3.3.4 Alkenes Flashcards
What are alkenes, what is their general formula?
Alkenes are unsaturated hydrocarbons.
General formula is CnH2n
Alkenes contain a carbon-carbon double bond somewhere in their structure.
The arrangement of bonds around the >C=C< is planar and has the bond angle 120°
What bonds does the C=C bond consist of and why does this make is susceptible to attack from electrophiles?
C=C double covalent bond consists of one sigma (σ) bond and one pi (π) bond.
π bonds are exposed and have high electron density.
They are therefore vulnerable to attack by species which ‘like’ electrons: electrophiles.
What is stereoisomerism?
Stereoisomers have the same structural formulae but have a different spatial arrangement of atoms.
Alkenes can exhibit a type of isomerism called E-Z stereoisomerism.
Why do E-Z isomers exist in alkenes?
E-Z isomers exist due to restricted rotation about the C=C bond.
E-Z stereoisomers arise when:
(a) There is restricted rotation around the C=C double bond.
(b) There are two different groups/atoms attached both ends of the double bond.
Explain why But-1-ene is a structural isomer of But-2-ene but does not show E-Z isomerism.
But-1-ene has two identical groups attached to one end of the restricted double bond - no E-Z isomers.
How do you name E-Z isomers?
First determine the priority groups on both sides of the double bond.
Priority group: The atom with the bigger atomic number is classed as the priority atom.
If the priority atom is on the same side of the double bond it is labelled Z.
If the priority atom is on the opposite side of the double bond it is labelled E.
Define an electrophile.
An electron pair acceptor.
What are electrophilic addition reactions?
- Alkenes are susceptible to attack by electrophiles (lone pair acceptors).
- This is because the C=C double bond is very electron rich due to the electron cloud of the π bond.
- In an addition reaction, the C=C double bond opens up and an atom or group of atoms joins onto each C of the C=C double bond.
- During the reaction, a carbocation is formed as an intermediate species in the mechanism.
- Cations are positive ions (anions are negative ions).
- Positive ions with the positive charge on the C atom are called carbocations.
- Carbocations are very unstable as the C atom only has 6 electrons around it in the outer shell.
- In terms of stability, tertiary carbocations (which have three alkyl groups bonded to the C+ atom) are the most stable (or least unstable).
(Most stable) tertiary carbocation > secondary carbocation > primary carbocation (Least stable) - Alkyl groups (e.g. methyl, ethyl, etc) are electron-releasing compared to hydrogen atoms (this is called the inductive effect). Therefore the more alkyl groups (rather than H atoms) on the C+ atom, the more stable the carbocation.
- When alkenes undergo electrophilic addition reactions and different carbocations can be formed, the main product formed will be from the more stable carbocation intermediate.
What are the conditions needed for electrophilic addition with Br2?
Aqueous (i.e. bromine water).
What are the conditions needed for electrophilic addition with HBr?
No specific conditions.
What are the conditions needed for electrophilic addition with H2SO4?
Concentrated H2SO4, cold (typically at room temperature).
What are the conditions needed for electrophilic addition with H2O?
H2O with strong acid (e.g. H2SO4, H3PO4)
What is the test for alkenes?
Add bromine water
- goes from orange to colourless in the presence of a double bond.
What are polymers?
Large molecules made from lots of repeating small molecules (monomers) joined together.
What type of polymers do alkenes produce and through what reaction?
Additional polymers.
Addition polymerisation.
Polymerisation = the process in which monomers join together.
Describe the uses of poly(chloroethene).
Poly(chloroethene) is a polymer that is water proof, an electrical insulator and doesn’t react with acids.
In its pure form it is a rigid plastic due to the strong intermolecular bonding between polymer chains prevents them moving over each other. In this un-plasticised form it is used make uPVC window frame coverings and guttering.
If a plasticiser is added the intermolecular forces are weakened which allows the chains to move more easily, resulting in more flexibility in the polymer. In this form PVC is used to make insulation on electrical wires, and waterproof clothing.
