14 - Alkenes

Question 1

What is an alkene?

Answer 1

An unsaturated hydrocarbon with one or more carbon-carbon double bond.

Question 2

What is the general formula of an alkene?

Answer 2

CnH2n

Question 2

Why is there no rotation about a double bond?

Answer 2

Because of the pi bond

Question 3

What is the simplest alkene?

Answer 3

Ethene - CH2=CH2
Planar molecule
Bond angles = 120

Question 4

What is a pi bond?

Answer 4

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.

Question 5

What is a sigma bond?

Answer 5

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.

Question 6

What is a position isomer?

Answer 6

The same structural formula with a pair of adjacent carbon atoms in different positions in the carbon chain.

Question 7

What is stereoisomerism?

Answer 7

The same structural formula with bonds arranged differently in space.

Question 8

What is E-Z notation?

Answer 8

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

Question 9

Does the double bond affect melting/ boiling points?

Answer 9

No, as there are the same number of carbons as alkanes.

Question 10

What are the intermolecular forces in alkene?

Answer 10

Van der Waal’s forces are the only forces acting between alkene molecules, so physical properties are similar to those of alkanes.

Question 11

What is the reactivity of alkenes like?

Answer 11

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.

Question 12

What are electrophiles?

Answer 12

Positively charged electron pair accepters.

Question 13

What is the combustion of alkenes like?

Answer 13

They will combust but aren’t used for fuels due to their reactivity and availability.

Question 14

Explain electrophilic addition.

Answer 14

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.

Question 15

Give 2 examples of electrophilic addition with alkenes.

Answer 15

Reacts with hydrogen halides
Reacts with halogens

Question 16

What is the reaction of alkenes and concentrated sulphuric acid like?

Answer 16

Occurs at room temperature

Question 17

What does it mean if an acid is concentrated?

Answer 17

High amount of acid per volume of water.

Question 18

What is the reaction of alkenes and steam like?

Answer 18

High temperature
High pressure
H3PO4 catalyst on silica

Question 19

What is the positive inductive effect?

Answer 19

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>)

Question 20

What is a major product?

Answer 20

The product most likely to be formed due to the carbocation being more stable.

Question 21

What is the test for an C=C bond?

Answer 21

Add bromine water
Reddish-brown colour turns colourless.

Question 22

What is a polymer?

Answer 22

A very large molecule built up from small monomers.

Question 23

What is a monomer?

Answer 23

A small molecule that can bind to other similar molecules to form a polymer.

Question 24

What is an addition polymer?

Answer 24

A polymer made from monomers based on ethene.

Question 25

What happens when monomers polymerise?

Answer 25

The double bond opens and the monomers bond together to form a backbone of carbon atoms.

Question 26

What are repeat units?

Answer 26

The smallest repeating structures within a polymer.

Question 27

How can the properties of polymer materials be modified?

Answer 27

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.

Question 28

What is an issue with polymer materials?

Answer 28

They aren’t biodegradable
As they have a non-polar bond they are very unreactive so they are not attacked by biological enzymes.

Question 29

What is mechanical recycling?

Answer 29

1) Separate the different types of plastics.
2) Wash and grind the plastic into small pellets.
3) Pellets are melted and remoulded.

Question 30

What is feedstock recyling?

Answer 30

1) Plastics are heated to a temperature that will break the polymer bonds and produce monomers.
2) Monomers are used to make new plastics.

Question 31

What is the issue with recycling?

Answer 31

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.

Question 32

How is low density polyethene made?

Answer 32

Polymerising ethene at high pressure and high temperature via a free-radical mechanism.

Question 33

What are the properties of low density polyethene?

Answer 33

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.

Question 34

How is high density polyethene made?

Answer 34

Made at temperatures and pressures slightly greater than room conditions.
Uses a Ziegler-Natta catalyst.

Question 35

What are the properties of high density polyethene?

Answer 35

Little chain branching.
High density and melting point due to the chains packing well together.

Question 36

What are the uses of low density polyethene?

Answer 36

Packaging
Cable insulation

Question 37

What are the uses of high density polyethene?

Answer 37

Buckets
Bottles