Alkanes

Question 1

Define aliphatic

Answer 1

A compound containing carbon and hydrogen joined together in straight chains, branched chains, and non-aromatic rings

Question 2

Define homologous series

Answer 2

A family of organic compounds having the same functional group but with each successive member differing by CH₂

Question 3

Define functional group

Answer 3

A group of atoms responsible for the characteristic reactions of a compound

Question 4

Define alicyclic

Answer 4

An aliphatic compound arranged in non-aromatic rings (with or without side chains)

Question 5

Define aromatic

Answer 5

A compound containing a benzene ring

Question 6

Define saturated

Answer 6

A compound containing only C-C single bonds

Question 7

Define unsaturated

Answer 7

A compound containing at least one mutiple C-C bond

Question 8

What is the general formula of an alkane?

Answer 8

C₂H_2n+2

Question 9

Define structural isomer

Answer 9

Compounds with the same molecular formula but different structural formula

Question 10

What are the 3 ways structural isomerism can occur?

Answer 10

• chain isomers
• position isomers
• functional group isomers

Question 11

Define hydrocarbon

Answer 11

A compound containing only carbon and hydrogen atoms

Question 12

Are alkanes saturated or unsaturated hydrocarbons?

Answer 12

Saturated

Question 13

What shape forms around the carbon atoms in an alkane?

Answer 13

Tetrahedral

Question 14

What 2 factors affect the boiling points of alkanes?

Answer 14

• chain length
• extent of branching

Question 15

As branch length increases, what happens to the boiling point?

Answer 15

Boiling point increases

Question 16

Why does boiling point increase with chain length?

Answer 16

• larger chain length means greater contact area betwen molecules
• meaning London forces between molecules will be greater
• so more energy is required to overcome the forces

Question 17

What happens to the boiling point as branching increases?

Answer 17

Boiling point decreases

Question 18

Why does boiling point decrease as branching increases?

Answer 18

• the more branched a molecule, the lower the contact area
• weaker London forces between molecules
• less energy is needed to overcome the intermolecular forces

Question 19

Why are alkanes relatively unreactive?

Answer 19

C-C bonds are non-polar

C-C and C-H bonds are strong

C-H bonds are non-polar as the elements have similar Pauling values

Question 20

What two reactions do alkanes undergo?

Answer 20

• combustion with oxygen
• reaction with halogens to form haloalkanes

Question 21

How can you tell complete combustion is occurring?

Answer 21

A blue flame is seen

Question 22

How can you tell incomplete combustion is occurring?

Answer 22

A yellow flame is seen and/or the formation of soot

Question 23

Give the equation for the complete combustion of methane

Answer 23

CH₄ + 2O2 → 2H₂O + CO₂

Question 24

When does incomplete combustion occur?

Answer 24

When there is insufficient oxygen

Question 25

Why is incomplete combustion dangerous?

Answer 25

It produces CO which is a toxic gas

Question 26

What type of reaction is the reaction between alkanes and halogens?

Answer 26

Free radical substitution

Question 27

What are the conditions required for the reaction between alkanes and halogens?

Answer 27

UV radiation

Question 28

Define radical

Answer 28

A species with an unpaired electron

Question 29

Define homolytic bond fission

Answer 29

The breaking of a COVALENT bond in which one of the bonded electrons goes to each atom, forming 2 radicals

Question 30

What are the 3 stages of free radical substitution called?

Answer 30

Initiation, propagation, termination

Question 31

Why does homolytic fission produce radicals?

Answer 31

The bond is split so the electrons are shared equally, i.e both atoms end up with an unpaired electron

Question 32

Why is UV light required for the production of radicals in initiation?

Answer 32

• provides activation energy
• to break the halogen bond homolytically

Question 33

What is the key feature of initiation in terms of radicals?

Answer 33

Radicals are generated

Question 34

What is the key feature of propagation in terms of radicals?

Answer 34

For every radical used in a reaction, a new one is produced

Question 35

What is the key feature of termination in terms of radicals?

Answer 35

Two radicals react together to form a compound which doesn’t contain a radical

Question 36

Explain why there is more than one product produced by free radical substitution

Answer 36

• Substitution of any of the hydrogens in the chain can occur
• Multiple substitutions can occur

Question 37

What is the overall equation for the reaction of C₅H₁₂ with Br₂ to produce C₅H₁₁Br

Answer 37

C₅H₁₂ + Br₂ →C₅H₁₁Br + HBr

Question 38

What are the 3 limitations of using free radical substitution in organic synthesis?

Answer 38

• Carbon chain length may vary
• Further substitution can occur
• Substitution at different positions of the carbon chain can occur

Question 39

What is the simplest alkane with at least one structural isomer?

Answer 39

Butane

Question 40

Why doesn’t the reaction of electrophilic substitution occur in the presence of visible light?

Answer 40

Visible light does not provide enough energy to homolytically break the halogen bond