12.2 Chemical Reactions Of The Alkanes Flashcards
Do alkanes have a high or low reactivity?
Low reactivity
Why do alkanes have a low reactivity?
The C-C and C-H sigma bonds are very strong and hard to break. These bonds have high bond enthalpy
The C-C and C-H binds are non-polar. There is little difference in electronegativity between C and H atoms
Why do alkanes make good fuels?
Because they react exothermically in oxygen to release heat energy
What are the products from alkanes in complete combustion?
Carbon dioxide and water
What are the products from alkanes in incomplete combustion?
Water and carbon monoxide
What happens in a reaction between an alkane and a halogen?
Radical substitution will occur
What is a radical?
A species with an unpaired electron
What are the three steps of radical substitution?
Initiation
Propagation
Termination
What happens in initiation? (Using the example CH4 + Cl2)
UV radiation provides energy for homolytic fission of chlorine molecules,
Producing highly reactive chlorine radicals
e.g. Cl2 —> Cl* + Cl*
What happens in propagation reaction 1?
Chlorine radicals react with methane to form HCL and methyl radicals
Cl* + CH4 —> HCl + CH3*
What happens in propagation reaction 2?
Methyl radicals react with chlorine to form CH3Cl and a chlorine radical
CH3 + Cl2 —> CH3 + Cl
What happens in termination?
Two radicals react together to form a new molecule.
The reaction stops there as there is no longer a constant supply of radicals
There are three possible termination reactions involving Cl* and *CH3
