Chapter 12: Alkanes

Question 1

Why dont alkanes react with most reagents?

Answer 1

Have strong single C–C and C–H sigma bonds and C–C/C–H bonds are non polar

Question 2

What is the definition of an alkane?

Answer 2

It is a saturated hydrocarbon containing single C–C and C–H bonds as sigma bonds

Question 3

What is a sigma bond?

Answer 3

The overlap of orbitals directly between the bonding atoms

Question 4

What is the molecular shape and bond angle around each carbon atom?

Answer 4

Tetrahedral shape

109.5⁰

Question 5

How does increasing the number of branches impact the boiling point of an alkane? Why?

Answer 5

Boiling point decreases
Less surface area / contact points for intermolecular forces because branches prevent molecules getting close together
Less total strength of attraction between alkane chains
Less energy / heat needed to break these forces

Question 6

How does the length of the carbon chain impact the boiling point of an alkane?

Answer 6

Boiling point increases
More surface area / contact points for intermolecular forces
More total strength of attraction (london forces) between alkane chains
More energy / heat needed to break all the forces

Question 7

Why are alkanes chemically unreactive?

Answer 7

Do not have any functional groups to take part in chemical reavtions

Question 8

What are the products of complete combustion?

Answer 8

Carbon dioxide
Water
Energy (as heat)

Question 9

What reactions do alkanes take part in?

Answer 9

Combustion

Halogenation

Question 10

What are the conditions needed for complete combustion?

Answer 10

Plentiful supply of Oxygen

Question 11

What are the products of incomplete combustion?

Answer 11

Carbon monoxide
Water
Energy as heat
Sometimes just carbon (as soot)

Question 12

What are the causes of the incomplete combustion of alkanes?

Answer 12

Closed system

Poor ventilation

Question 13

What are the three steps of halogenation?

Answer 13

Initiation
Propagation
Termination

Question 14

What kind of reaction halogentaion an example of?

Answer 14

Radical substitution

Question 15

What happens during Initiation?

Answer 15

Halogen molecule absorbs UV radiation

Halogen molecule undergoes homolytic fission to produce 2 reactive halogen radicals

Question 16

What conditions are required for Halogenation?

Answer 16

UV radiation

Question 17

What occurs during propagation?

Answer 17

A 2 step chain reaction:

1. A halogen radical attacks a H atom of an alkane to produce an alkane radical and a hydrogen halide compound
2. The alkane radical attacks another halogen molecule to produce a haloalkane compound and a halogen radical

Question 18

What happens during termination?

Answer 18

2 radicals run into each other
Unpaired electrons pair up to form a covalent bond
Results in stable molecules then radicala are taken out

Question 19

What are the limitations of radical substitution?

Answer 19

Further substitutions

Reactions at different positions in a carbon chain