13.3 Reactions of Alkenes, 13.4 Electrophilic Addition in Alkenes & 13.5 Polymerisation in Alkenes

Question 1

Why are alkenes more reactive than alkanes?

Answer 1

• The pi bond has relatively low bond enthalpy (not much energy is required to break it) as the electrons in a pi bond are more exposed than those in a sigma bond

Question 2

List 4 reactants alkenes can undergo addition reactions with.

Answer 2

• Hydrogen
• Halogens
• Hydrogen halides
• Steam

Question 3

List the reactants, products and conditions of the hydrogenation of alkenes.

Answer 3

• Alkene and hydrogen
• Nickel catalyst
• Alkane

Question 4

List the reactants, products and conditions of the halogenation of alkenes.

Answer 4

• Alkene and halogen
• Room temperature
• No catalyst
• Dihaloalkane

Question 5

Why is the halogenation of alkenes helpful?

Answer 5

• It can be used as a test for saturation
• This is because when bromine water is added to an alkene, it goes from orange to colourless
• However, when this is done to an alkane, there is no addition reaction, and therefore no colour change, so the bromine water stays orange

Question 6

List the products and conditions when hydrogen halides react with alkenes.

Answer 6

• Room temperature, no catalyst
• Concentrated hydrochloric or hydrobromic acid can be used
• Haloalkanes

Question 7

What happens when a hydrogen halide reacts with an unsymmetrical alkene?

Answer 7

• They form a mixture of 2 different products (positional isomerism)

Question 8

List the reactants, products and conditions of the hydration of alkenes.

Answer 8

• Alkene and steam
• Phosphoric acid (H3PO4) catalyst (or concentrated sulfuric acid)
• > 100°C (already indicated if you said steam/ wrote the gas state symbol)
• Alcohol (however, positional isomerism is possible again)

Question 9

What is an electrophile? Required.

Answer 9

• An electron pair acceptor
  (- They’re usually a positive ion or a molecule containing an atom with a partial positive charge)

Question 10

What is electrophilic addition?

Answer 10

• An addition reaction in which the first step is attack by an electrophile on a region of high electron density

Question 11

Describe why electrophiles take part in electrophilic addition.

Answer 11

• Electrophiles are atoms/ groups of atoms that are attracted to an electron rich-centre (in this case the high electron density of the pi-electrons) and accept an electron pair

Question 12

Which reactions are examples of electrophilic addition?

Answer 12

• The reaction between alkenes and halogens/ hydrogen halides

Question 13

Describe the mechanism for the reaction between an alkene and a hydrogen halide.

Answer 13

• The hydrogen halide molecule would be polar as halogens are more electronegative than hydrogen
• The electrons in the pi-bond are attracted to the H atom due to its partial positive charge, and shares its electrons with it, so the double bond breaks
• The H atom then bonds to one of the carbon atoms that was in the double bond
• The bond between the H atom and halogen atom breaks by heterolytic fission, and the electrons go to the halogen atom
• A halide ion and carbocation (C atom with a positive charge as it has only 3 bonds) are formed
• The halide ion reacts with the carbocation to form the final product

Question 14

What do curly arrows show in the reaction mechanism?

Answer 14

• The movement of a pair of electrons (including heterolytic fission)

Question 15

How should curly arrows be drawn?

Answer 15

• From the bond/ lone pair to the atom the electrons are being transferred to (always going form high electron density to low electron density)

Question 16

What is the electrophile in this example of electrophilic addition?

Answer 16

• The hydrogen halide (more specifically the hydrogen atom)

Question 17

Why do alkenes react with halogens?

Answer 17

• Halogen molecules aren’t polar, but as they approach the pi-bond, electrons are repelled, and a dipole is formed across the halogen molecule

Question 18

When a hydrogen halide reacts with an unsymmetrical alkene, how do you know which product more is formed of?

Answer 18

• According to Markownikoff’s rule, most of the time the halide will attach itself to the carbon atom bonded to more carbon atoms than the other one
• The product formed in this situation is the major product, while the other is the minor product

Question 19

Why does Markownikoff’s rule work?

Answer 19

• There are 3 types of carbocations; primary, secondary and tertiary
• Tertiary carbocations are bonded to 3 carbon atoms, secondary to 2 carbon atoms and primary carbocations to 1 carbon atom
• Tertiary carbocations are therefore the most stable, while primary carbocations are the least stable
• This is because alkyl groups can donate electrons

Question 20

What is a monomer?

Answer 20

• A small molecule which combines with many other monomers to form a polymer

Question 21

What is a polymer?

Answer 21

• A large molecule formed from many thousands of repeat units of smaller molecules (monomers)

Question 22

What is a repeat unit?

Answer 22

• The simplest unit of a polymer that is repeated many times to form the full polymer (drawn with brackets and n)

Question 23

How do alkenes form polymers?

Answer 23

• They undergo addition polymerisation to produce long saturated chains that contain no double bonds

Question 24

How are general equations for addition polymerisation reactions written?

Answer 24

• The monomer is the reactant, and has an n next to it
• The product is the repeat unit drawn inside square brackets with bonds extending beyond the brackets, and with an n outside the bracket

Question 25

How can the repeat unit be identified from a polymer chain?

Answer 25

• It consists of 2 carbons from the main chain, and the other atoms they are vertically bonded to (make sure to check to see which 2 carbons are being repeated)

Question 26

How are polymers named?

Answer 26

• Poly(monomer)
• For example, ethene would become poly(ethene)

Question 27

What are 2 sustainable ways that waste polymers can be processed?

Answer 27

• Combustion to produce energy
• Used as an organic feedstock to produce plastics and other organic chemicals

Question 28

What are biodegradable and photodegradable polymers?

Answer 28

• Biodegradable polymers can be broken down by microorganisms into water, carbon dioxide and biological compounds
• Photodegradable polymers contain bonds that are weakened when they absorb light, which causes them to degrade