Phase-Transfer and Micellar Catalysis in Two-Phase Systems - ACS

Chapter 6

Phase-Transfer and Micellar Catalysis in Two-Phase Systems F. S. Sirovski

Downloaded by UNIV OF PITTSBURGH on July 31, 2013 | http://pubs.acs.org Publication Date: February 1, 1997 | doi: 10.1021/bk-1997-0659.ch006

N. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, 47 Leninsky prospekt, 117913 Moscow, Russia

Quaternary ammonium salts with a long hydrocarbon chain are a kind of bifunctional catalysts as the reactions in their presence proceed by two catalytic pathways, i.e., phase-transfer and micellar one. The former is inhibited by lipophilic anions such as chlorate etc. The structure — activity relationship for quaternary salts can be described quantitatively using Hansch π-hydrophobicity constants.

The method of phase transfer catalysis (PTC) is applied in organic synthesis for 30 years. The micellar catalysis is much older and more developed, in spite of its more narrow application field. Till present these two types of catalysis were thought to be quite independent ones, although the onium salts are used as catalysts in both cases. Nevertheless, it is not only synthetic organic chemists, usually quite indifferent to the meanders of high theory, who do not realize the existence of the close enough link between these two types of catalysis, but also physical chemists. However, the existence of such link allows one to make a number of theoretical as well as practical conclusions. We intend to demonstrate this link and its practical consequences taking as an example some practically important reactions. First of them are the dehydrochlorination reactions, the kinetics of which we have studied for a number of years. These reactions are not very well investigated under PTC conditions in spite of their practical importance and also are interesting as a model ones. Organic compounds that are able to eliminate hydrogen halide are to a some extent also CH-acids. The deprotonation stage that is the first step of the elimination reaction (for E2H- and Elcbmechanisms) (7) is a common one also for such important reactions as the addition of CH-acids and dihalocarbenes to double bonds. The kinetics of these latter reactions under PTC conditions are not investigated altogether (2). The problems of catalyst activity—quaternary cation structure relationship and PTC—micellar catalysis link are adjoined quite closely. There are but a few attempts to link the catalytic activity of the quaternary ammonium cation (quat) with its structure. At the same time there are data on quat activity as extragents and physiologically active

© 1997 American Chemical Society

In Phase-Transfer Catalysis; Halpern, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.

6. SIROVSKI

Phase-Transer & Micellar Catalysis in Two-Phase Systems 69

substances. The presence of two immiscible liquid phases is common for these two types of activity. Also there are quite a few statistics on structure—activity relationship for physiologically active substances, and a developed mathematical formalism exists for the structure description (3—5). It seemed very interesting to use data from those different fields for obtaining the structure—activity model for phase-transfer catalysts.


1. Noncatalyzed elimination in liquid-liquid systems For a catalyzed dehydrochlorination in a liquid—liquid system (organochlorine com pound — aqueous NaOH) it is quite common to proceed together with an noncata lyzed one. We have studied its kinetics in order to assess its contribution into the overall reaction (6). The model compound was 1,1,2,2,3-pentachloropropane (PCP). In spite of non-homogeneity of the reaction system elimination proceeds not at the interface (as one can suppose on general consideration) but in the aqueous phase, that is confirmed by the first-order kinetics of the reaction in spite of its bimolecularity. Also, as shown in Figure 1 ( =[TCB] /[TCB] P = [NaOH] /[NaOHL OH

m

TCB

(14) (15) (16)

aq;

m

aq>

N a O H

m

where Ρ is the coefficient of the distribution of the reagents between the micellar and aqueous phases. 4

1

£ -10 /rw>l

L" S


obs

1

10 Ι

ο

«S

4

16

12

Ccai.l0 /mo1 L "

1

Fig. 6. Dependence of the observed rate constant of the dehydrochlorination of aHCCH on the concentration of the catalyst. (Reproduced with permission from ref. 7. Copyright 1995 Izvestiya Akademii Nauk, Ser. Khim. (Russ. Chem. Bull).) The reagents' material balance is described by the following equations: [NaOH] = (C , - CA/C)V]([NaOH] + [OH"] ) + V (C - CMC)(NaOH] + [OH"] (17) ca

aq

aq

cat

m

[C H C1 ] = [1 - ( C - C M C ) V][C H Cl ] + V ( C - C M C ) [C H Cl ] 6

6

6

c a t

6

6

6

aq

cat

6

6

6

m

m

(18)

Considering that (P - \)V = K, where Κ is the constant of the solubilization, we obtain: * Α Β * Ο Η ( 0 , , - CMC)[NaOH][HCCH]exp[-AC] {l + K (C TCB

at

- CMC)}{l + K (C 0H

m

- CMC)}


(19)

78

PHASE-TRANSFER CATALYSIS

If [HCCH] = const, and [NaOH] = const., than r = k exp[-XC], λ=4.3,


ohs

where C is the concentration of NaCl in the aqueous phase. The value of KJCB determined by the independent procedure is in quite satisfactorily agreement with the one obtained from the kinetic calculations. The obtained results demonstrate that the kinetic model of micellar catalysis that was developed for the pseudo-homogeneous systems is valid also for the heterogeneous ones. It is quite possible that catalytic effect of long-chain onium salts in alkaline dehydrochlorination at low concentrations of NaOH (up to 5 mass %) is brought about by micelles. Evidently at higher NaOH concentrations there are two catalytic pathways —the micellar and the phase transfer one. We had successfully applied this model for rationalizing the kinetic results of 3,4-dichlorobutene-l dehydrochlorination (29, 30). CH C1CHC1CH=CH + NaOH -> CH =CC1CH=CH 2

2

2

2

The kinetic curves of the substrate consumption consisted of two parts. At first there was a sharp fall in substrate concentration during short time, than it was slow consumed according to the first-order kinetics. This confirms our hypothesis about two catalytic pathways caused by the dualistic nature of the onium salt catalyst. The phasetransfer pathway is blocked in the course of the reaction by the evolving CI" ion. The micellar pathway is caused by the surfactant properties of the catalysts. The rate of the reaction neglecting the noncatalyzed one is the sum of rates along both pathways. The catalysts providing for the larger reaction rate along the micellar pathway are the most active. The rate of reaction along the phase transfer pathway shortly after the start sharply falls due to the effect of the so-called "chloride poisoning" of the catalyst. So the substrate is consumed mainly thanks to the reaction along the micellar pathway. Thus one can conclude that the long-chain quaternary onium salts are a kind of a missing link, which can bind PTC and micellar catalysis thanks to their "bifunctionality". In other words, they can be both micellar and phase transfer catalysts simultaneously. The obtained results allow us to propose a kind of phase diagram showing the different fields of catalysis by various quaternary ammonium salts (Fig. 7). The realm of pure PTC is confined between chain length of 2 to 8 carbon atoms and NaOH concentration higher 1 mole/1. The field of the purely micellar catalysis extends from the chain length of 8 carbon atoms and is bordered by NaOH concentrations from 0.01 to 1.5 mole/1. Between these two fields the scarcely investigated wilderness extends where two types of catalysis coexist. Surely, this map is very rough, but nevertheless it can be helpful in the choice of the catalyst and reaction conditions. This diagram also makes it clear that the long-chain quats are more active catalysts, as they display catalytic activity in a wider range of alkali concentrations due to their dualistic catalytic properties. So, in general, the elimination proceeds by three pathways: noncatalyzed (in water), PTC-catalyzed and micellar-catalyzed.


6.

SIROVSKI

Phase-Transer & Micellar Catalysis in Two-Phase Systems 79


[NaOH]

Fig. 7. Realms of catalysis: 1 — no catalysis; 2 — phase transfer catalysis; 3 — intermediate region; 4 — micellar catalysis. (Reproduced with permission from ref. 7. Copyright 1995 Izvestiya Akademii Nauk, Ser. Khim. (Russ. Chem. Bull).) 3. Some Practical Applications of the Catalytic Properties of the Long-Chain Quats. The dual catalytic nature of the long-chain quats sometimes leads to very peculiar consequences. Thus in the following reaction (EtD LP(S)ON

NaCl

that proceeds in the liquid—liquid system it was not TEBA-C1 that was the best catalyst (Yagniukova, Z., Sirovski F., Kalinina L., L'vova N., unpublished results) but surfactant quats. It is seen from Table Π that compounds with surfactant properties are better catalysts than a conventional quat. The specific properties of long-chain quats were exposed also in the liquid/solid system. Investigation of kinetics of two-phase catalyzed acetylation of vitanin A half-product showed that different results are obtained with TEBA-C1 and Katamin AB as catalysts.


80

PHASE-TRANSFER CATALYSIS

Table II. Activity of Different Catalysts in Phosphorylation (Yagniukova, Z., Sirovski F., Kalinina L., L'vova N., unpublished results) Catalyst

Initial rate, Catalyst concentration, mole-fl-minf % mass 2.83 1.0 3.90 1.0 3.25 3.0 1.19 1.0 0.66 0.5 1.73 3.0 1.90 1.0

1

+

2

[(C H —C H ) N —Me] S0 PhCH N Me2(C H25—C H )Cr PhCH N (CH CH OH) (C H —Ci H )Cr Et N CH PhCr C H C H40(CH CH 0) H (n=9—10) HO(CH CH 0) dicyclohexano-18-crown-6 8

17

10

21

3

2

4

+

2

12

16

33

+

2

2

2

2

12

25

6

37

+

3

2

9

19

6

2


2

2

2

n

5

Catalyst activ in relation to PEG-5 17.1 21.0 12.2 2.0 5.5 1.0 5.1

+

The dependence in case of the surfactant quat is shown in Figure 8. In case of TEBA the usual linear dependence was observed (Sirovski, F., Bobrova, E. unpublished results). r -HT\ mol (L-min)" - to

1

0

I

I

I

Ο

7

2

I 3

1 4

I 5

1 6

1 7

1 8

1 9

1

1

ΙΟ

C , % mol cat

Fig. 8. The catalyst influence in solid/liquid acetylation The surfactant quats also displayed different properties in the reaction of pxylene with NaOCl (31).



6. SIROVSKI

Phase-Transer & Micellar Catalysis in Two-Phase Systems

As shown in the above scheme the reaction proceeded along two pathways, de pending on pH. But this effect was observed only in the presence of Katamin AB. On the opposite, B U 4 N O H weakly catalyzed chlorination and did not at all effect oxyda tion. Possibly the micellar effects also influence the reaction direction as well as its rate. 4. Catalysts' Structure—Activity Relationship in PTC The problem of catalysts' structure—activity relationship is a central one in the theory of PTC. We would leave out the also very important question of the counter-ion influence and are going to concentrate the attention on the influence of a cation struc ture. Some investigators (2, 32) have operated with the coefficient of the distribution of the quat between the phases of the reaction mass using it for the estimation of the catalyst hydrophilic-lipophilic balance (33). Fukunaga and co-workers (34) proposed a fresh but arguable approach to the problem using for this aim the Hildebrand (35) solubility parameter 8. For the calculation of the catalyst hydrophilic-lipophilic balance D(

Phase-Transfer and Micellar Catalysis in Two-Phase Systems - ACS

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