Chapter 12 - Alkanes

Main components of natural gas and crude oil and used as fuels Homologous series of saturated hydrocarbons with the general formula CnH2n+2a

Very unreactive but do burn and react with halogens

• C-C and C-H bonds are strong • C-C and C-H bonds are non polar (similar electronegativity so no partial charges to attract any other molecules)

Z isomer in E isomers the polar bonds cancel out as they act in opposite directions

• longer chains means higher boiling point due to stronger London forces between molecules (more electrons in the molecule) • for isomers the more branched carbon chain has the lower boiling point as they have weaker London forces (molecules have less surface contact) THEN TALK ABT ENERGY IN RELATION TO BP

Separation by fractional distillation as different hydrocarbons within crude oil have different boiling points

• becomes more viscous • harder to ignite • less volatile (ability to turn into a gas) • higher boiling points

Thermal decomposition of longer alkanes to form smaller alkanes

Alkane + O2 → CO2 + H2O Highly exothermic, with enough oxygen

Alkane + O2 -> CO + H2O Alkane + O2 -> C + H2O

CO2 - greenhouse gas -> burn less fossil fuels CO - toxic -> ensure good supply of oxygen when burning fossil fuels C - respiratory problems, global dimming, blackens building -> ensure good supply of oxygen SO2 - acid rain -> remove S from fuel before burning (flue gas desulfurisation)

In the presence of sunlight an UV radiation alkanes react with halogens E.g. CH4 + Br2 -> CH3Br + HBr Substitution reaction

Series of steps of a reaction showing how pairs of electrons are thought to have moved

Free radical substitution

Initiation: covalent bond in a bromine molecule is broken by homolytic fission to form two highly reactive bromine radicals (Br-Br -> Br. + .Br)

Propagation Step 1: CH4 + Br. -> .CH3 + HBr - bromine radical reacts with a C-H bond in the methane forming a methyl radical and a molecule of hydrogen bromide Propagation Step 2: .CH3 + Br2 -> CH3Br + Br. - each methyl radical reacts with another bromine molecule, forming the organic product bromomethane together with a new bromine radical

As the new bromine radical formed after the second propagtion step it can react with another CH4 molecule like the first propagation stage and the two stages continue to cycle in a chain reaction In theory propagation could continue until all reactants are used up but in practice it is terminated when two radicals collide (estimated up to a million propagation cycles take place before termination)

Termination: two radicals collide forming a molecule with all electrons paired, but there are a number of of possible termination steps with different radicals in the reaction mixture Br. + .Br -> Br2 .CH3 + .CH3 -> C2H6 (unwanted product) .CH3 + .Br -> CH3Br

Can form numerous organic compounds

An electron pair acceptor