Orbital hybridisation (or hybridization) is the concept of mixing atomic orbitals to form new hybrid orbitals (with different energies, shapes, etc., than the component atomic orbitals) suitable for the pairing of electrons to form chemical bonds in valence bond theory.
For example, in a carbon atom which forms four single bonds, the valence-shell s orbital combines with three valence-shell p orbitals to form four equivalent sp3 mixtures in a tetrahedral arrangement around the carbon to bond to four different atoms.
Understanding carbon hybridization not only helps in predicting molecular shapes but also in understanding reaction mechanisms in organic chemistry, where the formation and breaking of bonds are central to the discipline.
Here’s a detailed look at the hybridization of carbon:
Ground State of Carbon:
- Carbon in its ground state has an electron configuration of 1s² 2s² 2p². This means it has two electrons in the 2s orbital and two in the 2p orbitals, but only two unpaired electrons in the p orbitals available for bonding.
Promotion and Hybridization:
- To form four bonds, as seen in many carbon compounds, one of the 2s electrons is promoted to the empty 2p orbital, resulting in four unpaired electrons (1s² 2s¹ 2p³). However, these orbitals are not equivalent in shape or energy, which leads to hybridization.
Types of Hybridization
Carbon, with its electronic configuration of 1s² 2s² 2p², can undergo hybridization to form different types of hybrid orbitals:
sp³ Hybridization:
- Example: Methane (CH₄)
- Process: One 2s orbital and three 2p orbitals mix to form four sp³ hybrid orbitals.
- Geometry: Tetrahedral, with bond angles of 109.5°.
- Properties: Each sp³ orbital has 25% s character and 75% p character. This results in strong bonds due to good orbital overlap.
sp² Hybridization:
- Example: Ethylene (C₂H₄)
- Process: One 2s orbital and two 2p orbitals hybridize to form three sp² orbitals, leaving one p orbital unhybridized.
- Geometry: Trigonal planar, with bond angles of 120°.
- Properties: The unhybridized p orbitals can form π bonds, which are weaker than σ bonds formed by sp² orbitals. This allows for double bonds.
sp Hybridization:
- Example: Acetylene (C₂H₂)
- Process: One 2s and one 2p orbital hybridize to form two sp orbitals, leaving two p orbitals unhybridized.
- Geometry: Linear, with a bond angle of 180°.
- Properties: This allows for the formation of triple bonds. The unhybridized p orbitals form two π bonds.
Bonding in Hybridized Carbon:
- σ (Sigma) Bonds: Formed by the end-to-end overlapping of atomic orbitals. In hybrid orbitals, these are strong and form the backbone of the molecule.
- π (Pi) Bonds: Formed by the side-to-side overlap of unhybridized p orbitals. These are present in double and triple bonds, making them weaker than σ bonds due to less effective overlap.
Bond Formation and Hybridization
The hybrid orbitals formed in carbon are used to form sigma (σ) bonds with other atoms. For example:
- In methane, the carbon atom forms four σ bonds with four hydrogen atoms using its four sp³ hybrid orbitals.
- In ethylene, the carbon atom forms three σ bonds with two hydrogen atoms and one carbon atom using its three sp² hybrid orbitals. It also forms a pi (π) bond with the other carbon atom using its remaining unhybridized p orbital.
- In acetylene, the carbon atom forms two σ bonds with one hydrogen atom and one carbon atom using its two sp hybrid orbitals. It also forms two π bonds with the other carbon atom using its two unhybridized p orbitals.
Implications of Hybridization:
- Stability and Reactivity: The type of hybridization affects the molecule’s stability and reactivity. For instance, compounds with sp² hybridization can participate in resonance, which can stabilize structures like benzene.
- Physical Properties: Hybridization influences the shape of molecules, which in turn affects physical properties like polarity, boiling point, and solubility.
Advanced Concepts:
- delocalization: In molecules like benzene, electrons in p orbitals are delocalized, leading to increased stability through resonance.
- Stereochemistry: The spatial arrangement due to hybridization impacts how molecules interact in 3D space, crucial in biological systems and drug design.