A responsive poly(N-isopropylacrylamide)/poly(ethylene glycol) diacrylate hydrogel microsphere

A responsive hydrogel microsphere, which is constituted by poly(N-isopropylacrylamide)/poly(ethylene glycol) diacrylate, was fabricated in an aqueous two-phase system based on the polymer–polymer immiscibility. Characteristics of the hydrogel microsphere, such as the particle size and the morphology of freeze-dried or hydrated natural microspheres in water, tetrahydrofuran (THF)/H₂O (1:1 in volume) or acetone/H₂O (1:1 in volume), were investigated. The results showed that the swelling ratio and the particle size of the hydrogel microspheres were highly dependent on solvent composition. In addition, these characteristics were dramatically reduced when THF or acetone was added into the aqueous media. Scanning electron microscopy and environmental scanning electron microscopy micrographs also visually demonstrated that the regular spherical shape of the microspheres in water turned to irregular in shape when the microspheres were immersed in THF/H₂O or acetone/H₂O mixtures instead of pure water.     

Mass transfer from theophylline hydrogels of a‐PVA/H2O and a‐PVA/NaCl/H2O system on heating

Theophylline hydrogels of atactic‐poly(vinyl alcohol) (a‐PVA)/H₂O and a‐PVA/NaCl/H₂O systems were prepared followed by cyclic freezing (?30°C for 16 hr)–thawing (at room temperature for 8 hr) and one cycle gelation (at ?20°C for 24 hr) processes, respectively. In order to prepare xerogels (dried hydrogels) of these hydogel systems, an apparently first‐order mass transfer phenomenon of water as evaporation was observed for a‐PVA/H₂O hydrogel system, while heating at 60°C. The rate of evaporation decreased with increasing time in hyperbolic fashion. The total surface area (both lateral and two end surfaces of hydrogel matrix disc) decreased linearly for the first 90 min and thereafter had a tendency towards the steady‐state. The total mass flux showed time dependent linear reduction phenomenon, which is a characteristic physical behavior for these hydrogel systems on heat treatment. When NaCl was included in a‐PVA/H₂O system mass transfer of water followed fourth‐order polynomial. But in consideration of a comparative study, sustained mass transfer was found from the hydrogel matrices of a‐PVA/H₂O/NaCl system (gelation at ?20°C). Copyright © 2003 John Wiley & Sons, Ltd.     

Two lanthanum(III) complexes containing η2‐pyrazolate and η2‐1,2,4‐triazolate ligands: intramolecular C—H...N/O interactions and coordination geometries

The lanthanum(III) complexes tris(3,5‐diphenylpyrazolato‐κ²N,N′)tris(tetrahydrofuran‐κO)lanthanum(III) tetrahydrofuran monosolvate, [La(C₁₅H₁₁N₂)₃(C₄H₈O)₃]·C₄H₈O, (I), and tris(3,5‐diphenyl‐1,2,4‐triazolato‐κ²N¹,N²)tris(tetrahydrofuran‐κO)lanthanum(III), [La(C₁₄H₁₀N₃)₃(C₄H₈O)₃], (II), both contain La^III atoms coordinated by three heterocyclic ligands and three tetrahydrofuran ligands, but their coordination geometries differ. Complex (I) has a mer‐distorted octahedral geometry, while complex (II) has a fac‐distorted configuration. The difference in the coordination geometries and the existence of asymmetric La—N bonding in the two complexes is associated with intramolecular C—H...N/O interactions between the ligands.     

Ultrafine hydrogel fibers with dual temperature‐ and pH‐responsive swelling behaviors

Ultrafine hydrogel fibers that were responsive to both temperature and pH signals were prepared through the electrospinning of poly(N‐isopropylacrylamide) (PNIPAAm) and poly(acrylic acid) mixtures in dimethylformamide. Both the diameters (700 nm to 1.2 μm) and packing of the fibers could be controlled through changes in the polymer compositions and PNIPAAm molecular weights. These fibers were rendered water‐insoluble by the addition of either Na₂HPO₄ or poly(vinyl alcohol) (PVA) to the solution, followed by the heat curing of the fibers. The fibers crosslinked with Na₂HPO₄ swelled to 30–120 times in water; this was significantly higher than the swelling of those crosslinked with PVA. The PVA‐crosslinked hydrogel fibers, however, exhibited faster swelling kinetics; that is, they reached equilibrium swelling in less than 5 min at 25 °C. They were also more stable after 1 week of water exposure; that is, they lost less mass and retained their fibrous form better. All the hydrogel fibers showed a drastic increase in the swelling between pH 4 and 5. The PVA‐crosslinked hydrogel fibers exhibited distinct temperature‐responsive phase‐transition behavior of PNIPAAm, whereas the Na₂HPO₄‐crosslinked hydrogel fibers showed altered two‐stage phase transitions that reflected side‐chain modification of PNIPAAm. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 6331–6339, 2004     

Hydrolysis and Condensation Processes of Titanium iso-Propoxide Modified with Catechol: An NMR Study

The hydrolysis and condensation processes of titanium iso-propoxide modified with catechol (C₆H₄(OH)₂; H₂cat) have been investigated by ¹H, ¹³C and ¹⁷O nuclear magnetic resonance spectroscopy. The hydrolysis reactions of the modified titanium iso-propoxide in the system with Ti:tetrahydrofuran (THF):H₂O = 1:20:x (x = 1, 2 and 5 in a molar ratio) are essentially completed in the initial stage (<1 h), and the condensation reactions also proceed significantly during this stage. Upon hydrolysis with H₂O/Ti = 1, the iso-propoxy groups are selectively hydrolyzed and the catecholate groups remain bound to titanium. With H₂O/Ti = 2 and 5, both the iso-propoxy and catecholate groups are hydrolyzed, and the hydrolysis of the iso-propoxy groups is relatively preferential. Approximately half the catecholate groups are stably bound to titanium, even after hydrolysis with H₂O/Ti = 5.     

Hydrogen‐bonded assemblies of 20‐(4‐pyridyl)porphyrin‐54,104,154‐tribenzoic acid with dimethyl sulfoxide and 4‐acetylpyridine in their dimethyl sulfoxide and tetrahydrofuran solvates

Crystals of the title compounds, 20‐(4‐pyridyl)porphyrin‐5⁴,10⁴,15⁴‐tribenzoic acid–dimethyl sulfoxide (2/5), C₄₆H₂₉N₅O₆·2.5C₂H₆OS, (I), and 20‐(4‐pyridyl)porphyrin‐5⁴,10⁴,15⁴‐tribenzoic acid–4‐acetylpyridine–tetrahydrofuran (1/2/10), C₄₆H₂₉N₅O₆·2C₇H₇NO·10C₄H₈O, (II), consist of hydrogen‐bonded supramolecular chains of porphyrin units solvated by molecules of dimethyl sulfoxide [in (I)] and 4‐acetylpyridine [in (II)]. In (I), these chains consist of heterogeneous arrays with alternating porphyrin and dimethyl sulfoxide species, being sustained by COOH...O=S hydrogen bonds. They adopt a zigzag geometry and link on both sides to additional molecules of dimethyl sulfoxide. In (II), the chains consist of homogeneous linear supramolecular arrays of porphyrin units, which are directly connected to one another via COOH...N(pyridyl) hydrogen bonds. As in the previous case, these arrays are solvated on both sides by molecules of the 4‐acetylpyridine ligand via similar COOH(porphyrin)...N(ligand) hydrogen bonds. The two crystal structures contain wide interporphyrin voids, which accommodate disordered/diffused solvent molecules, viz. dimethyl sulfoxide in (I) and tetrahydrofuran in (II).     

Rapid deswelling of sodium alginate/poly(N-isopropylacrylamide) semi-interpenetrating polymer network hydrogels in response to temperature and pH changes

In this study, temperature-/pH-responsive semi-interpenetrating polymer network (semi-IPN) hydrogels based on linear sodium alginate (SA) and cross-linked poly(N-isopropylacrylamide) (PNIPAAm) were prepared. The semi-IPN hydrogels reached an equilibrium deswelling state within 6 h in response to temperature or pH stimuli. Compared with the conventional PNIPAAm hydrogel, their dewelling rate in response to temperature was improved significantly, owing to the formation of a porous structure within the hydrogels in the presence of ionized SA during the polymerization process. Moreover, the deswelling process could be well described with a first-order kinetics equation and it is possible to design any hydrogel with the desired deswelling behavior through the control of the SA content in the semi-IPN hydrogels.     

The heteroscorpionate ligand 2,2‐bis(3,5‐dimethylpyrazol‐1‐yl)‐1,1‐diphenylethanol and an easy preparation of its tungsten complex

The heteroscorpionate ligand 2,2‐bis(3,5‐dimethylpyrazol‐1‐yl)‐1,1‐diphenylethanol, C₂₄H₂₆N₄O, features in the solid state an intramolecular O—H…N hydrogen bond. A heteroscorpionate tungsten complex, cis‐[2,2‐bis(3,5‐dimethylpyrazolyl)‐1,1‐diphenylethanolato]chloridodioxidotungsten(VI) tetrahydrofuran monosolvate, [W(C₂₄H₂₅N₄O)ClO₂]·C₄H₈O, was prepared by the simple mixing of solutions of the ligand and WOCl₄ in tetrahydrofuran. The tungsten complex was isolated after standing for several weeks. The complex exhibits a κ³N,N′,O‐coordination of the ligand. This simple synthetic procedure allows access to the cis isomer in high yield without additional purification steps. The Hirshfeld surface analysis shows a change of the intermolecular contacts due to the coordination of the WO₂Cl unit with the ligand molecule.     

A synthetic hydrogel with thermoreversible gelation,III: an NMR study of the sol–gel transition

The sol–gel transition mechanism of a thermoreversible hydrogel composed of a copolymer comprising poly(N-isopropylacrylamide) and poly(ethylene glycol) (PNIPAAm–PEG) was studied by NMR. The ¹H– and ¹³C–NMR spectra measured on a PNIPAAm–PEG solution in 99.9% D₂O showed a remarkable line width broadening of the PNIPAAm block of more than that of the PEG block, during thermally induced hydrogel formation. This result suggested that the mobility of the PNIPAAm block is more restricted than that of the PEG block during gelation. A crosslinked polymer network formation was ascertained by a sudden reduction in the spin-lattice relaxation time (T₁) of the residual HDO proton during gelation. The temperature dependency of the T₁ values for the PNIPAAm and PEG blocks revealed that the microscopic condition of the PNIPAAm block in water was drastically changed during gelation, while that of the PEG block was unchanged. The experimental results from NMR supported the following gelation mechanism; that an aggregation of PNIPAAm blocks in the separate copolymers caused by hydrophobic interaction forms crosslinking points to give an infinite three-dimensional network structure. The hydrated PEG chains in the copolymers provide the network with a swelling property in water, and prevent the aggregation from causing a macroscopic phase separation.     

Cobalt(II) chloride adducts with acetonitrile,propan‐2‐ol and tetrahydrofuran: considerations on nuclearity,reactivity and synthetic applications

High‐spin cobalt(II) complexes are considered useful building blocks for the synthesis of single‐molecule magnets (SMM) because of their intrinsic magnetic anisotropy. In this work, three new cobalt(II) chloride adducts with labile ligands have been synthesized from anhydrous CoCl₂, to be subsequently employed as starting materials for heterobimetallic compounds. The products were characterized by elemental, spectroscopic (EPR and FT–IR) and single‐crystal X‐ray diffraction analyses. trans‐Tetrakis(acetonitrile‐κN )bis(tetrahydrofuran‐κO )cobalt(II) bis[(acetonitrile‐κN )trichloridocobaltate(II)], [Co(C₂H₃N)₄(C₄H₈O)₂][CoCl₃(C₂H₃N)]₂, (1), comprises mononuclear ions and contains both acetonitrile and tetrahydrofuran (thf) ligands, The coordination polymer catena‐poly[[tetrakis(propan‐2‐ol‐κO )cobalt(II)]‐μ‐chlorido‐[dichloridocobalt(II)]‐μ‐chlorido], [Co₂Cl₄(C₃H₈O)₄], (2′), was prepared by direct reaction between anhydrous CoCl₂ and propan‐2‐ol in an attempt to rationalize the formation of the CoCl₂–alcohol adduct (2), probably CoCl₂(HOⁱPr)_m . The binuclear complex di‐μ‐chlorido‐1:2κ⁴Cl :Cl‐dichlorido‐2κ²Cl‐tetrakis(tetrahydrofuran‐1κO )dicobalt(II), [Co₂Cl₄(C₄H₈O)₄], (3), was obtained from (2) after recrystallization from tetrahydrofuran. All three products present cobalt(II) centres in both octahedral and tetrahedral environments, the former usually less distorted than the latter, regardless of the nature of the neutral ligand. Product (2′) is stabilized by an intramolecular hydrogen‐bond network that appears to favour a trans arrangement of the chloride ligands in the octahedral moiety; this differs from the cis disposition found in (3). The expected easy displacement of the bound solvent molecules from the metal coordination sphere makes the three compounds good candidates for suitable starting materials in a number of synthetic applications.     

The bimolecular structure of aquahexa‐μ‐chlorido‐μ4‐oxido‐tris(tetrahydrofuran‐κO)tetracopper(II)–hexa‐μ‐chlorido‐μ4‐oxido‐tetrakis(tetrahydrofuran‐κO)tetracopper(II)–tetrahydrofuran (2/1/4)

The title bimolecular structure, [Cu₄Cl₆O(C₄H₈O)₃(H₂O)]₂[Cu₄Cl₆O(C₄H₈O)₄]·4C₄H₈O, at 100 K has monoclinic (P2₁/c) symmetry. The structure contains nine symmetry‐independent molecules expressed in simplest molecular form as 6[Cu₄Cl₆O(C₄H₈O)₃(H₂O)·2(C₄H₈O)]:3Cu₄Cl₆O(C₄H₈O)₄. The compound exhibits a supercell (smaller than the unit cell based on weak reflections) structure due to pseudotranslational symmetry. The structure displays O—H...O hydrogen bonding between bound water ligands and tetrahydrofuran (THF) solvent molecules. The structure exhibits disorder for 12 of the THF molecules, of which seven are ligated to Cu and five are hydrogen bonded to H₂O ligands.     

The reactions and composition of the surface intermediate species in the selective catalytic reduction of NO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> with ethylene over Co-ZSM-5

A pretreatment-transient reaction product analysis method was applied to study the reactions and average composition of the possible surface intermediate species in selective catalytic reduction with ethylene of NO _x over Co-ZSM-5. The reactions of the surface species, formed by the pretreatment of Co-ZSM-5 in a NO/C₂H₄/O₂ mixture at 275°C, with the NO/O₂ flow produced much more N₂ than that with the individual NO or O₂ flow. The similarity of N₂/CO _x /H₂O product distribution generated from the above surface species-NO/O₂ reactions and that from the normal NO/C₂H₄/O₂ flow reactions implies that the surface species NC _a O _b H _c formed in the three-component pretreatment process is very likely the primary intermediate surface species generated during the real flow reactions. The in situ FT-IR (DRIFT) spectroscopy measurements of the surface species support the above conclusion.     

Nanofibrosis of uncrystallizable poly(lactide‐co‐glycolide) via phase separation

Poly(lactide‐co‐glycolide) (PLGA) copolymers are a kind of biocompatible and biodegradable materials being widely used in tissue engineering. However, phase separation had not been reported successfully in fabricating these amorphous polymers into nanofibrous matrix, although this technique had shown advantages over electrospinning in producing a nanofiber network. In this study, tetrahydrofuran (THF)/H₂O solvent pairs were found suitable solvents to induce the formation of uniform PLGA gel at selected gelation temperatures. The results indicated that fine nanofibrous structures with fiber diameter around 40–60 nm could be obtained following the steps of gel formation, solvent extraction, and freeze‐drying, by controlling the concentration of PLGA/THF/H₂O solution, THF/H₂O ratio, and gelation temperature. Copyright © 2009 John Wiley & Sons, Ltd.     

Structural aspects of two α‐dihydrazones displaying a complete survey of intermolecular interactions

The compounds (2′E,2′E)‐2,2′‐(propane‐1,2‐diylidene)bis[1‐(2‐nitrophenyl)hydrazine], C₁₅H₁₄N₆O₄, (I), and (2Z,3Z)‐ethyl 3‐[2‐(2‐nitrophenyl)hydrazinylidene]‐2‐[2‐(4‐nitrophenyl)hydrazinylidene]butanoate tetrahydrofuran hemisolvate, C₁₈H₁₈N₆O₆·0.5C₄H₈O, (II), are puzzling outliers deviating from a general synthetic route aimed at the preparation of substituted pyrazoles. Possible reasons for this outcome, which is exceptional in an otherwise firmly established synthetic procedure, are analyzed. Compound (I) is unsolvated, while compound (II) crystallizes with a tetrahydrofuran solvent molecule lying on an inversion centre. The ethoxycarbonyl chain of (II), in turn, appears disordered into two equally populated (50%) moieties. In both structures, a plethora of different commonly occurring weak intermolecular interactions [viz. π(phenyl)...π(phenyl), π(C=N)...π(C=N), π(phenyl)...π(C=N), N—H...O and C—H...O] appear responsible for the crystal stability. Much less common are the short O(nitro)...O(nitro) contacts which are observed in the structure of (I), an example of unusual `electron donor–acceptor' (EDA) interactions.     

From poly(N‐isopropylacrylamide)‐block‐poly(ethylene oxide)‐block‐poly(N‐isopropylacrylamide) triblock copolymer to poly(N‐isopropylacrylamide)‐block‐poly(ethylene oxide) hydrogels: Synthesis and rapid deswelling and reswelling behavior of hydrogels

Poly(N‐isopropylacrylamide)‐block‐poly(ethylene oxide)‐block‐poly(N‐isopropylacrylamide) (PNIPAAm‐b‐PEO‐b‐PNIPAAm) triblock copolymer was synthesized via the reversible addition‐fragmentation chain transfer/macromolecular design via the interchange of xanthate (RAFT/MADIX) process with xanthate‐terminated poly(ethylene oxide) (PEO) as the macromolecular chain transfer agent. The successful synthesis of the ABA triblock copolymer inspired the preparation of poly(N‐isopropylacrylamide)‐block‐poly(ethylene oxide) (PNIPAAm‐b‐PEO) copolymer networks with N,N′‐methylenebisacrylamide as the crosslinking agent with the similar approach. With the RAFT/MADIX process, PEO chains were successfully blocked into poly(N‐isopropylacrylamide) (PNIPAAm) networks. The unique architecture of PNIPAAm‐b‐PEO networks allows investigating the effect of the blocked PEO chains on the deswelling and reswelling behavior of PNIPAAm hydrogels. It was found that with the inclusion of PEO chains into the PNIPAAm networks as midblocks, the swelling ratios of the hydrogels were significantly enhanced. Furthermore, the PNIPAAm‐b‐PEO hydrogels displayed faster response to the external temperature changes than the control PNIPAAm hydrogel. The accelerated deswelling and reswelling behaviors have been interpreted based on the formation of PEO microdomains in the PNIPAAm networks, which could act as the hydrophilic tunnels to facilitate the diffusion of water molecules in the PNIPAAm networks. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Crystal Structures of [Mn2(H2O)4(bpy)2(C8H12O4)2] · 2 H2O and [Mn(H2O)2(bpy)(C8H12O4)2/2] · H2O with bpy = 2,2′‐bipyridine

Reaction of MnSO₄ · H₂O, 2,2′‐bipyridine (bpy), suberic acid and Na₂CO₃ in CH₃OH/H₂O yielded a mixture of [Mn₂(H₂O)₄(bpy)₂(C₈H₁₂O₄)₂] · 2 H₂O ( 1 ) and [Mn(H₂O)₂‐ (bpy)(C₈H₁₂O₄)_2/2] · H₂O ( 2 ). In both complexes, the Mn atoms are octahedrally coordinated by two N atoms of one bpy ligand and four O atoms of two trans positioned H₂O molecules and two suberato ligands (d(Mn–O) = 2.107–2.328 Å; d(Mn–N) = 2.250–2.330 Å). The bis‐monodentate suberato ligands bridge Mn atoms to form dinuclear [Mn₂(H₂O)₄(bpy)₂(C₈H₁₂O₄)₂] complex molecules in 1 and 1D [Mn(H₂O)₂(bpy)(C₈H₁₂O₄)_2/2] chains in 2 . Via the intermolecular hydrogen bondings and π‐π stacking interactions, the dinuclear molecules in 1 are assembled into 2D networks parallel to (100), between which the crystal H₂O molecules are sandwiched. The polymeric chains in 2 are linked together by interchain hydrogen bonding and π‐π stacking interactions into 3D networks with the crystal H₂O molecules located in tunnels along [010]. Crystal data for 1 : P2₁/c (no. 14), a = 10.092(1) Å, b = 11.916(2) Å, c = 17.296(2) Å, β = 93.41(1)° and Z = 2. Crystal data for 2 : P2₁/c (no. 14), a = 11.176(2) Å, b = 9.688(1) Å, c = 37.842(6) Å, β = 90.06(1)° and Z = 8.     

Supramolecular structure of racemic 5‐hydroxy‐4‐(4‐methoxyphenyl)‐10‐oxa‐4‐azatricyclo[5.2.1.02,6]dec‐8‐en‐3‐one

The structure of the title compound, C₁₅H₁₅NO₄, comprises a racemic mixture of chiral molecules containing five stereogenic centres. The cyclohexane ring tends towards a boat conformation and the two tetrahydrofuran rings adopt envelope conformations. Molecules are linked into sheets parallel to (100) by a combination of O—H...O, C—H...O and C—H...π hydrogen bonds, leading to a two‐dimensional supramolecular structure.     

In situ Cu(0) catalyzed SET‐LRP: The first attempt

In this work, single electron transfer‐living radical polymerization (SET‐LRP) was catalyzed by in situ Cu(0) generated from copper sulphate pentahydrate (CuSO₄·5H₂O) and hydrazine hydrate (N₂H₄·H₂O) at 25 °C. The polymerization occurred smoothly with moderate controllability: the polymerization rates increased by the increases of N₂H₄·H₂O, and the initiator concentration had an optimal value on the polymerization rate; the number‐average molecular weights (M_n,GPC) increased with monomer conversions and polydispersities were below 1.40. The M_n,GPC deviated much from theoretical ones with about 50% polymer chain‐end fidelities. Some side reactions stemming from the strong reduction performance of N₂H₄·H₂O were responsible for the mildly controlled polymerizations. This polymerization can be conducted in SET‐LRP unfavorable solvents or in bulk, such as toluene and tetrahydrofuran, owing to the H₂O contained in CuSO₄·5H₂O and N₂H₄·H₂O. On account of the utilization of CuSO₄·5H₂O, an inactive Cu(II) compounds in LRP area, this work confirmed from experimental level that it was Cu(0) which acted as activator and mediator in SET‐LRP. This work provided a first example of in situ Cu(0) catalyzing SET‐LRP with CuSO₄·5H₂O as a copper source. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Complexes of rare-earth metals with meconic acid

Two series of complexes of meconic acid (H₃ Mec) with rare-earths have been prepared by varying the preparative procedure. The compounds have the general formulae, [Ln(Mec) (H₂O)₂]·3 H₂O (whereLn=La, Ce, Pr, Nd, Sm, Ho and Y) and [Ln(HMec) (H₂ Mec) (H₂O)₂]·4 H₂O (whereLn=La, Pr, Nd and Sm). The infrared spectral data indicate that the carboxylate groups are bound to the rare-earth metal in a bidentate fashion. Thermal studies indicate that two water molecules are coordinated in each case. The complexes are probably polymeric.

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Komplexe von Seltenerd-Metallen mit Meconsäure

Zusammenfassung Es wurden zwei Reihen von Komplexen der Meconsäure (H₃ Mec) mit Seltenerd-Metallen mit den allgemeinen Formeln [Ln(Mec)(H₂O)₂]·3 H₂O (Ln=La, Ce, Pr, Nd, Sm, Ho, Y) und [Ln(HMec)₂(H₂ Mec) (H₂O)₂]·4 H₂O (Ln=La, Pr, Nd, Sm) hergestellt. Die IR-Spektren zeigen, daß die Carboxylat-Gruppen in zweizähniger Weise mit den Metallionen koordinieren; thermische Untersuchungen ergeben, daß in beiden Reihen jeweils zwei Wassermoleküle zusätzlich koordiniert sind. Die Komplexe weisen wahrscheinlich eine Polymerstruktur auf.

     

Echange d'oxygène entre des p-benzoquinones substituées et l'eau

The velocity of oxygen exchange between several substituted p-benzoquinones and H₂¹⁸O has been measured at 25°C in neutral tetrahydrofuran containing 2 to 5% water. Halogen substituents accelerate the exchange (Cl > Br), methyl groups slow it down. The reaction was found to be second or third order in [H₂O].