15 - Haloalkanes

Question 1

reactivity of haloalkanes

Answer 1

• halogen is more electronegative than hydrogen
• so electron pair is closer to the chlorine
  C-X bond is polar, C is delta +and can attract a species containing a lone pair - nucleophile

Question 2

what is nucleophile
- give examples

Answer 2

atom attracted to a electron deficient region
- it donates an electron pair

:OH-
H2O:
:NH3

Question 3

haloaklanes to alcohol

Answer 3

use NaOH ap
- under reflux
- a slow reaction at RTP so the mixture is heated under reflux

Question 4

Hydrolysis of a haloalkane

Answer 4

1) :OH- approaches the carbon attached to the halogen on the opposite side of the halogen
2) this direction of attack minimises repulsion from the delta negative Cl
3)a lone pair of electrons on the hydroxide ion is attracted to the delta positive carbon
4) a new bond is formed between oxygen atom of OH and the carbon
5) the carbon halogen bond breaks by heterolytic fission
6) halide ion is formed and an alcohol

Question 5

Hydrolysis and carbon-halogen bond strength

Answer 5

• in hydrolysis the C-X bond is broken and replaced by and OH
• the rate of hydrolysis depends on the strength of the C-X bond
  C-F is the strongest C-I is the weakest. SO less energy is needed to break the C-I bond
• so hydrolysis of haloalkanes with iodine is faster than those with fluorine
  rate of hydrolysis increases ->
  Cl, Br, I

Question 6

ozone layer

Answer 6

• ozone absorbs the damaging rays of the sun
• ozone is continually being formed and broken bt rhe action of UV radiation

oxygen into oxygen radicals
O2-> 2O

O2 + O <-> O3

Question 7

what are CFCs

Answer 7

• stable compound used ins refrigerants and aerosols
• it is stable because of carbon-halogen bonds
• CFCs remain stable until they reach stratosphere where they break down to form chlorine radicals- which catalyse the breakdown of the ozone layer

Question 8

How do CFCs deplete the ozone layer

Answer 8

• stability is due to the strength of C-X bond, they have a long residence time
• UV radiation provides energy to break the C-X bond in CFCs by homolytic fission to form radicals

Photodissociation
CF2Cl2 -> CF2Cl’ + Cl’

chlorine radical reacts with the ozone molecule breaking it into oxygen

Propagation 1
Cl’ + O3 -> ClO’ + O2

Propagation 2
ClO’ + O’ -> Cl’ + O2

overakk
O3 + O’ -> 2O2

other racidals can act like Cl’, ege NO’