Conformational Isomerism Preparation and Properties
→ Consider a simple hydrocarbon ethane, the free rotation of one carbon with respect to another gives rise to various arrangements of the atoms differing in relative positions of hydrogen atoms attached to these carbon atoms. These different spatial arrangements of atoms in space resulting due to free rotation around a single bond are called conformation or conformers or rotamers.
→ The energy required to show such isomerism is 3-15 kcal per mole which can be easily obtained at room temperature. These can be isolated because free rotation of carbon-carbon single bond takes place automatically at room temperature.
Conformation in Ethane
→ Ethane molecule (C2H6)contains a carbon-carbon single bond with each carbon atom attached to three hydrogen atoms. Infinite number of spatial arrangements of hydrogen atoms áttached to one carbon atom with respect to the hydrogen atoms attached to the other carbon atom.
→ There are two extreme cases. One such conformation in which hydrogen atoms attached to two carbons are as closed together as possible is called eclipsed conformation and the other in which hydrogens are as far apart as possible is known as the staggered conformation. Any other intermediate conformation is called as skew conformation. It may be remembered that in all the conformations, the bond angles and the bond lengths remain the same. Eclipsed and the staggered conformations can be represented by-
- Sawhorse formula and
- Newman projection formula
→ In a Newman projection formula, the molecule is viewed along an axis containing two atoms bonded to each other and the bond between them, about which the molecule can rotate. In a Newman projection formula, the “substituents” of each atom composing the bond, be the hydrogens or functional groups. can then be viewed both in front of and behind the carbon-carbon bond.
→ Specifically, one can observe the angle between a substituent on the front atom and a substituent on the back atom in the Newman projection, which is called the dihedral angle or torsion angle. Three front and three rear, total six sigma bonds are present.
→ In ethane, these are bonded by hydrogen bonding. Now if front three carbon atoms totally overlap rear three carbon atoms, it is known as eclipsed from. In staggered form of ethane, the electron clouds of carbon-hydrogen bonds are as far apart as possible.
→ In ethane, if the dihedral angle is o° and the hydrogens on the first carbon line up with or eclipse the hydrogens on the second carbon. When the dihedral angle is 0° and the hydrogens line up perfectly, ethane has adopted the eclipsed conformation.
→ The other extreme occurs when the hydrogens on the first carbon are as far away as possible from those on the second carbon; this occurs at a dihedral angle of 60° and is called the staggered conformation.
→ In staggered form of ethane, the electron clouds of carbon-hydrogen bonds are as far apart as possible. Thus, there are minimum repulsive forces, minimum energy and maximum stability of the molecule. In eclipsed conformation, the C—H bonds on the front and back carbons are aligned with each other with dihedral angles as 0°.
→ The eclipsed conformation of ethane has three such C—H eclipsing interactions, so one can infer that each eclipsed C—H ‘costs’ roughly 4.2 kJ mol-1 (1 kcallmol). Eclipsing interactions are an example of a general phenomenon called steric hindrance, which occurs whenever bulky portions of a molecule repel other parts of the same molecule. Because such hindrance causes resistance to rotation, it is also called torsional (or Pitzer) strain.
→ Magnitude of torsional strain depends upon the angle of rotation about C—C bond. This angle is also called dihedral angle or torsional angle. Among all the conformations of thane, the staggered form has the least torsional strain and the eclipsed form has maximum torsional strain. Thus it may be inferred that rotation around C—C bond in ethane is not completely free. The energy difference between the two extreme forms is of the order of 12.5 kJ mol-1 ,which is very small.
→ Even at ordinary temperature, the ethane molecule gains thermal or kinetic energy sufficient enough to overcome this energy barrier of 12.5 kJ mol-1 through intermolecular collision.
Newman’s Projection Formula of n-butane
- Stability : Fully staggered form > Gauche form > Eclipsed form > Fully eclipsed form
- Energy : Fully eclipsed form > Eclipsed form > Gauche form > Fully staggered form
→ Conformation In Cyclic System : In this series, we can take the example of cyclohexane. In cyclohexane, all the carbons are sp3 hybridised and thus configured at an angle of 109.28’. Therefore, it should not have planar hexagonal shape. Infact, cyclohexane exist, either in the shape of chair or boat. in 1918, Mohr suggested that cyclohexane can exist in two forms chair and boat form and the energy difference between the two forms is 6-7 kcallmol.
→ Hence these forms can be intercoverted at room temperature. The chair conformation is the most stable conformation. Chair form is free from torsion strain. In the chair conformer of cyclohexane, all the bond angles are 1110, which is very close, the ideal tetrahedral bond angle of 109.5°, and all the adjacent bonds are staggered.
→ Cyclohexane can also exist in a boat conformation. Like the chair conformer, the boat conformer is free of angle strain. However, the boat conformer is not as stable as the chair conformer because some of the bonds in the boat conformer are eclipsed, giving it torsional strain.
→ The boat conformer is further destablized by the close proximity of the flagpole hydrogens (the hydrogens at the “bow” and “stern” of the boat), which causes steric strain. Torsional strain and flagpole interactions cause boat conformation to have considerably higher energy than chair conformation. The chair form is more stable than the boat form by 44 kJ mol-1.
Newman’s Representation of Chair and Boat form of Cyclohexane.
- Stability : Chair form > Boat form
- Energy : Boat form > Chair form
Axial and Equatorial Hydrogen Bonds
→ In chair form of cyclohexane, there are two different types of C—H bonds, and thus two different types of hydrogen atoms as substituents. The C—H bonds, which point vertically upward or downward, are called axial hydrogen atoms (Hax).
→ There are six of these, three upward and three downward bonds, and they alternate up/downlup, etc., around the ring. The other six bonds which radiate away from the “equator” of the ring, are called equatorial hydrogeñ atoms (Heq). There are six of them, three of which are “slant up” and three of which are “slant down”, again alternating around the ring.