Revision: Organic Chemistry >> Hydrocarbons: Alkenes Chemistry (English Medium) ICSE Class 10 CISCE

Alkenes are unsaturated hydrocarbons containing at least one C=C double bond.

• General formula: CₙH₂ₙ (where n = 2, 3, 4…)
• Suffix for IUPAC naming: –ene
• e.g., Ethene (CH₂=CH₂), Propene (CH₃–CH=CH₂)
• The double bond consists of one σ bond and one π bond

• Ethene (C₂H₄) is the first member of the alkene series, found in natural gas and acts as a plant hormone responsible for fruit ripening.
• Each carbon in ethene forms two C–H single bonds and one C=C double bond, making a planar (flat) molecule.
• The bond angles in ethene (H–C–H and H–C=C) are approximately 120°, showing trigonal planar geometry.
• Structure of Ethene:
  

1. Solubility

• Alkenes are non-polar
• Insoluble in water
• Soluble in non-polar organic solvents (e.g., hexane, benzene)

2. Boiling Point (BP)

Increases with molecular mass

• More electrons → stronger London dispersion forces

Straight-chain > Branched-chain

• Straight chains have larger surface area → stronger intermolecular forces

Cis-alkenes > Trans-alkenes (usually)

• Cis is more polar → dipoles do not cancel → higher BP

Alkenes vs Alkanes (same number of carbons)

• Alkenes have slightly lower BP

Reason:

• π-bond leads to less effective electron distribution for dispersion forces
• Slightly weaker intermolecular attractions

3. Melting Point (MP)

Trans-alkenes > Cis-alkenes

• Trans is more symmetrical → packs better in crystal lattice → higher MP

Cis-alkenes

• Less symmetrical → poorer packing → lower MP

In trans-alkenes:

• Bond dipoles cancel → non-polar
• Leads to tighter packing in solid state

Alkenes undergo mainly electrophilic addition reactions due to the π bond (electron-rich site).

Reaction	Example
Addition of hydrogen (Hydrogenation)	\[\ce{H2C = CH2 ->[H2/Ni, Pt or Pd][523-573K] H3C - CH3}\]
Addition of halogen	\[\begin{array}{cc} \phantom{}\ce{H3CCH = CH2 + Cl — Cl ->[CCl4] CH3CH - CH2}\phantom{}\\ \phantom{....................................................}|\phantom{.........}|\phantom{}\\ \phantom{.....................................................}\ce{Cl}\phantom{.......}\ce{Cl}\phantom{} \end{array}\]
Addition of HX (Markovnikov's rule)	\[\begin{array}{cc} \phantom{..............................................................}\ce{Br}\phantom{}\\ \phantom{............................................................}|\phantom{}\\ \phantom{}\ce{\underset{(For unsymmetrical allkene-Markownikoff’s rule)}{H3CCH = CH2 + HBr} -> H3C - CH - CH3}\phantom{} \end{array}\]
Addition of HBr (Anti-Markovnikov / Kharasch effect)	\[\begin{array}{cc} \phantom{}\ce{\underset{(In presence of peroxide-reverse of Markownikoff’s rule)}{H3CCH = CH2 + HBr} ->[Peroxide] CH3CH2CH2Br}\phantom{}\\ \end{array}\]
Hydration (addition of H₂SO₄/H₂O)	\[\begin{array}{cc}  \ce{O}\phantom{..}\\ ||\phantom{..}\\  \phantom{}\ce{CH2 = CH2 + H - O - S - O - H -> C2H5HSO4}\phantom{}\\ ||\phantom{..}\\ \ce{O}\phantom{..}  \end{array}\]
Oxidation (KMnO₄/H⁺)	\[\begin{array}{cc} \phantom{..........................}\ce{O}\phantom{}\\ \phantom{..........................}||\\ \phantom{}\ce{H3C — CH = CH2 ->[{[O]}][KMnO4, {[H^{+}]}] H3C - C - OH + CO2 + H2O}\phantom{} \end{array}\]
Hydroxylation	\[\begin{array}{cc} \phantom{}\ce{H2C = CH2 + H2O + [O] ->[Dil.KMnO4][273K] CH2 - CH2}\\ \phantom{.....................................................}|\phantom{..........}|\phantom{}\\ \phantom{........................................................}\ce{OH}\phantom{.....}\ce{OH}\phantom{} \end{array}\]
Ozonolysis	\[\begin{array}{cc} \phantom{.....}\ce{H3C}\phantom{....................................}\ce{H3C}\phantom{............................}\\ \phantom{.....}\backslash\phantom{.........................................}\backslash\phantom{.....................}\\ \phantom{..........}\ce{C = CH2 + O3 ->[Zn/H2O] \phantom{.......}C = O + HCHO}\phantom{}\\ \phantom{......}/\phantom{..........................................}/\phantom{.....................}\\ \phantom{...............}\ce{H3C}\phantom{......................................}\ce{H3C}\phantom{......................................} \end{array}\]
Polymerisation
Hydroboration-oxidation	\[\begin{array}{cc}  \phantom{...............}\ce{H}\phantom{....}\ce{H}\phantom{.............................................}\ce{H}\phantom{....}\ce{H}\phantom{..........................}\ce{H}\phantom{....}\ce{H}\phantom{............................}\\ \phantom{.............}|\phantom{......}|\phantom{..............................................}|\phantom{......}|\phantom{............................}|\phantom{......}|\phantom{..........................}\\ \phantom{}\ce{6(H - C = C - H) + (BH3)2 ->[THF] 2(H - C - C)3 - B ->[H2O2][OH^Θ] H - C - C - H + B(OH)3}\phantom{}\\ \phantom{..........................................}|\phantom{......}|\phantom{............................}|\phantom{......}|\\ \phantom{..............................................}\ce{H}\phantom{.....}\ce{H}\phantom{..........................}\ce{H}\phantom{.....}\ce{OH}\phantom{.} \end{array}\]

• Used to make polythene and synthetic chemicals.
• Converted to ethanol for cosmetics and toiletries.
• Helps in fruit ripening.
• Used in epoxyethane and oxy-ethylene flame for detergents and metal cutting.