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Conformations of Alkanes

Conformation of a molecule represents the different molecular arrangement of atoms or groups caused by the free rotation around sigma bonds in the molecule. For example, free rotation around the carbon-carbon bond is responsible for numerous conformations. The hydrogen atom attached to one carbon atom in ethane can occupy several different positions with respect to the hydrogen atoms that are attached to the other carbon atom.

Some conformations are more stable than others. The staggered conformation is more stable than eclipsed conformation . In ethane, the energy difference between the staggered and eclipsed conformations is very small (12.6 kJ mol-1). Staggered conformation has a lower energy and is therefore more stable.
Depending on the angular relationship between hydrogens attached to the two carbon atoms in ethane, the molecule can have a large number of differing atomic arrangements. Two such arrangements are shown in Figure by the Newman projection. This representation is a convenient device for showing the three-dimensional arrangement of atoms about a single bond. In the projection formula, only the front carbon atom and the groups attached to both the carbon atoms are viewed and they look like the 'spokes of a wheel'. The carbon-carbon bond is in the line of view and thus the second carbon is not seen in the projection. The three bonds that are attached to the front carbon stem from the center of the circle and the three other bonds project from the edge or margin of the circle. In figure, ethane molecule is shown in the staggered arrangement in which the hydrogen atoms on the adjacent carbons are staggered and are as far apart as possible. In the other arrangement, three pairs of hydrogen atoms eclipse each other. Eclipsing leads to steric repulsion of hydrogen atoms that are not directly bonded. In the case of ethane, this non-bonded interaction (between the eclipsed hydrogen atoms) raises the energy of the eclipsed arrangement by about 3 kcal mol-1. The change from one arrangement to another in such steroisomers involves a rotation about the carbon-carbon bond.

Conformation of Cycloalkanes

Cycloalkanes are the alkanes (saturated hydrocarbons) in which carbon atoms are linked to form a ring system. Cycloalkanes are also called cycloparaffins.

The general formula of cycloalkanes having one ring system is CnH2n. Cycloalkanes are named by writing the prefix "cyclo-" before the name of the corresponding open-chain alkane.

The four valencies of a carbon atom are directed towards the four corners of a regular tetrahedron and bond angle is 109° 28'. However, the actual bond angles in cyclopropane (60° ) and cyclobutane (90° ) are quite different. As these rings are under strain, cyclopropane and cyclobutane are very reactive.

 Names of some cycloalkanes

The bond angle in cyclopentane (105° ) is quite near the bond angle in an sp3 hybridized carbon atom (109° 28'). Therefore, cyclopentane is quite stable, i.e. unreactive.

Cyclohexane and other higher members in the cyclohexane series are also unreactive and stable like cyclopentane, even though the calculated bond angle for cyclohexane is 120° which is quite different from 109° 28'.
The stability of cyclohexane and other higher members is because the ring systems are puckered (twisted) or folded in such a way that the bond angle remains 109° 28'. Thus the presence of puckered rings is responsible for the stability of the ring system containing six or more carbon atoms.

The ring system of cyclohexane can exist in two different ways. These give two different conformations of cyclohexane, namely the boat form, and the chair form. The chair form is more stable (or has less energy) than the boat form. The energy difference between the two conformations of cyclohexane is small (41 kJ mol-1) and, therefore, these do not exist as independent compounds.



Chair conformation of Cyclohexane

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