TOPIC 15 AND 16

Question 1

What is the carbon halogen bond?

Answer 1

• haloalkanes are polar and more reactive than alkanes due to the presence of the electronegative halogens
• halogens are generally more electronegative than carbon so the carbon-halogen bond is polar
• The delta/partially positive carbon is electron deficient which means it can be attacked by a nucleophile
• Halogenalkanes undergo substitution reactions with ammonia, hydroxide ions or cyanide ions
• A nucleophile is an electron pair donor- it can be a negative ion or an atom with a lone pair of electrons and therefore donates those electrons
• Nucleophiles are attracted to the nucleus
• OH-, CN- and NH3 are all nucleophile which react with haloalkanes
• Water is a nucleophile too but it reacts slowly

Question 2

What are haloalkanes hydrolysed into?

Answer 2

Form alcohols- halogen is substituted for another atom or group of atoms
Nucleophilic substitution reaction using warm aqueous alkali like sodium or potassium hydroxide
Heated under reflux
Halogen is displaced

Question 3

What haloalkane is hydrolysed the fastest?

Answer 3

• weaker carbon-halogen bond reacts fastest (iodoalkanes)

- fluroalkanes react slowest as their carbon halogen bond is strongest

Question 4

What happens when you mix a haloalkane with water and add silver nitrate solution?

Answer 4

• Mixing a haloalkane with water would result in an alcohol
• adding silver nitrate solution, silver ions react with halide ions to form a silver halide precipitate
• iodoalkanes—>yellow
• chloroalkanes—->white
• bromoalkanes—>cream

Question 5

What are chlorofluorocarbons?

Answer 5

• well known haloalkanes (CFC’s)
• Contain only chlorine, fluorine and carbon since all the hydrogens have been replaced
• Very stable, volatile, non-flammable and non toxic/chemically inert
• They were used in fridges, aerosol cans and making polymers and propellants etc. until scientists realised they were destroying the ozone layer.
• O2 —> 2O
• O2 + O —> O3 (in a reversible reaction)

Question 6

What is the ozone layer?

Answer 6

• acts as a chemical sunscreen
• Absorbs UV radiation which can cause skin cancer/sunburn risk/increased risk of cataracts
• Ozone is formed naturally when an oxygen molecule is broken down into 2 free radicals by ultraviolet radiation (chlorine radicals- chlorine itself is not bonded to another chlorine)
• The free radicals react with the ozone layer and break it down in a process called the ozone depletion process

How did the ozone layer get destroyed?

• holes in the ozone layer meant more harmful UV radiation could enter earth
• These holes formed because CFC’s in the upper atmosphere absorbed UV radiation and split to form chlorine radicals
• The process is called photodissociation as it is initiated by radiation
  
  • A typical CFC has the formula CFCl2
  • UV radiation breaks the covalent bond so each atom takes one electron from the bond

Question 7

What are the alternatives developed by chemists?

Answer 7

• HCFCs and HFCs are used as temporary alternatives to CFCs are used until safer products are developed
• Hydrocarbons also used
• Break down the atmosphere in 10-20 years, but the effect is smaller
• However, these are greenhouse gases which can result in the enhanced greenhouse effect