Chapter 15 Haloalkanes Flashcards
What are haloalkanes
compounds containing carbon hydrogen and at least one halogen
fluoro/ chloro/ bromo/ iodo/
Reactivity of haloalkanes
C-halogen bond is polar as halogen atoms are more electronegative than carbon atoms (electron pair is closer to halogens than carbon)
Carbon atom therefore is slightly positive & can attract species containing a lone pair of electrons
What is a nucleophile?
species that donate a lone pair of electrons
–> an atom or group of atoms that is attracted to an electron deficient carbon atom, where it donates a pair of electrons to form a new covalent bond
e.g. hydroxide ions, water molecules and ammonia molecules
Haloalkane + nucleophile reaction?
nucleophile replaces halogen in substitution reaction
–> new compound has a different functional group
–> nucleophilic substitution
Haloalkane + nucleophile reaction?
nucleophile replaces halogen in substitution reaction
–> new compound has a different functional group
–> nucleophilic substitution
Hydrolysis of haloalkane
halogen atom is replaced by hydroxyl group (nucleophilic substitution)
1. nucleophile OH approaches carbon on opposite side of haloalkane
2.lone pair of electrons from hydroxide ion is attracted and donated to delta positive carbon atom (C-halogen polar bond)
3. New bond is formed between oxygen atom of hydroxide ion and C atom
4. Carbon-halogen bond breaks by heterolytic (towards halogen)
5. New Alcohol is made and a halide ion is formed
What is hydrolysis?
chemical process involving water/ aqueous solution of hydroxide causing molecule to be split into two products
How else can haloalkanes be converted to alcohols
using aqueous sodium hydroxide and heated under reflux
haloalkane + sodium hydroxide –> alcohol + sodium halide
Hydrolysis and carbon-halogen bond strength
C-Halogen bond is broken and hydroxyl group replaces halogen during hydrolysis
–> rate of hydrolysis depends upon strength of C-halogen bond in haloalkane
C-F bond is strongest whilst C-I is weakest (less energy is required to break C-I bond)
–> can conclude that fluoroalkanes are unreactive
Measuring rate of hydrolysis of primary haloalkanes
C4H9X + H2O –> C4H9OH + (H+) + X-
- carry out in presence of aqueous silver nitrate (halide ions react with silver ions to form precipitate
Ag+ (aq) + X-(aq) –> AgX(s)
Results of hydrolysis of haloalkanes
chloroalkane forms a white precipitate very slowly
bromoalkane forms a cream precipitate faster than chloro but slower than iodo
iodoalkane forms a yellow recpipitate rapidly
chloro > bromo > iodo (carbon halogen bond strength)
Hydrolysis of primary/ secondary and tertiary haloalkanes
Tertiary haloalkane is hydrolysed first as a tertiary haloalkane reacts by a two step mechanism
–> 1. C-halogen bond is broken by heterolytic fission, forming a tertiary carbocation & halide ion
–>2. hydroxide ion attacks carbocation to form product
Primary haloalkane will react via a one-step mechanism
Organohalogen compounds
molecules that contain at least one halogen joined to a carbon chain
Ozone layer and UV-B
- found at the outer edge of the stratosphere (10-40km above surface)
- only tiny fraction of gases making up ozone layer is ozone but enough to absorb most biologically damaging UV radiation (UV-B) from sun’s rays
What is UV-B radiation linked to?
radiation linked to sunburns and harmful effects
–>depletion of ozone layer = more UV-B reaching earth = bad for living organisms due to increased genetic damage and greater risk of skin cancer
Formation of Ozone O3
ozone is continually formed and broken by UV radiation(initial very high UV breaks oxygen molecules into oxygen radicals O:
O2–> 2O
Steady state is set up involving O2 and oxygen radicals (rate of formation of ozone= rate at which it is broken down
O2 + O <=> O3
CFCs
Chlorofluorocarbons and HCFCs (used as refrigerants, air con units and aerosol propellants
–> very stable due to strength of carbon halogen bond
–> CFCs remain stable until the stratosphere where they break down into chlorine radicals (these catalyse the breakdown of ozone layer)
How does a CFC deplete the ozone layer
UV radiation in stratosphere is enough to break carbon-halogen bond by homolytic fission to form radicals
Photodissociation:
e.g. CF2Cl2 –> CF2Cl* + Cl*
Propagation steps of photodissociation
- Cl* + O3 –> ClO* + O2
- ClO* + O –> Cl* + O2
Step 2 regenerates a chlorine radical (can remove another molecule of ozone in step 1)
Overall reaction:
O3 + O –> 2O2
Other substances responsible for ozone depletion
Nitrogen oxide radicals (form naturally during lightning strikes and aircraft travel)
Propagation 1: NO* + O3 –> NO2* + O2
2. NO2* + O –> NO* + O2
Overall reaction: O3 + O –> 2O2
