3.4 Alkenes Flashcards
What are alkenes?
Alkenes are unsaturated hydrocarbons.
Alkenes contain a carbon-carbon double bond somewhere in their structure
What is the general formula for alkenes?
CnH2n
Draw the displayed formula for ethene.
(see page 1 in the chemrevise revision guide)
Draw the displayed formula for propene.
(see page 1 in the chemrevise revision guide)
Draw the displayed formula for but-1-ene.
(see page 1 in the chemrevise revision guide)
Draw the displayed formula for but-2-ene.
(see page 1 in the chemrevise revision guide)
What does a double bond consist of?
1 sigma bond
1 pi bond
Why are pi bonds vulnerable to attack by attacking species?
Pi bonds are exposed and have high electron density.
They are therefore vulnerable to attack by electrophiles.
What are electrophiles
An electron pair acceptor.
What isomerism can alkenes exhibit?
E-Z stereoisomerism
What are 2 reasons for why E-Z isomers arise?
- There is restricted rotation around the C=C bond.
2. There are two different groups/atoms attached both ends of the double bond.
Draw the displayed formula for Z-but-2-ene
(see page 1 in the chemrevise revision guide)
Draw the displayed formula for E-but-2-ene
(see page 1 in the chemrevise revision guide)
Draw the displayed formula for but-1-ene
(see page 1 in the chemrevise revision guide)
What is the priority group in E-Z isomers?
The atom with the bigger atomic number.
Draw the displayed formula for E-1,2-dichloroethene
(see page 1 in the chemrevise revision guide)
What is the bond angle of the arrangement of bonds around the >C=C
Planar and 120º
What mechanism reactions do alkenes undergo?
Electrohpilic addition.
The double bonds in alkenes are areas with high electron density. This attracts electrophiles and the alkenes undergo addition reactions.
Define an addition reaction
A reaction where two molecules react together to produce one.
What is the change in the functional group when alkenes react with Bromine?
alkene -> dihalogenoalkane
What is the reagent when alkenes react with Bromine?
What is the type of reagent?
Bromine
Electrophile: Br (partially positive)
What are the conditions when alkenes react with Bromine?
Room temperature
What is the mechanism when alkenes react with Bromine?
Electrophilic addition
Write the overall equation when alkenes react with Bromine.
(see page 2 in the chemrevise revision guide)
Write the mechanism equation when alkenes react with Bromine.
(see page 2 in the chemrevise revision guide)
What is the change in the functional group when alkenes react with Hydrogen Bromide?
alkene -> halogenoalkane
What is the reagent when alkenes react with Hydrogen Bromide?
What is the type of reagent?
HCl or HBr
Electrophile: H (partially positive)
What are the conditions when alkenes react with Hydrogen Bromide?
Room temperature
What is the mechanism when alkenes react with Hydrogen Bromide?
Electrophilic addition
Write the overall equation when alkenes react with Hydrogen Bromide.
(see page 2 in the chemrevise revision guide)
Write the mechanism equation when alkenes react with Hydrogen Bromide.
(see page 2 in the chemrevise revision guide)
Draw the two isomeric products when an unsymmetrical alkene react with Hydrogen Bromide?
(see page 2 in the chemrevise revision guide)
What is ‘Markownikoff’s Rule’?
In most cases, bromine will be added to the carbon with the fewest hydrogens attached to it.
(see page 2 in the chemrevise revision guide)
Why is HBr a polar molecule?
Br is more electronegative than H
What is the order of stability for carbocations?
- tertiary
- primary
- secondary
tertiary > secondary > primary
In electrophilic addition to alkenes, the major product is formed via the _______ carbocation intermediate.
In electrophilic addition to alkenes, the major product is formed via the more stable carbocation intermediate.
Read the bit labelled 1 on page 2 in the chemrevise revision guide.
got it?
What is a carbocation?
The intermediate formed, which has a positive charge on a carbon atom is called a carbocation.
How does Br2 become polar in the reaction with alkenes.
As the Br2 molecule approaches the alkene the pi bond electrons repel the electron pair in the Br-Br bond. This induces a dipole. Br2 becomes polar and electrophilic (Br partially positive)
What is the functional group change when alkenes react with sulphuric acid in stage 1?
alkene -> alkyl hydrogensulfate
What is the reagent when alkenes react with sulphuric acid in stage 1?
What is the type of reagent?
concentrate H2SO4
Electrophile: H2SO4
(acts as a catalyst as it is regenerated)
What are the conditions when alkenes react with sulphuric acid in stage 1?
Room temperature
What is the mechanism reaction when alkenes react with sulphuric acid in stage 1?
Electrophilic addition
What is the change in functional group when alkenes react with sulphuric acid in stage 2?
alkyl hydrogensulfate -> alcohol
What is the reagent when alkenes react with sulphuric acid in stage 2?
Water
What are the conditoins when alkenes react with sulphuric acid in stage 2?
Warm mixture
What is the type of reaction when alkenes react with sulphuric acid in stage 2?
Hydrolysis
Draw the mechanism reaction when alkenes react with sulphuric acid in stage 1.
(see page 3 in the chemrevise revision guide)
Draw the overall reaction when alkenes react with sulphuric acid in stage 2.
(see page 3 in the chemrevise revision guide)
Draw the displayed formula for H2SO4.
(see page 3 in the chemrevise revision guide)
Give the definition of hydrolysis.
A reaction where the molecule is split by the addition of water.
How are alkenes converted into alcohols in one step?
They are reacted with water in the presence of an acid catalyst.
What are the essential conditions when alkenes are converted into alcohols in one step?
- High temperature 300-600ºC
- High pressure 70atm
- Catalyst of conc H3PO4
What is the test for an alkene?
Bromine water.
Bromine water decolourises in the presence of a double bond. This can be used as a test for the presence of a double bond in a molecule. It can be used quantitatively to show the presence of multiple double bonds in compounds like polyunsaturated oils.
Why are poly(alkenes) unreactive?
They have strong C-C and C-H bonds
Draw the repeating units of polyethene
(see page 4 in the chemrevise revision guide)
Draw the monomer used to make polyethene
(see page 4 in the chemrevise revision guide)
Draw the monomer used to make poly(propene)
(see page 4 in the chemrevise revision guide)
Draw the repeating units of poly(ethene)
(see page 4 in the chemrevise revision guide)
When do you put the ‘n’ on a polymer diagram?
n - repeating units, lots of
no n - one repeating unit
(see page 4 in the chemrevise revision guide)
Draw the repeating unit for the polymer made from but-2-ene.
(see page 4 in the chemrevise revision guide)
What is poly(chloroethene) used for?
Poly(chloroethene) is a polymer that is:
- water proof
- an electrical insulator
- 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 h other. In this un-plasticised form it is used to make uPVC window from coverings and guttering.
Is 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.
Draw the repeating unit of poly(chlororethene)
(see page 4 in the chemrevise revision guide)
