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Module 4 Chapter 15 (15.1 The chemistry of haloalkanes (Reactivity (Have a…
Module 4
Chapter 15
15.1 The chemistry of haloalkanes
Naming
Prefix added to identify haloalkane e.g. Chloro bromo iodo
Prefix numbered to show position on carbon chain
Reactivity
Have a carbon-halogen bond
Halogen = more electronegative than carbon so electron pair is closer to the halogen atom making the bond polar
Nucleophile
-species that donate a lone pair of electrons
Carbon has a slightly positive charge and can attract species containing lone pairs (nucleophiles)
Nucleophile examples
- OH- (lone pair on Oxygen), H2O (lone pair on Oxygen), NH3 (lone pair on Nitrogen)
Nucleophilic substitution
- when a nucleophile replaces the halogen in the haloalkane
Hydrolysis
Hydrolysis
- chemical reaction involving water or an aqueous solution of hydroxide that causes the breaking of a bond in a molecule
Hydrolysis of haloalkane- halogen is replaced with OH-
Example
1)
Nucleophile OH- approaches carbon atom with halogen attached on opposite to side of halogen bond (reduces repulsion between nucleophile and δ- halogen)
2)
Lone pair from OH- is attracted and donated to the δ+ carbon
3)
New bond formed between oxygen of OH- and carbon atom
4)
Carbon-halogen bond breaks by heterolytic fission (electrons to halogen)
5)
New organic product is an alcohol. Halide ion also formed
Haloalkane + NaOH (aq) + reflux -> Alcohol + NaHalogen
Hydrolysis and carbon-halogen strength
Rate of hydrolysis depends on strength of carbon-halogen bond
C-F= strongest and C-I = weakest
Iodoalkanes will react quicker due to less energy required to break bond
Bromoalkanes will react faster than chloroalkanes
Fluoroalkanes are unreactive-lots of energy required to break
Rate of hydrolysis of primary haloalkanes
Compare rate of hydrolysis with 1-chlorobutane, 1-bromobutane and 1-iodobutane
General equation-
C4H9X + H20 -> C4H10O +H+ + X-
Carry out reaction in presence of silver nitrate (aq)
As reaction progresses, halide ions are produced that react with Ag+ forming silver halide precipitate
Haloalkanes are insoluble so the reaction is carried out in the presence of an ethanol solvent- allows water and haloalkane to mix
Chloro- white precipitate forms slowly C-Cl is strongest
Bromo- cream precipitate forms
Iodo- Yellow precipitate forms rapidly C-I is weakest
15.2 Organohalogen compounds
Uses
Organohalogen
- molecules containing at least 1 halogen atom joined to a carbon chain
General solvents- CHCl3
Dry cleaning solvent- C2H2Cl2, C2HCl
Making polymers- C2H3Cl , C2F4
Flame retardants- CF3Br
Refrigerants- F2CCl2, HCClF2, HCCl2F
Not found in nature as they are not broken down naturally by the environment
Ozone layer
Found at outer edge of the stratosphere (10-40km above earth's surface)
Ozone layer absorbs UV-B (biologically damaging radiation) from sun so only a small amount reaches earth's surface
UV-B linked with Sunburn, and some cancers
Further depletion will allow more UV-B = increased genetic damage and greater risk of skin cancer
Stratosphere ozone is continuously formed and broken down by UV radiation-
O2-> 2O
- high energy UV breaks oxygen into oxygen radicals
Steady state
- ozone made at same rate as broken down
Making ozone-
O2 + O⇌ O3
- not equilibrium as not closed system
Human activity- mainly production and use of chlorofluorocarbons (CFCs) has upset delicate equilibrium
CFCs and ozone
CFCs are very stable due to strength of carbon-halogen bond
CFCs are stable until they reach the stratosphere when they break down forming Cl radicals that catalyse the breakdown of ozone
Ozone depletion
CFCs have long residence time in troposphere due to stability
Takes them many years to reach stratosphere- once there UV radiation produces sufficient energy to break C-halogen bond
C-Cl bond breaks because it has the lowest bond enthalpy
Radiation initiates the breakdown so process called
photodissociation
Photodissociation- CF2Cl2 -> CF2Cl :black_small_square: + Cl :black_small_square:
Propagation
- Ozone reacts with chlorine radical breaking ozone down into oxygen
Step 1
- Cl :black_small_square: + O3 -> ClO:black_small_square: + O2
Step 2
- ClO:black_small_square:+ O -> Cl:black_small_square: + O2
Overall- O3 +O -> 2O2
Cycle because Cl radical formed which can react with ozone again
Are CFCs completely responsible?
No as other radicals can catalyse the breakdown e.g. Nitrogen oxide radicals (formed during lightning and aircraft) cause breakdown of ozone
Propagation
- Nitrogen
Step 1
- NO:black_small_square: +O3 -> NO2:black_small_square: + O2
Step 2
- NO2:black_small_square: + O -> NO:black_small_square: + O2
Overall - O3 +O -> 2O2
