Alkenes

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Alkenes — Key Facts for SLMC Medical (Sri Lanka)

• Alkenes are unsaturated hydrocarbons containing at least one C=C double bond
• General formula: CₙH₂ₙ (for one double bond)
• The double bond consists of one σ-bond and one π-bond
• Each carbon in the C=C is sp²-hybridized (120° bond angle)
• ⚡ Exam tip: Markovnikov’s rule and addition reactions are high-yield for SLMC

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Alkenes — SLMC Medical (Sri Lanka) Study Guide

What Are Alkenes?

Alkenes are hydrocarbons containing carbon-carbon double bonds. The double bond makes them chemically reactive compared to alkanes (which are saturated and inert). Their general formula for a single double bond is CₙH₂ₙ.

Nomenclature

Alkenes use the “-ene” suffix. Number the chain from the end that gives the double bond the lowest possible number:

• Ethene (C₂H₄): CH₂=CH₂
• Propene (C₃H₆): CH₂=CH–CH₃
• But-1-ene / But-2-ene (C₄H₈)
• For multiple double bonds: diene (2 double bonds), triene (3 double bonds)

Branched alkenes: if a double bond and a branch exist at the same position, the double bond takes priority in numbering.

Electronic Structure

Each carbon in a C=C double bond is sp²-hybridized:

• Three sp² orbitals form σ-bonds at ~120° angles (planar/trigonal geometry)
• The unhybridized p orbital on each carbon overlaps sideways to form the π-bond
• The π-bond is weaker and breaks first in reactions — this is why alkenes are more reactive

C=C bond length (~134 pm) is shorter than C–C single bond (~154 pm), and C=C bond energy (~611 kJ/mol) is stronger than a C–C single bond (~347 kJ/mol). However, the π-bond specifically (~268 kJ/mol) is weaker and more easily broken.

Physical Properties

Property	Trend
Boiling point	Increases with molecular size; trans alkenes generally have higher BP than cis
Solubility	Insoluble in water; soluble in organic solvents
Density	Less than water
State	Ethene, propene: gases; C₃–C₄: gases/liquids; C₅+: liquids

Cis-trans isomerism (geometric isomerism) arises when each carbon of the double bond has two different groups attached:

• Cis: identical groups on the same side
• Trans: identical groups on opposite sides

Chemical Properties — Addition Reactions

Alkenes undergo addition reactions at the π-bond. Key reactions:

1. Hydrogenation

Alkene + H₂ → Alkane (with Pt/Pd/Ni catalyst, heat)

• Ethene + H₂ → Ethane
• This reaction is exothermic (hydrogenation enthalpy ~ –136 kJ/mol for ethene)

2. Halogenation

Alkene + X₂ → Vicinal dihalide (X = Cl₂ or Br₂, in inert solvent)

• Ethene + Br₂ → C₂H₄Br₂ (1,2-dibromoethane)
• The reaction decolorizes bromine water (brown to colorless) — used as a test for unsaturation

3. Addition of Hydrogen Halides (HX)

Alkene + HX → Haloalkane

Markovnikov’s Rule: In unsymmetrical alkenes, the hydrogen adds to the carbon with more hydrogens already attached, and the halogen adds to the carbon with fewer hydrogens.

Example: Propene + HCl → 2-chloropropane (H adds to CH₂ end, Cl adds to CH end)

• The alternative (1-chloropropane) is formed in much smaller amount

4. Hydration (Addition of H₂O)

Alkene + H₂O → Alcohol (H⁺ catalyst, heat)

• Follows Markovnikov’s rule — water adds with H to the carbon with more H atoms
• Ethene + H₂O → Ethanol
• Propene + H₂O → Isopropanol (propan-2-ol)

5. Addition of Halogens in Water (HOX)

Gives halohydrin: halogen adds to one carbon, OH adds to the other. The halogen goes to the carbon with more H atoms (Markovnikov direction).

6. Ozonolysis

Alkene + O₃ → Ozonide → Cleavage products

• Alkene is cleaved at the double bond
• Products are aldehydes and/or ketones
• Used to determine the position of the double bond in an unknown alkene

Oxidation Reactions

Reagent	Product	Notes
Cold dilute KMnO₄	Diol (glycol)	Syn addition of two –OH groups
Hot concentrated KMnO₄	CO₂ + H₂O	Complete oxidation
O₃ (ozonolysis)	Aldehydes/ketones	Cleaves C=C bond

Alkene + cold dilute KMnO₄ → dihydroxylglycol (diol) — decolorizes purple KMnO₄

Alkene + acidified KMnO₄ (hot/concentrated) → CO₂ — complete cleavage of the double bond

Polymerization

Alkenes undergo addition polymerization:

• n CH₂=CH₂ → [–CH₂–CH₂–]ₙ (polyethylene)
• n CH₂=CH–CH₃ → [–CH₂–CH(CH₃)–]ₙ (polypropylene)

Common polymers: polyethylene (PE), polyvinyl chloride (PVC), polystyrene (PS), polymethyl methacrylate (PMMA).

Clinical and Medical Relevance

• Ethene (C₂H₄) is used as a plant hormone to ripen fruits — important in food science
• Propene is used to manufacture polypropylene (PP), used in medical syringes and packaging
• PVC is used in medical tubing, blood bags, and intravenous catheters
• Ozone (O₃) therapy: ozonolysis has medical applications in disinfection
• Halothane hepatitis: halogenated alkenes (like halothane) can cause severe hepatotoxicity after repeated exposure

Common SLMC Exam Traps

• Markovnikov’s rule applies ONLY to unsymmetrical alkenes — don’t apply it to symmetrical ones like ethene
• Cis-trans isomerism requires each double-bond carbon to have two different substituents — if either carbon has identical groups, no cis-trans exists
• “Dihydroxylglycol” (from KMnO₄ cold) should NOT be confused with glycol (HO–CH₂–CH₂–OH is ethylene glycol, a different compound)
• The product of halogen addition to an alkene is a vicinal (adjacent) dihalide — both halogens on adjacent carbons

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