4.1.3 Alkenes

Question 1

General formula of (aliphatic) alkenes.

Answer 1

CnH2n

Question 2

What are alkenes?

Answer 2

Unsaturated hydrocarbons.

Question 3

Describe the bonding in alkenes.

Answer 3

Double bond comprised of one sigma bond (from overlap of orbitals between bonding atoms) and one pi bond (sideways overlap of 2 p-orbitals with one electron from each carbon).
Pi bond causes a lack of rotation around the double bond.

Question 4

Describe the shape and bond angles in alkenes.

Answer 4

Trigonal planar shape due to: 3 regions of electron density around each of the carbons, repel eachother.
Bond angle around each carbon atom is 120.

Question 5

Definition of stereoisomer.

Answer 5

Compounds with same structural formula but different arrangement of the atoms in space.

Question 6

Definition of E/Z isomerism.

Answer 6

Type of stereoisomerism that occurs when there’s a C=C double bond and different groups attached to each carbon of the double bond in a molecule.

Question 7

Definition of cis-trans isomerism.

Answer 7

Special case of E/Z isomerism in which two of the groups attached to each carbon of the double bond are hydrogen atoms.

Question 8

How are compounds classed as E/Z?

Answer 8

E= entgegen, the two higher priority (higher atomic number) atoms are on opposite sides
Z= zusammen, the two higher priority (higher atomic number) atoms are on the same side

Question 9

How are compounds classed as cis/trans?

Answer 9

Cis= the two hydrogen atoms are on the same side
Trans= the two hydrogen atoms are on opposite sides

Question 10

Why are alkenes much more reactive than alkanes?

Answer 10

Presence of a pi bond, which has a lower bond enthalpy than a sigma bond meaning its weaker and therefore broken more easily also the electrons are more exposed because they’re on the outside of the double bond so alkenes can react easily.

Question 11

Conditions for addition reaction of alkenes with hydrogen.

Answer 11

Nickel catalyst and 423K

Question 12

Products of hydrogenation of alkenes.

Answer 12

Alkane

Question 13

Conditions for halogenation of alkenes.

Answer 13

Chlorine/bromine and room temperature.

Question 14

Products of halogenation of alkenes

Answer 14

Dihaloalkane

Question 15

How can an organic compound be tested for unsaturation?

Answer 15

Adding orange bromine water to alkenes makes a colourless solution but there will be no colour change with an alkane.

Question 16

Conditions for addition reaction of alkenes with hydrogen halides.

Answer 16

Hydrogen halide and room temperature.

Question 17

Products of addition reaction of alkenes with hydrogen halides.

Answer 17

Haloalkanes

Question 18

Conditions for hydration reactions of alkenes.

Answer 18

Steam and phosphoric acid catalyst (H3PO4)

Question 19

Products of hydration reactions of alkenes.

Answer 19

Alcohol

Question 20

Definition of an electrophile

Answer 20

Electron pair acceptor

Question 21

Mechanism for when alkenes take part in addition reactions to form saturated compounds.

Answer 21

Electrophilic addition.

Question 22

Why are electrophiles attracted to alkenes.

Answer 22

Double bond has a region of high electron density due to pi bond which attracts electrophiles.

Question 23

Describe the mechanism for but-2-ene and hydrogen bromide.

Answer 23

Bromine more electronegative so HBr is polar and has dipole, bromine - and hydrogen +
Electron pair in pi bond attracted to partially positive H atom, double bond breaks
Bond forms between H atom and carbon atom from double bond
HBr breaks by heterolytic fission, electron pair to bromine
Bromide ion and carbocation formed (+carbon atom)
Br- ion reacts with carbocation to form product.
(Electrophilic addition)

Question 24

Describe the mechanism for propene and bromine

Answer 24

Bromine is non-polar, when it approaches an alkene, the pi bond electrons interact with electron in Br-Br
Polarisation of Br2, one end partially+ and the other partially- (induced dipole)
Electron pair in pi bond attracted to +, double bond breaks
Bond formed between bromine and carbon
Br2 bond breaks by heterolytic fission, electron pair to Br-
Bromide ion and carbocation formed
Br- ion reacts with carbocation to form product

Question 25

What is a primary carbocation?

Answer 25

Positive charge is on a carbon at the end of a chain.

Question 26

What is a secondary carbocation?

Answer 26

Positive charge on a carbon with two carbons attached.

Question 27

How are carbocations more stable?

Answer 27

More alkyl groups attached to + carbon, the ion is more stable due to charge being spread out. Order of stability= tertiary (most), secondary, primary (least)

Question 28

Markownikoff’s rule

Answer 28

When unsymmetrical alkene reacts with an unsymmetrical molecule the hydrogen attaches itself to carbon with more hydrogens and smaller number of carbons. MAJOR PRODUCT

Question 29

What do alkenes undergo to become long saturated chains?

Answer 29

Addition polymerisation.

Question 30

What is a repeat unit?

Answer 30

Specific arrangement of atoms in the polymer molecule that repeats over and over again.

Question 31

Why are polymers used?

Answer 31

They’re readily available, cheap to purchase and unreactive.

Question 32

What are the issues with disposal of polymers?

Answer 32

Many are non-biodegradable and their unreactive nature makes them difficult to dispose of.

Question 33

Why is PVC disposal difficult?

Answer 33

High chlorine content, range of additives present. Landfill is not sustainable. When burnt, hydrogen chloride is released and other pollutants.

Question 34

How is PVC recycled?

Answer 34

Solvents dissolve the polymer. High-grade PVC recovered by precipitation from the solvent, solvent then reused.

Question 35

Why are waste polymers used for fuel?

Answer 35

They are derived from petroleum or natural gas and have high stored energy value.

Question 36

How are waste polymers used for fuel?

Answer 36

They are incinerated to produce heat which generates steam that drives a turbine producing electricity.

Question 37

What is feedstock recycling?

Answer 37

Chemical or thermal processes that can reclaim monomers, gases or oil from waste polymers.

Question 38

How can products of feedstock recycling be used?

Answer 38

Raw materials for the production of new polymers.

Question 39

Advantage of feedstock recycling.

Answer 39

Able to handle unwashed and unsorted polymers.

Question 40

What are bioplastics?

Answer 40

Produced from plant starch, cellulose, plant oils and proteins. Renewable and sustainable alternative to oil-based products.

Question 41

What are biodegradable polymers?

Answer 41

Polymers made form starch/cellulose. Broken down my microorganisms into water, CO2 and biological compounds. Leave no visible or toxic residues.

Question 42

What are photodegradable polymers?

Answer 42

Polymers that contain bonds that are weakened by absorbing light to start degradation.