4.2 - Alkanes

Question 1

What are alkanes mainly used for

Answer 1

Fules

Question 2

What is a single covalent bond known as

Answer 2

A sigma bond

Question 3

What is a sigma bond

Answer 3

the overlap of orbitals directly between the bonding atoms

A sigma bond is positioned on a line directly between bonding atoms

Question 4

What is the shape around a carbon atom in alkanes and bond angle

Answer 4

Teterahedral
109.5

Question 5

Explain the BP of alkanes

Answer 5

• Boiling point increases as chain length increases as with bigger chains there are more London forces therefore higher bp

Question 6

Explain the effect of chain length on boiling point

Answer 6

• London forces act between molecules that are in close surface contact
  
  • Bigger chain length = more SA = greater London forces = more energy required to overcome forces

Question 7

What happens to the bp of an alkanes when there is branching

Answer 7

• Isomers of alkanes have the same Mr but branched isomers have lower BP than straight chain isomers
  
  • There are fewer surface points of contact leading to less London forces leading to lower BP
  • Another reason is the shape of the molecules - branches get in the way and prevent the branched molecules getting as close to one another decreasing intermolecular forces further

Question 8

Why are alkanes not very reactive

Answer 8

• C-C and C-H sigma bonds are strong
  
  • C-C bonds are non-polar (no electronegativity)
  • C-H bond is also considered as non polar due to low electronegativity

Question 9

What is needed for the reaction of alkanes with halogens

Answer 9

UV radiation

Question 10

Alkanes + halogen->

Answer 10

Haloalkane + hydrogenhalide

Question 11

What mechanism do alkanes use

Answer 11

Radical substitution

Question 12

What are the 3 stages in radical substitution

Answer 12

Initiation
Propagation
Termination

Question 13

What happens in the initiation step of radical substitution

Answer 13

The covalent bond in a halogen molecule is broken by homolytic fission

Each of the haloge (br2) atoms take a electron and form 2 halogen radicals

uv is needed for the bond fission

Question 14

What happens in the propagation step of radical substitution

Answer 14

• Step 1:
  ○ A bromine radical reacts with a C-H bond in methane forming a methyl radical and a molecule of hydrogen bromide
  • Step 2:
    ○ Each methyl radical reacts with another bromine molecule forming the organic product bromomethane together with another bromine radical

Question 15

What happens in the termination step how many possibilities are there

Answer 15

2 radicals collide forming a molecule
3 possibilities

Question 16

What are the 2 limitations of radical substitution (4 marks)

Answer 16

Further substitution
- If CH3Br the final product collides with another bromine radical another hydrogen atom will be substituted forming a new molecule. This can continue until all hydrogen atoms have been replaced

Substitution at different points in a carbon chain
- With carbon chains of 3 or more you will get isomers formed