Alkenes 1

Question 1

What is the general formula of an alkene

Answer 1

CnH2n

Question 2

Describe the shape of an alkene

Answer 2

• 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.

Question 3

What is an electrophile

Answer 3

A lone pair acceptor

Question 4

In general, describe the mechanism for electrophilic addition in Alkenes

Answer 4

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.

Question 5

What is Markownikoffs rule for the electrophilic addition of alkenes

Answer 5

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

Question 6

What is a tertiary carbocation

Answer 6

A positively charged carbon atom that has three alkyl groups attached.

Question 7

What is a secondary carbocation

Answer 7

A positively charged carbon atom that is attached to two alkyl groups

Question 8

What is a primary carbocation

Answer 8

A positively charged carbon atom that is attached to one alkyl group

Question 9

From most stable to least, list the possible carbocations produced during electrophilic addition

Answer 9

Tertiary
Secondary
Primary

Question 10

Why does the halogen atom bond mostly to the carbon atom attached to the most alkyl groups (most stable carbocation)

Answer 10

Because alkyl groups have a tendency to release electrons which is a positive inductive effect.

Question 11

When Alkenes react with halogen molecules, what happens

Answer 11

• Halogen molecules are diatomic.
• The two halogen atoms from the molecule add across the Alkenes double bond.

Question 12

Describe what happens during the electrophilic addition of a hydrogen halide to an alkene

Answer 12

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.

Question 13

Describe what happens during the electrophilic addition of a halogen molecule to an alkene to form a dihalogenoalkane

Answer 13

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.

Question 14

How do you test for a double bond.

Answer 14

• 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.

Question 15

What type of reaction is the electrophilic addition of sulphuric acid to an alkene

Answer 15

Exothermic

Question 16

Why is the electrophilic addition of sulphuric acid to an alkene a useful reaction

Answer 16

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.

Question 17

Why are polyalkenes not biodegradable

Answer 17

• 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.

Question 18

Describe the structure and properties of low density polyethene

Answer 18

• 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.

Question 19

Describe the structure and properties of high density polythene

Answer 19

• 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.

Question 20

What is mechanical recycling

Answer 20

• 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.

Question 21

What is feedstock recycling

Answer 21

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.

Question 22

What are some issues that arise with recycling

Answer 22

• 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.

Question 23

Are Alkenes saturated or unsaturated

Answer 23

Unsaturated as they contain double bonds

Question 24

What types of bond are present in Alkenes

Answer 24

• Sigma bonds between carbon and hydrogen atoms
• The C=C bond consists of a sigma bond and a pi bond.

Question 25

Describe the key properties of a pi bond

Answer 25

• A pi bond is created by the sideways overlap of p orbitals
• A pi bond is non-rotational because any rotation would reduce the overlap of the p orbitals
• this means that the structure of an alkene across the double bond is effectively locked.

Question 26

Why can stereoisomerism be found in Alkenes

Answer 26

Because the pi bond ion the C=C bond is non-rotational

Question 27

Explain why Alkenes are highly reactive

Answer 27

• They are very reactive due to the double bond.
• The bond enthalpy of the pi bond is less than the sigma bond because the pi bond is a sideways overlap of orbitals whereas the sigma bond is a direct overlap.
• This means less energy is required to break the pi bond so it is more likely to react.
• the double bond contains to pairs of electrons so has a high electron density which makes it very reactive.

Question 28

What type of intermolecular force is found in a hydrogen halide molecule

Answer 28

Hydrogen halide molecules have a permanent dipole.

Question 29

What type of intermolecular force is found in halogen molecules when they undergo electrophilic addition

Answer 29

An induced dipole caused by then electron dense C=C bond

Question 30

Compare the electrophilic addition of a halogen molecule and a hydrogen halide

Answer 30

1) A halogen molecule has an induced dipole whereas a hydrogen halide has a permanent dipole.
2) When adding a halogen to an asymmetric alkene only one product is made whereas when you add a hydrogen halide to an asymmetric alkene, major and minor products are made.
3) When adding a halogen, a dihalogenoalkane is produced whereas with a hydrogen halide, a halogenoalkane is produced.

Question 31

What happens when you add bromine water to an unsaturated substance

Answer 31

The bromine will add across the double bond, decolourising the bromine water (which was brown/orange)

Question 32

What happens when you add bromine water to a saturated substance

Answer 32

No reaction occurs and the bromine water remains brown/orange.

Question 33

Why is the electrophilic addition of sulfuric acid an important reaction in industry

Answer 33

• Because if water is added to the product of this reaction, an alcohol is formed and the acid is reformed.
• Sulfuric acid can be used as a catalyst to the formation of alcohols from Alkenes and water.

Question 34

What are the two types of polymer

Answer 34

Addition polymers and condensation polymers.

Question 35

How are addition polymers named

Answer 35

Poly(monomer)

Question 36

Describe the bonding in addition polymers

Answer 36

Despite the monomer having had double bonds, addition polymers contain no double bonds in the carbon backbone.

Question 37

What is the key difference between the structure of the monomer and repeating unit of an addition polymer

Answer 37

• the monomer for an addition polymer always has a double bond between carbon atoms.
• the repeating unit never has a double bond between carbon atoms.

Question 38

What do plasticisers do

Answer 38

Make a polymer more flexible

Question 39

How do plasticisers make a polymer more flexible

Answer 39

• A plasticiser is a small molecule that fits between polymer chains, causing them to be further apart.
• this weakens the intermolecular forces (Van Der Waals) forces acting between the chains, making the polymer more flexible.