Chapter 15 (Haloalkanes)

Question 1

what is the reactivity of Haloalkanes compared to alkanes?

Answer 1

• haloalkanes are much more reactive than alkanes due to the electronegative halogens, the C-halogen bond is polar causing a partial positive and negative charge

Question 2

what type of reactions do haloalkanes undergo ?

Answer 2

• nucleophilic substitution reactions: a halogen is substituted for another atom or group of atoms
• elimination reactions: a hydrogen haliide is elimnated, the key product are alkenes

Question 3

What is a nucleophile ?

Answer 3

A species that donates a lone pair of electrons, an atom or group of atoms that is attracted to an electron deficient carbon atom

Question 4

What is hydrolysis ?

Answer 4

• it is a reaction involving water or an aqueous solution of OH ion that causes the breaking of a bond in a molecule

Question 5

What is mechanism of the hydrolysis of haloalkanes ?

Answer 5

• the reaction is slow, so you need to heat under reflux to get a better yeild

Question 6

What determines the rate of hydrolysis ?

Answer 6

• the rate is dependant on the strength of the carbon-halogen bond
• the strongest bond is C-F due to the great difference in electronegativity

Question 7

What is an organohalogen compound ?

Answer 7

• organohalogen compounds are compounds that contain at least one halogen atom joined to carbon chain

Question 8

What happens in the stratosphere ?

ozone

Answer 8

• Oxone is continuously being made and broken by UV
• very high energy UV breaks O2 to form oxygen radicals
• A steady state forms - rate of formation of ozone = rate at the breaking down for ozone
• O₂ + O -> O₃

Question 9

What happens with CFCs and the Ozone?

Answer 9

• CFC’s - Chlorofluorocarbons
• CFC’s remain stable until the stratosphere where they break down, forming Chlorine radicals
• these catalyse the breakdown of the ozone layer

Question 10

What is the reactions that happen in the ozone with CFC’s ?

Answer 10

• due to the stability of CFC’s, they can spend a long time in the troposphere
• once they reach the stratosphere, once here the UV gives the necessary energy to break C-halogen -> homolytic fission
• C-Cl has lowest bond enthalpy so breaks first
• photodissociation radiation starts the breakdown

Question 11

What are the radical steps in the ozone ?

Answer 11

CF₂Cl₂ -> CF₂Cl (radical) + Cl (radical)
- chlorine radical is formed which is very reactive
- it will react with an ozone molecule:
1) Cl (radical) + O₃ -> ClO(radical) + O₂
2) ClO (radical) + O -> Cl (radical) + O₂
3) overall: O₃ + O -> 2O₂

Question 12

What are the other radicals that catalyse the breakdown of ozone ?

Answer 12

• Nitrogen dioxide radical are formed naturally during a lighting strike and aircraft in stratosphere
• NO radical causes breakdown of ozone
  1) NO(radical) + O₃ -> NO₂ (radical) + O₂
  2) NO₂(radical) + O -> NO(radical) + O₂
  3) overall: O₃ + O -> 2O₂