Chapter 13 (4.1.3)

Question 1

What are alkenes?

Answer 1

Unsaturated hydrocarbons containing a C=C bond comprising a π-bond and a σ-bond.

Question 2

What is a π-bond?

Answer 2

Sideways overlap of adjacent p-orbitals above and below the bonding C atoms. It locks them in position, preventing rotation. (know how to draw) (3/4 C electrons involved in 3 σ-bonds, 1 in a p-orbital to the π-bond)

Question 3

Shape and angle of an alkene

Answer 3

Trigonal planar around each C atom of the double bond: since 3 regions of electron density repel each other as far apart as possible so bond angle is 120°, all atoms in the same plane.

Question 4

What is a stereoisomer?

Answer 4

A compound with the same structural formula but with a different arrangement of the atoms in space.

Question 5

E/Z isomerism conditions

Answer 5

• a C=C double bond
• there are different groups attached to each carbon atom of the double bond
  (restricted rotation about a double bond so groups attached to carbon are fixed relative to each other)

Question 6

Cis/trans isomerism conditions

Answer 6

• must satisfy E/Z these conditions

- one of the attached groups on each carbon atom of the double bond must be the same

Question 7

cis/trans e/z relation

Answer 7

cis=Z (same side group)

trans=E

Question 8

CIP priority groups

Answer 8

• priority given based on atomic number: higher atomic number = higher priority
• if groups of higher priority are on same side of double bond, compound is a the Z isomer
• if groups of higher priority are diagonally placed across the double bond, the compound is the E isomer

Question 9

Reactivity of alkenes

Answer 9

• π-bond on the outside of the double bond so these electrons are more exposed than electrons in the σ-bond
• so π-bond readily breaks + alkenes undergo addition reactions relatively easily
• π-bond has a lower bond enthalpy so breaks more readily than the σ-bond

Question 10

What happens in alkene addition reactions?

Answer 10

the addition of a small molecule across the double bond, causing the π-bond to break + for new bonds to form

Question 11

Hydrogenation

Answer 11

• alkene mixed with hydrogen + passed over a nickel catalyst 423K, an alkane is produced via an addition reaction
• hydrogen added across a double bond (all C=C bonds react with hydrogen in this way)

Question 12

Halogenation

Answer 12

• rapid addition with halogens to form dihaloalkanes at room temperature

Question 13

Hydrohalogenation

Answer 13

• alkenes react with gaseous hydrogen halides at room temperature to form haloalkanes
• if alkene is a gas, both gases are mixed. if alkene is a liquid, hydrogen halide is bubbled through it
• or solutions of hydrogen halides in water

Question 14

Hydration

Answer 14

• alkenes react with steam, H2O (g), in the presence of a phosphoric acids catalyst H3PO4 to form alcohols.
• steam adds across the double bond
• (or conc H2SO4)

Question 15

Test for unsaturation

Answer 15

When bromine water (orange) is added dropwise to a sample of alkene, bromine adds across the double bond. Orange colour disappears, indicating the presence of a C=C bond. No addition reaction and no colour change occurs is the test is carried out for a saturated compound. Any compound containing a C=C bond will decolourise bromine water.

Question 16

What is an electrophile?

Answer 16

An electron pair acceptor

Question 17

What is electrophilic addition?

Answer 17

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

Question 18

Why do alkenes undergo electrophilic addition?

Answer 18

The high electron density of the π-electrons attracts electrophiles.

Question 19

examples of electrophiles

Answer 19

Usually a positive ion or a molecule containing an atom with a partial positive charge, e.g. H 𝛿+

Question 20

Mechanism for electrophilic addition (e.g. for but-2-ene and hydrogen bromide) (must know the drawing - explanation is an aid)

Answer 20

1. HBr is polar = H𝛿+ — Br𝛿-
2. electron pair in π-bond is attracted to partially positive hydrogen atom, causing the double bond to break
3. bond forms between hydrogen of the H-Br molecule and a carbon atom that was part of the double bond
4. H-Br bond breaks by heterolytic fission, Br getting the electron pair
5. Br- ion and carbocation formed
6. Br- reacts with carbocation to form final product

Question 21

Electrophilic addition with non-polar molecules e.g. Br2

Answer 21

When bromine approaches an alkene, the π-electrons interact with the electrons in the Br-Br bond. This interactions causes polarisation of the Br-Br bond, with one end of the molecule becoming Br𝛿- and the other Br𝛿+, an induced dipole. Then same mechanism.

Question 22

Markownikoff’s rule

Answer 22

When a hydrogen halide reacts with an unsymmetrical alkene, the hydrogen of the hydrogen halide attaches itself to the carbon atom of the alkene with the greater number of hydrogen atoms and the smaller number of carbon atoms. This is the major product.

Question 23

What is a carbocation?

Answer 23

An ion that contains a positively charged carbon atom.

Question 24

Different carbocations

Answer 24

Classified by no. of alkyl groups attached to the positively charged carbon atom:
Primary: 1 alkyl group 2 hydrogens
Secondary: 2 alkyl groups, 1 hydrogen
Tertiary: 3 alkyl groups, 0 hydrogens

Question 25

Stability of carbocations

Answer 25

Tertiary carbocations = most stable, primary carbocations = least stable

Question 26

What is a polymer?

Answer 26

Large molecules formed from thousands of repeat units of smaller molecules called monomers

Question 27

What is addition polymerisation?

Answer 27

What unsaturated alkene molecules undergo to produce long saturated chains containing no double bonds.

Question 28

Conditions for addition polymerisation

Answer 28

In industry and high temperatures and pressures with catalysts.

Question 29

What is a repeat unit?

Answer 29

The specific arrangement of atoms in a polymer molecule that repeats over and over, written in square brackets with an n (know how to do addition polymerisation equation).

Question 30

Benefits for sustainability of processing waste polymers

Answer 30

• combustion for energy production: incineration to produce heat
• use as an organic feedstock for the production of plastics and other organic chemicals: using chemical and thermal processes to reclaim monomers, gases, or oil
• removal of toxic waste product e.g. PVC produces corrosive HCl gas when combusted. solvent can be used to dissolve the polymer, to prevent gas being produced.

Question 31

Benefits to the environment of development of biodegradable and photodegradable polymers

Answer 31

• biodegradable polymers are made from starch or cellulose (or additives) and are broken down by microorganisms into water, carbon dioxide + biological compounds
• contain bonds that are weakened by absorbing light to start degradation, or light absorbing additives used.