2023 Volume 22 Issue 1 Pages A8-A15
When each of the four identical carbon sp3 hybrid orbitals overlaps with the hydrogen 1s orbital, four identical C-H σ bonds are formed and methane (CH4) results. Because the four sp3 hybrid orbitals have a specific geometry, the angle formed by each H-C-H is 109.47°, the so-called tetrahedral angle. The structure of ethane (C2H6) is similarly explained. The bonds in methane and ethane are called single bonds because they result from the sharing of one electron pair between bonded atoms. Carbon atoms can also form double bonds by sharing two electron pairs or triple bonds by sharing three electron pairs. Ethylene (H2C=CH2) contains a carbon-carbon double bond. When two carbons with sp2 hybrid approach each other, they form a strong σ bond by sp2-sp2 overlap. The unhybridized p orbitals interact by sideways overlap to form a π bond, resulting the formation of a carbon-carbon double bond. Four hydrogen atoms form σ bonds with the remaining four sp2 orbitals to complete the structure of ethylene. The H-C=C or H-C-H bond angle in ethylene molecule is 121.3°or 117.4°respectively. Acetylene (HC ≡ CH) contains a carbon-carbon triple bond. When two carbons with sp hybrid approach each other, they form a strong σ bond by sp-sp overlap. The unhybridized py or pz orbitals interact by sideways overlap to form two π bonds (py − py and pz − pz), resulting the formation of a carbon-carbon triple bond. The two remaining sp hybrid orbitals each form a σ bond with hydrogen to complete the acetylene molecule. Acetylene is a linear molecule with H-C ≡ C bond angle of 180°. Difference electron density of HOMO or LUMO in ethylene molecule is visualized. The effect of π electron ring current for the chemical shift measurement in NMR (Nuclear Magnetic Resonance) spectroscopy is also discussed.
