Configurational Isomers In Butane

Stereoisomers

Stereoisomers are isomeric molecules that have same molecular formula and sequence of bonded atoms, but differ in the arrangement of the atoms in space. The consideration of such spatial aspects of molecular structure is known as stereochemistry.

Stereoisomers can be classified into two general categories:

• Conformational isomers

Isomers that differ in relative position of some of the atoms in the molecule in 3D-space due to the rotation about formally single bonds (sigma bonds).

• Configurational isomers

Isomers that arise due to some kind of rigidity in the molecule and they can be interconverted only by bond breaking or making.

Conformational Stereoisomers of Butane

Butane has a larger and more

Thus, the cyclohexane ring tends to assume certain non-planar (warped) conformations, which have all angles closer to 109.5° and therefore a lower strain energy than the flat hexagonal shape. The most important shapes are chair, half-chair, boat and twist-boat. The molecule can easily switch between these conformations and only two of them ---chair and twist-boat, can be isolated in pure form. The planar cyclohexane is not the ground state conformation, it is very high in energy and thus being neglected in the conformational analysis of cyclohexane. This is because there are a lot of torsional strain present in this form. Therefore, all six carbon-carbon bonds in planar form are eclipsed, therefore we can estimate the torsional strain as being minimum 6 times that in eclipsed ethane with one eclipsed C-C bond or 18 kcal/mole.

Cyclohexane’s ground state conformation is a staggered conformation shaped like a “chair” with a back, a seat and a footrest. This conformation does not have any torsional strain. Cyclohexane is known as the only cyclic hydrocarbon which is totally strain-free. Thus, the six-membered ring is very stable. However, the individually small torsional effects can affect the shapes of molecules tremendously, and shapes affect

Bonds C2-C3 and C5-C6 are thus parallel. In this form, the molecule has two perpendicular planes of symmetry and a C2 axis. The boat conformations have higher energy than chair conformations. The interaction between the two flagpole hydrogens generates steric strain. There is also torsional strain involving C2-C3 and C5-C6 bonds, which are eclipsed. Due to this strain, the boat conformation is unstable. The twist-boat conformation can be derived from the boat conformation by applying a slight twist to the molecule about the axes connecting the two unique carbons. The result is a structure with three C2 axes and no plane of symmetry. The concentration of twist-boat conformation at room temperature is very low (less than 0.1%) but at 1073 Kelvins it can reach 30%. Rapid cooling from 1073 K to 40 K will freeze in a large concentration of twist-boat conformation, which will then slowly convert to chair conformation upon heating. The half-chair conformation is a transition state with C2 symmetry generally considered to be on the pathway between chair as well as twist-boat. It involves rotating one of the dihedrals to zero such that four adjacent atoms are coplanar and the other two atoms are out of