CHAPTER 15: HALOALKANES

Question 1

What is nucleophilic substitution?

Answer 1

• Haloalkanes have polar carbon⎯halogen bonds.
• Halogens are more electronegative than Carbon.
• Electron pair in the bond is closer to the halogen.
• C atom has δ+ charge & halogen has δ- charge.
• δ+ atoms attract nucleophiles.
• Nucleophiles are electron pair donors.
• Nucleophile replaces halogen in substitution reaction.
• New compound made has different functional group.
• This is called Nucleophilic substitution.
• Substitution is when an atom/group of atoms is replaced by another atom/group of atoms.
• Nucleophilic substitution occurs in hydrolysis of primary haloalkanes with aqueous alkali.

Question 2

What happens in hydrolysis of haloalkanes?

Answer 2

• Bonds broken by water/aqueous hydroxide solutions.
Hydrolysis with Aqueous Alkali:
1) OH⁻ nucleophile is attracted to C atom attached to the halogen opposite to the side of the molecule from the hydrogen atom.
2) Direction of attack minimises repulsion between nucleophile & δ- halogen atom.
3) Lone pair of OH⁻ electrons is donated to C atom.
4) New bond formed between O of OH⁻ & C atom.
5) Carbon⎯halogen bond breaks by heterolytic fission.
6) Alcohol & halide ion is formed.

Question 3

What is the trend in rates of hydrolysis of primary haloalkanes?

Answer 3

• Rate of Hydrolysis depends on strength of the carbon⎯halogen bonds.
• Stronger bonds have larger electronegativity differences.
• Hydrolysis of Haloalkanes can occur with water.
• CH₃(CH₂)ₙX + H₂O → CH₃(CH₂)ₙOH + H⁺ + X⁻.
• Rate of reaction is followed with AgNO₃ (aq).
• Halide ions, X⁻, react with Ag⁺ & form silver halides.
• Haloalkanes are insoluble in water.
• Adding ethanol allows water & haloalkanes to mix.
• This gives a single solution rather than 2 layers.
• Time how long it takes for each precipitate to form.
• Iodoalkane reacts fastest as C-I bonds are weakest.

Question 4

How are halogen radicals produced?

Answer 4

• By UV radiation on CFCs in upper atmosphere.
• This catalyses breakdown of Earth’s ozone layer.

Breakdown of Ozone by Cl• :
• O₃ is formed & broken continually by UV radiation.
• UV energy breaks O₂ into O radicals: O₂ → 2O.
• Creates Dynamic equilibrium: O₂ + O ⇌ O₃.
• CFCs upset this dynamic equilibrium.
• UV light provides energy for photodissociation.
• C-Cl bond has lowest bond enthalpy, so it breaks.
• Homolytic fission produces chlorine radicals.

1) Initiation: CF₂Cl₂ → CF₂Cl• + Cl•.
2) Propagation 1: Cl• + O₃ → ClO• + O₂.
3) Propagation 2: ClO• + O → Cl• + O₂.
4) Overall equation: O₃ + O → 2O₂.

Breakdown of Ozone by NO• :
• NO radicals are formed by lightning strikes.
• They catalyse the breakdown of ozone.
1) Propagation 1: NO• + O₃ → NO₂• + O₂.
2) Propagation 2: NO₂• + O → NO• + O₂.
3) Overall equation: O₃ + O → 2O₂.

Question 5

Why were CFCs banned?

Answer 5

• Stable due to strength of carbon⎯halogen bonds.
• Remain stable until they reach stratosphere.
• Break down in stratosphere & form chlorine radicals.
• This catalyses breakdown of ozone layer.
• Ozone layer absorbs UV-B radiation from the Sun.
• UV-B is linked to sunburn & has harmful effects.
• It increases genetic damage & risk of skin cancer.