13 Alkenes Flashcards
What are the 2 types of bonds in alkenes?
Sigma and pi bonds
Why are there 2 bonds in alkene double bonds?
3 of 4 electrons in the outer shell are used for sigma bonds
The fourth electron occupies a p orbital
What is a pi bond?
- Formed by the sideways overlap of two p-orbitals, one from each carbon atom of the double bond
- Each C atom contributes 1 electron to the pair
- Electron density is above and below the line joining the nuclei of the bonding atoms
Why can atoms in the double bond not rotate?
Pi bond locks the 2 carbon atoms in position and prevents them from rotating around the double bond
What is the shape around a double bond?
- 3 regions of electron density
- Repel as far apart as possible
- Trigonal planar shape (120)
- All atoms are in the same plane
What is structural isomerism?
Molecules with the same molecular formula but a different structural formula
What is stereoisomerism?
Molecules with the same structural formula but a different arrangement of atoms in space
Why is there stereoisomerism around double bonds?
- The rotation about the double bond is restricted because of the electron density of the pi bond above and below the plane of the sigma bond
- The groups attached to each carbon atom are therefore fixed relative to each other
What are the conditions for E/Z isomerism?
- Contains a C=C double bond
- Each carbon in the double bond must be bonded to different groups
What is the test for unsaturation?
Addition reaction of alkene and bromine
Reacts with alkenes (orange to colourless)
Alkanes no reaction
What is the difference in reactivity between alkenes and alkanes and why?
Alkenes more reactive than alkanes
Have a pi bond where the pi electrons are outside of the bond so more exposed
What are the conditions for the hydrogenation of alkenes?
Nickel and 423K (150C)
Write an equation for propane and hydrogen
Propene + hydrogen –> Propane
Write an equation for alkenes with bromine
Ethene + bromine –> dibromoethane
Write an equation for the reaction of propene with hydrogen chloride
Propene + hydrogen chloride (HCl) –> haloalkane (chloropropane)
What are the 2 possible products of alkenes and hydrogen halides?
1-chloropropane –> minor product
2-chloropropane –> major product
What are the conditions for hydration of alkenes?
High temp (reacting with steam)
Phosphoric acid
Used to make ethanol from ethene
Write an equation of propene and steam
Propene + steam –> propan-1-ol or propan-2-ol
What is an electrophile?
An atom or molecule with a (full or partial) positive charge that can accept a pair of electrons
- Electron-pair acceptor
Draw the mechanism of the addition reaction of but-2-ene and hydrogen bromide and describe
- H in HBr is slightly positive due to permanent dipole
Arrow from double bond to H and from single bond to Br (heterolytic fission) - Br has a lone pair is bromide, C is positive
- Arrow from lone pair to positive C
- Br bonds to C
Alkenes –> Carbocation intermediate and Br- –> Alkane
What is a carbocation intermediate?
Has 3 bonds and a positive charge on the carbon
Is the HBr an electrophile or not and explain why?
Yes as accepts a pair of electrons from the pi bond
When do minor and major products form?
Unsymmetrical alkene
What happens to the H and Br on the major product?
Br attached to the C with less H
H has attached to the C with more H
Why does the major product form?
More stable carbocation intermediate forms major product (secondary carbocation more stable than primary)
Why are secondary carbocations more stable than primary?
sigma bond electrons stabilise the positive charge, more sigma donation happening so more product formed
What is a polymer?
A large molecule made from many small molecules called monomers
What do unsaturated alkene molecules undergo?
Addition polymerisation
Produce a long, saturated chain
What is a repeat unit?
The specific arrangement of atoms in the polymer that repeats over and over again, and is always drawn in square brackets
What are the 5 ways of disposing of polymers?
Recycling
PVC recycling
Using waste polymers as fuel
Feedstock recycling
Biodegradable/
photodegradable polymers
