Propane, bond rotation

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 ... 

Make a graph of potential energy versus angle of bond rotation for propane, and assign values to the energy maxima. 

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)... 

Cycl( propane, for example, must be a rigid, planar molecule l)ecause three points (the carbon atoms) define a plane. No bond rotation can take place around a cyclopropane carbon -carbon bond vvitliout breaking open the ring (k igure 4 1). 

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. 

FIGURE 14.10 (a) Condensed structure of propane, (b) Rotation around the bond indicated with the semicircular arrows does not result in different structures for propane. The three structures shown here are identical. 

Characteristics tetrahedral geometry and sp hybridization at C atoms all C-C and C-H single (< ) bonds rotation allowed about C-C single bonds Examples Methane, CHt and propane, CHJCH2CH3... 

Propane, the next higher member in the alkane series., also has a torsional barrier that results in hindered rotation around the carbon-carbon bonds. The barrier is slightly higher in propane than in ethane—a total of 14 kj/mol (3.4 kcal/mol) versus 12 kj/mol. 

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... 

It was noticed that the longer C-H distances are of little importance in the curve for the free-rotation model the curve calculated by ignoring all C-H terms except those for the bond distance and the next larger distance (which is unchanged by rotation) is qualitatively indistinguishable from the free-rotation curve, and the s values for the peaks of the two curves differ on the average by only 0.02. In our treatment of propane and isobutane we have made use of this simplification. 

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... 

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