Alkenes

Question 1

What are alkenes?

Answer 1

Unsaturated hydrocarbons

Made of carbon and hydrogen only and have one or more C=C double bonds

Question 2

Why do the double bonds of alkenes make them more reactive than alkanes?

Answer 2

Because of the high concentration of electrons (electron density) between the carbon atoms

Question 3

When are alkenes produced in large quantities?

Answer 3

When crude oil is thermally cracked

Question 4

What is the general formula of alkenes?

Answer 4

The homologous series of alkenes with one double bond has the general formula CnH2n

Question 5

Why can’t double bonds rotate?

Answer 5

As well as normal C—C single bond, there is a p-orbital (which contains a single electron) on each carbon.

These two orbitals overlap to form an orbital with a cloud of electron density above and below the single bond.

This is called a pi-orbital and its presence means the bond cannot rotate.

Question 6

What 3 types of isomers can alkenes form?

Answer 6

Chain

Position

Geometrical

Question 7

What is positional isomerism (alkenes)?

Answer 7

Double bond is in different positions.

The longer the carbon chain, the more possibilities there will be and therefore the greater the number of isomers.

Question 8

What is geometrical isomerism (alkenes)?

Answer 8

A form of stereoisomerism.

The two stereoisomers have the same structural formula but the bonds are arranged differently in space.

It occurs only around C=C double bonds.

Question 9

Draw Z-but-2-ene

Answer 9

P217

Question 10

Do the double bonds affect physical properties of alkenes?

Answer 10

No

Question 11

What intermolecular forces are present between alkene molecules?

Answer 11

van der Waals

Question 12

Why are the physical properties of alkenes very similar to alkanes?

Answer 12

Because the only intermolecular forces present between the molecules are van der Waals forces

Question 13

Are alkenes soluble in water?

Answer 13

No

Question 14

Why are alkenes more reactive than alkanes?

Answer 14

The double bond(s)

It forms an electron rich area in the molecule, which can be easily attacked by positively charged reagents (electrophiles)

Question 15

Most of the reactions of alkenes are ___________ ___________.

Answer 15

Electrophilic additions

Question 16

As alkenes are ___________ they can undergo _________ reactions

Answer 16

Unsaturated

Addition

Question 17

Define electrophile

Answer 17

An electron pair acceptor

Question 18

Why are alkenes not usually used as fuels (despite their combustion properties)?

Answer 18

Because their reactivity makes them very useful for other properties

Question 19

Electrophilic reactions

Answer 19

P220-223

Question 20

What is Markovnikov’s rule?

Answer 20

When hydrogen halides add on to alkenes, the hydrogen adds on to the carbon atom which already has the most hydrogens

Question 21

What is the positive inductive effect?

Answer 21

Alkyl groups have a tendency to release electrons

Question 22

What is the result of the positive inductive effect?

Answer 22

This electron-releasing effect tends to stabilise the positive charge of the intermediate carbocation. The more alkyl groups there are attached to the positively charged carbon, the more stable the carbocation is.

Question 23

What type of carbocation tend to be more stable?

Answer 23

A positively charged carbon atom which has 3 alkyl groups (tertiary carbocation) is more stable than one with 2 alkyl groups (secondary carbocation), which is more stable than one with just 1 (primary carbocation).

Question 24

What addition reaction is used to test for a C=C double bond?

Answer 24

Alkene + Br2

When a few drops of bromine solution (bromine water - reddish brown) are added to an alkene, the solution is decolourised because the products are colourless.

Question 25

What is the product of reacting alkene with water? What conditions are required for this reaction to occur?

Answer 25

Used industrially to make alcohols

Carried out with steam, at a suitable temperature and pressure, using an acid catalyst such as phosphoric acid

Question 26

What are polymers?

Answer 26

Large molecules that are built up from small molecules (monomers)

Occur naturally everywhere: starch, proteins, cellulose and DNA

Question 27

Give one way of classifying polymers

Answer 27

By the type of reaction by which they are made

Question 28

What are addition polymers?

Answer 28

Made from a monomer or monomers with a C=C double bond (alkenes)

Made from monomers based on ethene

Question 29

What is the general formula of a monomer that makes up an addition polymer?

Answer 29

P224

Question 30

What happens when monomers polymerise?

Addition polymers

Answer 30

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

Question 31

How are addition polymers named?

Answer 31

Systematically with the prefix ‘poly’ followed by the name of the monomer in brackets

Question 32

What are plasticisers?

Answer 32

Small molecules that get between the polymer chains forcing them apart and allowing them to slide across each other

Question 33

Give an example of use of plasticisers

Answer 33

PVC is rigid enough for use as drainpipes, but with the addition of plasticisers it becomes flexible enough for making aprons

Question 34

Why are polyalkenes not biodegradable?

Answer 34

They have a backbone which is a ling chain saturated alkane molecule.

Alkanes have strong non-polar C—C and C—H bonds. So they’re very unreactive molecules. They cannot be attacked by biological agents - like enzymes - and so they are not biodegradable.

Question 35

Explain what low density polythene is (including its uses)

Answer 35

Made by polymerising ethene at high pressures and high temperature via a free-radical mechanism.

This produces a polymer with a certain amount of chain branching. This is a consequence of the rather random nature of free-radical reactions.

The branched chains don’t pack together particularly well and the product is quite flexible, stretches well and has fairly low density. These properties make it suitable for packaging (plastic bags), sheeting and insulation for electrical cables.

Question 36

Explain what high density polythene is

Answer 36

Made at temperatures and pressures little greater than room conditions and uses a Ziegler-Natta catalyst.

This results in a polymer with much less chain branching (around one branch fro every 200 carbons on the main chain).

The chains can pack together well. This makes the density of the plastic greater and its melting temperature higher. Typical uses are milk crates, buckets and bottles for which low density polythene would be insufficiently rigid.

Question 37

What is mechanical recycling?

Answer 37

The simplest form of recycling

The first step is to separate the different types of plastics. The plastics are then washed and once they are sorted they may be ground up into small pellets. These can be melted and remoulded.

Question 38

What is feedstock recycling?

Answer 38

The plastics are heated to a temperature that will break the polymer bonds and produce monomers. These can then be used to make new plastics.