Pedro Cintas

Power ultrasound in organic synthesis: moving cavitational chemistry from academia to innovative and large-scale applications

Ultrasound, an efficient and virtually innocuous means of activation in synthetic chemistry, has been employed for decades with varied success. Not only can this high-energy input enhance mechanical effects in heterogeneous processes, but it is also known to induce new reactivities leading to the formation of unexpected chemical species. What makes sonochemistry unique is the remarkable phenomenon of cavitation, currently the subject of intense research which has already yielded thought-provoking results. This critical review is aimed at discussing the present status of cavitational chemistry and some of the underlying phenomena, and to highlight some recent applications and trends in organic sonochemistry, especially in combination with other sustainable technologies. (151 references.).

Green chemistry. The sonochemical approach

Although the applications of ultrasound have long been known in both industry and academy, the “green” value of the non-hazardous acoustic radiation has been recognised by synthetic and environmental chemists only recently. The chemical and physical effects of ultrasound arise from the cavitational collapse which produce extreme conditions locally and thus induce the formation of chemical species not easily attained under conventional conditions, driving a particular radical reactivity. This rationale, accessible in a non-mathematical manner, anticipates the advantages of using this technology in a variety of processes that include milder reactions with improved yields and selectivities, easy generation of reactive species and catalysts or replacement of hazardous reagents. Sonication enables the rapid dispersion of solids, decomposition of organics including biological components, as well as the formation of porous materials and nanostructures. This review summarises how ultrasound can be harnessed to develop an alternative and mild chemistry, which parallels the ability of acoustic waves to induce homolytic bond cleavage.

A new pilot flow reactor for high-intensity ultrasound irradiation. Application to the synthesis of biodiesel.

In recent years, chemistry in flowing systems has become more prominent as a method of carrying out chemical transformations, ranging in scale from microchemistry up to kilogram-scale processes. Compared to classic batch ultrasound reactors, flow reactors stand out for their greater efficiency and flexibility as well as lower energy consumption. This paper presents a new ultrasonic flow reactor developed in our laboratory, a pilot system well suited for reaction scale up. This was applied to the transesterification of soybean oil with methanol for biodiesel production. This reaction is mass-transfer-limited initially because the two reactants are immiscible with each other, then because the glycerol phase separates together with most of the catalyst (Na or K methoxide). In our reactor a mixture of oil (1.6 L), methanol and sodium methoxide 30% in methanol (wt/wt ratio 80:19.5:0.5, respectively) was fully transesterified at about 45 degrees C in 1h (21.5 kHz, 600 W, flow rate 55 mL/min). The same result could be achieved together with a considerable reduction in energy consumption, by a two-step procedure: first a conventional heating under mechanical stirring (30 min at 45 degrees C), followed by ultrasound irradiation at the same temperature (35 min, 600 W, flow rate 55 mL/min). Our studies confirmed that high-throughput ultrasound applications definitively require flow reactors.

Sonication‐Assisted Fabrication and Post‐Synthetic Modifications of Graphene‐Like Materials

Chemistry of and with graphene constitutes a rapidly evolving field that holds much promise for the generation of advanced materials with salient, and often unique, properties and potential applications in different fields. However, reliable, mild, and scalable methods to produce this layered carbonaceous material represent an important bottleneck and are critical for progress in this research area to continue. In this context, the use of ultrasound has become a routine and indispensable step in numerous synthetic protocols, even though most researchers usually overlook the science behind it. This minireview provides some fundamentals on the interaction of sound waves with matter and equally illustrates how sonication assists further synthetic decorations under benign conditions.

Chirality of Living Systems: A Helping Hand from Crystals and Oligopeptides

Left–right asymmetry is ubiquitous in nature. Recent studies reveal changes in the energy and growth rate of crystal surfaces to which D or L amino acids bind, with the binding itself being dictated by stereochemical matching. Likewise, oligomerization of amino acids appears to be a chiroselective process that enables the propagation of sequences with defined handedness.[[For a definition of chiroselective self-assembly, see: M. Bolli, R. Micura, A. Eschenmoser, Chem. Biol.1997, 4, 309–320.]] These results, along with related findings on symmetry breaking and further amplification of asymmetry at a supramolecular level, constitute new insights into the origin of homochirality in living species.

From parity to chirality: chemical implications revisited

Abstract Parity violation represents an essential property of particle and atomic handedness used to cope with the complex phenomenon of asymmetry in the universe. At the molecular level, however, numerous experiments suggest that parity-violating energy differences have not determined the amplification and propagation of homochirality. Asymmetric transformations conducted under far-from-equilibrium conditions reveal the existence of non-linear autocatalysis which is stochastic in nature. In any event and, globally considered, chirality appears as a unifying characteristic of our visible environment with evolutionary implications, thereby suggesting areas for productive research.

Nonlinear stereochemical effects in asymmetric reactions

Abstract A brief survey discussing the conceptual considerations of nonlinear effects in asymmetric reactions as well as their recent applications in catalysis and autocatalysis.

Harnessing mechanochemical effects with ultrasound-induced reactions

Chemical reactions may experience numerous and varied effects under the influence of ultrasound. This soft radiation, often viewed as a lab trick, induces and improves both physical and chemical transformations by means of efficient agitation, dissolution, mass and heat transfers, and reagents sonolysis, which all arise from the cavitational collapse. An empirical rationale that distinguishes between true chemical effects and mechanical ones, especially in heterogeneous reactions, was introduced more than two decades ago and has been a useful guidance on reporting sonochemical mechanisms. Recent studies have witnessed a truly remarkable data set that boosts sonochemistry over the wall of applied science. This perspective highlights the importance of the so-called false sonochemistry-closely related to mechanochemistry-in modern synthesis, thus illustrating the advantages of using pressure waves in chemistry. Emphasis is put on green transformations, mild polymerization reactions and the selective cleavage of functionalized polymers, which may have an effective impact in process chemistry and represent a realistic option in industry.

Chiral autocatalysis: where stereochemistry meets the origin of life

This article summarizes a series of recent and simple experiments to produce optically active substances from achiral precursors. These symmetry-breaking processes include either autocatalytic crystallization or asymmetric autocatalysis, and provide new insights into the origin of biomolecular homochirality. In addition, support from an extraterrestrial origin of chiral molecules has also come from recent findings.

Homochirality beyond grinding: deracemizing chiral crystals by temperature gradient under boiling

A single-chirality solid phase can be obtained in boiling solutions containing a racemic mixture of left- and right-handed enantiomorphous crystals due to dissolution-crystallization cycles induced by a temperature gradient. This phenomenon provides further insights into asymmetric amplification mechanisms under presumably prebiotic conditions.