Alicyclic compounds : Preparations of cyclo alkanes, Bayer strain theory and
orbital picture of angle strain.
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The orbital picture of angle strain in cyclic compounds provides a visual representation of the deviation from ideal bond angles and the resulting strain in the molecular structure. To understand this, let’s consider the example of cyclopropane, a cyclic compound known for its significant angle strain.
Cyclopropane consists of a ring of three carbon atoms, each bonded to two hydrogen atoms. In an ideal, planar cyclopropane molecule, the bond angles between the carbon atoms would be 120 degrees, similar to a trigonal planar geometry. However, due to the limited space available in the small cyclopropane ring, the actual bond angles deviate from this ideal value, resulting in increased angle strain.
From an orbital perspective, the deviation from ideal bond angles can be explained by considering the overlap of atomic orbitals involved in bonding. In cyclopropane, each carbon atom utilizes three sp2 hybrid orbitals to form sigma bonds with the other carbon atom and two hydrogen atoms. These sp2 hybrid orbitals are created by mixing one 2s orbital and two 2p orbitals.
In an ideal, planar geometry, the sp2 hybrid orbitals on each carbon atom would be oriented in a trigonal planar arrangement, with bond angles of 120 degrees. However, in cyclopropane, the carbon atoms are forced closer together due to the ring structure, resulting in a smaller bond angle. This leads to increased overlap of the sp2 hybrid orbitals, causing electron-electron repulsion and destabilization of the molecule.
The orbital picture of angle strain in cyclopropane shows that the overlapping sp2 hybrid orbitals on adjacent carbon atoms experience repulsive interactions due to the deviation from the ideal bond angles. This repulsion contributes to the strain in the molecule, increasing its potential energy and making it less stable compared to an acyclic molecule with ideal bond angles.
The angle strain in cyclopropane and other cyclic compounds can be reduced by adopting non-planar conformations that relieve the steric interactions and allow for larger bond angles. In the case of cyclopropane, the actual bond angles are slightly larger than the ideal 60 degrees, as the molecule adopts a puckered conformation known as the “envelope” or “twist-boat” conformation.
In summary, the orbital picture of angle strain in cyclic compounds, such as cyclopropane, involves the deviation from ideal bond angles due to the limited space in the ring structure. This deviation results in increased overlap of bonding orbitals, causing electron-electron repulsion and strain in the molecule. Understanding the orbital picture of angle strain helps explain the decreased stability and increased reactivity of cyclic compounds with significant angle strain.
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