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Title:The chemical effects of ultrasound
Author(s):Doktycz, Stephen Joseph
Doctoral Committee Chair(s):Kenneth S. Suslick
Department / Program:Chemistry
Discipline:Chemistry, Physical
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Chemistry, General
Chemistry, Organic
Chemistry, Physical
Abstract:The use of high intensity ultrasound to enhance the reactivity of heterogeneous, liquid-solid reactions has become a routine synthetic technique. Typically, ultrasound reduces reaction temperatures and increases rates of reaction while improving product yields. In spite of this, our understanding of the nature and range of sonochemistry is quite limited. The effects of ultrasound on liquid-solid interfaces include high speed liquid microjet impact, shock wave damage, enhanced mass transport near surfaces, and high velocity interparticle collisions.
The relative contributions of each of these physical mechanisms on increased heterogeneous reactivity is not known. In addition, investigations of cavitation on extended solid surfaces may not be relevant to the powders generally used in chemical reactions. To this end, we have focussed on understanding, on the atomic level, the origins of ultrasonic activation.
The effects of ultrasound on arene solvents commonly used in synthesis (i.e., benzene, toluene, m-xylene, and mesitylene) as well as the use of these solvents in the sonochemical synthesis of bis(arene) chromium complexes have been investigated. We have found that ultrasonic irradiation produces substantial decomposition, due to acoustic cavitation, of the arenes. Products, not unlike those observed in very high temperature pyrolysis of arenes are observed. In addition, up to 35-fold rate enhancements are observed for formation of bis(arene) chromium compounds with ultrasound.
We have also examined the effects of ultrasound on chemical reactivity, particle and surface morphology, and surface atomic composition of metal powders. Investigations with zinc (Reformatsky reaction, eq. 1) and copper (Ullman Coupling reaction, eq. 2) powders show that ultrasonic pretreatment of the powders substantially improves their reactivity ($>$50-fold).(UNFORMATTED TABLE OR EQUATION FOLLOWS)$$\eqalignno{&\rm RR\sp\prime C{=}O + Zn + BrCH\sb2CO\sb2R\sp{\prime\prime} \to RR\sp\prime C(OH)CH\sb2CO\sb2R\sp{\prime\prime}&\rm(eq.\ 1)\cr&\rm 2C\sb6H\sb4(NO\sb2)I + 2Cu \to (NO\sb2)H\sb4C\sb6{-}C\sb6H\sb4(NO\sb2) + 2\ CuI&\rm(eq.\ 2)\cr}$$(TABLE/EQUATION ENDS)We believe that the large increases in reactivity are due to high velocity interparticle collisions, which induce dramatic changes in surface morphology and composition of the powders. Ultrasonic irradiation of liquid-solid slurries creates shockwaves and turbulent flow which produce such collisions. We find that these shockwaves can accelerate metal particles up to speeds of 500 m/s, roughly half the speed of sound, which generates peak temperatures at the metal surface of 2600$\sp\circ$C to 3400$\sp\circ$C upon collision.
Issue Date:1990
Rights Information:Copyright 1990 Doktycz, Stephen Joseph
Date Available in IDEALS:2011-05-07
Identifier in Online Catalog:AAI9021674
OCLC Identifier:(UMI)AAI9021674

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