Noteworthy Chemistry

May 7, 2012

Take this route toward water-stable ion-exchange materials. To explore chemical cross-linking as a strategy for achieving hydrolytic stability, C. Subramanian and colleagues at the University of Connecticut (Storrs) and the University of Akron (OH) prepared electrospun nanofiber mats from highly sulfonated polystyrene (SPS) in the presence of various concentrations of poly(ethylene oxide) (PEO). Adding PEO to the SPS spinning dope improved spinnability and yielded bead-free, uniform nanofibers at optimal concentrations.

When the SPS–PEO electrospun nanofibers were heated at 130 °C, the crystalline PEO melted and shrank the fibers. This heat treatment also promoted chemical cross-linking between the hydroxyl groups (from PEO hydrolysis) and sulfonic acid groups to form sulfonate esters. The authors confirmed that the polymers cross-linked by forming hydrogels from them in water; the sol fraction contained pure PEO.

The PEO content determined the morphology of the treated mat. The structures ranged from fibrous at ≤30 wt% PEO to void-free films with trace amounts of fibers at 40 wt% PEO. The fibrillar structure remained in PEO-containing thermally treated mats after exposure to water. The authors believe that the sulfonated mats will be useful ion-exchange applications. (Macromolecules 2012, 45, 3104–3111; LaShanda Korley)

Prevent calcium scaling with a cyclodextrin. Calcium scaling is a costly problem for industries that use large volumes of water. Methods for combating the problem usually require environmentally harmful phosphorus-containing compounds. V. Derakhshanian and S. Banerjee* at Georgia Tech (Atlanta) report that β-cyclodextrin (1) can be used to prevent calcium scaling.

The authors used a turbidimeter to evaluate the antiscaling ability of β-cyclodextrin. They mimicked scaling by adding a mixture of Na2CO3 and CaCl2 to precipitate CaCO3 from aqueous solutions with and without the antiscalant. Turbidity measurements showed that 5–100 ppm 1 effectively reduces scaling. After filtering and examining the suspended solids, they concluded that β-cyclodextrin not only suppresses CaCO3 formation but also reduces crystal size.

For a more realistic test material, the authors used a waste solution from the pulping industry called black liquor. Black liquor contains mainly lignin and inorganic compounds; it must be boiled in an evaporator, which causes scaling. The authors immersed stainless steel coupons into untreated black liquor and into β-cyclodextrin-containing black liquor, dried the coupons, recorded the weight gained, scraped off the deposits, and reweighed the coupons. β-Cyclodextrin decreased the amount of deposits and made them easier to remove.

β-Cyclodextrin is a nontoxic, inexpensive additive for reducing scaling. The authors state that derivatized cyclodextrins might be more effective, but they would also cost more. (Ind. Eng. Chem. Res. 2012, 51, 4463–4465; JosÉ C. Barros)

Should the concept of hybrid atomic orbitals be removed from general chemistry courses? Several models that describe chemical bonding are taught in general chemistry. The models are theoretical interpretations of bonding and are used because the precise nature of atomic interactions is not known. Each model has applications in various types of molecules, complexes, or macromolecular structures.

One of the models, valence bond theory (VBT), uses the concept of the hybrid atomic orbital to explain experimental observations. A. Grushow at Rider University (Lawrenceville, NJ) notes that students in general chemistry courses have difficulty understanding orbital hybridization and that using this theory often fails to predict experimental results. (J. Chem. Educ. 2011, 88, 860−862) He suggests abandoning instruction on hybrid orbitals in undergraduate courses. His arguments generated a firestorm of letters to the Journal of Chemical Education.

Many professors disagree with Grushow and support current chemistry curricula. N. J. Tro at Westmont College (Santa Barbara, CA) states that the hybrid orbital is an inseparable part of VBT and, regardless of its deficiencies, discarding the whole theory would limit instructors’ ability to explain bonding. He does, however, thank Grushow for forcing instructors to examine why VBT is taught. (J. Chem. Educ. 2012, 89, 567−568)

R. L. DeKock* and J. R. Strikwerda at Calvin College (Grand Rapids, MI) believe that some statements in Grushow’s article are incorrect, but they agree that curricula could be simplified. (J. Chem. Educ. 2012, 89, 569) C. R. Landis* and F. Weinhold at the University of Wisconsin (Madison) insist that existing curricula are appropriate because they give students an opportunity to explore the various models. (J. Chem. Educ. 2012, 89, 570−572)

D. G. Truhlar at the University of Minnesota (Minneapolis) points out that hybrid orbitals are mathematically correct functions and are convenient for approximating molecular orbitals. (J. Chem. Educ. 2012, 89, 573−574) P. C. Hiberty* at the University of Paris-Sud (Orsay, France), F. Volatron at the Pierre and Marie Curie University (Paris), and S. Shaik* at the Hebrew University of Jerusalem share the opinion that it is proper to offer different theoretical models to students. (J. Chem. Educ. 2012, 89, 575−577)

In reply to these correspondents, Grushow repeats that general chemistry curricula should not be overcomplicated. He would remove VBT and even molecular orbital theory from introductory courses because they tend to be confusing and oversimplified. (J. Chem. Educ. 2012, 89, 578−579; Sally Peng Li)

Make the monohydrochloride salt of a dibasic molecule. GSK159797, a long-acting β-agonist, is a dibasic molecule that contains secondary amine and aniline groups. The compound also has a formamide function that is easily hydrolyzed under acidic conditions. Forming the dihydrochloride salt destabilizes the molecule.

D. P. Shapland and co-workers at GlaxoSmithKline (Stevenage, UK) describe a way to produce the monohydrochloride salt with better purity than conventional treatment with HCl. A monohydrochloride salt is formed at an earlier stage of the synthesis; the salt is then subjected to the final step, hydrogenolytic debenzylation.

The free base of the benzylated precursor in 1-pentanol solution is treated with 1.5 equiv HOAc and the solution is washed twice with 10% NaCl solution. A small amount of solvent is distilled off to ensure that the system is anhydrous. The solution is then cooled and seeded to produce the monohydrochloride salt in 86% yield. Analysis of the aqueous layers from the NaCl washes shows that the first wash contains 70% of the acetate and that the second wash contains another 15%. (Org. Process Res. Dev. 2012, 16, 518–523; Will Watson)

Use this flow reactor for photoredox catalysis with visible light. Visible-light photoredox catalysis is becoming popular as a synthetic tool. Standard batch-reaction setups, however, limit catalysis efficiency because light penetration decreases exponentially with increasing path length (the Beer–Lambert law). C. R. J. Stephenson and co-workers at Boston University solved this problem by developing a flow reactor that allows higher photon flux density and therefore more efficient reactions.

The authors chose commercially available perfluoroalkoxyalkane (PFA) tubing with 0.762 mm i.d. This path length guarantees almost complete irradiation absorption, and the tubing is resistant to most chemicals. A 105-cm length of PFA tubing with 479 μL of reaction volume was wrapped around a pair of glass test tubes in a figure-eight configuration. A peristaltic pump was used to pass reaction mixtures through the tubing. The tubing was irradiated by a blue light-emitting diode. A silver-mirrored flask mounted above the tubing reflected incident light back to the reaction medium for greater efficiency.

The authors’ first test reaction was the oxidation of N-aryltetrahydroisoquinolines (e.g., 1) catalyzed by Ru(bpy)3Cl2 under visible light with BrCCl3 as the terminal oxidant; bpy is 2,2′-bipyridine. The initial product was iminium salt 2, which could be efficiently trapped by a nucleophile such as MeNO2 or CN. The reaction medium was pumped through the tubing with a short residence time (tR) of 0.5 min, corresponding to a throughput rate of 5.75 mmol/h. This rate is much greater than obtained with a conventional batch reaction on the same scale (0.081 mmol/h).

Using the same system, the authors carried out several intra- and intermolecular radical reactions that they had developed previously. They were able scale up reactions that could not be scaled up with a traditional batch setup. The flow reactor can be used for many similar reactions, and the efficiency can be improved by using longer tubing. (Angew. Chem., Int. Ed. 2012, 51, 4144–4147; Xin Su)

Tune luminescence color by controlling hydrogen bonding. There is a great demand for luminescent materials with different light-emission colors. A common way to change a material’s emission is to modify its molecular structure chemically, but this sometimes requires painstaking synthetic efforts. N. Kawatsuki and co-workers at the University of Hyogo (Japan) used a simple physical method to change the luminescence color of a fluorophore–copolymer composite.

The fluorophore is a fluorene derivative with two pyridine substituents; the copolymer contains methacrylic acid blocks. Their composite films show green and blue luminescence in the presence and absence, respectively, of hydrogen bonds formed between the pyridine rings and the carboxylic acid side groups.

Annealing the films at various temperatures can reversibly tune their emission colors by forming or disrupting the hydrogen bonds. Annealing the films at 100 °C, for example, leads to green emission because hydrogen bonds are present; annealing at 200 °C destroys the hydrogen bonds and changes the emission color to blue. The green emission returns when the films are re-annealed at 100 °C. (Langmuir 2012, 28, 4534–4542; Ben Zhong Tang)

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