Hydrogen-bonded complexes of 1-chloro-2-methylpropan-2-ol with ketones

The interaction of 1-choro-2-methylpropan-2-ol (CMP) with acetone and methyl ethyl ketone was examined in dilute carbon tetrachloride solution. The strengths of the hydrogen-bonded complexes were compared with those of the corresponding complexes of trimethyl carbinol and 1,1,1-trichloro-2-methyl-propan-2-ol with ketones. The aim was to explain the inductive effect of the chlorine atom on the strength of carbinol-ketone hydrogen-bonded complexes. The monomer IR band of CPM appears in solution as a composite of five sub-bands, signifying that many conformers are present; this was confimed by quantum chemical calculations. Chlorine substitution increases the strength of the hydrogen bond.     

Enthalpies and entropies of vaporization of propan-2-ol-2-methylpropan-1-ol solutions

P-T-x dependences are measured for the solutions of a propan-2-ol-2-methylpropan-1-ol binary system and the enthalpies and entropies of vaporization are determined. Dimerization in propan-2-ol and 2-methylpropan-1-ol is rationalized and the contribution from energy introduced by isostructural methyl groups to the enthalpy of vaporization is determined. Structural and energy analyses of solutions with networks of specific interactions are performed. The formation of heterodimers in solutions and vapors with reduced hydrogen bond energies and specific interactions with the 2s ²(C) unshared electron pairs of the carbon atoms of terminal methyl groups in ethyl and propyl fragments of propan-2-ol and 2-methylpropan-1-ol, respectively, is substantiated. The hydrogen bond energy of heterodimers is estimated.     

Investigation of equilibria in solution. Determination of equilibrium constants with the HYPERQUAD suite of programs

A new suite of 10 programs concerned with equilibrium constants and solution equilibria is described. The suite includes data preparation programs, pretreatment programs, equilibrium constant refinement and post-run analysis. Data preparation is facilitated by a customized data editor. The pretreatment programs include manual trial and error data fitting, speciation diagrams, end-point determination, absorbance error determination, spectral baseline corrections, factor analysis and determination of molar absorbance spectra. Equilibrium constants can be determined from potentiometric data and/or spectrophotometric data. A new data structure is also described in which information on the model and on experimental measurements are kept in separate files.     

Catalytic properties of a mixed calcium-cobalt orthophosphate, I. Kinetics of propan-2-ol dehydrogenation

Kinetics of dehydrogenation of propan-2-ol on the most active catalyst found in the mixed calcium-cobalt orthophosphate system, Ca_3-xCo_x(PO₄)₂, with x=0.32, has been studied. The activation energy of the reaction was also determined.     

The biosynthesis of nornuciferine-I(2-methoxy-6aα-aporphine-1-ol)

The incorporation of (±)-norcoclaurine, (±)-coclaurine, (±)-N-methylcoclairine, (±)-N-methylnorcoclaurine into nornuciferine-I in Croton sparsiflorus morong has been studied, and the specific utilization of the (±)-N-methylcoclaurine demonstrated. The evidence supports the direct oxidative coupling of (+)-(S)-N-methylcoclaurine to give N-methylcrotsparine, which in turn is shown to be a specific precursor of nornuciferine-I. The experiments also show that N-methylcrotsparine is reduced to N-methylcrotsparinol and it is N-methylcrotsparinol-I which is preferentially dehydrated and rearranged to nornuciferine-I.     

Experimental and predicted excess molar enthalpies of binary mixtures containing 2,2′-oxybis[propane], benzene,butan-1-ol, 2-methylpropan-1-ol, 2-methyl-2-ene-1-propanol at 303.15 K

Excess molar enthalpies H^E have been measured for liquid binary mixtures of 2,2′-oxybis[propane] (diisopropylether ‘DIPE’), or, benzene + butan-1-ol, +2-methylpropan-1-ol (isobutanol), +2-methyl-2-ene-1-propanol (isobutenol), +n-heptane at 303.15 K and constant pressure using a C80, Setaram calorimeter. A Redlich–Kister type equation was used to correlate experimental results.     

Organic chemistry on cold molecular films: kinetic stabilization of SN1 and SN2 intermediates in the reactions of ethanol and 2-methylpropan-2-ol with hydrogen bromide

We prepared thin molecular films of ethanol and 2-methylpropan-2-ol on Ru(001) substrates at temperature of 100-150 K and examined their reactivity toward HBr. The reaction intermediates and products formed at the surfaces were unambiguously identified by the techniques of Cs(+) reactive ion scattering (RIS) and low-energy sputtering. The reaction on the ethanol surface produced protonated ethanol, which is stabilized on the surface and does not proceed to further reactions. On the 2-methylpropan-2-ol surface, protonated alcohol [(CH(3))(3)COH(2) (+)] and carbocation [(CH(3))(3)C(+)] were formed with the respective yield of 20 and 78 %. Alkyl bromides, which are the final products of the corresponding reactions in liquid solvents, have extremely small yields on these surfaces (< 0.3 % for ethyl bromide and 2 % for tert-butyl bromide). The results indicate that the reactions on frozen films are characterized by kinetic control, stabilization of ionic intermediates (protonated alcohols and tert-butyl cation), and effective blocking of the charge recombination steps in S(N)1 and S(N)2 paths. The implication of these findings for the molecular evolution process in interstellar medium is also discussed.     

Measurement of the rate constants for the interaction of diarylcarbonyl oxides with ketones by the flash photolysis technique

Rate constants for the interaction of a series of diarylcarbonyl oxides with various α-substituted ketones have been measured for the first time by the flash photolysis technique with rapid spectrophotometric detection. A comparative analysis of the dependence of the rate constants on the nature of the substituent in the substrate is presented.     

Projection maps for the determination of equilibrium constants

Projection maps are suggested for the construction of model functions to be compared with potentiometric measurements of the free ligand concentration. The theoretical curves are found to be a valuable guide in the choice of the optimal conditions for the evaluation of complex formation equilibria. As an example, measurements of [H⁺], with a quinhydrone electrode, have been planned with the help of model maps for the study of the protolysis of oxalic acid in 1 M(KCl) at 25°C.Projection maps are also proposed to interpret data obtained by the method of competitive reactions.     

Adiabatic ionization energies from charge transfer equilibrium constants; experimental determinations of entropy changes for charge transfer equilibria

Equilibrium constants have been measured as a function of temperature for the charge transfer equilibria in mixtures of NO with aromatic compounds, and mixtures of aromatic compounds. The experimentally determined entropy changes are in good agreement with entropy changes calculated from statistical mechanical considerations provided an intermolecular component is included. The enthalpy changes for the charge transfer reactions correspond to the differences in the adiabatic ionization potentials of the compounds.     

On the liquid junction potential for the determination of equilibrium constants by means of the potentiometric technique without constant ionic strength

A modified method for the calculation of liquid junction potentials based on the Henderson equation is proposed. The estimation of the activity coefficients and the conductivities is performed by means of the Modified Bromley Methodology [G. Borge, R. Castaño, M.P. Carril, M.S. Corbillón, J.M. Madariaga, Fluid Phase Equilibria 121 (1996) 85–98;G. Borge, N. Etxebarria, L.A. Fernández, M.A. Olazabal, J.M. Madariaga, Fluid Phase Equilibria 121 (1996) 99–109.] and the Extended Falkenhagen Equation [A. De Diego, Conductivity of concentrated electrolytic solutions: study of the dependence with concentration and temperature, Ph.D. Thesis, University of the Basque Country, Bilbao, 1996.]. The efficiency of the method has been tested in potentiometric titrations without constant ionic strength. Its applicability to potentiometric titrations with constant ionic strength is also discussed.     

Synthesis of a C(29)-C(51) subunit of spongistatin 1 (Altohyrtin A) starting from (R)-3-benzyloxy-2-methylpropan-1-ol

A protected C(29)-C(51) subunit ((+)-38) of spongistatin 1 has been obtained. Key steps involve the aldol condensation of (3S, 4R)-3-methyl-7-[(p-methoxybenzyl)oxy]-4-[(triethylsilyl)oxy]octan- 2-o ne ((-)-6) with (tert-butyl)dimethylsilyl 4-deoxy-2, 3-di-O-(methoxymethyl)-4-methyl-6-O-(tert-butyl)dimethylsilyl)-bet a-D -glycero-L-gluco-heptodialdo-1,5-pyranoside ((+)-7) and a C-glycosidation of (4R,7R&S,E)-7, 8-dichloro-2-methylidene-1-(trimethylsilyl)oct-5-en-4-yl p-methoxybenzoate (16). Aldehyde (+)-7 was derived from (R)-3-benzyloxy-2-methylpropan-1-ol ((+)-10) in 13 formal steps but requiring the isolation of five intermediate products only. The longest linear synthetic scheme converts (+)-10 into (+)-38 in 2% overall yield (isolation of 11 intermediate products).     

Application of RP-TLC technique for the determination of dissociation constants of 1-substituted pyrrolidin-2-one derivatives

The procedures for measuring dissociation constants (pK(a)) of twenty 1-substituted pyrrolidin-2-one derivatives are described. The dissociation constants of the compounds tested were determined using potentiometric titration, reversed-phase thin-layer chromatography (RP-TLC) and calculated using Pallas and Marvin programs. It was found that the RP-TLC method of determination of pK(a) could be considered as a feasible alternative to potentiometric titration. The Marvin program is a better tool for preliminary estimation of dissociation constant than the Pallas one.     

pH-metric titration technique for the study of metal-complexes in solution: A novel approach for the calculation of equilibrium constants

A new approach for the calculation of various parameters in metal-complex equilibria involving pH-titration curves is described. The calculations do not require a knowledge of the concentrations of either the mineral acid added to the system or of the titrant (alkali). Only the concentrations of the metal ions and the ligand are employed in obtaining the necessary relations. This simplifies the approach of Irving and Rossotti, and also the procedure described earlier by the present authors. It is further shown that for the complexation systems of monoprotonated ligands, the determination of protonation constant is not a prerequisite for obtaining the stepwise metal-ligand formation constants.     

Cyclization reactions of 1,3-dibromopropan-2-ol, 2,3-dibromopropan-1-ol and 1-bromomethyloxirane with 6-amino-2,3-dihydro-2-thioxo-4(1H)-pyrimidinone

The cyclization reactions, carried out in strongly- or weakly-basic media, are described. Sometimes, 7-amino-2,3-dihydro-3-hydroxymethyl-5H-thiazolo[3,2-a]pyrimidin-5-one is separated out, together with 8-amino-3,4-dihydro-3-hydroxy-2H,6H-pyrimido[2,1-b][1,3]thiazin-6-one, as the principal product. A mechanism of reaction, during which the cyclizating agents are changed into oxirane derivatives, is proposed. The results of single-crystal X-ray investigations on 8-amino-3,4-dihydro-3-hydroxy-7-nitroso-2H,6H-pyrimido[2,1-b][1,3]thiazin-6-one (R = 0.035 for 1013 reflections), and on 7-hydroxymethyl-6,7-dihydrothiazolo[3,2-a][1,2,3]triazolo[4,5-d]pyrimidin-9(1H)-one (R = 0.027 for 1607 reflections) are reported.