Module 4: Chapter 13 Alkenes

Question 1

What are the properties of alkenes?

Answer 1

Alkenes are unsaturated aliphatic hydrocarbons with double carbon-carbon bonds (C=C is the functional group)

Question 2

How do you names alkenes?

Answer 2

Start of alkene name (+a if there is more than one double bond) - at what position is the double bond - ENE (-di, -tri, -tetra etc added if more than one double bond is there)
e.g. buta-1,3-diene

• if there are any alkyl groups they go at the beginnings of the alkene name with regular rules

Question 3

What is the shape and bond angle of alkenes?

Answer 3

• three regions of electron density around each carbon atom
• 3 regions of electron density repel to be far apart as possible
• trigonal planar shape formed with a bond angle of 120°

Question 4

Explain the term ‘stereoisomer’?

Answer 4

compounds with the same structural formula but with a different arrangement in space

Question 5

Explain the term ‘E/Z isomer’?

Answer 5

an example of stereoisomerism that needs the following conditions:

1. no free rotations around the C=C bond
2. Different groups attached to each carbon atom of the C=C bond

· a E isomer has the the non-hydrogen groups diagonal and Z isomers have them both on the up/down side of the C=C bond

Question 6

Describe and draw a diagram of alkene bonding?

Answer 6

• a sigma bonds is formed between the carbon atoms
• a pi bonds is formed between the carbon atoms from the overlap of p-orbitals (the pi bond is above and below the plane but is only a single entity)

Question 7

Explain the term ‘Cis and Trans isomerism’?

Answer 7

a special case of E/Z isomerism in which two of the substituent groups attached to each carbon atom of the C=C group are the same.

• cis isomers (Z isomers) have the hydrogen atoms on each carbon in this double bond on the same side of the molecule
• trans isomers (E isomers) have hydrogen atoms diagonally opposite each other

Question 8

What is the Cahn-Ingold-Prelog Nomenclature rules?

Answer 8

• priority rule to work out wether a molecule is an E/Z isomer
• take each carbon apart of the C=C bonds in turn and the atom with the biggest atomic number is the high priority
• if the high priority atoms are on the same side then it’s a Z isomer, if they are diagonal it’s a E isomer

Question 9

Describe the reactivity of alkenes?

Answer 9

• alkenes tend to undergo addition reactions - part of the C=C bond breaks and an unsaturated substance is normally converted into a saturated one
• the pi bond is weaker than the sigma bond so breaks more easily

Question 10

What is a electrophile?

Answer 10

An electrophile is a species that is attracted to areas of negative charge. Electrophiles are able to accept a pair of electrons

Question 11

Describe the reaction with hydrogen

Answer 11

Hydrogenation
alkene + H-H -> alkane
nickel catalyst used
unsaturation to saturation

Question 12

Describe the reaction with halogens

Answer 12

• for example could form a 1,2-dichloroethane
• conditions: room temperature and pressure
• this reaction can be used to test for unsaturation (bromine water, Br2 in H2O, will change from orange to colourless on shaking)

Question 13

Describe the reaction with hydrogen halide

Answer 13

• produces a halogenalkane
• conditions: room temperature and pressure and inert solvent
• H-Cl or H-Br

Question 14

Describe the reaction with steam

Answer 14

alkene + H-OH -> alcohol

- conditions: (330°c, 600MPa) gaseous alkene and steam passed over a concentrated phosphoric acid catalyst

Question 15

What is an addition reaction?

Answer 15

An addition reaction has a single product - the reactants have ‘added’ together

Question 16

What is Markownikoff’s rule?

Answer 16

• asymmetrical alkenes produce 2 addition reaction products
• the major product is always the product that is produced from the most stable carbocation
• the more alkyl groups (carbons) attached to the carbocation the more stable the carbocation, tertiary carbocations are the most stable
• the attached alkyl group has an electron-donating ability, the more alkyl groups attached the more the donation and the greater the spreading the charge and the greater the stability

Question 17

Describe (and draw) the mechanism for the unsaturation test

Answer 17

• a test for alkenes is shaking with bromine water (the colour will change from orange to colourless)
  e. g. ethene + bromine -> 1,2-dibromoethane
• this is an electrophilic addition reaction
• bromine molecule becomes polarised as it approaches an alkene
• partially positively charged bromine behaves as an electrophile and reacts with the double bond
• one of the carbon atoms now has a positive charge (carbocation)
• carbocations react very rapidly with anything that has electrons to share such as the bromide ion also formed in this reaction
• as bromine water is used a lone pair of electrons on the oxygen of a water molecule can also react with carbocation forming bromoalcohol and a H+ ion (mixtures of products)

Question 18

What is addition polymerisation?

Answer 18

• alkenes tend to undergo addition reactions where monomer alkenes are joined together to form larger molecules
• conditions: high temp, high pressure and with a catalyst
• addition polymers have a high molecular mass, they are sometimes called macromolecules
• a repeat unit is used to show a section of the polymer

Question 19

What is LD and HD polyethene?

Answer 19

• low density polyethene (LDPE) are branched molecules in polymer, flexible and light (e.g. used in plastic bags)
• high density polyethene (HDPE) linear chain in polymer, strong and rigid (e.g. used to make plastic bottles)

Question 20

How is poly(chloroethene) created?

Answer 20

• also known as PVC or polyvinyl chloride
• used a chloroethene monomer (C2H3Cl)
• used in window frames and plastic bottles

Question 21

Name 4 ways that polymers can be disposed?

Answer 21

1. Recycling polymers - washed and melted for reuse, shredded for insulation/filling use or can be made into new products
2. PVC is dissolved in a solvent as it can’t be recycled (must involve as pure starting products as possible)
3. Incineration - polymers are made from the petroleum fraction of crude oil, so they readily burn
4. Feedstock - some polymers are thermally/chemically broken into monomers and become raw materials for new products (no washing or sorting for this)

Question 22

What are biodegradable polymers?

Answer 22

• some new polymers are biodegradable, microbes breakdown down the polymers into water, CO2 and biological compounds, they use starch or cellulose to link/make polymer chains
• compostable bags are made this way
• disposable cups cane be made of sugar cane fibre replacing conventional polystyrene cups

Question 23

What are photodegradable polymers?

Answer 23

Oil based polymer that break down when exposed to sunlight