Alkenes Flashcards
General formula of alkenes
CnH2n
General formula of cycloalkenes
CnH2n-2
What bonds are present in alkanes?
Only sigma bonds
What bond angle do alkanes have?
109.5 degrees
What shape do alkanes have?
Tetrahedral
What bonds are in alkenes?
Sigma bonds: overlap of orbitals directly between the bonding atoms.
Pi bonds: bond over two carbons. These form from sideways overlap of adjacent p-orbitals above and below the bonding C atoms which restricts rotation.
Draw a diagram to show how pi-bonds form
*Refer to alkenes notes
What bond angle and shape doe alkenes have?
120 degrees
Trigonal planar
Explain the shape caused by a double bond (3 points).
- There are 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 degrees.
- All of the atoms are in the same place.
Definition of stereoisomerism
Compounds with the same structural formula, but with a different arrangement of atoms in space.
To have E/Z isomerism, a molecule must have:
- C=C double bond which restricts rotation.
- Each C of the C=C double bond must be attached to two different groups.
Describe Cahn-Ingold-Prelog priority rules (8 points).
- Consider the two atomsattached to one carbon in the C=C group.
- If the atomic number of A is larger than the atomic number of B, then A is the higher pritority group of the two.
- Then consider the atoms attached to other side of the C=C.
- If D has a largerr atomic number than E, then D has the priority.
- Then compare the two highest priority atoms from the left and right side of the double bond.
- If the highest priority groups are both above or both below the C=C, then they are known as together (Z-isomer = same side).
- If they’re opposite, they are known as apart: E-isomer (opposite sides).
- If two atoms attached to the same side of the C=C have the same atoic number, then the atomic number of the atoms bonded to these atoms is compared.
Explain the relationship between cis-trans and E/Z isomers
Cis-trans isomers are always E/Z isomers but some E/Z isomers are not cis-trans isomers.
Explain how stereoisomerism arises around double bonds ( 2 points)
- Rotation about the double bond is restricted and the groups attached to each carbon atom are therefore fixed relative to each other.
- The reason for rigidity is the position of the pi-bond’s electron density above and below the plane of the sigma bond.
Explain the effect a hydrogen atom attached to each carbon in the C=C bond has on cis-trans isomerism
The cis isomer is the Z-isomer.
The trans isomer is the E-isomer.
Why are alkenes more reactive than alkanes?
The pi-bond has a relatively low bond enthalpy, making alkenes more reactive.
What is the name of the reaction that forms alkanes from alkenes?
Hydrogenation (adding H2- reduction).
What are the reagents, conditions and industrial importance for the hydrogenation of alkenes?
H2 and alkene
Ni catalyst
Manufacture of margarine from unsaturated veg oils.
What is the name of the reaction for formation of dihaloalkanes from alkenes?
Bromination (halogenation) ‘Adding a halogen’
What are the reagents, conditions, name of the mechanism and use for the bromination of alkenes?
Bromine- Br2(aq)- bromine water and alkene
Room temp
Electrophilic addition
Tests for presence of C=C. The bromine water decolourises from orange to colourless.
Describe the test for alkenes: C=C
Add bromine water at room temperature. Solution will turn orange to colourless.
What is the reagents, conditions and name of mechanism for the formation of haloalkanes from alkenes
Hydrogenhalide and alkene
Room temp
Electrophilic addition
What is the name of the reaction when alcohols are formed from alkenes?
Hydration ‘Adding steam’
What are the reagents, conditions and name of mechanism for the formation of alcohols from alkenes?
Steam- H2O(g) and alkene
High temp and pressure. Concentrated H3PO4(aq) catalyst
Electrohilic addition
Definition of an electrophile
A species that accepts a pair of electrons.
Draw the mechanism for electrophilic addition of ethene into bromoethane
*Refer to alkenes notes
Draw the mechanism for the formation of dibromoethane from ethene reacting with bromine
*Refer to alkene notes
What is the order of the stability of carbocations?
Tertiary > secondary > primary
Explain how to identify the major product (1 point).
The most stable carbocation
What is Markownikoff’s law?
Used to predict which of the products by electrophilic addition is major.
What is an addition polymer?
A long chain, high molecular mass saturated molecule.
Write the general equ of polymerisation
*Refer to alkene notes
What is a repeat unit?
The specific arrangement of atoms in the polymer molecule that repeats over and over again.
What are the two things that polyethene is used for?
Plastic bags and shampoo bottles
What is polychloroethene used for?
Window frames
What are the two uses for polyphenylethene?
- Packaging.
- Insulation.
What are the 3 uses of polypropene?
- Carpet firbres.
- Ropes.
- Food packaging.
Why are alkene-based addition polymers unreactive? (2 points).
- C-C and C-H bonds have high bond enthalpies.
- Non-polar.
What are the 2 uses of addition polymers?
- Storing food.
- Storing chemicals safely.
What are the three options of disposing of polymers?
- Recylcing- sorting and reclamation.
- Combustion- NOT ENVIRONMENTALLY FRIENDLY.
- Organic feedstock.
What do bioplastics degrade into naturally?
CO2(g)
H2O(l)
Biological compounds
5 ways that chemists work to minmise the environmental damage caused by disposal of polymers
- Developing biodegradable or compostable polymers.
- Developing ways of making polymers from plant-based materials.
- Developing the use of polymers as chemical feedstock.
- Design processes with high atom ecomony / reduce waste products.
- Develop ways of sorting and recycling polymers.
Describe how biodegrable polymers are broken down
Micro-organisms break them down into water, carbon dioxide and biological compounds.
Describe how biodegrable polymers are broken down
Micro-organisms break them down into water, carbon dioxide and biological compounds.
What are biodegradable compounds?
Made from starch or cellulose, or contains additives that alter the structure of traditional polymers so that micro-organisms can break them down.
How do compostable polymers break down?
These degrade and leave no visible or toxic residues.
