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Hydrocarbons: Alkenes & Alkynes revision notes

Every idea in the Hydrocarbons: Alkenes & Alkynes chapter for JEE Main, with the usual mistakes and the fast methods. Topic 1 is open to everyone; sign in free to read all 13 and download the PDF.

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Structure, stability and geometrical isomerism

C=C and C≡C bonds; alkene stability
4× asked, last 2026
C−C 154, C=C 134, C≡C 120 pm;348, 681, 823 kJ mol−1RX2C=CRX2>RX2C=CHR>RCH=CHR (trans>cis)>RCH=CHX2>CHX2=CHX2\begin{gathered}\ce{C-C}\ 154,\ \ce{C=C}\ 134,\ \ce{C#C}\ 120\ \text{pm};\qquad 348,\ 681,\ 823\ \text{kJ mol}^{-1}\\ \ce{R2C=CR2} > \ce{R2C=CHR} > \ce{RCH=CHR}\ (\textit{trans} > \textit{cis}) > \ce{RCH=CH2} > \ce{CH2=CH2}\end{gathered}
C=C is one σ + one weaker π; C≡C is σ + 2π. More alkyl groups on the C=C: more hyperconjugation, more stable alkene.
154C–C134C=C120C≡Clength (pm)348C–C681C=C823C≡Cbond enthalpy (kJ mol⁻¹)sheet values
C=C is less than twice C–C (681 < 2 × 348): the π bond is weaker than σ
butanebut-1-ene−127cis-but-2-ene−120trans-but-2-ene−116
Heats of hydrogenation to butane (standard values, kJ mol⁻¹): the more stable alkene sits lower and releases less
  • Heat of hydrogenation runs opposite to stability: the more stable alkene releases less. |ΔH|: but-1-ene > cis-but-2-ene > trans-but-2-ene.
  • Not cis vs trans first. Count alkyl groups on the C=C first; cis/trans only breaks a tie. cis-But-2-ene (2 groups) still beats propene (1).
Geometrical isomerism
3× asked, last 2026
abC=Cde: a≠b, d≠e;cis-but-2-ene μ=0.33 D, bp 277 K;trans μ=0, bp 274 K\ce{abC=Cde}:\ a \ne b,\ d \ne e;\qquad \textit{cis}\text{-but-2-ene } \mu = 0.33\ \text{D, bp } 277\ \text{K};\quad \textit{trans}\ \mu = 0,\ \text{bp } 274\ \text{K}
Turning about C=C breaks the π bond, so groups are fixed. Like groups on one side: cis. E/Z: higher-priority groups apart = E.
CH₃HCH₃Hcis: dipoles add, μ = 0.33 DCH₃HCH₃Htrans: they cancel, μ = 0
C–CH₃ bond dipoles (orange) summed as vectors: only cis has a net dipole (red), so it boils higher
  • cis is polar (bond dipoles add; trans with like groups has μ = 0), so cis boils higher and dissolves better in polar solvents. trans packs better, so it melts higher (but-2-ene: trans 168 K, cis 134 K).

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Practise Hydrocarbons: Alkenes & Alkynes

11 more concepts in 4 topics

Preparation of alkenes; Electrophilic addition to alkenes; Oxidation and ozonolysis; and Alkynes: preparation, acidity and addition. Read the first part free. Sign in free with Google to read the rest and download the PDF.

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Preparation of alkenes

  • Partial reduction of alkynes7×
  • Elimination: alkyl halides and alcohols6×

Electrophilic addition to alkenes

  • Addition of HX: Markovnikov and peroxide3×
  • Carbocation rearrangements4×
  • Addition of halogens3×
  • Also remember1

Oxidation and ozonolysis

  • H₂ uptake and degree of unsaturation5×
  • KMnO₄: cold dilute vs hot acidic6×
  • Ozonolysis and working back15×

Alkynes: preparation, acidity and addition

  • Preparation of alkynes4×
  • Acidity of terminal alkynes4×
  • Addition to alkynes: HX, Br₂, water3×
  • Also remember1

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