Alkenes: Properties and Isomerism

• Alkenes are hydrocarbons containing one or more carbon-carbon double bonds.
• They are also known as olefins.
• The general formula for alkenes with one double bond is CₙH₂ₙ.

Properties of Alkenes

• Alkenes are nonpolar compounds due to the similar electronegativity of carbon and hydrogen.
• Physical properties are similar to alkanes with the same number of carbon atoms.
• Alkenes are insoluble in water but soluble in organic solvents.
• Alkenes exhibit van der Waals forces.
• Boiling points increase with increasing molecular weight due to increased van der Waals forces.
• Branched alkenes have lower boiling points compared to straight-chain alkenes due to reduced surface contact.
• Alkenes are more reactive than alkanes due to the presence of the pi (π) bond.
• The double bond is a region of high electron density, making alkenes susceptible to electrophilic attack.

Isomerism in Alkenes

• Alkenes exhibit both structural and stereoisomerism.
• Structural isomers have the same molecular formula but different arrangements of atoms.
• Stereoisomers have the same connectivity but different spatial arrangements.
• Geometric isomerism (cis-trans isomerism) occurs in alkenes due to restricted rotation around the double bond.
• Cis isomers have substituents on the same side of the double bond.
• Trans isomers have substituents on opposite sides of the double bond.
• Cis-trans isomerism requires that each carbon of the double bond must have two different groups attached.
• The Cahn-Ingold-Prelog (CIP) priority rules are used to assign E/Z configurations to alkenes.
• If the higher priority groups are on the same side of the double bond, it is the Z isomer (from German zusammen, meaning together).
• If the higher priority groups are on opposite sides of the double bond, it is the E isomer (from German entgegen, meaning opposite).

Alkene Reactions

• Alkenes undergo addition reactions in which atoms or groups of atoms add to the carbon-carbon double bond.
• Electrophilic addition is a characteristic reaction of alkenes.
• Common electrophilic addition reactions include:
  • Hydrogenation: Addition of hydrogen (H₂) across the double bond in the presence of a metal catalyst (e.g., Pt, Pd, Ni).
  • Halogenation: Addition of halogens (e.g., Cl₂, Br₂) across the double bond.
  • Hydrohalogenation: Addition of hydrogen halides (e.g., HCl, HBr) across the double bond, following Markovnikov's rule.
  • Hydration: Addition of water (H₂O) across the double bond in the presence of an acid catalyst (e.g., H₂SO₄).
• Markovnikov's rule states that in the addition of HX to an alkene, the hydrogen atom adds to the carbon with more hydrogen atoms already attached.
• Peroxide effect (anti-Markovnikov addition): In the presence of peroxides, HBr adds to alkenes in an anti-Markovnikov fashion, where the hydrogen atom adds to the carbon with fewer hydrogen atoms.
• Oxymercuration-Demercuration: A reaction sequence used to hydrate alkenes, following Markovnikov's rule, without carbocation rearrangements. It involves the addition of mercury(II) acetate followed by reduction with sodium borohydride.
• Hydroboration-Oxidation: A reaction sequence used to hydrate alkenes in an anti-Markovnikov fashion. It involves the addition of borane (BH₃) followed by oxidation with hydrogen peroxide (H₂O₂) and hydroxide (OH⁻).
• Epoxidation: Reaction of an alkene with a peroxyacid (e.g., m-CPBA) to form an epoxide (oxirane).
• Ozonolysis: Reaction of an alkene with ozone (O₃) followed by reduction with zinc or dimethyl sulfide to cleave the double bond and form aldehydes or ketones.
• Polymerization: Alkenes can undergo polymerization to form polymers.

Alkene Synthesis

• Alkenes can be synthesized through various methods, primarily elimination reactions.
• Dehydration of alcohols: Removal of water (H₂O) from an alcohol, typically using an acid catalyst (e.g., H₂SO₄, H₃PO₄) at high temperatures.
• Dehydrohalogenation of alkyl halides: Removal of a hydrogen halide (HX) from an alkyl halide, typically using a strong base (e.g., KOH, NaOH, alkoxides).
• E2 mechanism is favored for dehydrohalogenation.
• Zaitsev's rule states that the major product is the more substituted alkene (the alkene with more alkyl groups attached to the double-bonded carbons).
• Cracking of alkanes: Breaking down large alkane molecules into smaller, more useful alkenes and alkanes at high temperatures.
• Wittig Reaction: Reaction of an aldehyde or ketone with a Wittig reagent (phosphorus ylide) to form an alkene.
• Hofmann Elimination: Elimination reaction of quaternary ammonium hydroxides to form alkenes. The less substituted alkene is the major product (anti-Zaitsev or Hofmann product).

Industrial Applications of Alkenes

• Alkenes are important building blocks in the chemical industry.
• Ethene (ethylene) is used to produce polyethylene, a widely used plastic.
• Propene (propylene) is used to produce polypropylene.
• Alkenes are used in the synthesis of various organic compounds, including alcohols, aldehydes, ketones, and carboxylic acids.
• Alkenes are used as monomers in the production of polymers and as intermediates in the synthesis of pharmaceuticals, agrochemicals, and other fine chemicals.