Organic 1 Section 3 - Unit 13: Halogenoalkanes
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Explain how a single Cl radical can cause the decomposition of many molecules of ozone (1 mark)
Cl• is regenerated and causes a chain reaction in the decomposition of ozone
Explain why the halogenoalkane can be attacked by the nucleophile in nucleophilic substitution (3 marks)
• The halogen is more electronegative than carbon • The carbon is partially positive • So a lone electron pair on the nucleophile is donated to the partially positive carbon and therefore the halogenoalkane is attacked
State the two equations which show how a single Cl radical can damage the ozone layer (2 marks)
• Cl• + O3 → ClO• + O2 • ClO• + O3 → Cl• + 2O2 • Cl• + O → Cl• + O2
Explain why iodomethane has a higher boiling point than fluoromethane even though the electronegativity of iodine is less than that of fluorine (3 marks)
• Iodine is bigger than fluorine so the van der Waals forces between CH3I molecules are stronger than those between CH3F molecules • The dipole-dipole forces between CH3F molecules are stronger than those between CH3I molecules • However the van der Waals forces are stronger than the dipole-dipole forces so these dominate
State the initiation equation for free radical substitution with methane and chlorine (1 mark)
Cl2 → 2Cl•
State the propagation equations for free radical substitution with methane and chlorine (2 marks)
CH4 + Cl• → HCl + •CH3 • CH3 + Cl2 → Cl• + CH3Cl
State the termination equations for free radical substitution with methane and chlorine (3 marks)
Cl• → Cl2 • CH3 + •CH3 → C2H6 • CH3 + Cl• → CH3Cl
State the term for the positively charged species formed when ammonia reacts with halogenoalkanes (1 mark)
• Intermediate
Define nucleophile (1 mark)
• Electron pair donator
Explain a factor that affects the rate of substitution reactions
• The halogen in the bond • The bond enthalpy of the halogen decreases down the group so the C-X bond is weaker • Iodoalkanes are therefore the fastest to substitute
Change in functional group, reagent and conditions for nucleophilic substitution with aqueous hydroxide ions (3 marks)
• Halogenoalkane → alcohol • Potassium/Sodium Hydroxide in aqueous solution (OH-) • Heat under reflux
Change in functional group, reagent and conditions for nucleophilic substitution with cyanide ions (3 marks)
• Halogenoalkane → nitrile • KCN dissolved in ethanol/water mixture (:CN-) • Heat under reflux
Change in functional group, reagent and conditions for nucleophilic substitution with ammonia (3 marks)
• Halogenoalkane → amine • NH3 dissolved in ethanol (:NH3) • Heat under pressure
Change in functional group, reagent and conditions for elimination with alcoholic hydroxide ions (3 marks)
• Halogenoalkane → alkene • Potassium/Sodium Hydroxide in ethanol (OH-) • Heat under reflux
State the environmental problem caused by the breakdown of ozone by CFCs (1 mark)
Smog formation