the f i r s t t r i p l e t state to account f o r the s e n s i t i z a t i o n and i n h i b i t i o n experiments ( 2 9 ) · 1
3
3
3
3
Q
In Photochemistry of Environmental Aquatic Systems; Zika, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
14
PHOTOCHEMISTRY OF ENVIRONMENTAL AQUATIC SYSTEMS
Downloaded by UNIV OF MICHIGAN ANN ARBOR on March 8, 2013 | http://pubs.acs.org Publication Date: December 8, 1987 | doi: 10.1021/bk-1987-0327.ch002
cf'
From the s e n s i t i z e d phosphorescence of b i a c e t y l , the quantum y i e l d of inter-system crossing (
