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Noteworthy Chemistry

July 20, 2015


Polymer-supported buffers reduce phosphate pollution. Buffering agents, primarily phosphates, are used in many processes. They are not consumed in the reaction, but they usually are discharged in wastewater and may be hazardous to freshwater ecosystems.

To reduce the impact of phosphate effluents on the environment, Y. Li and co-workers at the Shanghai Institute of Technology developed polymer-supported phosphonic acids (PSPAs) to replace phosphate buffers. They prepared the PSPAs (3 in the figure) from chloromethyl-substituted polystyrene beads (1) by treating them with trimethyl phosphite [(MeO)3P] and hydrolyzing the methylated product (2).

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Preparation of PSPAs

The authors investigated the buffering action of the PSPAs by potentiometrically titrating them with NaOH. The buffering curves had breaks at pH ranges 4–6 and 8–10, similar to other phosphonic acid resins.

After use, the PSPAs can be removed by filtration and recycled 10 times without loss of buffer activity. They then can be regenerated and used for another 10 cycles.

The authors examined three enzyme-catalyzed reactions run with PSPA buffers: dephosphorylation catalyzed by acid phosphatase, urease-catalyzed urea hydrolysis, and the horseradish peroxidase–catalyzed reaction of hydrogen peroxide with 2,2’-azinobis(3-ethylbenzothiazoline-6-sulfonic acid). In all reactions, the polymer-supported buffer performed similarly to phosphate buffers. (Green Chem. DOI: 10.1039/c5gc00801h; José C. Barros)

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Do unconventional oil and gas processes pollute groundwater? This is a multimillion dollar question that scientists in many fields are trying to unravel, but there are no easy or simple answers. The variables are seemingly infinite.

The Barnett shale formation in Texas underlies 13,000 km2 of land, 17 counties, and the city of Fort Worth. It is thought to be the largest onshore source of natural gas in the United States. Currently, there are ≈20,000 unconventional oil and gas (UOG) wells in the basin.

The groundwater in this area is vulnerable to contamination from urban and rural sources. The Trinity and Woodbine aquifers, which overlie the Barnett shale, historically have been classified as “good”, with low levels of heavy metals and naturally elevated levels of total dissolved solids (TDS).

In a study of 100 private drinking water wells, K. A. Schug and colleagues (Environ. Sci. Technol. DOI: 10.1021/es4011724) reported higher levels of heavy metals compared with historical data. At least two other studies identified elevated amounts of methane and heavy metals associated with UOG extraction in Pennsylvania and other parts of Texas.

In the continuing effort to determine the effects of UOG activities on drinking water, Z. L. Hildebrand, K. A. Schug, and coauthors at Inform Environmental (Dallas), the University of Texas at Arlington, Tarleton State University (Stephenville, TX), the University of North Texas (Denton), and the University of Houston analyzed water samples from 550 wells in the Barnett shale region. At each well, the researchers measured basic water quality parameters such as temperature, dissolved oxygen, conductivity, TDS, salinity, and pH. They then analyzed the samples by several chromatographic techniques, including gas chromatography–mass spectrometry, headspace gas chromatography, and ion chromatography.

The authors used multiple regression analyses of the water quality parameters as a function of distance from the nearest UOG wells and depth of groundwater. Depth was a slightly better predictor of overall water quality, but the relationship was weak. They found elevated TDS and alkalinity throughout the sampled counties; but, as reported in other studies, this is not unusual for the two aquifers.

Elevated levels of nitrate and fluoride are most likely associated with agricultural activities and natural sources in the area. In several counties, beryllium, iron, and molybdenum levels exceeded maximum contaminant levels (MCLs); but because there is no historical precedent for these elements, more research is needed to determine their significance.

In several counties where UOG activity is the highest, the authors detected the presence of higher-than-expected levels of methanol, ethanol, dichloromethane, and the BTEX compounds (benzene, toluene, ethylbenzene, and xylene). In Montague County, 55 of the 66 samples contained a BTEX compound. This county is the site of underground injection wells for drilling-waste disposal from central Texas and Oklahoma. This “abundance of BTEX . . . is consistent with the characterization of produced water,” according to the authors.

The authors conclude that although this is the largest study so far of underground water quality in a UOG-active region, “the detection of numerous volatile organic compounds in aquifers above the Barnett shale does not necessarily implicate . . . UOG extraction as the source of contamination; however, it does provide an impetus for further monitoring and analysis of groundwater quality in this region.”

Do UOG processes contaminate groundwater? To be continued . . . . (Environ. Sci. Technol. DOI: 10.1021/acs.est.5b01526; Beth Ashby Mitchell)

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Gauge silver–π interactions with molecular torsion balances. Metal–π electron interactions are common classic noncovalent interactions between charged species and π systems. These interactions play an important role in assembling supramolecular materials and are crucial for mediating a variety of catalytic reactions. Accurately measuring the strengths of metal–π interactions, however, is usually challenging, especially for weaker ones such as silver–π interactions.

K. D. Shimizu and co-workers at the University of South Carolina (Columbia) designed and synthesized a series of molecular torsion balances that can be used to measure the energies of Ag–π interactions directly. They formed the molecular balances (13 in the figure) on the N-arylimide bicyclic framework, in which the aryl group orientation can change by rotating around the C–N bond.

Molecular torsion balances and their folding actions

In the solid state, balances 1 and 2 exist exclusively in the unfolded conformation (4a) because of the energetically favored CH–π interaction between the methyl group and the phenyl ring. But in the presence of silver fluoroborate (AgBF4), Ag+ coordinates to the pyridyl nitrogen atom; and both balances crystallize into the folded conformation (5b) as the result of Ag–π interactions.

The authors next used NMR spectroscopy to examine the folding–unfolding equilibria of balances 1 and 2 in solution as a function of Ag+ concentration. Based on energetic analysis with balance 3 (which does not exhibit Ag–π interactions) as a reference, they found that the energies of Ag–π interactions are –1.34 to –2.63 kcal/mol in methylene chloride. This energy range is comparable with noncovalent interactions of charged species. The strength of Ag–π interactions decreases as solvent polarity increases, possibly because of competing solvent binding.

These molecular torsion balances allow direct experimental measurement of the strengths of Ag–π interactions and provide new insights into metal–π interactions. This design can be extended to other metal–π interactions as a general strategy to quantify the thermodynamics of these noncovalent interactions experimentally. (J. Am. Chem. Soc. DOI: 10.1021/jacs.5b04554; Xin Su)

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Optimize the methylation of a ketone to an alcohol. An early step in the synthesis of a spleen tyrosine kinase inhibitor converts 4-acetylbenzonitrile to the corresponding tertiary alcohol via a Grignard reaction with methylmagnesium bromide (MeMgBr). When MeMgBr also acts as a base, however, the aldol impurities cannot be reduced to below 13% in the crude product and 5% in the purified product.

F. J. Weiberth and co-workers at Sanofi US R&D (Waltham, MA) modified the reaction conditions or used MeMgCl to make the Grignard reagent, but these changes did not solve the problem. Attempts to suppress enolate formation by adding 10% zinc chloride or stoichiometric lanthanum chloride–lithium chloride led only to modest decreases in the aldol byproduct.

Eventually, the authors used methyllithium and titanium tetrachloride to prepare methyltitanium trichloride (MeTiCl3), which chemoselectively produced the carbinol when they added 4-acetylbenzonitrile. They investigated various conditions for preparing this reagent and found that cumene–tetrahydrofuran is the best solvent, and –10 to 0 ºC is the preferred temperature range. (Org. Process Res. Dev. DOI: 10.1021/op5003769; Will Watson)

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A photo-Finkelstein reaction cleanly produces aryl iodides. Aryl iodides are important building blocks in organic chemistry. But the photo-iodination of organic compounds has been thought impossible because the product is photolabile. C.-J. Li and co-workers at McGill University (Montreal) report that aryl iodides can be made from other aromatic halides via a photochemical Finkelstein reaction.

The authors chose bromobenzene as a substrate and screened several variables such as iodide salt reactant, additive, and solvent. They found that sodium iodide in acetonitrile (MeCN) at room temperature is the best combination because the salt is soluble in MeCN, but the sodium bromide byproduct is not. This drives the equilibrium toward forming the product (see figure).

Photo-Finkelstein reaction to prepare aryl iodides and its proposed mechanism

The authors propose a reaction mechanism that is based on

  • homolytic scission of the C–Br bond to form phenyl and bromine radicals;
  • bromine oxidation of the iodine anion to the iodine radical; and
  • reaction of the iodine and phenyl radicals to produce iodobenzene (see figure).

In a side reaction, the phenyl radical reacts with MeCN to make benzene. Therefore, the authors added 10 mol% molecular iodine to the reaction mixture, which improved the yields to as high as 91%. The quantum efficiency (iodobenzene molecules produced per photon consumed) at 254 nm is 21%.

The authors tested several bromobenzenes and bromoheterocycles as substrates. Because the reaction is run under mild conditions, most polar substituents such as ester, nitrile, or hydroxyl groups are tolerated. Chlorobenzenes and vinyl bromides are also tolerated. This is a practical, green route to iodobenzenes. (J. Am. Chem. Soc. DOI: 10.1021/jacs.5b03220; José C. Barros)

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