OZ - Reaction mechanisms *01 (and sustainability) Flashcards
what is a substitution reaction?
when a functional group in a compound is replaced by another functional group
what is a nucleophile?
something that is attracted to the slightly +ve carbon atom
is usually -ve
What does the relative reactivities of haloalkanes depend on?
The carbon halogen bond strength determines reactivity
- decreases down group
C-F bond the strongest (despite being the most polar) as it has the highest bond enthaply
What can you test the relative reactivities of the haloalkanes?
react the haloalkanes with water in the presence of silver nitrate
- see which reacts the fastest.
- put a chloroalkane, bromoalkane, and iodoalkane in three different test tubes
- add silver nitrate solution and some ethanol
Ag+ + X- —> AgX (s)
- the silver halide forms a precipitate
- the precipitate forms fastest with iodoalkane - so must be the most reactive
what is bond fission?
breaking a covalent bond
bond fission - heterolytic
two different substances are formed
- a positively charged cation
- a negatively charged anion
bond fission - homolytic
two electrically uncharged radicals are formed
eg.
X-Y —> X* + Y*
what are chain reactions?
When a product causes more reactions to take place
chain reactions - initiation
free radicals are produced
chain reactions - propagation
free radicals react with molecules and produce new radicals
these go on to react with more molecules, producing even more radicals
chain reactions - termination
two radicals react together to form a stable molecule
Radical mechanism for reaction of alkanes with halogen - initiation
- sunlight provides enough energy to break the Cl-Cl bond - photodissociation
- the bind splits equally - homolytic fission
- the molecule form two highly reactive free radicals
Cl-Cl —UV—> Cl* + Cl*
Radical mechanism for reaction of alkanes with halogen - propagation
- Cl* attacks a methane molecule, forming a hydrogen halide and a methyl radical
Cl* + CH4 –> HCl + *CH3
- the new methly radical, *CH3, can attack another Cl2 molecule
CH3 + Cl-Cl —–> CH3Cl + Cl
the new Cl* can attack another CH4 and so on until all the Cl2 and CH4 molecules are wiped out
Radical mechanism for reaction of alkanes with halogen - termination
- if two free radicals join together they make a stable molecule
- there are lots of possible termination reactions
eg.
* CH3 + Cl* —–> CH3Cl
*CH3 + *CH3 —> C3H6
what is ozone?
in the stratosphere
contains ozone molecules, O3
How is Ozone formed?
when UV radiation from the sun hits oxygen molecules
O2 + hV —-> O + O
O + O2 —-> O3
how is the ozone layer constantly being replaced?
O2 + O < —> O3
it is a reversible reaction
ozone is continuously being destroyed and replaced as UV hit the molecules
an equilibrium is set up so concentrations stay fairly constant
Why is ozone important?
protects us from UV radiation
- UVB can damage DNA in cells and cause skin cancer (also sunburn)
however in small amounts UV needed
- vitamin D
Ozone at ground level (troposphere) causes and effects
due to the effect of sunlight on mixtures of nitrogen dioxide and hydrocarbons
- from vehicle engine and power stations
in industrialised areas ozone mixes with solid particles of carbon to create PHOTOCHEMICAL SMOG
- can cause respiratory problems
- damages plants and materials
- ozone itself is toxic to humans
How is Ozone broken down in the stratosphere
by CFCs (chlorofluorocarbons) - haloalkanes that have all their hydrogens replaced by Cl and F atoms
the C-Cl bond can be broken down by high energy UV in the stratosphere to form chlorine radicals
- these act as catalysts in the breakdown of ozone
ease of photodissociation of haloalkanes
all haloalkanes have carbon and halogen atoms - UV can break these bonds
- the carbon-halogen bonds split homolytically to create two free radicals
the ease it is broken depends on the halogen
- carbon-iodine most likely to break because it has the lowest bond enthalpy
- carbon-fluorine lest likely to break because it has the highest bond enthalpy
How is the ozone layer destroyed by homogeneous catalysis
Chlorine free radicals formed when CFCs are broken down by high energy UV in the stratoshpere
- these free radicals are catalysts
INITATION - CCl3F3 (g) + hv —> CCl2F3 (g) + Cl (g)
These radicals react with ozone to form an intermediate ClO* and an oxygen molecule
PROPAGATION :
Cl* + O3 —> O2 + ClO*
ClO* + O —-> O2 + Cl* (the Cl* radical is regenerates and goes onto destroy more ozone)
the reaction will only terminate if two radicals react together
TERMINATION: Cl* + Cl* —> Cl2
OVERALL REACTION: O3 + O —-> 2O2 ….. and Cl* is the catalyst
what other free radical can destroy ozone?
NO from nitrogen oxides
eg. NO2 + hv —> NO* + O
