Alkenes (13) Flashcards
What are alkenes?
Alkenes are unsaturated hydrocarbons which means they have at least one double bond.
What is the general formula for alkenes?
CnH2n
Which alkenes don’t obey the general formula?
Cyclic alkenes and alkenes with more than one double bond.
What creates the double bond?
A sigma bond and a pi bond create the double bond
What is a pi bond?
The sideways overlap of 2 p orbitals and its electron density is above and below the bonded atoms
Which is stronger, sigma or pi bond?
Sigma bonds are stronger because they have a stronger electron density and a direct overlap which means the attraction between the nuclei and electron pair is stronger. Pi bonds are weaker because they have a their electron density above and below the sigma bond which makes the attraction between the nucleus and electrons weaker.
Are alkenes reactive?
They are more reactive than alkanes because of their pi bond. They have electrons above and below the sigma bonds which are likely to be attacked by an electrophile
Can alkenes rotate freely?
Alkenes cannot rotate freely as the pi bond locks the carbon atoms in position and prevents them rotating.
What are the shapes of alkenes?
Alkenes have 3 sigma bonds and one pi bond which means they have 3 regions of electron density which repel equally so they form trigonal planars and have bond angles of 120 degrees.
Stereoisomers
These are molecules that have the same structural formula but a different arrangement of atoms in space
Why does stereoisomerism occur?
It occurs because of the double bonds in alkenes which doesn’t allow them to rotate.
What conditions are needed for E/Z isomerism to occur?
A C=C bond
Different groups attached to each carbon atom in the double bond e.g a H and a methyl group attached to one carbon
Z isomer
The same groups positioned across
E isomer
The same groups opposite from each other
What is cis-trans isomerism?
E/Z isomerism where at least one of the groups is the same on each carbon e.g. both carbons have an H
Cis
The same principle as Z, the same groups positioned across
Trans
The same principle as E, the same groups are opposite each other
What is the Cahn Ingold prelog?
This is used when there are different groups on each carbon. You have to find which of the groups attached to the carbon has the highest priority
How do you find the priority of groups on C=C bonds?
You find the rfm. The group with the highest rfm on each atom is the high priority the other one is low priority
How can you tell which one is E or Z using the Cahn Ingold Prelog?
If you read across the molecule and you see high-high, it’s a Z isomer. If you read across and see a low-low its an E isomer.
What kind of reactions can alkenes undergo easily and why?
They can undergo addition reactions easily because of the pi bond which exposes the electrons to electrophiles
How can you work out the enthalpy of a pi bond?
If you know the enthalpy of the double bond and single bond just do double - single
What is hydrogenation of alkenes?
This is when H2 is added to an alkene. The hydrogen is added and it breaks the double bond and forms an alkane. All double bonds will be broken, even if there are multiple, you will just have to put a higher number of hydrogens
What are the conditions for a hydrogenation?
Nickel catalyst and 150-degree temp
What is the halogenation of alkenes?
This is where a halogen is added to an alkene e.g Cl2. and you form a haloalkane
What are the conditions for a halogenation?
Room temperature
Describe what the addition of hydrogen halides are
You add a gaseous hydrogen halide e.g HCl to an alkene and will form two products if the alkene is unsymmetrical. An electrophilic addition is needed for this. It will form a haloalkane.
What are the conditions for a hydrogen halide addition?
Must be a gaseous hydrogen halide or a concentrated hydrochloric or hydrobromic acid
Describe what the hydration of an alkene is
It is when alkenes react with steam (water) and form two products depending on where the H and where the OH are placed. It forms an alcohol.
What are the conditions for a hydration?
A phosphoric acid catalyst (H3PO4) is needed as well as steam.
How can you test for alkenes?
You can add bromine water which is orange to an alkene. It will undergo a bromination if it is an alkene and the double bond breaks. This is shown by the solution turning from orange to colourless.
What is an electrophilic addition?
An addition reaction occurs where the alkene is attacked by an electrophile which is attracted to electrons.
What is an electrophile?
An atom or groups of atoms that is attracted to an electron rich centre and accepts an electron pair. It usually has a positive or partially positive charge.
What happens if the electrophile used in the electrophilic addition is non-polar?
When the non-polar molecule approaches the alkene, it interacts with the electrons in the pi bond of the alkene and this creates an induced dipole in the non-polar molecule making it an electrophile. The pair of electrons will move closer to the furthest atom in the electrophile.
What kind of carbocations are there?
Primary, secondary and tertiary
Primary carbocations
C+ attached to 2 hydrogens and an R group
Secondary carbocations
C+ attached to 1 hydrogen and 2 R groups
Tertiary carbocations
C+ attached to 3 R groups
Which is the most stable carbocation?
The tertiary carbocation is the most stable because the positive charge is spread out over the R-groups so the charge doesn’t build up in one place and it is more stable. The alkyl groups also push electrons towards the carbocation to make it more stable.
What is the major product in Markownikoffs rule?
The product formed where the hydrogen attaches to the carbon that has the least hydrogens.
What is the minor product in Markownikoffs rule?
The product formed where the hydrogen attaches to the carbon with the most hydrogens.
What are polymers?
Extremely large molecules formed from thousands of small units called monomers
What can alkenes undergo to make them polymers?
They undergo addition polymerisation where their double bond is broken to produce long unsaturated chains
What are the rules of addition polymerisation?
You break the double bond and draw square brackets around your alkane.
The horizontal bonds have to be coming out of the square bracket
You write an ‘n’ which is the number of repeat units
How do you find a repeat unit of a polymer?
Look at the polymer and see where it repeats. You draw a line and rewrite that section of the polymer, putting a double bond in the middle.
Name 5 polymers and their uses
Polyethene ( supermarket bags), polychloroethene (PVC) (pipes), polypropene (children toys), polystyrene (packaging foam), polytetrafluoethane ( Teflon pans)
What does polyethene look like?
It’s a C=C with 4 Hs attached
What does PVC look like?
It’s a C=C with a Cl attached to one carbon and 3 Hs
What does propene look like?
It’s a C=C with a CH3 and 3 Hs attached
What does polystyrene look like?
It has a benzene ring and 3 Hs attached
What does tetrafluoroethene look like?
It’s a C=C bond with 4 fluorines attached
Why is PVC not safe to dispose of?
Because it has high chlorine content, so when burnt it releases HCl along with other toxins that pollute. You can recycle it by grinding it and dissolving it.
Why can you use polymers as fuel?
They are alkanes so can be incinerated to produce heat and energy.
Feedstock recycling
This is the chemical and thermal process that recovers monomers and other useful resources from waste polymers. These recovered materials can then be used as raw material for other polymers.
Bioplastics
Scientists have created bioplastics made from starch and cellulose that are biodegradable so are more sustainable. If biodegradation isn’t possible, they also made photodegradable polymers.
