A Mathematical Model for Concerted Proton Transfer in the Hydrolysis of Ethylphenylbenzaldehyde Acetals

Abstract

L a b o r a t o i r e de Chimie Organique Physique U n i v e r s i t e des Sciences e t Techniques du Languedoc Place Eugene B a t a i l l o n , 34060 MONTPELLIER Cedex. FRANCEThe h y d r o l y s i s o f seven e t h y l s u b s t i t u t e d -p h e n y l benzaldehyde a c e t a l s , 1, has -been examined i n some d e t a i l s I i t i s a g e n e r a l -a c i d c a t a l y z e d r e a c t i o n and t h e c a t a l y t i c c o n s t a n t s o f s i x b u f f e r a c i d s IpKa r a n g i n g from 2.6 t o 81, as w e l l as t h o % o f hydronium i o n and water have been measured . T h i s system has been chosen because i t i s p o s s i b l e t o m o d i f y t h e t y p e o f c a t a l y s i s i n changing t h e n a t u r e of t h e s u b s t i t u e n t x . A good B r i i n s t e d c o r r e l a t i o n i s o b t a i n e df o r a l l componds. t h e a c o e f f i c i e n t d e c r e a s i n g r e g ul a r l y from 0 . 7 t o 0. when t h e b a s i c i t y o f t h e l e av i n g group i n c r e a s e s . On t h e c o n t r a r y t h e Hammett p l o t s o b t a i n e d f o r each a c i d a r e bad, excepted those c o r r e s p o n d i n g t o hydronium i o n and water.On t h e b a s i s o f t h e e x p e r i m e n t a l r e s u l t s a model of c o n c e r t e d p r o t o n t r a n s f e r i s presented w h i c h extends and p u t s i n a m a t h e m a t i c a l form t h e p r e v i o u s models proposed by Jencks, Kreevoy and G u t h r i e . W i t h t h i s model wa can e x p l a i n most o f t h e e x p e r i m e n t a l r e s u l t s and demonstrate t h a t : il The h y d r o l y s i s of a g i v e n compound can b s e i t h e r a s p e c i f i c -a c i d o r a g e n e r a la c i d c a t a l y z e d r e a c t i o n depending upon t h e s t r e n g t h of t h e c a t a l y s t . F o r t h e above compounds t h e h y d r o l y s i s i s g e n e r a l -a c i d c a t a l y z e d i n t h e presence of t h e weak a c i d s and s p e c i f i c -a c i d c a t a l y z e d i n t h e presence o f t h e hydronium i o n except f o r X = p-NOZ f o r w h i c h t h e hydronium i o n c a t a l y z e d h y d r o l y s i s i s a l s o o f t h e g e n e r a l -a c i d t y p e .i i l The a B r t i n s t e d c o e f f i c i e n t i s n o t a measure of t h e '"degree o f p r o t o n t r a n s f e r i n t h e t r a i l s i t i o n s t a t e " b u t i s r a t h e r a measure o f t h e " e a s i n e s s o f cleavage o f t h e -C -O A r bond" .iiil I t i s p o s s i b l e t o p r e d i c t t h e k i n d of c a t a l y s i s w h i c h a r e a c t i o n w i l l be s u b m i tt e d t o . when we know t h e two r a t e c o n s t a n t s , ko and kH+. f o r t h e "spontaneous" and t h e hydro- -350 -