Organic 1 Section 3 - Unit 13: Halogenoalkanes

Question 1

Explain how a single Cl radical can cause the decomposition of many molecules of ozone (1 mark)

Answer 1

Cl• is regenerated and causes a chain reaction in the decomposition of ozone

Question 2

Explain why the halogenoalkane can be attacked by the nucleophile in nucleophilic substitution (3 marks)

Answer 2

• 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

Question 3

State the two equations which show how a single Cl radical can damage the ozone layer (2 marks)

Answer 3

• Cl• + O3 → ClO• + O2
• ClO• + O3 → Cl• + 2O2
• Cl• + O → Cl• + O2

Question 4

Explain why iodomethane has a higher boiling point than fluoromethane even though the electronegativity of iodine is less than that of fluorine (3 marks)

Answer 4

• 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

Question 5

State the initiation equation for free radical substitution with methane and chlorine (1 mark)

Answer 5

Cl2 → 2Cl•

Question 6

State the propagation equations for free radical substitution with methane and chlorine (2 marks)

Answer 6

CH4 + Cl• → HCl + •CH3

•CH3 + Cl2 → Cl• + CH3Cl

Question 7

State the termination equations for free radical substitution with methane and chlorine (3 marks)

Answer 7

Cl• → Cl2
•CH3 + •CH3 → C2H6
•CH3 + Cl• → CH3Cl

Question 8

State the term for the positively charged species formed when ammonia reacts with halogenoalkanes (1 mark)

Answer 8

• Intermediate

Question 9

Define nucleophile (1 mark)

Answer 9

• Electron pair donator

Question 10

Explain a factor that affects the rate of substitution reactions

Answer 10

• 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

Question 11

Change in functional group, reagent and conditions for nucleophilic substitution with aqueous hydroxide ions (3 marks)

Answer 11

• Halogenoalkane → alcohol
• Potassium/Sodium Hydroxide in aqueous solution (OH-)
• Heat under reflux

Question 12

Change in functional group, reagent and conditions for nucleophilic substitution with cyanide ions (3 marks)

Answer 12

• Halogenoalkane → nitrile
• KCN dissolved in ethanol/water mixture (:CN-)
• Heat under reflux

Question 13

Change in functional group, reagent and conditions for nucleophilic substitution with ammonia (3 marks)

Answer 13

• Halogenoalkane → amine
• NH3 dissolved in ethanol (:NH3)
• Heat under pressure

Question 14

Change in functional group, reagent and conditions for elimination with alcoholic hydroxide ions (3 marks)

Answer 14

• Halogenoalkane → alkene
• Potassium/Sodium Hydroxide in ethanol (OH-)
• Heat under reflux

Question 15

State the environmental problem caused by the breakdown of ozone by CFCs (1 mark)

Answer 15

Smog formation