Unreactive: C-C bonds non- polar. C- H non- polar as electronegativity of C and H similar.
• Complete Combustion: Alkanes burn completely with plentiful supply of oxygen to produce
carbon dioxide and water to give out heat. C gives CO2 and half of H gives H2O.
• Incomplete Combustion: In insufficient supply of oxygen, hydrogen atoms in alkane always
oxidised to water. Combustion of carbon incomplete so forms toxic carbon monoxide gas or
carbon (soot/ black smoke).
• Alkanes and Halogens: Fluorine would cause explosion and iodine not reactive enough.
• Methane is CH4 not CH3.
• Radical Substitution: Bromination of Methane. Makes haloalkanes from alkanes. Radicals
are very reactive as the unpaired electron wants to pair up. Must name the three steps.
• Overall equation- CH4 + Br2 -> CH3Br + HBr. To work out overall equation, cancel out the radicals
that appear in both equations of propagation.
1- Initiation (1 step). Using UV radiation, bromine covalent bond broken by homolytic fission to
form two bromine radicals. Br2 -> 2∙Br. IBr -> ∙I + ∙Br.
2- Propagation (2 steps). 1) CH4 + ∙Br -> ∙CH3 + HBr. 2) ∙CH3 + Br2 -> CH3Br + ∙Br. Chain reaction as
after step 2, bromine radical reacts with another CH4 molecule as in step 1. These two steps
cycle.
1) CH4 + ∙Br -> ∙CH3 + HBr. 2) ∙CH3 + IBr -> CH3I + ∙Br
3- Termination (3 steps). Two radicals collide to form molecule which stops reaction.
1) ∙Br + ∙Br -> Br2. 2) ∙CH3 + ∙CH3 -> C2H6. 3) ∙CH3 + ∙Br -> CH3Br.
1) ∙CH3 + ∙Br -> CH3Br. I + I -> I2
• More complicated radical substitution, form many products, so only write the ones questions
mentions, usually the products are formed in both the propagation steps.
• Another product when methane reacts with bromine- CH2Br2.
• Why there are many organic products? Problems of this?
– More than one hydrogen substituted. Forms many compounds. E.g. in propagation, CH3Br can
have H substituted to CH2Br2 due to collision with bromine radical- cycle.
– Lots of termination steps leading to mixture of products.
– Monosubstituted isomers by substitution of halogen at different positions in carbon chain.