12.2 Chemical Reactions of Alkanes
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Do alkanes generally have a high or low reactivity?
low
Why do alkanes have a low reactivity?
• CーC + CーH σ-BONDS STRONG • NON-POLAR CーC • NON-POLAR CーH
Which are weaker, non-polar or polar bonds?
polar (covalent bonds break heterolytically when shared pair of electrons are taken by 1 of the bonded atoms. In polar bonds, the shared pair is already closer to one of the bonded atoms. So, polar bonds are closer to breaking than the non-polar and therefore weaker.)
What is the general word equation for the combustion of an alkane?
Alkane + Oxygen ⟶ Carbon Dioxide + Water
What is the general word equation for the incomplete combustion of an alkane?
alkane + oxygen ⟶ carbon monoxide/carbon + water
What are the products of complete combustion of an alkane?
carbon dioxide + water
What are the products of incomplete combustion of an alkane?
carbon dioxide and/or carbon + water (carbon is formed in even lower levels of oxygen)
What are the conditions of combustion?
High temperature High pressure
How do the conditions of complete and incomplete combustion differ?
complete combustion = plentiful O2 incomplete combustion = limited supply of O2, not enough for reactants to form carbon dioxide
What is the general equation for the reaction of alkanes with halogen?
ALKANE + HALOGEN ⟶ HALOALKANE + HYDROGEN HALIDE (using UV light)
Under what conditions do alkanes react with halogens?
UV light
What is the reaction mechanism for the reaction for the halogenation of alkanes?
radical substitution
Methane + Bromine ⟶
Bromomethane + Hydrogen Bromide
Outline the reaction mechanism for the following reaction & label the steps: CH4 + Br2 ⟶ CH3Br + HBr
INITIATION: Br2 ⟶ 2Br• using UV light PROPAGATION: Br• + CH4 ⟶ •CH3 + HBr • CH3 + Br2 ⟶ CH3Br + Br• TERMINATION: 2Br• ⟶ Br2 2•CH3 ⟶ C2H6
What are termination reactions in radical substitution?
reactions which take radicals out of system
The propagation step in the halogenation of alkanes is described as a “chain reaction”. Why?
the Br radical reacts to form new radicals, which allows for further/subsequent reactions to take place.
The propagation step in the halogenation of alkanes can carry on producing products until…
all radicals are taken out of the system.
What is the limitation of using radical substitution to produce a particular isomer haloalkane?
Br radical can be substituted at different points on the chain forming different isomers. or multiple substitutions can occur.
Outline the mechanism for which C3H6Cl2 is converted to C3H5Cl3:
RADICAL SUBSTITUTION INITIATION: Cl2 ⟶ 2Cl• PROPAGATION: C3H6Cl2 + Cl• ⟶ •C3H5Cl2 + HCl • C3H5Cl2 + Cl2 ⟶ C3H5Cl3 + Cl• TERMINATION: 2Cl• ⟶ Cl2 2•C3H5Cl2 ⟶ C6H10Cl4 • C3H5Cl2 + Cl• ⟶ C3H5Cl3