14 - Alkenes Flashcards
What is an alkene?
An unsaturated hydrocarbon with one or more carbon-carbon double bond.
What is the general formula of an alkene?
CnH2n
Why is there no rotation about a double bond?
Because of the pi bond
What is the simplest alkene?
Ethene - CH2=CH2
Planar molecule
Bond angles = 120
What is a pi bond?
A type of covalent bond formed by the sideways overlap of the p-orbital on each carbon, forming an orbital with a high electron density above and below the nuclei.
What is a sigma bond?
A type of covalent bond formed by the direct overlap of atomic orbitals along the axis connecting two nuclei, forming a high electron density bond between the nuclei.
What is a position isomer?
The same structural formula with the functional group in a different postion.
What is stereoisomerism?
The same structural formula with bonds arranged differently in space.
What is E-Z notation?
They’re based on which side the two atoms with the greatest atomic number is on around the double bond.
E - different side
Z - same side
Does the double bond affect melting/ boiling points?
No, as there are the same number of carbons as alkanes.
What are the intermolecular forces in alkene?
Van der Waal’s forces are the only forces acting between alkene molecules, so physical properties are similar to those of alkanes.
What is the reactivity of alkenes like?
Their double bond makes them more reactive than alkanes as the C=C bond creates an area of high electron density which is highly likely to be attacked by electrophiles.
What are electrophiles?
Positively charged electron pair accepters.
What is the combustion of alkenes like?
They will combust but aren’t used for fuels due to their reactivity and availability.
Explain electrophilic addition.
The electrophile is attracted to the double bond.
They accept a pair of electrons from the double bond.
A positive carbocation is formed
An anion forms a bond with the carbocation.
Give 2 examples of electrophilic addition with alkenes.
Reacts with hydrogen halides
Reacts with halogens
What is the reaction of alkenes and concentrated sulphuric acid like?
Occurs at room temperature
What does it mean if an acid is concentrated?
High amount of acid per volume of water.
What is the reaction of alkenes and steam like?
High temperature
High pressure
H3PO4 catalyst on silica
What is the positive inductive effect?
Alkyl groups have a tendency to release electrons which tends to stabilise the positive charge of the intermediate carbocation.
The more alky groups the more stable the carbocation. (tertiary>)
What is a major product?
The product most likely to be formed due to the carbocation being more stable.
What is the test for an C=C bond?
Add bromine water
Reddish-brown colour turns colourless.
What is a polymer?
A very large molecule built up from small monomers.
What is a monomer?
A small molecule that can bind to other similar molecules to form a polymer.
What is an addition polymer?
A polymer made from monomers based on ethene.
What happens when monomers polymerise?
The double bond opens and the monomers bond together to form a backbone of carbon atoms.
What are repeat units?
The smallest repeating structures within a polymer.
How can the properties of polymer materials be modified?
By the use of additives like plasticisers, which are small molecules that go between polymer chains forcing them apart, allowing them to slide across each other, making them flexible.
What is an issue with polymer materials?
They aren’t biodegradable
As they have a non-polar bond they are very unreactive so they are not attacked by biological enzymes.
What is mechanical recycling?
1) Separate the different types of plastics.
2) Wash and grind the plastic into small pellets.
3) Pellets are melted and remoulded.
What is feedstock recyling?
1) Plastics are heated to a temperature that will break the polymer bonds and produce monomers.
2) Monomers are used to make new plastics.
What is the issue with recycling?
It can only be done a limited number of times because at each heating some of the chains break and become shorter, degrading the plastics properties.
How is low density polyethene made?
Polymerising ethene at high pressure and high temperature via a free-radical mechanism.
What are the properties of low density polyethene?
Chain branching due to the random nature of free-radical reactions.
Low-density, flexible and stretches well because the branched chains do not pack together well.
How is high density polyethene made?
Made at temperatures and pressures slightly greater than room conditions.
Uses a Ziegler-Natta catalyst.
What are the properties of high density polyethene?
Little chain branching.
High density and melting point due to the chains packing well together.
What are the uses of low density polyethene?
Packaging
Cable insulation
What are the uses of high density polyethene?
Buckets
Bottles
