4.1.3 Alkenes Flashcards
(28 cards)
What are the bonding properties of alkenes?
- Unsaturated hydrocarbons
- General formula CnH2n
- Contains C=C
- Comprises of a pi-bond and sigma-bond
- Restricted rotation of the pi-bond
What is a pi-bond?
Sideways overlap of adjacent p-orbitals above and below the bonding C atoms
What are the shape properties of alkenes?
- There are 3 regions of electron density around each C in the C=C
- Electron repulsion creates a trigonal planar shape around each C
- All atoms are on the same plane
- Bond angle is 120°
What is stereoisomers?
Compounds with the same structural formula but with a different arrangement in space
What is E/Z isomerism?
An example of stereoisomerism, in terms of restricted rotation about a double bond and the requirement for 2 different groups to be attached to each C atom of the C=C group
What is cis-trans isomerism?
A special case of E/Z isomerism in which 2 of the substituent groups attached to each C atom of the C=C group are the same
What are the conditions for E/Z isomerism?
- Restricted rotation around the double bond
- Different groups must be attached to each C atom
What are the CIP rules?
- Atoms attached to each C are given a priority based on their atomic number
- Z isomers = higher priority groups on the same side
- E isomers = higher priority groups on different sides
- The higher the atomic number, the higher the priority
- If you cannot assign priority 1 bond away, move one bond further away
What are the conditions for cis-trans isomerism?
- Each C must be attached to different groups
- Must be a matching group across the double bond
- Cis isomers = same groups on same side
- Trans isomers = same groups on opposite sides
Why are alkenes reactive?
- Electrons in the pi-bond are more exposed than those in the sigma-bond
- Pi-bonds break more readily so have a low bond enthalpy
What are the reagents and conditions for the hydrogenation of alkenes?
Reagents = alkene, H2
Conditions = 423K, nickel catalyst
What is the hydrogenation of alkenes?
- Addition reaction to form alkanes
- Hydrogen is added across the double bond
What are the reagents and conditions for the halogenation of alkenes?
Reagents = alkene, halogen (Br or Cl)
Conditions = room temperature
What is the halogenation of alkenes?
- Addition reaction
- Halogen adds across the double bond
How can reactions with Br demonstrate saturation?
- Bromine water is an orange solution
- When added to an alkene, bromine adds across the bond
- This decolourises bromine water
- No addition reaction with a saturated compound
What are the reagents and conditions for addition with hydrogen halides?
Reagents = alkene, gaseous hydrogen halide
Conditions = room temperature
What is the addition of alkenes with hydrogen halides?
- Addition reaction to form haloalkanes
- Multiple products can form if unsymmetrical alkene
What are the reagents and conditions for the hydration of alkenes?
Reagents = alkene, steam
Conditions = 300°C, 60-70atm, phosphoric acid catalyst
What is the hydration of alkenes?
- Steam adds across the C=C bond to form alcohols
- Widely used to produce ethanol form ethene
What is an electrophile?
An electron pair acceptor
Why does the electrophilic addition mechanism happen?
- High electron density of the pi-electrons attracts electrophiles
- Electrophile is either a positive ion or molecule with an atom of partial positive charge
How does the electrophilic addition mechanism happen?
- Curly arrow from double bond to partially positive charged atom
- Curly arrow from bond of dipole to partially negative charged atom
- Heterolytic fission of dipole
- Forms a carbocation and curly arrow from ion to positive C
How do halogens become an induced dipole?
Interaction with C=C electrons causes polarisation
Why are hydrogen halides a dipole?
- Halogen is more electronegative than H
- Polar
