Hydration Dehydration Research Articles

Importance of bound water in hydration–dehydration behavior of hydroxylated poly( N-isopropylacrylamide)

In this study, a differential scanning calorimetric analysis was performed to investigate the role of water existing around the polymer chains on their thermoresponsive behaviors using the novel functional temperature-sensitive copolymer, poly( N-isopropylacrylamide- co-2-hydroxyisopropylacrylamide) (poly(NIPAAm- co-HIPAAm)). The HIPAAm comonomers were incorporated into the polymeric chains as hydrophilic parameters, and then the hydration states of poly(NIPAAm- co-HIPAAm) with various HIPAAm compositions were examined. Bound water, which is affected by the polymeric chains to some extent, was produced by adding the copolymers to the water, and the temperature due to the melting of the bound water decreased as the HIPAAm content increased. On the basis of this result, we considered that the interaction between the bound water and the polymeric chains is reinforced by the increasing HIPAAm composition. In addition, the cloud points of the copolymers shifted to a higher temperature, and the endothermic enthalpy for the phase transition decreased with increasing the HIPAAm content, suggesting that the number of water molecules disassociated from the polymeric chains decreased. Based on these results, we postulate that the changes in the interaction between the thermosensitive polymer and bound water exert a strong influence on its phase transition and/or separation, such as the cloud point or dehydration behavior.

Regioselective Synthesis of 6‐Prenylpolyhydroxyisoflavone (Wighteone) and Wighteone Hydrate with Hypervalent Iodine

The oxidative rearrangement of 3′‐iodotetraalkoxychalcone with [hydroxyl(tosyloxy)iodo]benzene, followed by cyclization of the resultant acetal gave 6‐iodotrialkoxyisoflavone. The coupling reaction of the isoflavone with 2‐methyl‐3‐butyn‐2‐ol gave 6‐alkynylisoflavone, whose hydrogenation gave wighteone hydrate. Wighteone was synthesized by dehydration of wighteone hydrate.

Aspects of Electrochemical Behavior of Aldehydes and Ketones in Protic Media

Reduction of carbonyl group in aldehydes and ketones, as well as oxidation of numerous aldehydes is discussed, as well as those reductions of organic compounds where the CO group activates cleavage of an adjacent CX bond where X is a good leaving group like halogen, OH, NH2 or SR or activates hydrogenation of an adjacent CC group. Survey involves aliphatic and aromatic aldehydes, aryl alkyl and diaryl ketones, as well as α-ketoacids, 1,2-diketones and compounds where the carbonyl group is a part of a ring. The role of acid–base, hydration–dehydration and in some cases keto–enol equilibria on electrochemical behavior is pointed out, as well as the role of buffer kind and concentration and the nature of the cation of supporting electrolyte. Better understanding of these factors promises finding of best conditions for electroanalytical procedures.

Characterization of water status in primed seeds of tomato (Lycopersicon esculentum Mill.) by sorption properties and NMR relaxation times

The enhanced laboratory and field emergence characteristics of osmo- and halo-primed tomato seeds (cv. Pusa Ruby) were related to changes in hydration–dehydration kinetics, modified sorption properties and nuclear magnetic resonance (NMR) relaxation behaviour of humidified and imbibed seeds. Water sorption isotherms were constructed for primed and unprimed seeds by equilibrating to different water activities (aw) at 25°C. Analysis of the isotherms by the D'Arcy–Watt equation revealed that priming reduced the number of strong binding sites and the associated water content, and increased significantly the number of weak binding sites and the associated water content. This redistribution of water, which increased the availability of seed water, may be the reason for the higher speed of germination of primed seeds. The changes in transverse relaxation time (T2) of seed water and its components, measuredin vivousing nuclear magnetic resonance spectroscopy, showed interesting differences between primed and unprimed seeds. With an increase in humidification time, the T2of primed seeds could be resolved into three components with varying mobilities, while the control seeds had only two components until 10 d of humidification. During imbibition, the third component appeared after 2 and 6 h in primed and control seeds, respectively. This component disappeared after the germination process started in all treatments. The third fraction, with very low molecular mobility, which accounted for about 40% of the proton population, was assigned to hydration water of macromolecules. Hence, we propose that better performance of primed seeds may be attributed to the modifications of seed water-binding properties and reorganization of seed water during imbibition, so as to increase the macromolecular hydration water required for various metabolic activities related to the germination process.

Effect of dioxane on the structure and hydration–dehydration of α-chymotrypsin as measured by FTIR spectroscopy

A new experimental approach based on FTIR spectroscopic measurements was proposed to study simultaneously the adsorption/desorption of water and organic solvent on solid enzyme and corresponding changes in the enzyme secondary structure in the water activity range from 0 to 1.0 at 25 °C. The effect of dioxane on the hydration/dehydration and structure of bovine pancreatic α-chymotrypsin (CT) was characterized by means of this approach. Dioxane sorption exhibits pronounced hysteresis. No sorbed dioxane was observed at low water activities ( a w < 0.5) during hydration. At a w about 0.5, a sharp increase in the amount of sorbed dioxane was observed. Dioxane sorption isotherm obtained during dehydration resembles a smooth curve. In this case, CT binds about 150 mol dioxane/mol enzyme at the lowest water activities. Three different effects of dioxane on the water binding by the initially dried CT were observed. At a w < 0.5, water adsorption is similar in the presence and absence of dioxane. It was concluded that the presence of dioxane has little effect on the interaction between enzyme and tightly bound water at low a w. At a w > 0.5, dioxane increases the amount of water bound by CT during hydration. This behavior was interpreted as a dioxane-assisted effect on water binding. Upon dehydration at low water activities, dioxane decreases the water content at a given a w. This behavior suggests that the suppression in the uptake of water during dehydration may be due to a competition for water-binding sites on chymotrypsin by dioxane. Changes in the secondary structure of CT were determined from infrared spectra by analyzing the structure of amide I band. Dioxane induced a strong band at 1628 cm −1 that was assigned to the intermolecular β-sheet aggregation. Changes in the intensity of the 1628 cm −1 band agree well with changes in the dioxane sorption by CT. An explanation of the dioxane effect on the CT hydration and structure was provided on the basis of hypothesis on water-assisted disruption of polar contacts in the solid enzyme. The reported results demonstrate that the hydration and structure of α-chymotrypsin depend markedly on how enzyme has been hydrated — whether in the presence or in the absence of organic solvent. A qualitative model was proposed to classify the effect of hydration history on the enzyme activity- a w profiles.

Effect of hydration and dehydration cycles on seed germination of Aster kantoensis (Compositae)

Seeds of Aster kantoensis Kitamura (Compositae) were experimentally exposed to different cycles of hydration and dehydration: 3H1D (cycles of 3-d hydration and 1-d dehydration periods), 2H3D, 2H1D, 1H3D, 1H2D, and 1H1D. Under continuous hydration (control), all viable seeds germinated within 9 d. However, all viable seeds exposed to the 3H1D, 2H3D, and 2H1D cycles germinated within 36, 50, and 36 d of the start of the experiment, respectively. Not all viable seeds exposed to the 1H3D, 1H2D, and 1H1D cycles germinated during the experimental period. Compared with the control, the number of days required for 50% germination increased in seeds exposed to the hydration–dehydration treatments except for those seeds exposed to the 3H1D and 2H1D cycles. In addition, seeds treated with a 1-d hydration period required a larger number of cumulative hydration days for 50% germination than those seeds exposed to one of the other three treatments or the control. These results indicate that sporadic germination of A. kantoensis seeds during an extended period of several months in the gravelly flood plain where they are found results because the dispersed seeds are exposed to various cycles of hydration and dehydration depending upon the rainfall pattern and spatial heterogeneity of evaporation rates at the sand surface resulting from shading by gravel.Key words: Aster kantoensis, dehydration tolerance, gravelly flood plain, hydration–dehydration cycles, seed germination.

Electroreduction and oxidation of terephthalaldehyde and the role of hydration

Electroreduction of terephthalaldehyde (benzene-1,4-dicarboxaldehyde) (I) in protic media is complicated by acid–base and hydration–dehydration equilibria. At pH 0–5.5 the diprotonated form of I is reduced in a reversible two-electron step to a diradical-quinonemethide. This species is at pH 1–3 converted into an aldol (4-HOCH 2C 6H 4CHO), which is further reduced at a more negative potential. At pH 5.5–8.5 the monoprotonated form is reduced. The protonated formyl group is reduced first and then, at a more negative potential, the second formyl group in the aldol, formed in the first step, is further reduced. As terephthalaldehyde is in aqueous solutions present in about 15% as a monohydrate, the limiting currents in this pH range are controlled by the hydration–dehydration equilibrium, the establishment of which is acid and base-catalyzed. The reduction of the unprotonated form occurs at pH > 8.5. At pH > 10 addition of OH − ions takes place. The formation of a geminaldiol anion results in a decrease of reduction waves of the aldehyde, but an increase in the oxidation anodic wave of the diol. Dependence of anodic waves on activity of OH − ions indicates a consecutive addition of OH − ions to the two formyl groups. The understanding of the sequence of chemical (protonation, hydration) and electrochemical reaction steps enabled application of changes of polarographic i–E curves with time as an analytical method for following (in real time) concentration changes of the parent compound, its monohydrazones and monoximes as well as dihydrazones and dioximes.

Pedogenic transformation of magnetic minerals in Pliocene–Pleistocene palaeosols of the Siwalik Group, NW Himalaya, India

Abstract We present preliminary rock magnetic and geochemical analyses of thirty-one pedogenic intervals from six palaeosol profiles in a Pliocene–Pleistocene fluvial sequence of the Siwalik Group in NW Himalaya. The palaeosols are characterised by low magnetic susceptibility (χlf mean=9×10−8 m3/kg), low frequency dependent susceptibility (χfd mean=0.93%), high coercivity of remanence ((B0)cr mean=447 mT) and high mean hysteresis ratios (Mrs/Ms mean=0.6) as a result of the predominance of imperfect antiferromagnetic minerals, hematite and goethite. Stepwise IRM acquisition to 7 T and backfield demagnetisation to 1 T suggest the predominance of goethite in several intervals. The relative contributions of goethite and hematite are derived from differential rates of IRM acquisition, which are further confirmed using stepwise thermal demagnetisation of orthogonal IRM components acquired at 4, 1 and 0.3 T. The absence of a significant superparamagnetic fraction, and the apparent depth control of χfd%, are attributed to post-depositional processes of compaction and burial diagenesis. Anomalously high coercivities within the goethite-enriched horizons may have resulted from TRM acquisition due to the high crustal heat flows due to cumulative burial to depths of over 1 km. Pedogenically induced magnetic mineral production and transformation within the palaeosol profiles are inferred using a range of rock magnetic parameters, geochemical ratios (e.g. Rb/Sr and Al/Si), and percentage organic/inorganic carbon content. We use magnetic ratios based on the relative contributions of goethite, hematite and soft ferrimagnetic minerals to infer climate-sensitive processes of oxidation–reduction, hydration–dehydration, hydroxylation–dehydroxylation and humification. In the earliest Pliocene palaeosol, we infer the pedogenic depletion of magnetic susceptibility due to the transformation of parental ferrimagnetic minerals mainly into antiferromagnetic minerals. In contrast, the youngest (middle Pleistocene) profile shows significant modification of parental antiferromagnetic minerals into ferrimagnetic forms. We suggest that further detailed rock magnetic and geochemical studies using a higher sampling density and at a regional scale would help reconstruct the pedogenic transformation of iron oxides in the tropical–subtropical Siwalik soils and the effect of post-depositional and burial processes upon them.

Pedogenic transformation of magnetic minerals in Pliocene–Pleistocene palaeosols of the Siwalik Group, NW Himalaya, India

We present preliminary rock magnetic and geochemical analyses of thirty-one pedogenic intervals from six palaeosol profiles in a Pliocene–Pleistocene fluvial sequence of the Siwalik Group in NW Himalaya. The palaeosols are characterised by low magnetic susceptibility ( χ lf mean=9×10 −8 m 3/kg), low frequency dependent susceptibility ( χ fd mean=0.93%), high coercivity of remanence (( B 0) cr mean=447 mT) and high mean hysteresis ratios ( M rs/ M s mean=0.6) as a result of the predominance of imperfect antiferromagnetic minerals, hematite and goethite. Stepwise IRM acquisition to 7 T and backfield demagnetisation to 1 T suggest the predominance of goethite in several intervals. The relative contributions of goethite and hematite are derived from differential rates of IRM acquisition, which are further confirmed using stepwise thermal demagnetisation of orthogonal IRM components acquired at 4, 1 and 0.3 T. The absence of a significant superparamagnetic fraction, and the apparent depth control of χ fd%, are attributed to post-depositional processes of compaction and burial diagenesis. Anomalously high coercivities within the goethite-enriched horizons may have resulted from TRM acquisition due to the high crustal heat flows due to cumulative burial to depths of over 1 km. Pedogenically induced magnetic mineral production and transformation within the palaeosol profiles are inferred using a range of rock magnetic parameters, geochemical ratios (e.g. Rb/Sr and Al/Si), and percentage organic/inorganic carbon content. We use magnetic ratios based on the relative contributions of goethite, hematite and soft ferrimagnetic minerals to infer climate-sensitive processes of oxidation–reduction, hydration–dehydration, hydroxylation–dehydroxylation and humification. In the earliest Pliocene palaeosol, we infer the pedogenic depletion of magnetic susceptibility due to the transformation of parental ferrimagnetic minerals mainly into antiferromagnetic minerals. In contrast, the youngest (middle Pleistocene) profile shows significant modification of parental antiferromagnetic minerals into ferrimagnetic forms. We suggest that further detailed rock magnetic and geochemical studies using a higher sampling density and at a regional scale would help reconstruct the pedogenic transformation of iron oxides in the tropical–subtropical Siwalik soils and the effect of post-depositional and burial processes upon them.

Studies of processes occurring during alkoxide derived V–O–W unsupported catalyst formation

The aim of this paper is to investigate the processes occurring during V–O–W unsupported catalyst formation in order to gain the knowledge necessary to design supported catalysts. Sol–gel synthesis of V–W hydroxo-oxide hydrate (W:V=9:1) from tungsten and vanadyl isopropoxides is described. The hydrate structure is shown to be related to WO3·0.33H2O. The role of vanadium in the formation of this structure is discussed. Hydrate dehydration is shown to result in the formation of V–W hydroxo-oxide isostructural with hexagonal WO3. Further heating of hydroxy-oxide causes a loss of hydroxy groups accompanied by the formation of V–W oxide bronze isostructural with tetragonal WO3. Annealing the bronze in an air atmosphere results in the removal of vanadium from the bronze crystallites as a results of its surface segregation. The formation of the surface vanadia-like species on WO3-related crystallites is found to be a result of vanadium segregation. Increasing the annealing temperature causes recrystallisation of the surface species. The synthesis of a V–W/TiO2 oxide catalyst via a mixed oxide formation in the presence of titania sols followed, by surface vanadium segregation in an air containing atmosphere, is proposed. The application of the knowledge gained during our investigation of the formation of the unsupported V–O–W catalyst to the discussion of the morphology of the V–O–W/Ti(Sn)O2 (rutile) catalyst is also demonstrated.

Fiber-Cement Extrudates with Perlite Subjected to High Temperatures

Short fiber reinforced cement composites with large volumes of perlite have been investigated for their resistance to high temperature exposures. The composites were manufactured by the extrusion technique. The extruded products, thin plates and cylinders, were subjected to different thermal treatments with the maximal exposure temperatures, \IT\dm\N, of 200, 300, 450, and 600°C, and exposure periods, \IT\dh\I, of 1 and 10 h. After that, the residual mechanical properties of those extrudates, such as flexural strength, ƒ\i\dt, compressive strength, ƒ\i\dc, and modulus of elasticity, \iE were measured and normalized to the corresponding values obtained from specimens without thermal treatments. The weight loss ratios, ω, of the composites were also monitored during the thermal treatment process. In addition, the microstructure of these composites was studied using a scanning electron microscope and the pore structure was examined by mercury intrusion porosimetry. It was found that ƒ\i\dt, ƒ\i\dc, and \iE significantly decreased as \IT\dm\N, \IT\dh\N, or both increased. The property deterioration could be attributed to fiber melting or softening, dehydration of cement hydrates, and porosity increasing during the heating process. Flexural strength, ƒ\i\dt, decreased more remarkably than compressive strength, ƒ\i\dc, and modulus of elasticity, \iE. The thin sheets reinforced by polyvinyl acetate (PVA) fibers showed more significant reduction in flexural strength than those reinforced by glass fibers due to the much lower melting point of PVA fiber. The composites, incorporating silica sand as aggregates, showed greater degeneration of mechanical properties than those using perlite as aggregates. The improvement of thermal resistance of perlite specimens could be attributed to the greater specific heat, lower thermal conduction coefficient, and the better thermal insulation property of perlite.

Hydration testing in collegiate wrestlers undergoing hypertonic dehydration.

Because dehydration (DEH) violates assumptions used in the assessment of body composition, hydration testing has become an integral part of minimal weight (MW) assessment. To determine the accuracy of hydration tests for the detection and quantification of hypertonic DEH. Twenty-five male collegiate wrestlers (mean +/- SD, age: 20.0 +/- 1.4 yr, height: 175.0 +/- 7.1 cm, body mass: 81.7 +/- 15.3 kg) had their hydration assessed under well-controlled conditions of euhydration (EUH) and DEH. The DEH phase occurred on the same day as EUH, after subjects acutely dehydrated 2-6% of body weight through fluid/food restriction and exercise in a hot environment. All hydration tests except plasma potassium significantly increased from EUH to DEH, and meaningful cutoff values could be established for most tests. Cutoff values for urine tests were 586 mOsm.L(-1) for osmolality and 71 mEq.L(-1) for potassium. Plasma cutoff values were 293 mOsm.L(-1) for osmolality, 140 mEq.L(-1) for sodium, 103 mEq.L(-1) for chloride, and 3.5 pg.mL(-1) for arginine vasopressin. For ratio tests, a urine:plasma osmolality of 2.06 and an extracellular:intracellular water of 0.533 measured by the bioelectrical impedance spectroscopy were cutoff values. For urine specific gravity, a cutoff value of 1.020 g.mL(-1) had a sensitivity and specificity of 96% each for the automated harmonic oscillation technique and 87% and 91% (respectively) for the dipstick technique. Protein (by dipstick) was detected in 5% of subjects in EUH, and 100% of subjects in DEH. Correlations between hydration tests and dehydration were only low to moderate. This study supports a specific gravity cutoff of 1.020 g.mL(-1) for the identification of hypertonic DEH. Future research should test the cutoff values established in this study and explore the relationship between DEH and urine protein.

Structural modifications of Callovo-Oxfordian argillite under hydration/dehydration conditions

The clay minerals are among the first silicated minerals to be transformed physically and chemically during changes of temperature, pressure, relative humidity and fluid composition. One of the first physical processes, extremely rapid, is the hydration and/or dehydration of swelling clays minerals. These processes could generate changes of the clay structure and consequently the texture of an argillaceous rock. In order to observe in situ structural modifications on a deep argillaceous rock “argillite” (Parisian basin, in France) according hydration–dehydration cycles, an Environmental Scanning Electron Microscope (ESEM) was used. This instrument allows the possibility to observe geological samples in their natural state without preliminary preparation or modification. Two types of samples were prepared, parallel and perpendicular to the lithology. Then, each sample was submitted to three water condensation/evaporation cycles. The observed samples were from the Callovo-Oxfordian formations in HTM 80743, HTM 983, HTM 02618 and EST 2159 cores. ESEM observations show that the water sensitivity of these bulk samples depends directly on the clay mineral proportion in the rock, and on the clay family present. It is also obvious that the water sensitivity depends on the anisotropy of the pore structure and the particle size and total porosity. The main structural modifications observed were the cracking of the surface, particles aggregation/disaggregating and opening/closing of pores and/or cracks.

Effect of Hydration–Dehydration Processes on the Structure and Porosity of Detemplated Silica Fibers

A study was carried out on the conditions of template removal (calcination or extraction by solvents) and hydration–dehydration processes on the hexagonal structure and porosity of detemplated fibers. The hydration–dehydration cycle leads to a decrease in fiber porosity, especially in fiber detemplation by extraction with surfactant solvents such as ethanol and acetic acid.

Kinetics of thermal dehydration and decomposition of hydrated chlorides of some 3d transition metals (Mn-Co series). Part-III. Dehydration and decomposition of CoCl2 hydrates

The kinetics of dehydration of NiCl 2 .2H 2 O have been studied by isothermal method in air and by non-isothermal method in air as well as in flowing nitrogen atmosphere. While in air (or self-generated atmosphere) the dehydration apparently takes place in single step, in nitrogen atmosphere two steps, corresponding to the losses of about 1.46 and 0.48 moles of H 2 O, can be identified in the TG curves. Whereas in static air three-dimensional diffusion (D3 or D4 model) appears to be the best fit mechanistic model, in nitrogen atmosphere the three-dimensional progress of the reaction phase boundary (R3) followed by the first order decay of partially dehydrated porous particles (Fl ) appear to be the best fit models for the two steps of dehydration. When the hydrated salt is calcined at temperature above 450, rapid dehydration renders the material porous and therefore, subsequent dechlorination also follows first order decay law (Fl). Generally, there is a broad agreement in mechanistic model as well as in Arrhenius parameters between isothermal and non-isothermal methods of dehydration.

Kinetics of thermal dehydration and decomposition of hydrated chlorides of some 3d transition metals (Mn-Cu series). Part-I. Dehydration of MnCl2.4H2O

The thermal dehydration of MnCl 2 hydrate has been studied by using simultaneous TG/DTA under the static air (or self-generated atmosphere) and also under the flowing nitrogen condition at different heating rates. In either of the above condition the salt dehydrates in three steps, with the losses of about 1.0, 1.2 and 1.1 moles of water in static air and 0.9, 2.1 and 1.1 moles of water in flowing nitrogen atmosphere. The heats of the three steps of dehydration in air have been estimate2d from area under the corresponding DTA peaks at the heating rate of 10°/min. The first step of dehydration is accompanied with the melting of the salt and therefore does not follow any kinetic model of solid state decomposition. For the other two steps of dehydration changes in the mechanism are observed and attempts have been made to identify these changes. All these have been possible by converting non-isothermal time into isothermal time which is more appropriate in the kinetic analysis. It is observed that either nucleation and growth or progress of the reactant/product interface is generally the most suitable kinetic model for the dehydration of manganese chloride hydrate.

Electrochemical permeability measurements of hydrophilic and hydrophobized montmorillonite films

Abstract In the first part of our electrochemical (cyclic voltammetric) study of montmorillonite-modified electrodes, results about film permeability have been presented. The structure of the porous aerogel–hydrogel and the effect of layer-thickness have been discussed. The more compact structure and less permeable Na- and H-montmorillonite result in smaller peak currents characteristic of the permeability of the clay film, than with more permeable Ca-montmorillonite. The thicker the film the less organized the structure and the value of the peak current decreases at increasing thickness.The kinetics of the hydration–dehydration, swelling–shrinking (“break-in/leach-out”) processes and the effect of hydrophobization were demonstrated, too. Kinetical measurement data provide information about diffusional transport via macro, meso-, and micropores, and thus characterize pore structure. Permeability decreases with increasing extents of hydrophobization. The rate constants of structure-dependent macro- and microdiffusion have been calculated.

Effect of hydration–dehydration cycles on germination of seven Calligonum species

A greenhouse experiments were conducted to study the effects of imbibition phase and hydration–dehydration cycles on the germination of seven Calligonum species, dominant shrubs in mobile sand dunes and stabilized sand field in the northern desert of China. In August 1998, seeds of Calligonum were collected and were treated by different hydration–dehydration cycles. The results show that there is not relationship between the imbibition phase and germination rate. Compared to the control, the minimum germination time for seven species was unaffected until seeds were treated to at least three hydration–dehydration cycles, cycles with delays did not alter the minimum germination time from that achieved with no hydration–dehydration cycles, all cycles treatments decreased the complete germination percentage of Calligonum rubicundum, C. gobicum, C. mongolicum, C. caput-medusae and C. arborescens.

Effects of Hydration-Dehydration and Iodination on Seed-Borne Fungi of Duma Wheat Variety

The drought resistant Duma seeds of wheat was subjected to hydration –dehydration (HD) and Iodination (I) treatment to enhance the seed quality as well as protect it\'s deterioration during aging. The effect of the treatments was investigated on external and internal seed-borne fungi, levels of free-fatty acids, moisture content and vigor index. The treatments could effectively reduce both external and internal seed-borne fungi. Reduced levels of free fatty acids and moisture content and high value of vigor index also manifested the effect of the treatments. These findings strongly suggest that the hydration-dehydration and iodination treatments effectively improved the quality of the wheat seeds. (Af. J. of Science and Technology: 2003 4(1): 62-66)

Equilibrium constants for dehydration of water adducts of aromatic carbon-carbon double bonds.

Equilibrium constants (K(de)) are reported for the dehydration of hydrates of benzene, naphthalene, phenanthrene, and anthracene. Free energies of formation of the hydrates (DeltaG(o) (f)(aq)) are derived by combining free energies of formation of the parent (dihydroaromatic) hydrocarbon with estimates of the increment in free energy (DeltaG(OH)) accompanying replacement of a hydrogen atom of the hydrocarbon by a hydroxyl group. Combining these in turn with free energies of formation of H(2)O and of the aromatic hydrocarbon products furnishes the desired equilibrium constants. The method depends on the availability of thermodynamic data (i) for the hydrocarbons from which the hydrates are derived by hydroxyl substitution and (ii) for a sufficient range of alcohols to assess the structural dependence of DeltaG(OH). The data comprise chiefly heats of formation and standard entropies in the gas phase and free energies of transfer from the gas phase to aqueous solution (the latter being derived from vapor pressures and solubilities). They also include experimental measurements of equilibrium constants for dehydration of alcohols, especially cyclic, allylic, and benzylic alcohols. In general DeltaG(OH) depends on whether the alcohol is (a) primary, secondary, or tertiary; (b) allylic or benzylic; and (c) open chain or cyclic. Differences in geminal interactions of the hydroxyl group of the alcohol with alpha-alkyl and vinyl or phenyl groups account for variations in DeltaG(OH) of 5 kcal mol(-1). Weaker variations which arise from beta-vinyl/OH or beta-phenyl/OH interactions present in the aromatic hydrates but not in experimentally studied analogues are estimated as 1.0 kcal mol(-1). Equilibrium constants for dehydration may be expressed as their negative logs (pK(de)). Reactions yielding the following aliphatic, aromatic, and antiaromatic unsaturated products then have pK(de) values: +4.8, ethene; +15.0, ethyne; +22.1, cyclopropene; +28.4 cyclobutadiene; -22.2, benzene; -14.6, naphthalene; -9.2, phenanthrene; -7.4, anthracene. Large positive values are associated with formation of strained or antiaromatic double bonds and large negative values with aromatic double bonds. Trends in pK(de) parallel those of heats of hydrogenation. The results illustrate the usefulness of a substituent treatment for extending the range of currently available free energies of formation. In addition to hydroxyl substituent effects, DeltaG(OH), values of DeltaG(pi) for substitution of a pi-bond in a hydrocarbon are reported.