C12 alkanes

• alkanes are saturated hydrocarbons which contain only C-C and C-H bonds
• alkanes can be unbranched chains, branched chains, or rings
• the general formula for alkanes that are chains is $C_nH_{2n+2}$
• the general formula for alkanes that are rings is $C_nH_{2n}$
• to name alkanes, use the number of carbons in the longest carbon chain as the root, then name the side chains using prefixes and numbers to show which carbon the side chains are attached to
• alkanes are almost non-polar because carbon and hydrogen have very similar electronegativities, so the only intermolecular forces they have are weak van der Waals forces
• melting/boiling points of alkanes increase as chain length increases because the van der Waals forces are stronger in larger molecules, alkanes with branched chains have lower melting/boiling points than alkanes with unbranched chains because the branched molecules cannot pack as closely together so the van der Waals forces are weaker
• alkanes are insoluble in water because the hydrogen bonds in water are stronger than the van der Waals forces in alkanes, but they do mix with non-polar liquids
• alkanes are relatively unreactive, they do not react with oxidising agents, reducing agents, acids or bases, but they do react with halogens and will burn in oxygen
• crude oil is a mixture of mostly alkanes, both branched and unbranched, with small amounts of other compounds from the original plants and animals it was formed from
• crude oil can be separated into fractions by fractional distillation as the different chain lengths of molecules mean they all have different boiling points
• the process of fractional distillation is:

  • mixture is vaporised and fed into fractionating column
  • column is hottest at the bottom and coldest at the top
  • vapours rise, cool and condense
  • at each layer the condensed products are siphoned off
• hydrocarbons with short carbon chains have lower boiling points so rise higher up the column before condensing, and are siphoned off nearer the top of the column
• hydrocarbons with long carbon chains have higher boiling points so rise less far up the column before condensing, and are siphoned off nearer the bottom of the column
• longer chain hydrocarbons are less useful than shorter chain hydrocarbons, so they are broken down by cracking
• cracking of hydrocarbons breaks C-C bonds
• the two types of cracking are thermal and catalytic
• thermal cracking produces a high proportion of alkanes and alkenes
• thermal cracking requires high pressure and high temperature
• catalytic cracking produces a high proportion of aromatic hydrocarbons and motor fuels
• catalytic cracking requires a slight pressure, high temperature and zeolite catalyst
• alkanes make good fuels as they release a lot of energy when burnt
• when alkanes are burnt in sufficient oxygen, they are completely combusted to produce carbon dioxide and water
• when alkanes are burnt in insufficient oxygen, they are incompletely combusted to produce carbon monoxide and water
• carbon monoxide and nitrogen oxides are toxic products of combustion of alkanes, they can be removed from systems using catalytic converters

  catalytic converters remove toxic products by converting them into more stable products such as carbon dioxide and water, using a rhodium catalyst
• carbon particulates can be produced from incomplete combustion, they are small fragments which pollute the air, causing cancer and exacerbating the lungs
• impurities in the fuels that are burnt which contain sulfur can lead to the production of sulfur dioxide, which leads to the acidification of water if it escapes into the atmosphere

  the impurities can be removed from waste products by flue gas desulfurisation which uses calcium oxide or calcium carbonate which react with the sulfur dioxide and oxygen/water to form calcium sulfite

  calcium sulfite can be oxidised to calcium sulfate which is a useful product
• alkanes react with halogens in the presence of UV light to form halogenoalkanes
• UV light breaks down the bonds between the halogen atoms, producing reactive free radicals which attack the alkanes
• a free radical substitution has three main steps: initiation, propagation, and termination
• in the initation step of a free-radical substitution, the halogen is broken down into free radicals
• in the propagation step of a free-radical substitution, a free radical reacts with the alkane to form a new molecule and a new free radical, then the new free radical reacts with the original halogen to form the product and the original free radical
• in the termination step of a free-radical substitution, two free radicals react together to form a stable molecule
• the propagation step of a free-radical substitution can happen many times, creating a chain reaction
• the free radical substitution of methane with chlorine:

  initiation = $Cl_2$-> $2Cl\bullet$

  propagation = $Cl\bullet$+ $CH_4$-> $\bullet CH3$+ $HCl$then $\bullet CH3$+ $Cl_2$-> $CH_3Cl$+ $Cl\bullet$

  termination = $2Cl\bullet$-> $Cl_2$or other combination