March's Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 7th Edition (2013)
Part I. Introduction
Chapter 7. Irradiation Processes in Organic Chemistry
7.B. Sonochemistry
Sonochemistry (chemical events induced by exposure to ultrasound) occupies an important place in organic chemistry.70 The chemical effects of high-intensity ultrasound were extensively studied in aqueous solutions for many years,71 but are now applied to a variety of organic solvents. The origin of sonochemistry is acoustic cavitation: the creation, growth, and implosive collapse of gas vacuoles in solution by the sound field. Acoustic cavitation is the phenomenon by which intense ultrasonic waves induce the formation, oscillation, and implosion of gas bubbles in liquids.72 Liquids irradiated with high-power ultrasound undergo chemical decomposition and emit light.73 These phenomena occur near the end of the collapse of bubbles expanded many times their equilibrium sizes. Chemistry (sonochemistry), light emission (sonoluminescence), and cavitation noise often accompany the process of acoustic cavitation.74
Sonochemistry generates gas vacuoles in situ. The collapse of gas vacuoles generates transient hot spots with local temperatures of several thousand Kelvin, and pressures of hundreds of atmospheres. A sonochemical hot spot forms where the gas- and liquid-phase reaction zones have effective temperatures of 5200 and 1900 K, respectively.75 The high temperatures and pressures that are achieved in the bubbles during the quasiadiabatic collapse76 lead to the generation of chemistry and to the emission of light, most probably coming from molecular excited states and molecular recombination. Note that work has been done that shows the commonly held view that bubbles are filled with saturated gas is inconsistent with a realistic estimate of condensation rates.77 The alternative view of extensive solvent vapor supersaturation in bubbles uniformly heated to a few thousand kelvin, depending on the conditions, is in accord with sonochemical rates and products.78
There is a correlation between sonochemical and sonoluminescence measurements, which is usually not observed. Sonoluminescence is the consequence that both the sonochemical production (under air) of oxidizing species, and the emission of light reflect the variations of the primary sonochemical acts, which are themselves due to variations of the number of “active” bubbles.79 Pulsed ultrasound in the high-frequency range (>1 MHz) is extensively used in medical diagnosis, and the effects of pulsed ultrasound in the 20-kHz range using an immersed titanium horn has been reported.80
The chemical effects of ultrasound have been studied for >50 years,81 and applied to colloid chemistry in the 1940s.82 Modern interest in the chemical uses of ultrasound involves chemistry in both homogeneous83 and heterogeneous84 systems. Organic solvents (e.g., alkanes) support acoustic cavitation and the associated sonochemistry. This leads to carbon–carbon bond cleavage and radical rearrangements, with the peak temperatures reached in such cavities controlled by the vapor pressure of the solvent.85
It is often difficult to compare the sonochemical results reported from different laboratories (the reproducibility problem in sonochemistry).86 The sonochemical power irradiated into the reaction system can be different for different instruments. Several methods are available to estimate the amount of ultrasonic power entered into a sonochemical reaction,86 the most common being calorimetry. This experiment involves measurement of the initial rate of a temperature rise produced when a system is irradiated by power ultrasound. It has been shown that calorimetric methods combined with the Weissler reaction can be used to standardize the ultrasonic power of individual ultrasonic devices.87
Sonochemistry has been used to facilitate or assist many organic reactions,88 and there are other applications.89 The scope of reactions studied is beyond this work, but some representative examples will be listed. Ultrasound has been used to promote lithiation of organic compounds,90 for the generation of carbenes,91 and reactions of metal carbonyls where sonochemical ligand dissociation has been observed, which often produces multiple CO substitution.92 The influence of ultrasound on phase-transfer catalyzed thioether synthesis has been studied.93
Sonochemistry has been applied to acceleration of the Reformatsky reaction,94 Diels–Alder reactions,95 the arylation of active methylene compounds96 nucleophilic aromatic substitution of haloarenes,97 and to hydrostannation and tin hydride reduction.98 Other sonochemical applications involve the reaction of benzyl chloride and nitrobenzene,99 an SRN1 reaction in liquid ammonia at room temperature,100 and Knoevenagel condensation of aromatic aldehydes.101 Iodination of aliphatic hydrocarbons can be accelerated,102 and oxyallyl cations have been prepared from α,α′-diiodoketones using sonochemistry.103 Sonochemistry has been applied to the preparation of carbohydrate compounds.104 When sonochemistry is an important feature of a chemical reaction, this fact will be noted in the reactions presented in Chapters 10–19.