Chapter 3
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Mechanism of Phase-Transfer Catalysis: The Omega Phase Charles L. Liotta, Edward M. Burgess, Charles C. Ray, Elzie D. Black, and Barbara E. Fair School of Chemistry, Georgia Institute of Technology, Atlanta, GA 30332 The use of polyethers and quaternary salts as liquid-liquid and solid-liquid phase transfer catalysts has been well-documented in the literature.1,2,3 It has been shown that (1) the catalyst functions as a vehicle for transferring the anion of a metal salt from the aqueous or solid phase into the organic phase where reaction with an organic substrate ensues, (2) the rate of reaction is proportional to the concentration of the catalyst in the organic phase, 4 and (3) small quantities of water have a significant effect on the catalytic process.5,6,7 This Communication specifically addresses the role of cyclic polyethers as phase transfer catalysts and the influence of water with regard to the location of the catalyst. The rates of reaction of benzyl bromide and benzyl chloride with potassium cyanide were studied as a function of added water, in the presence and absence of 18-crown-6 (Equation 1). These heterogeneous reactions were carried out in toluene (50 mL) at 85°C and 25°C. (1)
The data for the reactions of potassium cyanide with benzyl halides at 85°C and 25°C are summarized in Tables I-III and graphical representations of these data are shown in Figures 1-3. The reactions carried out at 85°C were followed to 70% completion, while those at 25°C were followed to 50% completion. In general, excellent first-order kinetic plots were obtained. Each point on the graphs represents an average of at least three kinetic determinations. It is interesting to note that in the absence of added water (solidliquid phase transfer catalysis), the rates of benzyl halide disappearance were more accurately described by zero-order kinetics. The data in Tables I-III clearly indicate that the amount of added water had a marked effect on the rate of reaction. In the presence of 18-crown-6, addition of minute quantities of water caused a dramatic increase in rate. Beyond this maximum, as the quantity of 0097-6156/87/0326-0015$06.00/0 © 1987 American Chemical Society
In Phase-Transfer Catalysis; Starks, C.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
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16
PHASE-TRANSFER CATALYSIS
Table I. The Rate of Reaction of Benzyl Bromide with Potassium Cyanide at 85°C In the Presence and Absence of 18-Crown-6 as a Function of Added Hater
Volume of Water (mL)
k x l ( T sec
0 1 10 20 30 40 50
0.02 13.9 11.9 6.8 4.4 3.2 2.8
( a ) 1
c
( b )
k x l ( F sec
0.0 0.0 0.0 0.6 1.0 1.3 2.1
1
k
x l
cat °
s e c
0.02 13.9 11.9 6.2 3.4 1.9 0.7
a. C o n d i t i o n s : 0.05 mol b e n z y l bromide, 0.0025 mol 18-crown-6, 0.15 mol KBr, 0.15 mol KCN. b. 18-Crown-6 o m i t t e d . c. Zero o r d e r (M sec~^)
In Phase-Transfer Catalysis; Starks, C.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
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3.
LIOTTA ET A L .
17
The Omega Phase
Table II· The Rate of Reaction of Benzyl Chloride with Potassium Cyanide at 85°C In the Presence and Absence of 18-Crown-6 as a Function of Added Water
Water (mL)
0.0 0.36 0.50 1.0 2.0 10.0 15.0 20.0 30.0 40.0 50.0 75.0
kxlO
3
sec
3.2 9.2 9.4 11.6 14.7 10.2 6.9 5.8 5.0 3.9 4.2 4.8
1
C
kxl9
3
sec
0.0 0.0 0.0 0.0 0.0 0.0 0.7 1.3 1.7 1.9 2.5 3.2
1
k
.xlO
J
sec
3.2 9.2 9.4 11.6 14.7 10.2 6.2 4.5 3.3 2.0 1.7 1.6
a. C o n d i t i o n s : 0.05 mol b e n z y l c h l o r i d e , 0.010 mol 18-crown-6, 0.15 mol KC1, 0.15 mol KCN. b) 18-Crown-6 o m i t t e d . c) Zero o r d e r (M sec""*)
In Phase-Transfer Catalysis; Starks, C.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
Downloaded by UNIV OF MASSACHUSETTS AMHERST on September 7, 2015 | http://pubs.acs.org Publication Date: December 19, 1987 | doi: 10.1021/bk-1987-0326.ch003
18
PHASE-TRANSFER CATALYSIS
Table III· The Sate of Reaction of Benzyl Bromide with Potassium Cyanide at 25°C In the Presence of 18-Crown-6 as a Function of Added Water
Volume o f Water (mL)
( ) a
kxlO s e c "
0.0 0.5 1.2 1.4 1.5 1.6 1.75 2.0 2.5 3.0 3.5 4.0 6.0
2.06 1.71 2.54 2.84 9.40 5.10 5.34 5.12 3.36 3.50 3.26 2.20 1.49
b
a.
C o n d i t i o n s : 0.05 mol b e n z y l bromide, 0.010 mol 18-crown 6, 0.15 mol K B r , 0.15 mol KCN.
b.
Zero o r d e r (M sec""*)
In Phase-Transfer Catalysis; Starks, C.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
3.
LIOTTA ET
AL.
19
The Omega Phase 7
water i n c r e a s e d , the f i r s t o r d e r r a t e c o n s t a n t d e c r e a s e d . It is i m p o r t a n t to note t h a t i n the absence of crown, no r e a c t i o n took p l a c e d u r i n g the same time p e r i o d i n the presence of l e s s than c a . 10 mL of water. W i t h the a d d i t i o n of l a r g e r q u a n t i t i e s of w a t e r , h y d r o l y s i s of the b e n z y l h a l i d e s becomes a measurable p r o c e s s i n b o t h the presence and absence of crown. I n order to e l i m i n a t e t h i s u n i n t e r e s t i n g pathway from the a n a l y s i s k was d e f i n e d as the d i f f e r e n c e i n the r a t e c o n s t a n t s between the c r o w n - c a t a l y z e d r e a c t i o n s and those r e a c t i o n s w i t h no crown p r e s e n t . The v a l u e s f o r k a r e a l s o t a b u l a t e d i n Tables I and I I . The s t u d i e s conducted a t 25 C ( T a b l e I I I ) employed o n l y s m a l l q u a n t i t i e s of water where the u n c a t a l y z e d (absence of 18-crown-6) r e a c t i o n r a t e s were n e g l i g i b l y slow.
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c a t
I n order to address the r e l a t i o n s h i p between the r a t e p r o f i l e s ( F i g u r e s 1, 2 and 3) and the l o c a t i o n of the 18-crown-6, the f o l l o w i n g s e r i e s of experiments were conducted. 18-Crown-6 (0.004 mol) was d i s s o l v e d i n 10 mL of t o l u e n e c o n t a i n i n g a m i x t u r e of p o t a s s i u m c h l o r i d e and p o t a s s i u m c y a n i d e (0.027 mol e a c h ) . Varying q u a n t i t i e s of water were added. At each a d d i t i o n , the system was a l l o w e d to e q u i l i b r a t e a t room temperature and the q u a n t i t y of 18crown-6 i n the o r g a n i c phase was measured by c a p i l l a r y gas chromatography. The r e s u l t s are summarized i n T a b l e IV and a g r a p h i c a l r e p r e s e n t a t i o n i s shown i n F i g u r e 4. I n i t i a l l y , a l l the crown was p r e s e n t i n the t o l u e n e . A d d i t i o n of the s a l t m i x t u r e produced a s l i g h t decrease i n the amount of crown i n the o r g a n i c phase. Q u i t e s u r p r i s i n g l y , a d d i t i o n of minute q u a n t i t i e s of water ( t o t a l l i n g 0.08 mL) r e s u l t e d i n a 97% d e p l e t i o n of the c a t a l y s t from the o r g a n i c phase. That the 18-crown-6 was t r a n s l o c a t e d onto the s u r f a c e of the s a l t was proven by f i l t e r i n g the s o l i d s a l t , d r y i n g i t , and e x t r a c t i n g i t w i t h methylene c h l o r i d e . A l l of the 18-crown-6 was recovered. I t i s e v i d e n t from F i g u r e s 1-3 t h a t the g r e a t e s t c a t a l y t i c a c t i v i t y observed i n the r e a c t i o n of the b e n z y l h a l i d e s w i t h c y a n i d e i o n takes p l a c e at v e r y low c o n c e n t r a t i o n s of water. I n c o n j u n c t i o n w i t h t h i s , F i g u r e 4 i n d i c a t e s t h a t o n l y minute q u a n t i t i e s of the c a t a l y s t i s p r e s e n t i n the o r g a n i c phase (
