Trans conformation butane

The trans conformation corresponds to a torsion angle of 180°, the gauche(+) conformation to oni + 60° and the gauche -) conformation to -60°. These approximately correspond to the torsion anj of the three minimum energy conformations of butane. 

Examination of other systems (Scheme 5) shows the importance of the third major factor, ring fusion strain, in influencing the position of conformation equilibria in these types of systems. The change in position of conformational equilibrium from 73% trans-conformation in perhydro-oxazolo[3,4-fl]pyridine (468 469) to 14% trans-conformation in perhydro-oxazolo[3,4-(/][l, 4]oxazine (472 473) [74JCS(P2)1419] may at first sight be attributed to a reduction in the magnitude of the indicated flMc/ie-butane-type interaction in the cis-fused conformation, since Me/ heteroatom interactions are less severe than CH2/CH2 interactions. The swing back to 84% trans-conformer in the perhydro-oxazolo[3,4-[Pg.262]

Bicyclo[1.1.0]butane is usually a side product of the photocyclization of butadiene to cyclobutene (Srinivasan, 1963) in isooctane, the quantum yield ratio is I 16 (Sonntag and Srinivasan, 1971). It becomes the major product in systems in which the butadiene moiety is constrained near an s-trans conformation and bond formation between the two terminal methylene groups that leads to cyclobutene is disfavored. An example is the substituted diene 88 in Scheme 30, for which the bicyclobutane is the major product a nearly orthogonal conformation should result from the presence of the 2,3-di-r-bu-tyl substituents (Hopf et al., 1994). 

With acyclic dienes, the quantum yield for cyclobutene formation (cb) rarely exceeds ca 0.1, the expected result of the fact that the planar s-trans conformer normally comprises the bulk (96-99%) of the conformer distribution at room temperature. However, cb is often significantly larger than the mole fraction of s-cis form estimated to be present in solution. For example, 1,3-butadiene, whose near-planar (dihedral angle 10-15° 05,i06 s-cw con fonner comprises ca 1% of the mixture at 25 °C, yields cyclobutene with <1>CB = 0.04 " , along with very small amounts of bicyclo[1.1.0]butane. A second well-known example is that of 2,3-dimethyl-l,3-butadiene (23 ca 4% gauche s-cis at 25 °C ), which yields 1,2-dimethylcyclobutene (25) with cB = 0.12 (equation 16) ". Most likely, these apparent anomalies can be explained as due to selective excitation of the s-cis conformers under the experimental conditions employed, since it is well established that s-trans... 

Fig. 3.6 Gauche and trans n-butane looking down the 2-3 bond axis. The gauche conformation is on the left.

Figure 2.1 Conformations produced by rotation about C2-C3 bond of n-butane molecule. Each carbon atom in this molecule, being sp hybridized, is tertrahedral with bond angles of 109.50°. The planar cis conformation, corresponding to the closest approach of the two bulky methyl groups, is the least stable. Conversely, the planar trans conformation, where the bulky groups are farthest apart, is the most stable. (After Young and Lovell, 1990.)...

The term conformation (Koenig, 1980) is used to describe the spatial arrangements of various atoms in a molecule that may occur because of rotations about single bonds. The planar cis and planar trans conformations of a n-butane molecule produced by rotations about the bond joining carbons 2 and 3 are illustrated in Fig. 

For butane and most compounds of the form YCH2CH2X, antiperiplanar conformers are the most stable ones. The long alkyl chains of fatty acids are usually also all-anti or all-trans conformers in molecular assemblies (Fig. 1.2.3). If the substituents are, however, electronegative atoms, in particular oxygen, the synclinal (= gauche) form usually dominates. Important gauche effects (= the tendency to adopt conformations with the maximum number of gauche interactions between adjacent electron pairs and/or polar bonds) are found in peroxides. 

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