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The use of sonochemistry in organic reactions.

It is not very often that the opportunity arises for a chemist to move into a brand new field of research. An increasing number of scientists, however, are now becoming interested in just such a new field-sonochemistry-used to describe the effect of ultrasonic sound waves on chemical reactivity. The drawbacks to most new scientific developments are that they require expensive or specialized apparatus and a considerable degree of expertise in a particular field of study. What makes research in sonochemistry so appealing is that it is quite inexpensive to get started, and power ultrasound has found applications over the whole range of chemistry.

Ultrasound is defined as sound of a frequency beyond that to which the human ear can respond. The normal range of hearing is between 16Hz (Hz = Hertz = cycle per second) and 16kHz and ultrasound is generally considered to lie between 2OkHz and 50OMHz (see Figure 1). For many years ultrasound has found a wide variety of uses in engineering, science and medicine but its applications to chemistry have only recently come to the fore.(1) Several reviews on the chemical applications of ultrasound have been published recently.(2,3,4)

The Chemical and Physical Effects of Power Ultrasound

Power ultrasound produces its effects through the phenomenon of cavitation. Like any sound wave ultrasound is transmitted via waves which alternately compress and stretch the molecular structure of the medium through which it passes. During each stretching' phase (rarefaction), provided that the negative pressure is strong enough to overcome intermolecular binding forces, a fluid medium can be literally torn apart producing tiny cavities (microbubbles). In succeeding compression cycles, these cavities can collapse violently with the release of large amounts of energy in the immediate vicinity of the microbubbles. It has been estimated that temperatures of up to several thousand Kelvin and pressures of several hundred atmospheres are produced during this collapse. The mechanical and chemical effects of the collapsing bubble will be felt in two distinct regions: a) within the bubble itself which can be thought of as a highpressure and temperature microreactor; and b) in the immediate vicinity of the bubble where the shockwave produced on collapse will create enormous shear forces.

Sonochemistry is mainly concerned with reactions involving a liquid component within which cavitation can be induced. The effects of power ultrasound on such systems are best summarised in the terms of three different reaction types.

Homogeneous Reactions

The microbubble (or cavitation bubble) formed in the rarefaction cycle does not enclose a vacuum-it contains vapour from the solvent or any volatile reagent present so that, on collapse, these vapours are sujected to the enormous increases in both temperature and pressure referred to above. Under such extremes, the solvent and/or reagent suffers fragmentation to generate reactive species of the radical or carbene type. Thus, if water is sonicated then the extreme conditions generated on collapse of the cavitation bubbles are sufficient to cause rupture of the 0-H bond itself with the formation of radical species and the subsequent production of oxygen gas and hydrogen peroxide (see Scheme 1).(5)

Any species dissolved in the water is clearly going to be subject to chemical reaction with these ultrasonically produced radicals and/or hydrogen peroxide, thus if iodide ion is present in solution elemental iodine will be liberated.

The shock wave produced on bubble collapse, can disrupt solvent structure and this can influence reactivity by altering solvation of the reactive species present. An example of this is to be found in the ultrasonically assisted reaction of 2-chloro-2-methylpropane in aqueous alcoholic media (see Scheme 2) where sonochemical rate enhancements of up to 20 fold have been reported.(6) The sonochemical effect increases with an increase in ethanol content of the solvent and this is consistent with the destruction of solvent structure (which also increases with ethanol content).

As with many sonochemical studies the effect of irradiation in this solvolysis decreases as the reaction temperature is increased. This general observation is related to a decrease in the energy of cavitational collapse as the solvent vapour pressure is raised. In simple terms, the more vapour which enters the cavitation bubble the more of a cushion it will provide against violent collapse.

Heterogeneous Reactions Involving a Solid/Liquid Interface

There are two types of reaction involving solid/liquid interfaces: i) in which the solid is a reagent and is consumed in the process, and ii) in which the solid-often a metal-functions as a catalyst.

Solid as Reagent: a classic use of ultrasound is in the initiation and enhancement of organometallic reactions. One such example is the preparation of a Grignard reagent-an organomagnesium halide (see Scheme 3). A long-standing problem associated with Grignard reagent synthesis is that, in order to facilitate reaction between the organic halide and the metal in an ether solvent, all of the reagents must be dry and the surface of the magnesium must be clean and oxide free. Such conditions are difficult to achieve and so, many methods of initiating the reaction have been developed. Most rely on adding activating chemicals to the reaction mixture. The modern method of initiating the reaction is by sonication avoiding the need to add chemical activators. Even in damp, technical-grade ether ultrasonic irradiation can initiate the reaction in under four minutes whereas with conventional methodology initiation required several hours.(7) This is potentially of great economic importance to industry, indicating that in some situations sonication may remove the need to employ super-pure chemicals. Scheme 3

When examined by electron microscopy, surfaces of metals which have been subjected to ultrasonic irradiation reveal 'pitting' looking not unlike craters on the moon.(8) This pitting serves both to expose new surface to the reagents and to increase the effective area available for reaction. The pitting is the result of two processes: i) the implosion of cavitation bubbles formed from gases or impurities on the surface of the metal, and ii) the generation of a jet of solvent which impinges on the surface when a cavitation bubble collapses close to it. If the metal reagent is in the form of a the other hand, you are about to purchase a bath, then the scientific supply houses offer a dauntingly wide choice. I would recommend that you start with the type of bath which has been used for the experiments described in this paper-a Kerry Pulsatron 60. This bath operates at 38kHz, is rated at 50 watts and has internal dimensions of 14 x 15 cm and a depth of 10 em (see Figure 2). For reproducible results the ultrasonic bath should be thermostatted either with circulating coolant or by the addition of ice. Another solution to the problem of temperature stability is to determine the equilibrium' temperature of the bath (ie. the maximum temperature which the bath water attains and maintains under continuous running conditions) and perform most reactions under these conditions. A further aid to reproducibility is the addition of detergent to the bath water (we use 5% Decon 90) - the detergent permits much more even cavitation, and therefore energy transfer in the bath.

Demonstrations of the Physical Effects of Ultrasound

Here are four quick and easy demonstrations of some aspects of sonochemistry which can be used to illustrate sonochemical processes. In these examples the bath can be used from cold-no special heating or thermostatting is required.

Reactions involving metal surfaces

We have referred to the pitting' of metal surfaces when exposed to ultrasonic irradiation and the perforation of foil provides a perfect example of this effect. Take a piece of ordinary kitchen foil about six inches square and dip it into the bath (do not immerse your fingers). After 30 seconds remove the foil and you should find that it is liberally perforated. In fact, this is an ideal test to determine whether your bath is powerful enough for sonochemistry.

Reactions involving Powders

Place a few pieces of blackboard chalk in water 150cm3) in a 250cm3 conical flask. When the flask is dipped into the ultrasonic bath clouds of chalk are seen to be produced from the surface of the solid. Very quickly the liquid becomes cloudy illustrating the way in which ultrasound can erode surfaces and break down particles to a much smaller size.

Emulsion reactions

Ultrasound is known to generate extremely fine emulsions from mixtures of immiscible liquids. This can be shown by placing some water 100cm3) and toluene 50CM3) in a 250cm3 conical flask and dipping the base of the flask in the ultrasonic bath. By carefully adjusting the depth of immersion, the clearly defined phase boundary between the two immiscible liquids is first seen to become agitated. Shortly afterwards, a cloudiness appears in the interfacial region. Substantial emulsification will have been caused within one minute. The demonstration is made even more dramatic if some dye is added to make one of the phases brightly coloured.

Ultrasonic Degassing

The cavitational effects, the basis of sonochemical action, are also the reason for the extremely effective use of ultrasound to degas liquids. Any dissolved gases or gas bubbles in the medium act as nuclei for the formation of cavitation bubbles. Such bubbles are not easily collapsed in the compression cycle of the wave. They contain gas and will continue to grow on further rarefaction cycles, filling with more gas and eventually floating to the surface. Since the rarefaction cycles are taking place extremely rapidly (38,000 times per second using the Kerry bath) the bubbles grow so quickly that degassing appears to occur almost instantaneously.

To produce a gasified liquid place some granular zinc metal in 4M hydrochloric acid (250 cm3) in a 500cm3 conical flask and allow the effervescence to start, swirl occasionally and dilute with water to modify the reaction. After about one minute the liquid will become opaque with liberated hydrogen. Now place the base of the flask onto the surface of the water in the ultrasonic bath. As if by magic the cloudiness is cleared by a front which rises from the base. Obviously the process can be repeated as long as the zinc remains.

Laboratory Experiments Using Ultrasound The two experiments below have been chosen for their simplicity in terms of experimental procedures, availability of equipment and availability of chemicals.

1. A Heterogeneous reaction: the Wurtz coupling of bromobenzene using lithium metal.

This is an ideal experiment for the demonstration of the effect of ultrasound on organometallic chemistry. It is interesting for the student because various colour changes occur during this reaction making it visually appealing. The work was originally reported by Boudjouk(l3) (see Scheme 6) but, perhaps due to the use of a more effective bath for our experiments, we have been able to reduce the reaction times considerably.

Method

Bromobenzene (15.7g, 0.1 mole), lithium shot (0.75g, 0.11 mole, washed with petroleum ether to remove oil), naphthalene (0.3g, added to assist organolithium formation) and tetrahydrofuran 25cm3) were placed in a 50cm3 quickfit conical flask equipped with a condenser (as a precaution against any excessive exotherm).

The flask was placed in the Kerry ultrasonic bath filled to within one centimetre of the top with water containing 5% Decon 90. The flask was immersed to a depth of a centimetre or two so the reaction could be readily observed and maximum agitation of the contents achieved. The level of sonication of the flask contents takes several forms depending on position. Ideally, the surface should be sufficiently disturbed to 'shoot' small jets of solution upwards and against the walls of the vessel. Sometimes this is accompanied by visible pulsing' of vapours in the upper flask. At other stages, the reaction mixture may be seen to seethe or rotate violently. If little or no surface activity is visible, the flask should be repositioned and, in fact, this must be done several times during the reaction as the ultrasonic bath water warms up.

In the first few moments, a blue colour is seen to stream from the surface of the lithium metal colouring the whole solution. Within a minute the reaction becomes blue/green and after about two minutes it becomes quite hot (although it does not achieve reflux temperature) and the reaction mixture turns dark brown. Thereafter the reaction temperature falls and remains moderate with the reaction itself turning dark green after about 12 minutes.

Sonication must be maintained through the total reaction time of about 30 to 40 minutes and the end of the reaction is signaled by a change in reaction colour from dark green to caramel brown.

At the end of the reaction, the mixture is filtered through glass wool to remove any residual lithium and then quenched by the addition of a few drops of water. After the tetrahydrofuran had been removed from the orange liquid by rotary evaporation, crystallisation of the residue afforded crude biphenyl which was purified by recrystallisation from methanol. The yield is typically around 4g, ie. 55%.

2. A Homogeneous reaction: the rate of solvolysis of 2-chloro-2-methylpropane in aqueous ethanol This is based upon a standard undergraduate experiment in which the first order hydrolysis of a haloalkane is followed by monitoring the change in conductivity of the solution as HC1 is liberated (see Scheme 2). The experiment can be performed at several temperatures and in this way, it can be demonstrated that the effect of ultrasound increases as the temperature of the reaction is lowered.(14) As with any kinetics experiment involving ultrasound, there is a problem over obtaining absolute reproducibility but this degree of accuracy is probably unnecessary for the purposes of this class experiment. It should be noted, however, that accuracy and reproducibility are certainly possible when following sonochemical reactions although more sophisticated apparatus is required.(15)

Method

Conductivity water and absolute ethanol may be used without further purification to make up the required solvent compositions by mass. A stock solution of 2-chloro-2-methylpropane was prepared by dissolving lcm.sup.3 of freshly distilled material in 9cm.sup.3 absolute ethanol. In the case of normal (non-ultrasonic) runs, 25cm.sup.3 of the appropriate solvent mixture was placed in a reaction vessel immersed in a constant temperature bath and allowed to obtain thermal equilibrium. Twenty-five of the stock substrate solution was injected, and the change in conductivity of the reaction was monitored using platinum electrodes. The reaction was stirred throughout using a magnetic stirrer follower and a submersible stirrer. (Frequent swirling of the mixture is an inexpensive alternative to mechanical stirring.) The reaction was generally followed for at least one half life. First order rate constants are determined with an infinity value or by using the Guggenheim method.(16)

All ultrasonic reactions were performed using precisely the same method except:

a) the constant temperature bath was replaced by an ultrasonic cleaning bath;

b) since the temperature inside the reaction vessel is always a few degrees above that of the bath, it was particularly important to allow the system to reach some form of equilibrium. Physical measurement of the reaction temperature is best made with a thermocouple;

c) ultrasonic agitation was assumed to be sufficient to replace mechanical stirring;

d) The ultrasonic bath temperature was held approximately constant by the addition of ice.

Approximate values for the first-order rate constants obtained from these experiments under ideal conditions are given in the table. The enhancement in reactivity due to ultrasound is clearly seen to be inversely related to the temperature of the system.

References

1. T.J. Mason and J.P. Lorimer, Sonochemistry - the theory, applications and uses of ultrasound in chemistry, Ellis Horwood, Chichester (1989).

2. K.S. Suslick, Scientific American 62 (1989).

3. T.J. Mason and J.P. Lorimer, Endeavour, 13,123 (1989).

4. Advances in Sonochemistry, Volume 1, Ed T.J. Mason, JAI press, London (1990).

5. K.Makino, M.M. Mossoba and P. Riesz, J. Phys. Chem., 87, 1369 (1983).

6. T.J. Mason, J.P. Lorimer and B.P. Mistry, Tetrahedron, 26, 5201 (1985).

7. J.D. Sprich and G.S. Lewandos, Inorg. Chim. Acta., 76, 1241 (1982).

8. J.C. Barboza, C. Petrier and J.L. Luche, J. Organic Chem., 51, 55 (1988).

9. J. Lindley, P.J. Lorimer and T.J. Mason, Ultrasonics, 24, 292 (1986).

10. D.H. Shin and B.H. Han, Bull. Korean Chem. Soc., 6, 247 (1985).

11. R.S. Davidson, A. Safdar, J.D. Spencer and D.W. Lewis, Ultrasonics, 25, 35 (1987).

12. T.J. Mason, J.P. Lorimer and J. Moorhouse, Education in Chemistry, 26, 13 (1989).

13. B.H. Han and P. Boudjouk, Tetrahedron Letters, 22, 2757 (1981).

14. If the solvolysis is carried out under a variety of conditions by different students it is possible to pool the results for class comparisons and discussion.

15. Accurate measurements are possible using a cup-horn system (see ref 6).

16. J.P. Lorimer and T.J. Mason, Education in Chemistry, 22, 19 (1985).
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Title Annotation:effect of ultrasonic sound waves on chemical reactivity
Author:Mason, Timothy J.
Publication:Canadian Chemical News
Date:Mar 1, 1991
Words:2807
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