Conformation echpsed

It is clear that this representation of c/5 -dimethyl-cyclohexane shows a plane of symmetry, and we can deduce it to be a meso compound. No such plane of symmetry is present in the representation of fran -dimethylcyclohexane. Why does this approach work Simply because the transformation of planar cyclohexane (with echpsed bonds) into a non-planar form (with staggered bonds) is a conformational change achieved by rotation about single bonds. The fact that cyclohexane is non-planar means we may have to invoke the conformational mobihty to get the three-dimensional picture. 

Torsional energy and torsional strain Torsional energy is the energy required for rotating about the C-C a bond. In ethane, this is very low (only 3 kcal). Torsional strain is the strain observed when a conformer rotates away from the most stable conformation (i.e. the staggered form). Torsional strain is due to the slight repulsion between electron clouds in the C-H bonds as they pass close by each other in the echpsed conformer. In ethane, this is also low. 

Red circles gauche 60° and 300°. Red circles anti 180°. Gauche and anti relationships occur only in staggered conformations therefore, ignore the echpsed conformations (0°, 120°, 240°, 360°). 

All the staggered conformations of propane are equivalent to one another, and all its echpsed conformations are equivalent to one another. The energy diagram resembles that of ethane in that it is a symmetrical one. 

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 echpsed 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. 

Only the most stable conformer, the one with the caiboityl oxygen echpsing the methyl group, was observed in the free jet millimetre-wave spectrum. 

Ligand redistribution in liquid Me2SbBr (no solvent) leads to the formation of the salt [Me3SbSbMe2]2 MeBr2Sb(p-Br)2SbBr2Me] which contains ions 19.56 and 19.57. The proposed pathway is given in scheme 19.97. The echpsed conformation of cation 19.58 is probably determined by close cation-anion interactions in the solid state. 

Furthermore, the role of hyperconjugation is expected to increase even more in anionic species. However, the difference between staggered and echpsed conformation is only slightly higher in ethyl anion (2.3kcal at the CCSD(T)/6-311+G(d,p//B3LYP/6-311++G(3df,2p) level of theory) than in methyl amine and much lower than in ethane. ... 

The strain induced in the echpsed conformation of ethane is an example of torsional strain. Torsional strain (also called echpsed interaction strain) is strain that arises when nonbonded atoms separated by three bonds are forced from a staggered conformation to an eclipsed conformation. 

Eclipsed conformation Conformation in which bonds on adjacent atoms are aligned with one another. For example, the C—H bonds indicated in the structure shown are echpsed. 

The rotational barrier in methylsilane (Table 3.4, entry 5) is significantly smaller than that in ethane (1.7 versus 2.88 kcal/mol). This reflects the decreased electron-electron repulsions in the echpsed conformation resulting from the longer carbon-sihcon bond length (1.87 A) compared to the carbon-carbon bond length (1.54 A) in ethane. 

There are two families of conformations available to terminal alkenes. These are the echpsed and bisected conformations shown below for propene. The echpsed conformation is more stable by about 2kcal/mol/ ... 

The origin of the preference for the echpsed conformation of propene can be explained in MO terms by focusing attention on the interaction between the double bond and the n component of the orbitals associated with the methyl group. The dominant interaction is a repulsive one between the filled methyl group orbitals and the filled n orbital of the double bond. This repulsive interaction is greater in the bisected conformation than in the echpsed conformation. ... 

Conformations A and B are of the echpsed type, whereas C and D are bisected. It has been determined by microwave spectroscopy that the echpsed conformations are more stable than the bisected ones and that B is about 0.15 kcal more stable than MO calculations at the 6-31G level have found relative energies of 0.00, —0.25, 1.75, and 1.74kcal/mol, respectively, for A-D. ... 

Further substitution can introduce additional factors, especially nonbonded repulsions, which influence conformational equilibria. For example, methyl substitution at C-2, as in 2-methyl-1-butene, introduces a methyl-methyl gauche interaction in the conformation analogous to B, with the result that in 2-methyl-1-butene the two echpsed conformations are of approximately equal energy. Increasing the size of the group at... 

The difference in energy between the staggered and eclipsed conformations is 12 kJ/mol (2.9 kcalAnol), a small enough difference that the rotation is still very rapid at room temperature, andj e conformatiim ot-he separated. Because three echpsed C-H bonds increase the end mraOTTOrmaBonlty C-Hbond results in an increase in energy... 

The staggered conformations (1, 3, and 5) are lower in energy than the echpsed conformations (2, 4, and 6), but how do the energies of the individual staggered and eclipsed conformations compare to each other The relative energies of the individual staggered conformations (or the individual eclipsed conformations) depend on their steric strain. 

Torsional strain (Sections 4.8B, 4.9, and 4.10) The strain associated with an eclipsed conformation of a molecule it is caused by repulsions between the ahgned electron pairs of the echpsed bonds. 

Converting a Newman projection to a perspective formula. First convert the staggered conformer to an eclipsed conformer, and then convert the echpsed conformer to a Fischer projection as you did in Example 8. Once you have the Eischer projection, you can convert it into a perspective formula as described in Examples 6. 

Three conformations with respect to internal rotation around the C-C bond were found, viz. 56% of a conformation with Cj molecular symmetiy and the C-Cl bond echpsed with the C=0 bond, 30% of a conformation with the CH2CI group rotated 131° from the former position and the remaining 14% of a conformation with 79° rotation of the CH2CI group. The chain of atoms Cl-C-C-O-H is planar and zig-zag in conformer of symmetiy [1]. The nozzle temperature was 170 °C. 

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