Alkanes Conformational isomerism

Conformational isomerism in propane Propane is a three-carbon- (sp -hybridized) atom-containing linear alkane. All are tetrahedrally arranged. When a hydrogen atom of ethane is replaced by a methyl (CH3) group, we have propane. There is rotation about two C-C cr bonds. 

Conformational isomerism in cyclohexane Cyclohexane (C6H12) is a six-carbon cyclic alkane that occurs extensively in nature. Many pharmaceutically important compounds possess cyclohexane rings, e.g. steroidal molecules. If we consider cyclohexane as a planar and regular hexagon, the angles are 120° (cf. 109.5° for sp hybrids). 

Increasing the chain length of the alkanes, conformational analysis of a series of methyl substituted hexanes indicates that the important potential energy profiles can be identified with those bonds which contain the methyl substituent. This has important consequences for polymers. The theoretical rotational isomeric potential profile for 2-methylhexane is shown in Figure 3.8. 

The same conformational isomerism has been found to occur in all of the 1-halogenated normal alkanes starting with the propanes and gives rise to a... 

The conformational interconversion of diazabicyclo [k.l.m] alkanes (Figure 6 Section 5.21.3.1.2) has been investigated by variable temperature H NMR (68JA2428). The bis-deuterochloride salt of l,10-diazabicyclo[8.8.8]hexacosane showed a slow isomerization from exo-exo to endo-endo forms during which the NH proton resonance shifted from 8.0 p.p.m. (exo-exo) to 4.6 p.p.m. (endo-endo). The equilibrium constant for the process Ke —endo-endo / exo-exo) was greater than 100. Similar measurements on differently lengthed diazabicyclo[k, l. m ]alkanes revealed that even chain Iengthed species prefer the... 

Quantitative determination of conformational disorder, in addition to assignments of the spectral features, requires accurate calibrations of relative absorptivities (extinction coefficients) for the various bands. These were calculated with the aid of the rotational isomeric state model of Flory (22), a statistical mechanical determination of conformational disorder in alkanes. The model requires an estimate for the energies of gauche bonds, chosen from the best available experiments to be 508 +/- 50 cal/mole, and an energy for the bond pairs E, chosen to be 3000 cal/mole. The... 

Summary Rules for Naming Alkanes 94 3-4 Physical Properties of Alkanes 95 3-5 Uses and Sources of Alkanes 97 3-6 Reactions of Alkanes 99 3-7 Structure and Conformations of Alkanes 100 3-8 Conformations of Butane 104 3-9 Conformations of Higher Alkanes 106 3-10 Cycloalkanes 107 3-11 Cis-trans Isomerism in Cycloalkanes 109 3-12 Stabilities of Cycloalkanes Ring Strain 109 3-13 Cyclohexane Conformations 113... 

Ono and Ware"" have measured the absorption, emission, and excitation spectra, the fluorescence decay times, and the quantum yields of a series of substituted diphenylmethylenes in rigid matrices at low temperatures. Acean-thrylene shows S2- So emission in hexane with a yield of 0.017 and lifetime of 4.3 ns. The low-temperature fluorescence spectra of bis-2-naphthyl-alkanes and their derivatives have been studied. Excimer formation is an activated process. The fluorescence and absorption spectra of 1,1-diphenyl-ethylenes have been analysed in some detail by Gustav and Bolke. " The S — Si transitions in trans isomers of phenylnaphthylethylenes have been assigned by picosecond absorption spectroscopy. Effects of solvent viscosity and the role of conformers in the mechanism of isomerization are elucidated. The production of non-equilibrium conformer concentrations in glassy solutions of diarylethylenes at 77 K due to restrictions imposed by the solid matrix has also been reported. Free jet excitation and emission spectra of diphenyl-butadiene show clearly the lowest excited Ag state and give a lifetime of 52.8 ns for 0-0 excitation.Electric field-induced charges in the optical... 

It is easy to list the various chemical and biological events influenced by flexibility, but unfortunately efforts to quantitate this structural attribute have been few. Mann analyzed the conformation of alkanes by modifying the number of gauche arrangements with Pitzer s steric partition function. Luisiranked alkanes on a. scale of conformational rigidity based on three-states rotational isomerism. Unfortunately, these schemes are designed for acyclic hydrocarbons and have no inherent capability to be adapted to heteroatomic molecules. 

Sasanuma, Y., Conformational analysis of chain molecules in liquid crystalline phases by a rotational isomeric state scheme with maximum entropy method I. H— H dipolar couplings from n-alkanes dissolved in a nematic solvent, Polym. J., 32, 883-889 (2000a). 

Sasanuma, Y, and Abe, A., Conformational anisotropy of n-alkanes dissolved in a nematic solvent a rotational isomeric state analysis of proton-proton dipolar coupling and deuterium quadrupolar splitting data, Polym. J., 23, 117-125 (1991). 

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