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CHEM - 4.1.3 Alkenes > Properties > Flashcards

Flashcards in Properties Deck (4)
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What are alkenes?

- Alkenes are unsaturated hydrocarbons. They have at least 1 carbon to carbon double bond.
- General formula CnH2n


Double bonds

- Double bond is made up of a sigma bond and a pi bond.
- Sigma bond is formed by two orbitals overlapping in a straight line directly between the bonding atoms.
- Pi bond is formed by a sideways overlap of adjacent p-orbitals above and below the bonding C atoms.
- This restricts the rotation of the pi bond.


Why are pi bonds weaker than sigma bonds?

- They are weaker because the electron density is spread out above and below the nuclei.
- This means the electrostatic attraction between nuclei and shared pair of elect. is weaker.
- So pi bonds have low bond enthalpy.


Shape around double bond

- Shape around each of the carbon atoms in the double bond is trigonal planar.
- 3 regions of electron density around each of the carbon atoms.
- 3 regions repel each other as far apart as possible, so bond angle is 120.
- All of the atoms are in the same plane.