Cyclohexane constant

Substituent effects (electronegativity, configuration) influence these coupling constants in four-, five- and seven-membered ring systems, sometimes reversing the cis-tmns relationship so that other NMR methods of structure elucidation, e.g. NOE difference spectra (see Section 2.3.5), are needed to provide conclusive results. However, the coupling constants of vicinal protons in cyclohexane and its heterocyclic analogues (pyranoses, piperidines) and also in alkenes (Table 2.10) are particularly informative. 

Neighbouring diaxial protons of cyclohexane can be clearly identified by their large coupling constants 11-13 Hz, Table 2.10) which contrast with those of protons in diequatorial or axial-equatorial configurations ( Jee 2-4 Hz). Similar relationships hold for pyranosides as oxy-... 

A solution of 1 g of (239) in 80 ml cyclohexane is irradiated as described above. After 3 hr e reaches a constant value of 4700. The solvent is evaporated and the residue chromatographed on alumina (40 g). Elution with pentane-benzene (3 1) gives 0.14 g unreacted starting material... 

This is most readily studied with cyclohexane- /n in which 11 of the 12 protons are replaced with deuterium. The spectrum of cyclohexane- /n resembles the behavior shown in Fig. 4-8 at about — 100°C (the slow exchange regime) two sharp lines are seen these broaden as the temperature is increased, reaching coalescence at — 61.4°C, and becoming a single sharp line at higher temperatures. (The deuterium nuclei must be decoupled by rf irradiation.) Rate constants t for the conversion were measured over the temperature range — 116.7°C to — 24.0°C by Anet and Bourne. It is probable that the chair-chair inversion takes place via a boat intermediate. 

Remember that e is the dielectric constant of the solvent. The examples show the value for cyclohexane (2.0). 

Total Ionic Strength Adjustment Buffer (TISAB). Dissolve 57 mL acetic acid, 58 g sodium chloride and 4g cyclohexane diaminotetra-acetic acid (CDTA) in 500 mL of de-ionised water contained in a large beaker. Stand the beaker inside a water bath fitted with a constant-level device, and place a rubber tube connected to the cold water tap inside the bath. Allow water to flow slowly into the bath and discharge through the constant level this will ensure that in the... 

The quantitative solution of the problem, i.e. simultaneous determination of both the sequence of surface chemical steps and the ratios of the rate constants of adsorption-desorption processes to the rate constants of surface reactions from experimental kinetic data, is extraordinarily difficult. The attempt made by Smith and Prater 82) in a study of cyclohexane-cyclohexene-benzene interconversion, using elegant mathematic procedures based on the previous theoretical treatment 28), has met with only partial success. Nevertheless, their work is an example of how a sophisticated approach to the quantitative solution of a coupled heterogeneous catalytic system should be employed if the system is studied as a whole. 

Cyclohexane (C) and methylcyclopentane (M) are isomers with the chemical formula C6H12. The equilibrium constant for the rearrangement C M in solution is 0.140 at 25°C. (a) A solution of 0.0200 mol-L 1 cyclohexane and 0.100 mol-I. 1 methylcyclopentane is prepared. Is the system at equilibrium If not, will it will form more reactants or more products (b) What are the concentrations of cyclohexane and methylcyclohexane at equilibrium (c) If the temperature is raised to 50.°C, the concentration of cyclohexane becomes 0.100 mol-L 1 when equilibrium is reestablished. Calculate the new equilibrium constant, (d) Is the reaction exothermic or endothermic at 25°C Explain your conclusion. 

In addition to the use of cyclohexane, dimethylformamide (DMF) was used as a good solvent. So as to pick up the thiol-modified terminal, gold-coated cantilevers were used. The nominal values of their spring constant ki were 30 or 110 pN nm . A typical force-extension curve measured in cyclohexane is shown in Figure 21.4. The solvent temperature was kept at about 35°C, which corresponded to its temperature for PS chains. Thus, a chain should behave as an ideal chain. The slope at the lowest extension limit (dashed line in Figure 21.4) was 1.20 X lO" N m . ... 

FIGURE 21.4 Nanofishing of a single polystyrene (PS) chain in cyclohexane. The solvent temperature was about 35°C (0 temperature). A cantilever with a 110 pN nm spring constant was used. The worm-like chain (WLC), solid line, and the freely jointed chain (FJC), dashed line models were used to obtain fitting curves. (From Nakajima, K., Watabe, H., and Nishi, T., Polymer, 47, 2505, 2006.)... 

In a poor solvent (cyclohexane at 5°C), a polymer chain takes on a condensed globular state because constituent molecules are repulsed by the solvent molecules. Nanofishing of this chain revealed a perfectly different force-extension curve, as shown in Figure 21.5. It was observed that constant force continued from about 30 to 130 nm after nonspecific adsorption between a... 

FIGURE 21.5 Nanofishing of a single polystyrene (PS) chain in cyclohexane at 5°C (poor region). A cantilever with a 30 pN nm spring constant was used. The coil-strand coexistence was revealed in the extension range of 30-130 nm. 

Solvent effects on the rate of the decarbonylation of MeCOMn(CO)5 were examined by Calderazzo and Cotton (50) and are presented in part in Table IV. In general they are very small, and no regular trends can be discerned. This virtual lack of dependence of the rate on the nature of the solvent and very little correlation between the rate and the dielectric constant of the solvent are typical of substitution reactions of metal carbonyls (J). In the light of the foregoing, a qualitative observation that CpFe(CO)2-COMe decarbonylates much more readily on treatment at reflux in nonpolar heptane or cyclohexane than in polar dioxane is somewhat intriguing 219). 

Typical forces profdes measmed between glass surfaces in ethanol-cyclohexane mixtures are shown in Fig. 2. Colloidal probe atomic force microscopy has been employed. In pure cyclohexane, the observed force agrees well with the conventional van der Waals attraction calculated with the nometarded Hamaker constant for glass/cyclohexane/glass. 

FIG. 2 Interaction forces between glass surfaces upon compression in ethanol-cyclohexane mixtures. The dashed and solid lines represent the van der Waals force calculated using the nonretarded Hamarker constants of 3 X 10 1 for glass/cyclohexane/glass and 6 X 10 J for glass/ethanol glass, respectively. 

Transfer constants for polystyrene chain radicals at 60° and 100°C, obtained from the slopes of these plots and others like them, are given in the second and third columns of Table XIII. Almost any solvent is susceptible to attack by the propagating free radical. Even cyclohexane and benzene enter into chain transfer, although to a comparatively small extent only. The specific reaction rate at 100°C for transfer with either of these solvents is less than two ten-thousandths of the rate for the addition of the chain radical to styrene monomer. A fifteenfold dilution with benzene was required to halve the molecular weight, i.e., to double l/xn from its value (l/ rjo for pure styrene (see Fig. 16). Other hydrocarbons are more effective in lowering the degree of polymerization through chain transfer. 

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