Alkenes 1 Flashcards
What is the general formula of an alkene
CnH2n
Describe the shape of an alkene
- Alkenes are planar (flat) molecules and this makes the angles between each bond roughly 120 degrees.
- Unlike the C—C bond in alkanes, there is no rotation around the C=C double bond and so if a hydrogen is replaced by a different molecule, the alkene, the molecule will still have the same flat shape.
What is an electrophile
A lone pair acceptor
In general, describe the mechanism for electrophilic addition in Alkenes
1) The electrophile is attracted to the double bond
2) Electrophiles are positively charged and accept a pair of electrons from the double bond. The electrophile may be a positively charged ion or have a positively charged area.
3) A positive carbocation is formed.
4) A negatively charged ion forms a bond with the carbocation.
What is Markownikoffs rule for the electrophilic addition of alkenes
When a hydrogen halide reacts with an alkene, it is more likely to bond with the carbon atom that is attached to the most alkyl groups
What is a tertiary carbocation
A positively charged carbon atom that has three alkyl groups attached.
What is a secondary carbocation
A positively charged carbon atom that is attached to two alkyl groups
What is a primary carbocation
A positively charged carbon atom that is attached to one alkyl group
From most stable to least, list the possible carbocations produced during electrophilic addition
Tertiary
Secondary
Primary
Why does the halogen atom bond mostly to the carbon atom attached to the most alkyl groups (most stable carbocation)
Because alkyl groups have a tendency to release electrons which is a positive inductive effect.
When Alkenes react with halogen molecules, what happens
- Halogen molecules are diatomic.
- The two halogen atoms from the molecule add across the Alkenes double bond.
Describe what happens during the electrophilic addition of a hydrogen halide to an alkene
1) The halogen is more electronegative than the hydrogen, so a polar bond is formed.
2) The electrophile is the delta positive H+ from the H—X bond.
3) The delta positive H is attracted to the Carbon=Carbon double bond because of the double bonds high electron density.
4) One of the pairs of electrons from the C=C forms a bond with the delta positive H to form a positive carbocation and at the same time the electrons in the H—X bond are drawn towards the halogen.
5) The bond in the hydrogen halide breaks heterolytically. Both electrons from the shared pair go to the bromine atom because it is more electronegative than than hydrogen leaving a X- ion.
6) The :X then attaches to the positively charged carbon atom from the carbocation and forms a bond with one of its electron pairs.
Describe what happens during the electrophilic addition of a halogen molecule to an alkene to form a dihalogenoalkane
1) At any instant a halogen molecule is likely to have an instantaneous dipole.
2) the delta positive atom in this dipole is attracted to the electron rich C=C double bond and the X2 halogen molecule becomes the electrophile.
3) The electrons in the double bond are attracted to the delta positive halogen atom and repel the electrons in the X—X double bond. And this strengthens the dipole in the halogen molecule.
4) Two of the electrons from the double bond form a bond with the delta positive halogen atom and the the other halogen atom becomes a X- ion.
5) This leaves a carbocation, in which the carbon atom from the double bond that isn’t bonded to an X atom has a positive charge.
6) the X- ion now forms a bond with the carbocation.
How do you test for a double bond.
- Using the electrophilic addition of a halogen molecule to a alkene.
- When a few drops of bromine water ( which has an orange/brown colour) are added to an alkene,the solution is decolourised because the products are colourless.
What type of reaction is the electrophilic addition of sulphuric acid to an alkene
Exothermic
Why is the electrophilic addition of sulphuric acid to an alkene a useful reaction
Because if water is added to the product of the reaction, an alcohol is formed and sulphuric acid reforms. This means that the overall effect is add water across the double bond with sulphuric acid as a catalyst.
Why are polyalkenes not biodegradable
- Polyalkenes have a backbone that consists of an alkane chain.
- Alkanes have strong, non polar C—C and C—H bonds which means that they are not attacked by biological agents such as enzymes and therefore are not broken down.
Describe the structure and properties of low density polyethene
- Low density polyethene is made by polymerising ethene at high pressure and high temperatures via a free radical mechanism.
- This produces a polymer with some chain branching and these branched chains do not pack together well.
- This makes the product quite flexible, stretch well and has a fairly low density.
- This makes it perfect for plastic packaging, sheeting and insulation for electrical cables.
Describe the structure and properties of high density polythene
- high density polyethene is produced at temperatures little higher than room temperature, with a Ziegler-Natta catalyst.
- This results in a polymer with much less chains branching, meaning the chains pack together well and the density of the plastic is greater and the melting and boiling points are higher.
- Typical uses are milk crates and bottles etc where low density polyethene would be impractical.
What is mechanical recycling
- Mechanical recycling is the simplest form of recycling.
- The different plastic types are separated out.
- The plastics are then washed and once they are sorted can be ground up into small pellets.
- These can be melted and remoulded.
What is feedstock recycling
In feedstock recycling, the plastics are heated to a temperature that will break the polymer bonds and produce monomers that can be used to make new plastics.
What are some issues that arise with recycling
- Some polymers can only be melted and remoulded a certain number of times
- This is because at each heating, some of the chains break and become shorter, thus degrading the plastics properties.
Are Alkenes saturated or unsaturated
Unsaturated as they contain double bonds
What types of bond are present in Alkenes
- Sigma bonds between carbon and hydrogen atoms
- The C=C bond consists of a sigma bond and a pi bond.
