Aggregates prepared under fully destabilized conditions by the action of Brownian motion were exposed to an extensional flow generated at the entrance of a sudden contraction. Two noninvasive techniques were used to monitor their breakup process [i.e. light scattering and three-dimensional (3D) particle tracking velocimetry (3D-PTV)]. While the first one can be used to measure the size and the morphology of formed fragments after the breakage event, the latter is capable of resolving trajectories of individual aggregates up to the breakage point as well as the trajectories of formed fragments. Furthermore, measured velocity gradients were used to determine the local hydrodynamic conditions at the breakage point. All this information was combined to experimentally determine for the first time the breakage rate of individual aggregates, given in the form of a size reduction rate K(R), as a function of the applied strain rate, as well as the properties of the formed fragments (i.e., the number of formed fragments and the size ratio between the largest fragment and the original aggregate). It was found that K(R) scales with the applied strain rate according to a power law with the slope being dependent on the initial fractal dimension only, while the obtained data indicates a linear dependency of K(R) with the initial aggregate size. Furthermore, the probability distribution function (PDF) of the number of formed fragments and the PDF of the size ratio between the largest fragment and the original aggregate indicate that breakage will result with high probability (75%) in the formation of two to three fragments with a rather asymmetric ratio of sizes of about 0.8. The obtained results are well in agreement with the results from the numerical simulations published in the literature.
Aggregates grown in mild shear flow are released, one at a time, into homogeneous isotropic turbulence, where their motion and intermittent breakup is recorded by three-dimensional particle tracking velocimetry (3D-PTV). The aggregates have an open structure with a fractal dimension of ∼2.2, and their size is 1.4 ± 0.4 mm, which is large, compared to the Kolmogorov length scale (η = 0.15 mm). 3D-PTV of flow tracers allows for the simultaneous measurement of aggregate trajectories and the full velocity gradient tensor along their pathlines, which enables us to access the Lagrangian stress history of individual breakup events. From this data, we found no consistent pattern that relates breakup to the local flow properties at the point of breakup. Also, the correlation between the aggregate size and both shear stress and normal stress at the location of breakage is found to be weaker, when compared with the correlation between size and drag stress. The analysis suggests that the aggregates are mostly broken due to the accumulation of the drag stress over a time lag on the order of the Kolmogorov time scale. This finding is explained by the fact that the aggregates are large, which gives their motion inertia and increases the time for stress propagation inside the aggregate. Furthermore, it is found that the scaling of the largest fragment and the accumulated stress at breakup follows an earlier established power law, i.e., dfrag ∼ σ(-0.6) obtained from laminar nozzle experiments. This indicates that, despite the large size and the different type of hydrodynamic stress, the microscopic mechanism causing breakup is consistent over a wide range of aggregate size and stress magnitude.
An unprecedented external oxidant-free electrochemical protocol for 1, 3-oxohydroxylation of donor-acceptor cyclopropane is disclosed. The strategy encompasses the activation of the labile π-electron cloud of the aryl ring to cleave the strained C sp 3 À C sp 3 bond of cyclopropane to afford the β-hydroxy ketones via insertion of molecular oxygen. More significantly, based on the detailed mechanistic investigations and cyclic voltammetry experiments, a plausible mechanism is proposed.The magnificent journey of the most strained carbocycle named cyclopropane began long back in the 1970s. [1] The fundamentals laid by the frontiers have enabled the scientific community to exploit them for the construction of several complex molecular architectures cleverly. Bearing the ring strain of 115 KJ mol À 1 with an inefficient orbital overlap induces a significant amount of π-character in the bent CÀ C bonds of Cylclopropanes. [2] Further, the introduction of the donor and acceptor groups at the vicinal position of these CÀ C bonds induces a push-pull effect which polarizes the cycloalkane. [3] The previous decade has encountered a huge renaissance in the area of donor-acceptor cyclopropanes (DACs). [4] However, most of these documented protocols rely on the concept of Lewis [5] or Bronsted acid [6] catalysis, which cleaves the CÀ C bond in a heterolytic fashion and renders a zwitterionic species. [4] These 1,3-dipole species have been utilized for several cycloadditions, [7] rearrangements, [8] and ring-opening reactions. [9] Among them, 1,3-bifunctionalization of these DACs are still rare and relatively underexplored. However, there are several reports which demonstrate that these 1,3-dipoles can be seized with amines, [10a] azides, [10b] phenols, [10c] thiols [10d] etc. wherein, the ensuing anion traps the proton. In 2011, Sparr and Gilmour manifested the enantioselective 1,3-dichlorination of cyclopropane carbaldehyde in an organocatalytic fashion. [11] Later, in 2014, Werz and coworkers disclosed the ring-opening 1,3-dichlorination in the presence of iodobenzene dichloride (Scheme 1a). [12a] Followed by this, in 2017, the same group documented the reactivity of DACs with chalcogenyl chloride and bromides (Scheme 1b). [12b-c] In continuation to the endeav-[a
The synthesis of paracetamol still relies on multistep protocols involving the utilization of a stoichiometric amount of oxidizing/reducing or other corrosive agents. Herein we report a regioselective electrochemical Ritter-type reaction at the C(sp 2 )−H of unprotected phenol toward the environmentally benign and direct synthesis of paracetamol. The reaction proceeds under exogenous oxidant-and catalyst-free conditions. The protocol is scalable, can be deployed to a variety of phenols, and offers a sustainable alternative for the synthesis of paracetamol.
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