3.3 Halogenoalkanes

Question 1

How does the chlorine free radical atoms catalyse the

decomposition of ozone?

Answer 1

Cl• + O3 → ClO• + O2
ClO• + O3 → 2O2 + Cl•
Overall equation:
2 O3 → 3 O2

Question 2

Why were CFCs replaced by HFCs as fridge refrigerant?

Answer 2

• HFCs (Hydro fluoro carbons) e.g. CH2FCF3 are now used for refrigerators and air-conditioners. These are safer as they do not contain the C-Cl bond.
• The C-F bond is stronger than the C-Cl bond and is not affected by UV

Question 3

What are the conditions for Elimination reaction of halogenoalkanes with alcholic hydroxide ions

Answer 3

• Change in functional group: halogenoalkane → alkene
• Reagents: Potassium (or sodium) hydroxide
• Conditions: In ethanol and Heat
• Mechanism: Elimination
• Type of reagent: Base, OH-

Question 4

What compound will react in a nucleophilic substitution faster, a compound containing the bond C–Cl or compound containing C–Br

Answer 4

The compound with the bond C—Br because the C—Br bond is weaker than the C—Cl bond thus the C—Br bond breaks more easily.

Question 5

What is the role of chlorine atoms in the depletion of Ozone (O3)

Answer 5

It acts as a catalyst

Question 6

What is the meaning of the term structural isomer

Answer 6

Same molecular formula but different structural/displayed formula

Question 7

Give one reason why bonds in molecules such as carbon dioxide absorb infrared radiation.

Answer 7

the bonds vibrate/stretch/bend or bonds are polar

Question 8

How do you form an alcohol from a halogenoalkane, and what are the conditions and reagents?

Answer 8

Reagent: potassium (or sodium) hydroxide
Conditions: In aqueous solution; Heat under reflux
Mechanism: Nucleophilic Substitution
Type of reagent: Nucleophile, OH-

Question 9

How do you form a nitrile from a halogenoalkane, what are the conditions and reagents?

Answer 9

Reagent: KCN dissolved in ethanol/water mixture
Conditions: Heating under reflux
Mechanism: Nucleophilic Substitution
Type of reagent: Nucleophile, :CN-

Question 10

How do you form an amine from a halogenoalkane, what are the conditions and reagents?

Answer 10

Reagent: NH3 dissolved in ethanol
Conditions: Heating under pressure (in a sealed
tube)
Mechanism: Nucleophilic substitution Type of reagent: Nucleophile, :NH3

Question 11

How do you form an alkene from a halogenoalkane, what are the conditions and reagents?

Answer 11

Reagents: Potassium (or sodium) hydroxide In ethanol
Conditions: Heat
Mechanism: Elimination
Type of reagent: Base, OH-