Propane, bond rotation conformations

Sketch a potential energy diagram for rotation around a carbon-carbon bond in propane. Clearly identify each potential energy maximum and minimum with a structural formula that shows the conformation of propane at that point. Does your diagram more closely resemble that of ethane or of butane Would you expect the activation energy for bond rotation in propane to be more than or less than that of ethane Of butane ... 

All the conformations so far discussed have involved rotation about sp -sp bonds. Many studies have also been made of compounds with sp -sp bonds. " For example, propanal (or any similar molecule) has four extreme conformations, two of which are called eclipsing and the other two bisecting. For propanal the eclipsing conformations have lower energy than the other two, with P favored over Q by... 

The activation energy for bond rotation in propane is expected to be somewhat higher than that in ethane because of van der Waals strain between the methyl group and a hydrogen in the eclipsed conformation. This strain is, however, less than the van der Waals strain between the methyl groups of butane, which makes the activation energy for bond rotation less for propane than for butane. 

The rotational barrier is now slightly higher than for ethane 14 kJ mol-1 as compared to 12 kJ mol-1. This again reflects the greater repulsion of electrons in the coplanar bonds in the eclipsed conformation rather than any steric interactions. The energy graph for bond rotation in propane would look exactly the same as that for ethane except that the barrier is now 14 kJ 1110I-1. 

Normally w-amino-a-arylalkanes can adopt a conformation which produces exciplex fluorescence by a process involving C—C-bond rotation which is very fast. Both 3-(4-dimethylaminophenyl)-l-(9-anthracenyl)propane and 3-(4-dimethylaminophenyl)-l-(l-pyrenyl)propane form fluorescent exciplexes, which by means of picosecond time-resolved fluorescence spectroscopy have been shown to take a few nanoseconds to be formed (Migita et al., 1980, 1981). The rate of intramolecular fluorescent exciplex formation has also been shown to be dependent upon the length of the linking chain, the polarity of the solvent (the build up time decreases as solvent polarity is increased)... 

Figure 3-9 shows a graph of the torsional energy of propane as one of the carbon-carbon bonds rotates. The torsional energy of the eclipsed conformation is about 13.8 kJ/mol (3.3 kcal/mol), only 1.2 kJ (0.3 kcal) more than that required for ethane. 

Pentadienyl radical, 240 Perturbation theory, 11, 46 Propane, 16, 165 n-Propyi anion conformation, 34 n-Propyl cation, 48, 163 rotational barrier, 34 Propylene, 16, 139 Protonated methane, 72 Pyrazine, 266 orbital ordering, 30 through-bond interactions, 27 Pyridine, 263 Pyrrole, 231... 

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. 

In addition to conformational isomerism about the 2,3 bond in butane, rotations about the 1,2 bond and die 3,4 bond are possible. The energy changes here are much smaller and are comparable to those found in propane. 

Butane provides a more complex example. Here there are two different types of carbon-carbon bonds. Analysis of the conformations available by rotation about the bond between carbon 1 and carbon 2 (or carbon 3 and carbon 4) is very similar to the analysis of propane, with the difference that there is an ethyl group on one carbon rather... 

Torsional energy of propane. When a C—C bond of propane rotates, the torsional energy varies much like it does in ethane, but with 13.8 kJ/mol (3.3 kcal/mol) torsional energy in the eclipsed conformation. 

Nitrocyclopropane. Skancke reported that in the bisected conformer of nitrocyclo-propane, the vicinal bonds are longer and the distal bond is shorter than the CC bonds of 1. This is a result of charge transfer from the orbital of 1 to the empty pyr orbital of the nitro group. Rotation of the nitro group leads to an energy increase by 3.2-4.7 kcal mol 5,146 same time, the vicinal bond lengths are decreased while the distal bond... 

In practical evaluation of force-fields parameters the H... H and C... C non-bonded potentials were chosen first. Calculations of staggered and eclipsed rotamers of ethane, propane and n-butane then revealed the contribution of the non-bonded interactions to the total rotation barrier. For example, the van der Waals parameters proposed by Alhnger et al. in their first paper 40) on calculation of conformations were such that the repulsion between hydrogens on adjacent carbon atoms accounted for about 31% of the barrier in ethane. The remainder was accoimted for as a quantity which was added by considering the torsion interaction to be zero for all... 

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