Alkenes Flashcards
(35 cards)
What are alkenes?
CnH2n
They have atleast one C=C double covalent bond.
They are unsaturated.
How are alkenes named?
The carbon the double bond starts on is numbered in the name.
If it has two double bonds the suffix is diene.
What is the structure of double bond in alkenes?
It is made of a sigma bond and a pi bond.
What is a pi bond?
Sideways overlap of adjacent p-orbitals above and below the bonding C atoms.
What is the strength of pi bonds?
Pi bonds are weaker than sigma because electron density is spread out above and below the nuclei.
This means the electrostatic attraction is weaker, and have a low bond enthalpy.
What is the reactivity of alkenes?
There’s two pairs of electrons in the bond, so it has very high electron density - very reactive.
The pi bond also sticks out of the molecule, so is more likely to be attacked by electrophiles.
What are electrophiles?
Electron pair acceptors.
They have a low electron density.
What is the rotation of alkenes?
Can’t rotate because the p-orbitals have to stay in the same position to overlap and form a pi bond.
Double bonds are rigid.
Things can still rotate around the single bonds though.
What are stereoisomers?
The same stuctural formula but different arrangement of atoms in space.
Alkenes have stereoisomers because there’s a lack of rotation around the C=C double bond.
What is E/Z isomerism?
When the double bonded carbons atoms each have two different atoms or groups attached to them.
What are priority groups?
If the two groups have their ‘higher priority groups’ on opposite sides, it’s an E isomer.
If they are on the same side, it’s a Z isomer.
What are the CIP priority rules?
The highest priority group on each of the C=C is the one with the atom with the highest atomic number directly bonded to the Carbon atom.
If the atoms directly bonded to carbon are the same, look to the next atom along.
What is Cis/Trans isomerism?
A type of E/Z isomerism where the two carbon atoms either side of the double bond have at least one group in common.
The cis isomer is the one with two identical groups on the same side of the C=C bond.
The trans isomer has the two identical groups on opposite sides of the C=C bond.
What are electrophilic addition reactions?
The double bond in an alkene opens up and atoms are added to carbon atoms.
This happens because the double bond has plenty of electrons and is easily attacked by electrophiles.
The double bond is also nucleophilic - it is attracted to places with not enough electrons.
How are alkanes produced from alkenes?
Hydrogenation:
Alkenes react with hydrogen gas to form alkanes.
At the temperature of 150 c with a nickel catalyst.
How do alkenes react with halogens?
They form dihaloalkanes.
The halogens add across the double bond, and each carbon ends up bonded to a halogen atom.
It’s an electrophilic addition reaction.
What is the test for unsaturation?
If the hydrocarbon is an alkene, the bromine water will go from orange to colourless.
This is because bromine is added across the double bond to form a colourless dibromoalkane, by electrophilic addition.
How do you synthesise alcohols from alkenes?
Hydrating alkenes with steam at 300 c and 60-70 pressure.
The reaction needs catalysing by a strong acid such as phosphoric or sulfuric.
It can produce ethanol from ethene.
What is the equation for synthesising alcohols?
H2C=CH2 (g) +H2O (g) <–> CH3CH2OH (g)
It is reversible and the reaction yield is low.
How are haloalkanes synthesised from alkenes?
Alkenes undergo electrophilic addition reactions with hydrogen halides to form haloalkanes.
What is Markownikoff’s rule?
When a hydrogen halide reacts with an unsymmetrical alkene, the hydrogen attaches itself to the carbon with the greater number of hydrogens and smaller of carbons.
What are the three possible carbocations?
Primary - 1 R group
Secondary - 2 R groups
Tertiary - 3 R groups
R is an alkyll group, an alkane with a hydrogen removed.
How does the stability of carbocations affect reactions?
The more stable carbocations are more likely to form than less stable ones.
This is because the first step of the mechanism is more likely to lead to the formation of the most stable carbocation.
So more product will be formed via the more stable carbocation.
This can explain what the major product will be when a hydrogen halide reacts with an unsymmetrical alkene.
What is the carbocation stability dependent on?
Each alkyl group donates and pushes electrons towards the positive charge of the carbocation.
The positive charge is spread over the alkyl groups.
The more alkyl groups attached to the carbocation, the more the charge is spread out, making the ion more stable.
