4.2 - Alkenes Flashcards
(57 cards)
What are Alkenes ?
They are a group of unsaturated hydrocarbons with the general formula CnH2n
What type of bonds are Alkenes composed of ?
The C=C carbon double bond consists of one sigma bind and one pi bond.
How is a sigma bond formed ?
A sigma bond is formed by the overlap of 2 atomic orbitals between thr nuclei of two bonded atoms
How is a pi bond formed ?
A pi bond is formed by the sideways overlap of two p-orbitals above and below the plane of a molecule
Draw a diagram showing sigma and pi bonds in an Alkene ?
How is the geometry of Alkenes different from Alkenes ?
- In an alkene, the ( pi ) π-bond locks the two carbon atoms in position and prevents them from rotating around the double bond.
- In an alkane, rotation is possible around every atom.
What is the shape and bond angle of Alkenes ?
- There are three bonding pairs surrounding each carbon atom in the C=C carbon double bond which repel meaning it has a trigonal planar shape
- The bond angle is 120o
Define the term ‘stereoisomers’ ?
Stereoisomers - Isomers that have the same structural formula but a different spatial arrangement of atoms
What are the two types of stereoisomers ?
- E/Z isomerism
- Optical isomerism
What are the conditions for E/Z isomerism ?
- Must contain a C=C carbon double bond that does not rotate/ has restricted rotation
- Must have different groups attached to each carbon atom of the double bond
What is cis/trans isomerism ?
A special case of E/Z isomerism in which two of the substituent groups attached to each carbon atom of the C=C group are the same
What determines whether atom/molecules attached to carbon atoms are high/ low priority ?
Atoms / molecules attached to each carbon atom in a double bond are given priority based on atomic number
Describe how you can identify whether an alkene is an E or Z stereoisomer ?
- — takes priority over — on the LHS/RHS due to higher atomic number
- — group takes priority over — group on the LHS/RHS
When are isomers Z or E isomers ?
- If groups of high priority are on the same side of the double bond, it is a Z isomer
- If groups of high priority are on different sides of double bond, it is an E isomer
When are isomers cis or trans isomers ?
- If the same groups are on the same side of the double bond, it is a cis isomer
- If the same groups are on different sides of the double bond, it is a trans isomer
Do you know how to identify E/Z isomerism ?
Yes
Do you know how to identify cis/trans isomerism ?
Yes
Explain the reactivity of Alkenes ?
- Alkenes are more reactive than Alkenes due to the presence of the pi bonds
- The electron density in the pi bond is concentrated above and below the plane of the sigma bond, making the pi electrons more exposed than sigma electrons
- This means the pi bond is ‘weaker’ than the sigma bond and can easily break
What is an addition reaction ?
A reaction where two or more reactants result in just one product
What reactants can Alkenes undergo addition reactions with ?
- Hydrogen
- Halogens
- Hydrogen halides
- Steam
What is the addition reaction with hydrogen called ? What are the conditions ?
- ‘Hydrogenation’
- Reagents = Hv2 (g)
- Conditions = Nickel catalyst, 423k
What is the addition reaction with halogens called ? What are the conditions ?
- ‘Halogenation’
- Reagents = Brv2, Clv2 or Iv2
- Conditions = Room temperature
What is the addition reaction with hydrogen halides called ? What are the conditions ?
- ‘’ / no specific name
- Reagents = HBr (g), HCl (g), HI (g)
- Conditions = Room temperature
What is the addition reaction with steam/ water called ? What are the conditions ?
- ‘Hydration’
- Reagents = Hv2O (g)/ steam
- Conditions = Phosphoric acid or concentrated sulphuric acid catalyst
