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Alkenes are a class of unsaturated hydrocarbons that contain at least one carbon-to-carbon double bond (C=C) in their structure. This double bond reduces the number of hydrogen atoms that can bond with the carbon chain compared to alkanes. The general formula for alkenes is CₙH₂ₙ, where n is the number of carbon atoms in the molecule.
For example:
Ethene (C₂H₄)
Propene (C₃H₆)
Butene (C₄H₈)
In each case, the number of hydrogen atoms is twice the number of carbons, which matches the CₙH₂ₙ formula. This pattern occurs because the double bond uses two electrons between carbon atoms, reducing available positions for hydrogen bonding compared to alkanes, which follow the formula CₙH₂ₙ₊₂.
Alkenes are chemically more reactive than alkanes due to the presence of the double bond. This bond is an area of high electron density, making alkenes undergo addition reactions with halogens, hydrogen, and other reagents. These reactions are widely used in the production of plastics, alcohols, and many other industrial compounds.
Other given options are incorrect:
CₙH₂ₙ₊₁: Represents alkyl radicals, not stable hydrocarbons.
CₙH₂ₙ₊₂: Represents saturated alkanes with single bonds.
CₙH₂ₙ₋₂: Represents alkynes with triple bonds or cyclic alkenes with two double bonds.
Therefore, the correct general formula for simple open-chain alkenes with one double bond is CₙH₂ₙ.
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