A carbon-carbon double bond forms when two carbon atoms share two pairs of electrons, resulting in a strong and reactive bond.
The Process of Double Bond Formation
Here's a breakdown of how a carbon-carbon double bond is formed:
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Electron Sharing: Each carbon atom contributes two electrons to the bond. This shared pair of electrons constitutes one covalent bond. Because two pairs of electrons are shared, it's a double bond.
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Orbital Hybridization: Typically, the carbon atoms involved in a double bond undergo sp2 hybridization. This means that each carbon atom has three sp2 hybrid orbitals and one unhybridized p orbital.
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Sigma (σ) Bond: One of the shared electron pairs forms a sigma (σ) bond. This bond results from the direct, head-on overlap of two sp2 hybrid orbitals, one from each carbon atom. It's a strong and stable bond.
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Pi (π) Bond: The second shared electron pair forms a pi (π) bond. This bond arises from the sideways overlap of the unhybridized p orbitals on each carbon atom. The π bond is weaker than the σ bond.
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Planar Geometry: The sp2 hybridization results in a trigonal planar geometry around each carbon atom involved in the double bond. Consequently, the atoms directly bonded to the double-bonded carbons lie in the same plane.
Characteristics of Carbon-Carbon Double Bonds
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Strength: Double bonds are stronger than single bonds (because they involve two shared pairs of electrons), but they are not twice as strong. The π bond is weaker than the σ bond.
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Reactivity: Double bonds are more reactive than single bonds due to the presence of the weaker π bond, which is more easily broken in chemical reactions.
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Rigidity: Rotation around a double bond is restricted compared to a single bond. This rigidity leads to cis-trans isomerism (also called geometric isomerism) in molecules with different substituents on each carbon atom of the double bond.
Example: Ethene (Ethylene)
Ethene (C2H4) is a simple example of a molecule containing a carbon-carbon double bond. Each carbon atom is sp2 hybridized, forming a σ bond with each other and with two hydrogen atoms. The remaining p orbitals overlap to form the π bond, completing the double bond.
In summary, carbon-carbon double bonds are formed through the sharing of two pairs of electrons between two carbon atoms, resulting in a strong and reactive bond comprising a sigma (σ) bond and a pi (π) bond.
