Chapter 13 Alkenes

Question 1

Alkenes

Answer 1

• Unsaturated hydrocarbons (at least 1 C=C bond)

Question 2

General formula of alkenes

Answer 2

• CnH2n

Question 3

The nature of the C=C bond

Answer 3

• C has 4 electrons and 3 are used in sigma bonds (1 to the other C and the other 2 for other atoms)
• The remaining electron is a pi bond

Question 4

pi bond formation

Answer 4

Formed by the sideways overlap of 2 electrons in the p-orbitals (one from each C of the C=C bond)

Question 5

Where is the pi-electron density concentrated

Answer 5

above and below the line joining the nuclei of the bonding atoms

Question 6

What does the pi bond do to the C atoms

Answer 6

Locks the C atoms in positions and prevents them from rotating around the C=C bond -> geometry of alkenes is different to that of alkanes

Question 7

Alkenes in the natural world

Answer 7

• Carotene = makes flamingos pink
• Limonene = causes scent of oranges and lemon

Question 8

Stereoisomers

Answer 8

Have the same structural formula but a different arrangement of the atoms in space

Question 9

Types of stereoisomerism

Answer 9

• E/Z isomerism
• Optical isomerism

Question 10

Why does stereoisomerism occur?

Answer 10

The rotation around the C=C bond is restricted so the groups attached to those Cs are therefore fixed relative to eachother

Question 11

When will a molecule have E/Z isomerism

Answer 11

When the molecule has:
* A C=C bond (restricted rotation)
* 2 different groups attached to each C atom of C=C bond

Question 12

Cis-trans isomerism

Answer 12

• Special case of E/Z isomerism
• Same requirements as E/Z isomerism but one of the attached groups on each C atom of the C=Cmust be the same

Question 13

Relationship between cis/trans isomerism and E/Z isomerism

Answer 13

When there is a H on each C=C bond
* Cis isomer = Z isomer
* Trans isomer = E isomer

Question 14

Cahn-Ingold-Prelog Rules

Answer 14

• Z isomer: If the groups of highest priority are on the same side of the C=C bond
• E isomer: If the groups of highest priority are diagonally places across the C=C bond

Question 15

How to assign priority

Answer 15

• As atomic number increases, priority increases
• If it is a large atom, find the first point of difference and use this to compare priority

Question 16

Why are alkenes much more reactive than alkanes

Answer 16

pi bond -> the electrons in the pi bond are more exposed than the electrons in the sigma bond. The pi bond readily breaks and alkenes undergo addition reactions relatively easily

Question 17

Addition reactions of the alkenes

Answer 17

• hydrogen in the presence of a nickel catalyst
• halogens
• hydrogen halides
• steam in the presence of an acid catalyst

Question 18

Hydrogenation of alkenes

Answer 18

Alkene + hydrogen -> alkane

Question 19

What is the product of an addition reaction?

Answer 19

Always saturated

Question 20

Halogenation of alkenes

Answer 20

Alkene + bromine -> dihaloalkane

Question 21

Test for unsaturation (C=C bond)

Answer 21

Add bromine water
orange -> colourless

Question 22

Hydrogen halides

Answer 22

Hydrogen + halogen
e.g. HCl

Question 23

Alkenes + hydrogen halides

Answer 23

haloalkane

Question 24

Hydration of alkenes

Answer 24

Alkene + Water -> alcohol

Question 25

Electrophilic addition

Answer 25

Reaction mechanism for addition reactions of alkenes

Question 26

Electrophile

Answer 26

• Electron pair acceptor (attracted to an electron-rich centre)
• Usually a positive ion, or a molecule containing an atom with a δ+ charge

Question 27

Reaction between but-2-ene + hydrogen bromide

Answer 27

1. Bromine is more electronegative than hydrogen, so hydrogen bromide is polar (contains dipole Hδ+ - Brδ-)
2. The electron pair in the pi bond is attracted to the Hδ+ -> causes the double bond to break
3. A bond forms between the H of the H-Br and a C that was part of the C=C bond
4. The H-Br bond breaks by heterolytic fission, with the electron pair going to the bromine atom
5. A Br- ion and carbocation are formed. These react together to form the addition product

Question 28

How does polarisation of bromine occur (bromine is a non-polar molecule)

Answer 28

The pi electrons interact with the electrons in the Br-Br bond -> causes Brδ+ -Brδ-

Question 29

Markownikoff’s rule

Answer 29

When a hydrogen halide reacts with an unsymmetrical alkene, the hydrogen from the hydrogen halide attaches to the carbon with more hydrogen and less carbon

Question 30

Steps of electrophilic addition

Answer 30

Sometimes, 3 different carbocations can be produced:
* Primary carbocation (has 1 alkyl group)
* Secondary carbocation (has 2 alkyl groups)
* Tertiary carbocation (has 3 alkyl groups)

Question 31

R

Answer 31

Alkyl group

Question 32

Polymers

Answer 32

Extremely large molecules from many repeat units (monomers)

Question 33

What polymerisation do alkenes undergo?

Answer 33

Addition polymerisation -> produces long chains of saturated chains with no C=C bonds

Question 34

Characteristics of addition polymers

Answer 34

• High molecular masses
• Have prefix poly-

Question 35

Examples of synthetic addition polymers

Answer 35

• Poly(ethene)
• Poly(chloroethene)
• Poly(propene)
• Poly(styrene)
• Poly(tetrafluoroethene)

Question 36

Environmental concerns of polymers

Answer 36

• Disposing of waste polymers
• Recycling
• PVC recycling
• Using waste polymers as fuel
• Feedstock recycling

Question 37

Biodegradable and photodegradable polymers

Answer 37

• Biodegradable polymers
• Photodegradable polymers

Question 38

Environmental concerns of polymers: Recycling

Answer 38

Reduces environmental impact by conserving finite fossil fuels and decreasing amount of waste going to landfill

Question 39

Environmental concerns of polymers: PVC recycling

Answer 39

Disposal and recycling of PVC is hazardous due to high chlorine and additive content

• New technology uses solvents to dissolve the polymer

Question 40

Environmental concerns of polymers: Using waste polymers as fuel

Answer 40

As polymers are derived from natural gas, they have a high stored energy value -> so can be incinerated to produce heat -> electricity

Question 41

Environmental concerns of polymers: Feedstock recycling

Answer 41

Describes the chemical + thermal processes that can reclaim monomers, gases or oil from waste polymers

Question 42

Advantage of feedstock recycling

Answer 42

Can handle unsorted and unwashed polymers

Question 43

Bioplastics

Answer 43

• Produced from plant starch, cellulose, plant oils and proteins
• Offer a renewable + sustainable alternative to oil-based product, and protects our environment and conserves our valuable oil reserves

Question 44

Biodegradable polymers

Answer 44

• Broken down by microorganisms into water, carbon dioxide and biological compounds

Question 45

What are compostable polymers

Answer 45

Based on poly(lactic acid)

Question 46

Photodegradable polymers

Answer 46

Contain bonds that are weakened by absorbing light to start the degradation

Question 47

Catalyst for hydration of alkenes

Answer 47

Phosphoric acid

Question 48

Differences between sigma bonds and pi bonds

Answer 48

• pi bond is weaker than sigma bonds
• pi bond is above and below the bonding atoms whereas sigma bond is between bonding atoms

Question 49

When there is a possibility for different haloalkanes to be produced in a reaction, which one will be produced in greater quantity?

Answer 49

The tertiary haloalkane because it is more stable than the secondary and primary haloalkane

Question 50

Advantages of combusting polymers

Answer 50

Produces electricity

Question 51

Disadvantages of combusting polymers

Answer 51

Produces Hcl or chlorine (however this can be neutralised using an alkali)

Question 52

Bonding angle around addition polymer

Answer 52

Tetrahedral
109.5