13 Alkenes Flashcards
What is an unsaturated hydrocarbon? (1 mark)
A hydrocarbon with at least one double or triple bond, or ring in it’s chain.
What is a pi bond? (3 marks)
The overlap of two p-orbitals.
The pi-electron density is concentrated above and below the line joining the nuclei of the bonding atoms.
Can pi bonds rotate, why can/can’t they? (2 marks)
No, it locks the 2 carbon atoms in position.
What is the shape around each carbon atom in an Alkene, why? (3 marks)
Trigonal planar.
Three regions of electron density, repel equally (120 degrees).
What is the general formula for an Alkene? (1 mark)
Cn H2n
Why are alkenes more reactive than alkanes? (2 marks )
The double bond is an area of high electron density.
The pi bond is weaker.
What are dienes? (1 mark)
Hydrocarbons that contain two C=C bonds.
Compare and explain the boiling points of an alkene to it’s corresponding alkane. (4 marks)
The alkene had a boiling point that is a few number of degrees lower than the corresponding alkane.
Each alkene has 2 fewer electrons than the alkane.
So the induced dipole-dipole interactions are slightly weaker.
Less energy is needed to overcome interactions.
Therefore, lower boiling point.
What is sp^2 hybridisation? (2 marks)
The mixing of two electrons from a p orbital and one from an s orbital.
This creates three new hybrid orbitals at the same every level.
Why is a pi bond not as strong as a sigma bond? (1 mark)
Because they are sideways, rather than forward.
What must a compound have to have E/Z isomers? (2 marks)
C=C double bond
Different groups attached to each carbon atom in the double bond.
What is cis-trans isomerism? (3 marks)
Special case of E/Z isomerism
C=C bond
Same groups stretched to each carbon atom in the double bond.
Explain the Cahn-Ingold-Prelong rules. (2 marks)
The higher the atomic number, of the element directly attached to the C, the higher the priority.
What is stereoisomerism? (2 marks)
Same molecular and structural formula
Different spatial arrangement
Why do Z isomers have a higher boiling point than E isomers? (2 marks)
Z isomers are polar molecules
This means there are permanent dipole-dipole and induced dipole-dipole forces.
E isomers only have induced.
Why do E isomers have a higher melting point than Z isomers? (2 marks)
E isomers have a linear shape
So the pack together better than the Z isomers
Therefore have stronger intermolecular forces when solid.
What is the hydrogenation of an alkene? (2 marks)
Mixed with hydrogen
Passed over a nickel catalyst
Forms an alkane
What is the halogenating of an alkene? (2 marks)
Mixed with Cl or Br
At room temperature
Forms a haloalkane
Describe the reaction between an alkene and hydrogen halide. (2 marks)
Mixed with a hydrogen halide
At room temperature
Forms a haloalkane
What is the hydration of an alkene?
React with steam
Phosphoric acid catalyst
Forms an alcohol
What is an electrophile? (2 marks)
Attracted to regions of negative charge (that can accept a paid of electrons).
Donate electrons.
What mechanism are the addition reactions of alkenes? (1 mark)
Electrophilic addition reactions
What is the chemical test for the presence of C=C bonds? (1 mark)
Aqueous bromine
Orange —> colourless
What do electrophilic addition reactions of unsymmetrical alkenes result in? (2 marks)
Carbocation intermediates —> possibility of two different products
What does Markovnikov’s rule state, explain? (4 marks)
The stability of carbocation intermediates depend on the number of alkyl groups attached.
Alkyl groups are electron donating groups.
Electron donating groups decrease the size of the positive charge on the carbon.
So the carbocation becomes more stable.
How are long saturated chains (addition polymers) formed from alkenes? (2 marks)
Polymerisation- repeated addition reactions
In what conditions is industrial polymerisation conducted in? (2 marks)
High pressure
High temperature
What is a repeat unit, and what mustn’t you forget about them? (3 marks)
The part that repeats in the polymer over and over again.
Always in square brackets
Put an n after the bracket (shows that there is a large number of repeats)
What mechanism is the polymerisation of alkenes? (1 mark)
Free radical addition
How can chemists modify the strength of intermolecular forces between polymer chains? (1 mark)
By including different substituent groups
What does including plasticisers do to a polymer, why? (3 marks)
Lower the melting point and tensile strength.
Increase flexibility.
The plasticisers are relatively small so they can penetrate between the polymer chains (reducing the strength of the intermolecular forces between them)
What does increasing the chain length of a polymer chain do to a polymer? (2 marks)
Increase melting point and tensile strength
Decrease flexibility
What does branching do to polymers? (2 marks)
Lower density, melting point, tensile strength and rigidity because it stops the molecules lining up regularly.
Give two ways that waste polymers can be reused? (2 marks)
Melted and remoulded
Cracked and provide organic feeds
What is an E isomer? (1 mark)
The priority groups are diagonal.
What is a Z isomer? (1 mark)
The priority groups are on the same side.
What is a cis isomer?
The same groups are on the same side (make a C shape)
What is a trans isomer? (1 mark)
The same groups are on diagonal sides.
