Alkenes

Question 1

What are Alkenes? (2)

Give the basic definition and general formula.

Answer 1

• Alkenes are unsaturated hydrocarbons which contain at least one C=C bond.
• Their general formula is Cn2n.

Question 2

Describe the Double Bond in Alkenes. (3)

Explain in terms of types of covalent bond.

Answer 2

• Double bonds are made up of a sigma (σ) bond and a pi (π) bond.
• Sigma bond = direct overlap of p orbital, pi bond = sideways overlap of p orbital
• Pi bonds are weaker than sigma bonds, but increase bond enthalpy.

Question 3

Why are Alkenes More Reactive than Alkanes? (3)

Explain their feature and what reaction can happen as a result.

Answer 3

• Alkenes’ double bond is an area of high electron density, and pi bonds are easy to break.
• This makes them a target for electrophiles to undergo electrophilic addition reactions.

Question 4

What are Electrophiles? (1)

Give the basic definition.

Answer 4

• Electrophiles are species that are attracted to areas of high electron density. They accept an alectron pair.
• (electro = negative, phile = loving)

Question 5

Describe the First Step of Electrophilic Addition. (3)

Explain the process step by step, and what is formed.

Answer 5

• There is a difference in electronegativity between two atoms in a molecule - the δ+ is attracted to the double bond, with high electron density.
• This atoms accepts an electron pair, breaks the π bond and forms a covalent bond with the carbon.
• This forms a carbocation as an intermediate.

Question 6

Describe the Second Step in Electrophilic Addition. (2)

Explain the rest of the process.

Answer 6

• The rest of the initial molecule is negative, and is attracted to the positive carbocation.
• This then bonds to the other carbon, forming the final product.

Question 7

How is the Electrophilic Addition Mechanism Drawn? (3)

Describe how the curly arrows are drawn.

Answer 7

• In the first step, a curly arrow is drawn from the pi bond in the alkene to the δ+ atom on the reactant.
• A curly arrow is also drawn from the covalent bond to the** δ- atom**.
• Finally, in step 2, a curly arrow is drawn from the electron pair on the negative ion to the positive carbon in the carbocation.

Question 8

Describe how Halogens (e.g. Bromine) can Act as an Electrophile. (2)

Explain how the molecules interact.

Answer 8

• When bromine gets close to the alkene, the high density of electrons in the double bond repels electrons.
• This polarises the bromine molecule, allowing it to react with the double bond.

Question 9

How does Bromine Water Test for Alkenes? (2)

Describe the reaction and positive test.

Answer 9

• Bromine reacts with the alkenes by electrophilic addition. This consumes the bromine removing the orange colour.
• Therefore, if alkenes are present, bromine water turns from orange to colourless.

Question 10

How does Symmetry of Alkenes affect Products Formed? (2)

Describe how this affects products formed.

Answer 10

• In symmetrical alkenes, the double bonded carbons are identical, leading to a single product.
• However, in asymmetrical alkenes, different position isomers can be formed

Question 11

Describe Electrophilic Addition with Water. (3)

Explain what reaction occurs and the conditions required.

Answer 11

• Forms alcohols from alkenes - known as hydration
• Requires 300’C, 60-70 atm, and a phosphoric acid catalyst.

Question 12

Describe Electrophilic Addition with Sulfuric Acid, and its’ Use. (3)

Describe what can be produced in both stages.

Answer 12

• Forms alkyl hydrogen-sulfates from alkenes, from concentrated sulfuric acid and room temperature.
• Alkyl hydrogen sulfates can then form alcohols and sulfuric acid in a nucleophilic substitution (hydrolysis) reaction, using water.

Question 13

What are the Classifications of Carbocation? (3)

Name each classification and define them.

Answer 13

• Primary carbocation - one alkyl group attached to positive carbon.
• Secondary carbocation - two alkyl groups attached to positive carbon.
• Tertiary carbocation - three alkyl groups attached to positive carbon.

Question 14

What is Markovnikov’s Rule? (2)

Describe and state the most stable carbocation.

Answer 14

• Markovnikov’s rule states that in electrophilic addition, the most stable carbocation is favoured and produced more, leading to major and minor products.
• In terms of stability:
• Tertiary > Secondary > Primary

Question 15

Why are Tertiary Carbocations more Stable? (2)

Describe and explain stabilities.

Answer 15

• Tertiary carbocations have more neighbouring alkyl groups.
• These provide a stronger positive inductive effect, which stabilises the positive charge making them more stable.

Question 16

What Effect do Stability of Carbocations have on Products? (2)

Explain and provide a relevant example.

Answer 16

• Tertiary/secondary carbocations are more stable, so the major product will have the halogen bonded to this carbon.
• E.g. In a reaction between propene and hydrogen bromide, 2-bromopropane will be the major product over 1-bromopropane.

Question 17

What are Addition Polymers? (2)

Give the basic definition and relate to alkenes.

Answer 17

• Polymers are large, complex molecules composed of long chains of monomers joined together by covalent bonds.
• Addition polymers are made by breaking the double bond in alkenes to form polyalkenes.

Question 18

Describe how Addition Polymers are Drawn. (4)

Name all four important features of drawing polymers.

Answer 18

• Draw the monomer unit without the double bond, and place it in brackets.
• Extend bonds out beyond the brackets.
• Draw an n to show the macromolecular nature of the polymer.

Question 19

How are Polymers Named? (2)

Describe the process of naming addition polymers.

Answer 19

• The name of the alkene is placed in brackets.
• The prefix ‘-poly’ is then added to the front to give the name of the polymer.

Question 20

What are the Main Properties of Polymers? (3)

State three beneficial properties of polymers for industrial use.

Answer 20

• High melting/boiling point
• Inert (chemically unreactive)
• Strong / rigid

Question 21

Why do Polymers have High Melting Points? (3)

Describe features and explain why.

Answer 21

• Polymers are macromolecules, with large numbers of atoms and therefore electrons.
• This mean they experience strong Van der Waal’s forces, which require lots of energy to overcome.

Question 22

Why are Polymers Inert? (3)

Describe and explain why.

Answer 22

• Polymers contain large numbers of C-C bonds. These are strong and non-polar.
• They are also saturated molecules.
• This means they require lots of energy to be broken and form new products.

Question 23

Why do some Polymers have Higher Melting points than Others? (3)

Give reasons and explain.

Answer 23

• Some polymers contain polar bonds (e.g. poly(chloroethene) or poly(ethenol).
• This means they can experience permanent dipole-dipole forces or hydrogen bonds, which are much stronger and require more energy to overcome than VdW forces.

There are also ranges in size of polymers which affects strength of VdW forces.

Question 24

What is PVC? (3)

Describe the material and its’ properties.

Answer 24

• PVC stands for poly(vinyl chloride), which is poly(chloroethene).
• It contains VdW and permanent dipole-dipole forces making it an incredibly strong material. Its’ rigidity also gives it many applications.

Question 25

Explain how Plasticisers Work. (3) | Give the basic definition and explain how they work.

Answer 25

* Plasticisers are small molecules positioned **between polymer chains**, which **increase flexibility** of polymers. * They increase distance between chains, **reducing VdW forces**, allowing them to slide more easily.