Haloalkanes

Question 1

Reaction 1 occurs via a nucleophilic substitution mechanism.
Explain why the halogenoalkane is attacked by the nucleophile in this reaction.

Answer 1

• Bromine is more electronegative than carbon
• C is partially positive / electron deficient M2
• Lone/electron pair (on the nucleophile) donated to the partially positive carbon

Question 2

identify the radical produced from this CFC that is responsible for the depletion of ozone in the atmosphere.
Explain, with the aid of equations, why a single radical can cause the decomposition of many molecules of ozone.

Answer 2

radical = Cl.

Cl. + O3 –> ClO. +O2

ClO.+O3 —> Cl.+2O2

Cl. is regenerated (and causes a chain reaction in the decomposition of ozone)

Question 3

state why ozone layer is beneficial for organisms

Answer 3

absorbs/prevents harmful UV

Question 4

state how CFCS form chlorine atoms in upper atmosphere

Answer 4

C-Cl bonds broken

Question 5

Give equations to show how chlorine atoms catalyse the decomposition of ozone. [2 marks]

Answer 5

Cl.+O3 → ClO.+O2

ClO. + O3 → Cl. +2O2

Question 6

Hydrochlorofluorocarbons (HCFCs) have been used in place of CFCs.
In the mechanism to make an HCFC from a fluoroalkane, two incomplete steps are shown.
Complete each step in the mechanism.

give the name of the type of step shown by these equations (3 marks)

equation 1

—————– —> .CHF2 + HCl

equation 2
.CHF2 + Cl2 —> ——————

Answer 6

equation 1

CH2F2 + Cl. —> .CHF2 + HCl

equation 2
.CHF2 + Cl2 —> Cl. + CHF3

name: propagation

Question 7

pentane is used as a refrigerant. state environmental problem chlorodifluoromethane can cause. give one reason pentane does not cause this problem.

Answer 7

environmental problem
- causes ozone depletion

why pentane does not cause this problem
- pentane does not have C-Cl bonds