Alkenes Flashcards
What is the general formula for alkenes
Cn H2n
What are the properties of alkenes
Unsaturated hydrocarbons and have at least one C=C double bond
More reactive than alkanes because of the double bond
The C=C double bond is the functional group in an alkene
What is the shape of an alkene
Triagonal planar around each carbon of the double bond
Three bonding regions and no lone pairs
So bonding regions repel
C=C
What is a double bond made up of
A sigma bond and a pi bond
What are the features of sigma bonds
Head on overlap of 2 atomic orbitals directly in a line between the 2 bonded atoms
Occur between C-H and C-C
What are the features of the pi bond
Formed through the sideways overlap between 2 adjacent p orbitals
Explain how the bonding differs in alkanes and alkenes (2)
Ethane is an alkane so has a single C-C bond madeup of a sigma bond
Ethene is an alkene so has a double C=C bond made up of a sigma bond and a pi bond
Why are alkenes more reactive than alkanes
Alkenes will be more reactive than alkanes because the double bond has a high electron density and because the pi bond has a low bond enthalpy so it more easily broken than the C-C sigma bond in ethane
How can alkenes form stereoisomers
Single bonds have free rotation but double and triple bonds have restricted rotation
What are stereoisomers
Compounds with the same structural formula but a different arrangement of atoms in space
When does stereoisomers occur
When 2 double bonded carbon atoms each have 2 different atoms or groups attached to them
The molecule must contain a C=C double bond which restricts rotation
What are the 2 types of stereoisomers
E/Z stereoisomerism
Optical
What happens if an alkene is an E isomer
Has the groups apart on opposite sides (one above one below)
What happens if an alkene is a Z isomer
Has the groups together on the same side
How do you use the Cahn Ingold Prelog priority rules to determine E/Z stereoisomers
The atom or group on each side of the double bond with the higher Ar or Mr is given the higher priority
When can you use Cis/trans isomers
When the carbon atoms have at least one group in common
What is a cis isomer
The same groups are on teh same side of the double bond
What is a trans isomer
The same groups are on opposite sides of the double bond
Where may the cis/trans isomer s not work
The carbon atoms have different groups attached so theres no way of deciding which isomer is cis and which is trans
What are electrophiles
Electron pair acceptors
What are the main compounds that react with alkenes
Br2
HBr
How does bromine become slightly polar during electrohilic addition
When bromine approaches an alkene a dipole is induced in the bromine molecule because the bond electrons repel the electrons in the Br-Br bond. So its slightly polar
How does the double bond break during electrophilic addition
The electron pair in the pi bond is attracted to the slightly positive atom
What is heterolytic fission
The bond breaks and both electrons go to one atom
How does a bromide ion and carbocation form
The Br-Br bond breaking by heterolytic fission with the electron pair going to the bromine atom
What is a carbocation
An organic ion containing a positively charged carbon atom
How do you use bromine water to test for carbon double bonds
When you shake an alkene with orange bromine water the solution will quickly decolourise
This is because bromine is added across the double bond to form the colourless dibromoalkane
What happens during electrophilic addition with hydrogen halides
Bromine is more electronegative than hydrogen so H-Br is polar
The pi bond breaks so its electron pair is donated to the H atom in HBr
A covalent bond will form between a C from the double bond and the H
What happens during electrophilic addition for hydrogen halide (2)
The H-Br bond breaks by heterolytic fission and the carbocation forms
What does asymmetrical mean
It is a mixture of 2 products. A major and minor product
Why are carbocations with more alkyl groups the most stable
Because the alkyl groups feed electrons towards the positive charge
Which carbocations are the most stable
The primary carbocations with one alkyl group are the least stable
Secondary with 2 alkyl is more
Tertiary with 3 alkyl groups is the most stable
How do you explain why which is a major/ minor product
_________ is the major product because it is formed from a ________ carbocation intermediate which is more stable than the ____________ carbocation
What is a monomer
A small molecule that combines with other molecules to form a polymer
What is a polymer
A long moleculer chain built up from monomer units
Why can an alkene behave as a monomer
Each alkene can break in double bonds joining alkenes together to make a long carbon chain
What is the reaction that occurs between alkene monomers
Addition Polymerisation
How must a repeating unit be shown
Brackets
The use of n
Extended bonds through brackets
What is the reaction called when a halogen is added to alkene
Halogenation
Why are alkenes not soluble in water
They have non polar bonds
EXAM QUESTION- Explain the bonding in a C=C double bond Use the orbital overlap model (2)
Sideways overlap of p orbitals (1)
Forming a pi bond (1)
include diagram
EXAM QUESTION- Suggest why the average bond enthalpy of a C=C double bond is not twice the bond enthalpy of a C-C bond (1)
The pi bond is weaker than the sigma bond OR VICE VERSA
What happens during hydrogenation
Adding hydrogen to an alkene
Need a nickel catalyst and 150 degrees
What happens in a hydration reaction
Adding water to an alkene in steam
Need phosphoric acid and makes an alcohol
EXAM QUESTION- Explain why one of the organic products forms in much greater quantity than the other organic products (2)
The major product would be formed in much greater quantity as the intermediate in the mechanism would contain a tertiary carbocation which is much more stable than a secondary carbocation
