3.3.4 Alkenes

Question 1

What are alkenes?

Answer 1

1. Alkenes have the general formula C_nH_2n
2. All alkene molecules have a double carbon covalent bond (C=C) therefore they are unsaturated
3. They can make more bonds in addition reactions

Question 2

Why are alkenes reactive?

Answer 2

There are two pairs of electrons in the C=C bond, therefore it is very electronegative, and can attract electrophiles

Question 3

What are electrophiles?

Answer 3

Electrophiles are electron pair acceptors. They are electron deficient, therefore are attracted to areas where there is a high electron density - e.g. C=C double bond

Examples of electrophiles:
* Positively charged molecules - e.g. H⁺ or NO₂⁺
* Polar molecules - the δ+ atom is attracted to high electron densities

Question 4

Explain how bromine water can be used to test for unsaturation (alkenes)

Answer 4

When an alkene is shaken with orange bromine water, the solution quickly decolourises

Question 5

Describe electrophillic addition using bromine water and ethene as an example

Answer 5

1. The double bond of repels the electrons in Br₂, polarising Br-Br
2. A pair of electrons in the double bond attracts the Br^δ+ and forms a bond with it. This repels electrons in the Br-Br bond further, until it breaks
3. A positively charged carbocation intermediate is produced, to which the Br^- ion is attracted to the positve carbon
4. Next a bond forms between the postive carbon and the Br^- ion
5. The product is now 1,2-dibromoethane

Question 6

How can adding a hydrogen halide to an unsymetrical alkene form two products?

Answer 6

• The amount of each product formed depends on how stable the carbocation intermediate is
• Carbocations with more alkyl groups are more stable because the alkyl groups feed electrons towards the postive charge
• The more stable carbocation is more likely to form

Question 7

Explain how alkenes can react with cold concentrated sulfuric acid, and cold water to then be warmed to produce an alcohol

Answer 7

1. Cold conc. sulfuric acid reacts with an alkene in an electrophillic addition reaction - e.g. H₂C=CH₂ + H₂SO₄ -> CH₃CH₂OSO₂OH
2. Cold water is then added, and the mixture is warmed, it then hydrolyses to form an alcohol - e.g. CH₃CH₂OSO₂OH + H₂O -> CH₃CH₂OH + H₂SO₄
3. The sulfuric acid isn’t used up - it acts as a catalyst

Unsymetrical alkenes will produce a mixture of products

Question 8

What is a polymer?

Answer 8

• Polymers are long chain molecules formed when lots of monomers (small molecules) join together
• They can be synthetic or natural

Question 9

What is an addition polymer?

Answer 9

Polymers made from many alkenes due to the double bond “opening up” to join with another alkene, creating a longer molecule of repeating units

Question 10

Why are addition polymers unreactive?

Answer 10

• Addition polymers are saturated (only contain single bonds)
• The carbon chain is also non-polar

These factors make addition polymers unreactive

Question 11

How do the intermolecular properties of polyalkenes depend on intermolecular forces?

Answer 11

• Polyalkenes are usually non-polar, therefore chains are only held together by Van der Waals forces
• The longer the polymer chains are the and the closer together they can get, the stronger the Van der Waals forces between chains will be
• This means that polyalkenes made up of long, straight chains tend to be strong and rigid, while short, branched chain alkenes tend to be weaker and more flexible

Question 12

What does plasticiser do to polymers?

Answer 12

Plasticisers makes polymers more flexible. The plasticiser molecules get between the polymer chains and weaken the intermolecular forces between the chains, so they can slide, therefore the polymer is more flexible