Alkenes Flashcards
How do alkenes have a double bond? (2)
- Have sigma bonds and pi bonds
- Pi bond formed by the sideways overlap of two p-orbitals
What is the shape around a double bond? (3)
- Three regions of electron density around each of the carbon atoms
- The three regions repel each other as far apart as possible, so the bond angle around each carbon atom is 120°
- All the atoms are in the same plane
What is a stereoisomer?
Have the same structural formula but different arrangement in space
When can there be E/Z or cis/trans isomerism? (3)
- There is a C=C bond
- Different groups are attached to each carbon atom of the double bond
- One of the two substituent groups are the same
What is cis trans isomerism? (2)
- Cis isomerism is same as Z
- Trans isomerism is same as E
How do you use the Cahn- Ingold- Prelog rules? (2)
- If the high priority groups are on the same side of the double bond, the compound is the Z isomer
- If the high priority groups are on different sides of the double bond, the compound is the E isomer
How do you assign priority to a group around a double bond?
Higher atomic number means higher priority
Why are alkenes more reactive than alkanes? (4)
- Contain a pi bond
- So electron density is above and below the molecule
- Electrons more susceptible to react
- Pi bond breaks more readily
What are the addition reactions that alkenes can undergo? (4)
- Hydrogen in the presence of a nickel catalyst
- Halogens
- Hydrogen halides
- Steam in the presence of an acid catalyst
How do you test for unsaturation?
Unsaturated molecules decolourise bromine water
Why can alkenes undergo electrophilic addition reactions? (2)
- Double bond is an area of high electron density due to sigma and pi bonds
- High electron density attracts electrophiles
What is an electrophile?
An atom or group of atoms that is attracted to an electron rich centre and accepts an electron pair
What is Markownikoff’s rule?
When a hydrogen halide reacts with an alkene, the hydrogen attaches to the carbon with the greatest number of hydrogen atoms
Why does stability vary between primary, secondary and tertiary carbocations? (4)
- Linked to the electron donating ability of alkyl groups
- Each alkyl group donates and pushes electrons towards positive carbon
- This spreads out positive charge, making it more stable
- More alkyl groups mean more electrons and so more stability
What is a polymer?
Extremely large molecule made up of repeat units called monomers
