15.1 Chemistry of Haloalkanes
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What is a haloalkane?
An alkane with at least 1 hydrogen substituted for a halogen
What is a primary haloalkane?
Haloalkane where halogen is bonded to a carbon which is bonded to 1 other carbon atoms.
What is a secondary haloalkane?
Haloalkane where halogen is bonded to a carbon which is bonded to 2 other carbon atoms.
What is a tertiary haloalkane?
Haloalkane where halogen is bonded to a carbon which is bonded to 3 other carbons.
Haloalkanes undergo hydrolysis reactions. Explain how reactivity changes with different haloalkanes:
Lower carbon-halogen bond enthalpy = faster rate of hydrolysis - MORE SHELLS in halogen atom - larger ATOMIC RADIUS - weaker ELECTROSTATIC ATTRACTION between shared pair of electrons + nuclei of bonded atoms - less ENERGY required to break bond - FASTER rate of hydrolysis
Why is there a difference in reactivity of haloalkanes?
BOND ENTHALPY in carbon-halogen bond changes depending on halogen used.
If a haloalkane has a halogen which has a LARGE ATOMIC RADIUS, how reactive would the haloalkane be?
more reactive, weaker bond enthalpy
If a haloalkane has a halogen which has a SMALL ATOMIC RADIUS, how reactive would the haloalkane be?
less reactive, weaker bond enthalpy
Why are fluoroalkanes very unreactive?
fluorine has only 2 shells, small atomic radius»_space; stronger electrostatic attraction between shared pair of electrons + nuclei of bonded atoms»_space; bond enthalpy is too high
What is a nucleophile?
a species ATTRACTED to an ELECTRON-DEFICIENT CENTRE where it DONATES an ELECTRON PAIR forming a NEW COVALENT BOND
What are some different types of nucleophiles?
:OH- H2O: :NH3
Outline the mechanism for nucleophilic substitution:
• NUCLEOPHILE ATTRACTED to 𝛅+ carbon • approaches from opposite 𝛅- halogen»_space; minimise repulsion • LONE PAIR on nucleophile DONATED to 𝛅+ carbon • New COVALENT BOND formed between Carbon + nucleophile. • Carbon-Halogen bond breaks by HETEROLYTIC fission Products = halide + molecule
What is nucleophilic substitution?
Substitution reaction where nucleophile is ATTRACTED to ELECTRON DEFICIENT centre where it DONATES an electron PAIR + REPLACES a group of atoms
How do you measure the rate of hydrolysis of haloalkanes?
• Dissolve haloalkane in ETHANOL. • Put in water bath @ 60°C. Put Silver Nitrate test tube in water bath. • Start stopwatch + add AgNO3 to haloalkane + record time taken for precipitate to form + cross underneath to disappear. • AgX(s)
Do bromoalkanes react faster than iodoalkanes?
no bromoalkanes have a smaller ATOMIC RADIUS (stronger electrostatic attraction) so stronger bond ENTHALPY so slower RATE of reaction.
Do Iodoalkanes react faster than bromoalkanes?
yes Iodialkanes have a bigger ATOMIC RADIUS (weaker electrostatic attraction) so weaker bond ENTHALPY so faster RATE of reaction
What factor(s) determine the rate of hydrolysis of haloalkanes?
atomic radius / bond enthalpy