Properties of alkenes

🔗 Structure of Alkenes

• Alkenes are unsaturated hydrocarbons – they contain at least one C=C double bond.

• General formula of aliphatic (non-cyclic) alkenes:
  CₙH₂ₙ

• First three members:

  • Ethene: CH₂=CH₂

  • Propene: CH₂=CH–CH₃

  • But-2-ene: CH₃–CH=CH–CH₃

🌀 Branched, Cyclic and Polyunsaturated Alkenes

• Branched and polyunsaturated alkenes still obey the general formula CₙH₂ₙ, but cyclic alkenes do not.

• Examples:

  • 2-methylpropene

  • buta-1,3-diene

  • cyclopentene (cyclic, so general formula doesn't apply)

⚛️ Bonding in the Double Bond

• Each carbon atom in the double bond uses:

  • 3 electrons to form sigma (σ) bonds: 2 single C–H bonds and 1 C–C σ bond

  • 1 electron in a p-orbital to form a π-bond by sideways overlap

• So, a double bond = 1 σ-bond + 1 π-bond

The π-bond:

• • Electron density lies above and below the plane of the bonded nuclei

  • Prevents rotation around the double bond – the bond is locked in position

Want to find out more about the origin of atomic orbitals? Click here to read my blog about Schödinger's Journery to Atomic Orbials.

🔺 Shape Around a Double Bond

• Around each C in the C=C bond:

  • There are three bonding regions (2 C–H and 1 C–C)

  • Electron-pair repulsion theory: repulsion between 3 bonding regions gives a trigonal planar shape

  • Bond angles = 120°

  • All atoms lie in the same plane

🧠 Worked Example: But-2-en-1-ol

Structure contains both C=C and OH group.

• Atom a. (a carbon in the double bond):

  • Has 3 bonding pairs → trigonal planar, 120°

• Atom b. (a carbon with 4 single bonds):

  • Has 4 bonding pairs → tetrahedral, 109.5°

• Atom c. (an oxygen in OH):

  • 2 bonding pairs + 2 lone pairs → bent shape

  • Lone pairs repel more → bond angle ~ 104.5°