HYDROCARBON CHEMICAL REACTION OF ALKENES

Описание: The chemical reactivity of alkenes is primarily driven by the presence of the carbon-carbon double bond (
). This double bond consists of one strong sigma (
) bond and one weaker, electron-rich pi (
) bond. Because the
electrons are loosely held and exposed above and below the plane of the molecule, they act as a nucleophile, attracting "electron-loving" species called electrophiles.
1. Electrophilic Addition Reactions
This is the most characteristic reaction of alkenes. In these reactions, the
bond breaks, and two new
bonds are formed with the attacking atoms.
Hydrogenation: Adding hydrogen (
) in the presence of a metal catalyst (like Pt, Pd, or Ni) converts an alkene into a saturated alkane. This is used industrially to turn vegetable oils into solid fats like margarine.
Halogenation: Alkenes react with halogens (like
or
) to form vicinal dihalides.
Example: The decolourization of orange bromine water is a standard laboratory test to detect the presence of a double bond.
Hydrohalogenation: Adding hydrogen halides (like
or
) produces alkyl halides.
Markovnikov's Rule: In the addition of
to an unsymmetrical alkene, the
atom attaches to the carbon that already has more hydrogen atoms.
Hydration: Adding water (
) in the presence of an acid catalyst (like
) yields alcohols. Industrially, this is a major route for producing synthetic ethanol.
2. Oxidation Reactions
Alkenes can be oxidized to form different oxygen-containing functional groups depending on the reagents used.
Hydroxylation: Reaction with cold, dilute potassium permanganate (
), known as Baeyer’s Reagent, produces vicinal glycols (diols). The purple color of the reagent disappears, making this another common test for unsaturation.
Ozonolysis: Reaction with ozone (
) followed by a reduction step cleaves the double bond entirely, producing aldehydes or ketones. This is often used to locate the position of a double bond in an unknown molecule.
Oxidative Cleavage: Stronger oxidizing agents (like hot, acidic
) can break the carbon chain at the double bond to form carboxylic acids or ketones.
3. Polymerization
Alkenes undergo addition polymerization, where thousands of monomer units link together by breaking their
bonds to form long saturated chains.
Polythene (Polyethylene): Formed from ethene monomers.
Polypropylene: Formed from propene monomers.
4. Combustion
Alkenes burn in the presence of oxygen to produce carbon dioxide, water, and heat. However, they often burn with a smoky flame compared to alkanes due to their higher carbon-to-hydrogen ratio, which leads to more frequent incomplete combustion.
If you are interested in a specific reaction, I can provide a step-by-step mechanism or explain how to predict products for unsymmetrical molecules using Markovnikov's rule.