Journal of Molecular Catalysis, 31 (1985) 81 - 88 81

CATALYST POISONING AND SELECTIVITY CONSTANTS IN POLYETHYLENE GLYCOL CATALYZED PHASE TRANSFER CATALYSIS

RONNY NEUMANN and YOEL SASSON*

Casali Institute of Applied Chemistry, School of Applied Science and Technology, Hebrew University, Jerusalem 91904 (Israel)

(Received July 3, 1984)

Summary

Polyethylene glycol was found to undergo anionic ‘catalyst poisoning’ by lipophilic anions in a manner similar to that of quaternary ammonium salts, even though the interaction between alkali salts and the catalyst is generally considered to be influenced only by the cation. From reaction profiles which indicated catalyst poisoning, an exact kinetic equation de- scribing the conversion as a function of time could be used to compute selectivity constants, K,, by graphic iterations.

Introduction

The greater affinity of lipophilic anions as compared to hydrophilic anions to quaternary onium cations which are commonly used as phase transfer catalysts is a welldocumented phenomenon [l - 41. The different degree of lipophilicity of the counter-anion can cause what is usually called ‘catalyst poisoning’. For example, in a SN2 type reaction, (eqn. (1))

a leaving anion, X-, significantly more lipophilic than the attacking anion, Y-, will poison the catalyst after a certain conversion is obtained. Various aspects of this poisoning have been discussed, taking into account numerous parameters [5]. The catalyst poisoning is due in essence to the varying degrees of extractability of the different anions, and therefore it is common to define the relative extractability by a selectivity constant, K,, eqn. (2)

K = [QYlor,Wla, ’ [QXl,,,Wl,,

*Author to whom correspondence should be addressed.

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Selectivity constants smaller than unity show preference of the catalyst for the leaving anion and therefore catalyst poisoning can be expected.

For the second class of phase transfer catalysts, such as crown ethers, cryptates and polyethylene glycols, the selectivity of the cation-complexing catalyst towards various anions and consequently the ‘poisoning effect has yet to be studied. In fact, it is not obvious a priori that such studies should be necessary, since generally the interaction of these catalysts with metal salts is considered to be strictly cationic. Although Pederson [6] noted when first preparing crown ether-alkali salt complexes that the anion of the com- plexed salt had an effect on the degree of compiexation, these effects have never been quantified in a manner similar to that of quaternary ammonium salts, and the phenomenon of catalyst poisoning has never received serious attention.

In recent years PTC reactions catalyzed by polyethylene glycol (PEG) have become quite common [7], due to the attractiveness of this catalyst as a low-cost non-toxic substitute for crown ethers. In our investigations of this catalyst we have found that polyethylene glycol can be poisoned by lipo- philic anions in much the same way as quaternary onium salts. In this paper we wish to give some examples of this catalyst poisoning and present a novel method for the computation of selectivity constants, In this case the select- ivity constant, KS, is defined as follows (eqn. (3)), and was computed by use of kinetic me~urements.

K = [PEG-MYl,,,IMfX-la,

’ [PEG-MXl,,,[M+Y-1,~ (3)

M I= alkali metal

Results and discussion

Catalyst poisoning As stated previously, the effect of the anion on the extractability of

the various alkali salts by PEG is unknown in the literature. In order to gain some understanding of the anion effect, equimolar amounts of potassium salts and PEG-400 were mixed until equilibrium was reached and the amount of extracted (complexed) salt was measured (see Table 1). It is clear from the results that the extraction is anion-dependent; however, it is equally apparent that the lipophilicity of the counter-Zion is not of major impor- tance in the complexation. Hydrophilic anions such as fluoride and hy- droxide were well extracted, whereas iodide was extracted less efficiently than bromide. From these results it seemed possible that polyethylene glycol-catalyzed reactions using the hydroxide anion as base would not suffer from the severe catalyst poisoning found in the quaternary onium salt- catalyzed system. In order to test this hypothesis we used the Williamson ether synthesis and p-elimination reaction as reference reactions. Both have

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TABLE 1

Degree of complexation of potassium salts of polyethylene glycol

salt Complexation (mol%)

KOH 38.7 KF 24.3 KCI 15.8 KBr 38.3 KI 30.5

Time, hours

Fig. 1. Comparison of reaction profiles for Williamson ether synthesis using quaternary ammonium salts and polyethylene glycol as PT catalyst. Reaction conditions: 0.01 mol n-butanol, 0.01 mol n-butylbromide, 0.01 mol 50% NaOH and 0.001 mol catalyst at 60 “C. TBAHS = tetrabutylammoniumhydrogen sulphate.

been previously investigated using PEG as a catalyst, but no mention has been made of possible catalyst poisoning because an excess of base was used

[6,91. When mixing equimolar amounts of n-butanol, n-butyl bromide and

50% NaOH or 60% KOH at 60 “C, eqn. (4),

cat BuOH,,,,, + BuBr(,,,) + NaOH(,,, - BuOBu,,,,, + NaBr(,,, (4)

we found similar reaction profiles for the polyethylene glycol and qua- ternary ammonium salt-catalyzed reactions (see Fig. 1). In both cases the final conversion was similar and one can conclude that catalyst poisoning oc- curred. However we found this reaction unsuitable for further investigation, because it is known to proceed through n-butoxide which is formed in an equilibrium reaction between the hydroxide anion and n-butanol. We there- fore chose a simpler reaction, the p-elimination of /3-bromoethylbenzene, eqn. (5).

PEG PhCH,CH,Br(,,,, + NaOH,,,, -+ PhCH=CH,,,,,, + NaBrtaPj + HZ0 (5)

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30% NaOH

Time,minuies

Fig. 2. Reaction profiles for P-elimination of fl-bromoethylbenzene. Reaction conditions: 0.01 mol fl-bromoethylbenzene, 0.01 mol 25% or 30% NaOH, 0.001 mol PEG and 10 ml toluene; T = 70 “C, stirring speed 1200 rpm.

This reaction was found to be very fast even in dilute solutions (25 - 30%) of sodium hydroxide. The aqueous phase must be sufficiently dilute initially so that the water formed will not dilute it, which would reduce the reaction rate [lo]. Here again, mixing equimolar amounts of /3-bromoethylbenzene and 25% or 30% sodium hydroxide gave reaction profiles which indicated catalyst poisoning, Fig. 2. The reaction halted almost completely after 8 h.

Differential and integral analyses of the reaction profile showed no simple order for the reaction. Increasing the molar ratio NaOH@bromo- ethylbenzene to 5/l gave a reaction profile which corresponded to second order in the organic substrate. Such a reaction order cannot be explained by any known mechanism, and led us to believe that this result was a coin- cidence caused by a combination of catalyst poisoning partially offset by excess sodium hydroxide. It could be argued that the stalling of the reaction was due to some chemical decomposition or deactivation (e.g. chain degrada- tion) of the catalyst itself. However, an initial reaction mixture containing a ratio of sodium bromide and hydroxide such as found when the reaction stalls totally inhibited the reaction. Furthermore, addition of sodium hy- droxide to a stalled reaction caused it to restart. Therefore, it can be safely argued that the reaction comes to a standstill because of anionic catalyst poisoning. The sodium bromide released during the reaction is complexed to the PEG catalyst more efficiently than in the sodium hydroxide substrate.

Selectivity constant The concept of catalyst poisoning is a qualititative one. The selectivity

constants represent the relative extractability or complexation from an aqueous phase of alkali salts with the polyethylene glycol catalyst. The con- stant gives a quantitative measure of the concentrations of the complexes and aqueous ‘anions in the reaction system. Knowledge of the selectivity constant will enable prediction of catalyst poisoning and reaction profiles. We have used this principle in the reverse order, computing selectivity con- stants from measured reaction profiles.

The reaction can be divided into two stages, an extraction stage, eqn. (6), and a chemical reaction stage in the organic phase, eqn. (7).

KS PEG-NaBr(,,,, + NaOH(,,, 1_ PEG-NaOH(,,,, + NaBr(,,) (6)

orC-Y+XSC-X+Y

PEG-NaOH,,,, + PhCH,CH,Br -% PhCH=CH, + PEG-NaBr,,,,, + H,O

orC-X+A---tB+C-Y +D (7)

The general rate equation for the number of moles of styrene, NB, formed (N = number of moles) can be written as follows (eqn. (8)):

where n is the reaction order in the organic substrate. The total amount of catalyst active in the reaction system, N,, is the sum of the catalyst in the NaBr form, Nc--N, and in the NaOH form, N,_,. This assumes that all the catalyst originally complexed remains so, i.e. there is no decomplexation during the reaction. Therefore since

N, = N,_, + N,_, (9)

and

NC-, = &NC-,N,lN, (10)

rearrangement will give the amount of catalyst in the hydroxide form as eqn. (ll), and the rate equation may be rewritten as eqn. (12):

NC_, = KsNJ’Jx NY + K,N,

WFi b* -=NAo -=

kKsNcN,N,” kd’+“x NA” dt dt Ny +K,Nx = N, + K,N,

(11)

(12)

where ;YA is the conversion and NAo the initial number of moles of /3-bromo- ethylbenzene.

In order to solve the above differential equation, the reaction order in the organic substrate, n, must be known. It has been postulated that for slow chemical reactions compared to fast extraction steps (chemical reaction- controlled reactions) the reaction is of the first order, n = 1 and the catalyst complex is at equilibrium concentration [ll]. If the chemical reaction is very fast compared to the extraction stage, then the reaction is diffusion- controlled, of zero order in the organic substrate and the catalyst complex is only approximately at its equilibrium concentration. Only at maximal stirring rates will the catalyst complexes be at equilibrium concentration. From results published previously [9] we suspected the chemical reaction

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was fast compared to the extraction stage. We verified this by performing the reaction at several stirring rates. Only at high stirring rates (12 1200 rpm) was the reaction rate (from the profile) independent of the stirring rate. This, together with the low activation energy 6.5 f 0.5 kcal mol;’ found for the reaction, indicated that it was diffusion~ontrolled with the extraction stage rate-controlling. Therefore one may assume the reaction to be of zero order in the substrate. In fact, careful inspection of concentration us. time plots at low conversions (no significant poisoning) revealed that the reaction was indeed of zero order in the organic substrate. This was double checked by using the method of initial rates [ 121 which yielded the same results.

Since n = 0 and for equimolar ratios of sodium hydroxide and P-bromo- ethylbenzene NAO = Nxo, eqn. (12) can be rewritten as eqn. (13), and solved

(eqn. (14)).

(13)

NAO (Ks - 13xA - N,o ln(l -x ) = k b t

A 08 for R = 0 KS KS

(14) N AO=Nxo

With the aid of eqn. (14) the selectivity constant, K,, can be found. Graphs of

NAO(&--t&)x, -NAo/K, (ln(1 -%A)) US time for various KS will give a linear plot with a slope of kobs only at the correct KS. In this manner KS can be found by graphic interation, Fig. 3. The following values were found:

KS NaOH/NaBr = 0.010

KS NaOH/NaCl = 0.10 (from reaction with fl-chloroethylbenzene)

KS NaCl/NaBr = 0.10

100 200 300

Ti~,minu~es

Fig. 3. Graphic iteration for computation of KS NaOH/NaBr.

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There remain two questions, the answers to which are interrelated. First, one may ask why the selectivity constants found above showed a large preference for Bf and Cl- versus OH-, even though OH- was well extracted from solid potassium hydroxide by polyethylene glycol. Second, why can the select- ivity constant not be measured directly from an aqueous mixture of alkali salts which undergo extraction by polyethylene glycol into an organic phase ? The OH- anion undergoes extremely strong hydration by water relative to Br- and Cl- anions. Therefore, even though in the absence of water it is well extracted by PEG, the addition of water causes the OH- to exhibit a large preference for the aqueous phase. This is why it is also dif- ficult to measure the amount of hydroxide anion extracted de facto into an organic phase from an aqueous phase by direct analytical methods such as titrations. (We attempted direct measurements and found very large devia- tions.)

Conclusions

Polyethylene glycol effectively complexes alkali metal salts; however, similar to the case of quaternary onium salts, the extractability of the alkali salt is a function of the anion. We have found that polyethylene glycol undergoes anionic catalyst poisoning, which causes the stalling of reactions which are not equilibrium reactions such as the Williamson ether synthesis and p-elimination. Using the reaction profiles of &elimination, the selectivity constants of various alkali salts complexed by polyethylene glycol were measured. This approach can be used as a general method with other reac- tions to compute the selectivity constants between other alkali salts.

Experimental

Extractions 0.0125 Mol of alkali salt were mixed for 24 h with 0.0125 mol of

PEG-400 and the uncomplexed salt was filtered off. The PEG phase was analyzed by titration of the anion after decomplexation by addition of an excess of water.

Reactions Reactions were performed in a 50 ml round-bottom flask equipped

with a mechanical stirrer and 1.4 inch Teflon@ blade. Samples were taken from a side arm and analyzed by GLC. The flask was immersed in a 10 1 water bath heated to the required temperature. A typical reaction procedure involved mixing 0.01 mol sodium hydroxide with 1.2 g HZ0 (25% NaOH), 0.001 mol (10% mol) PEG-400 and 10 ml toluene as solvent. After 0.5 h of stirring to allow for full complexation of the sodium hydroxide, 0.01 mol of /3-bromoethylbenzene was injected. Samples were taken and quenched by dilute HCl and then analyzed.

Analysis Analysis was carried out by GLC using a Packard Model 427 gas chro-

matograph equipped with an FID. The column used was a 2 m X + in glass column packed with 5% OV-17 on Chromosorb W acid-washed. The column temperature was 150 - 180 “C with Nz as carrier gas at 40 ml min-‘.

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