4.1.3: Alkenes Flashcards
General formula (alkenes)
CnH2n
What is an alkene?
An unsaturated hydrocarbon with a C=C double bond which comprises of 1 σ bond and 1 𝜋 bond.
𝜋 bond
sideways overlap of adjacent p orbitals, above and below σ bonds in the plane.
Which is stronger, a σ bond or a 𝜋 bond?
- A σ bond is stronger
* Because a 𝜋 bond is further from the nucleus and easier to break
σ bond
Head-on overlap of atomic or hybrid orbitals directly between the bonding atoms. All single bonds are σ bonds.
Electrophile
Electron pair acceptor
Angle between two hydrogens bonded to a C in a C=C double bond
120⁰
Angle between two hydrogens bonded to a C in a C–C single bond
109.5⁰
Conditions for hydrogenation
- 150⁰C (423K)
* Nickel catalyst
Hydrogenation reagents
- Hydrogen gas
* Alkene
What type of reaction is hydrogenation?
Addition reaction: adds across the double bond
Polymerisation (mechanism)
Addition polymerisation
Conditions for polymerisation
- Heat (200⁰C)
- Pressure (2000 atmospheres)
- Presence of O₂ initiator
What is the shape around each of the carbon atoms in a C=C double bond?
Trigonal planar.
Describe rotation around the double bond
- Rotation is restricted because the 𝜋 bond locks the two carbon atoms in position, preventing them from rotating.
- Contrast to in alkanes, where rotation is possible around every atom.
Conditions for E/Z isomerism
- a C=C double bond
* 2 different groups attached to each carbon in the C=C
How do you determine the “sides” of the double bond?
Draw the split in the same direction that the bond lines go.
What does the ‘E/Z’ in ‘E/Z isomerism’ mean?
- E = Entgegen: the two higher priority groups are on (diagonally) opposite sides of the double bond.
- Z = Zusammen: the two higher priority groups are on the same side of the double bond.
What determines priority in E/Z isomerism?
- Mᵣ = priority
- Higher Mᵣ = higher priority
- Work along until the atoms have different Mᵣs. The one with the highest Mᵣ wins priority.
What are the Cahn-Ingold-Prelog rules?
- If the groups of higher priority are on the same side of the double bond, the compound is a Z isomer.
- If the groups of higher priority are diagonally placed across the double bond, the compound is an E isomer.
What is cis-trans isomerism?
A case of E/Z isomerism where one of the groups attached to each carbon of the C=C double bond is a hydrogen.
Conditions for halogenation of alkenes
Room temperature
Describe the bromine water test
- Test for the presence of a double bond
- Bromine water = orange
- If double bond present, turns colourless
Alkenes can be hydrated to form
alcohols
Conditions for hydration of alkenes
• H₃PO₄ (phosphoric acid catalyst)
Hydration (reagents)
- Alkene
- Steam
- Phosphoric acid catalyst
What is Markownikoff’s rule?
• When a hydrogen halide reacts with an unsymmetrical alkene the hydrogen of the hydrogen halide attaches itself to the carbon atom of the alkene with the greater number of hydrogen (and the smaller number of carbon) atoms.
Why is Markownikoff’s rule correct?
- Where the H from the hydrogen halide is added depends on the stability of the carbocation
- Due to the inductive effect, alkyl R groups push e- density onto the carbocation, making it more stable
- The more alkyl groups, the more stable the carbocation; tertiary carbocations are the most stable, primary carbocations the least stable
- Less stable = more attractive to H, so H will be added to the carbon atom with the least alkyl groups
