Alkanes

What are alkanes?

  • Main components of natural gas and crude oil.
  • Amongst the most stable organic compounds, and their lack of reactivity has allowed crude oil deposits to remian in the earth for many millions of years.
  • Have the general formula CnH2n+2
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Bonding in alkanes

  • Saturated hydrocarbons, containing only carbon and hydrogen atoms joined together by single covalent bonds.
  • Each carbon atom is joined to four other atoms by single covalent bonds called sigma bonds.
  • A sigma bond is the result of the overlap of two orbitals, one from each atom.
  • Each overlapping orbital contains one electron so the sigma bond has two electrons which are shared between bonding atoms. 
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Boiling point of alkanes

  • Boiling point increases because of the weak intermolecular forces known as London Forces. The greater these forces are, the higher the boiling point. 
  • As the chain length increases, the molecules have a larger surface area and therefore more surface contact. The London forces will be greater so more energy is required to overcome these forces. 
  • Branched molecules have fewer points of contact giving fewer London forces so less energy is needed to overcome the forces so boiling point decreases. 
  • Branches also prevent the molecules getting close together further decreasing the number of intermolecular forces. 
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Combustion of alkanes

  • All alkanes react with a plentiful supply of oxygen to produce carbon dioxide and water. 
  • All combustion processes give out heat and alkanes are used as fuels becasue they are readily available, easy to transport, and burn in a plentiful supply of oxygen without releasing toxic waste products. 
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Incomplete combustion of alkanes

  • In a limited supply of oxygen, there is not enough oxygen for complete combustion.
  • When oxygen is limited the hydrogen atoms are always oxidised to water, but combustion of the carbon may be incomplete forming carbon monoxide or even carbon itself as soot. 
  • Carbon monoxide is a colourless, odourless and highly toxic gas. 
  • Combines irreversibly with haemoglobin to form carboxyhaemoglobin, which prevents haemoglobin from transporting oxygen around the body. 
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Reactions of alkanes with halogens

  • In the presence of sunlight, alkanes react with halogens.
  • The high energy UV radiation present in the sunlight provides the initial energy for a reaction to take place. 
  • This is a substitution reaction as a hydrogen atom in tha alkane has been substituted by a halogen atom. 
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Mechanism for bromination of alkanes

  • The mechanism for the bromination of methane is an example of radical substitution. 

Step 1: initiation 

  • The treaction is started when the covalent bond in a bromine molecule is broken by homolytic fission. 
  • Each bromine atom takes one electron from the pair, forming two highly reactive bromine radicals. The energy for this bond fission is provided by UV readiation. 

Br - Br ---> Br. + Br. 

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Mechanism for bromination of alkanes

Step 2: propagation 

Propagation step 1: CH4+Br. ---> .CH3+ HBr

Propagation step 2: .CH3+Br2 ---> CH3Br+Br.

  • In the first propagation step, a bromine radical reacts with a C-H bond in the methane, froming a methyl radical and a molecule of hydrogen bromide 
  • In the second propagation step, each methyl radical reacts with another bromine molecule forming bromomethane, and a brand new bromine radical. 
  • The new bromine radical then reacts with another CH4 molecule as in the first propagation step and the two steps can continue to cycle through in a chain reaction. 
  • Propagation is terminated when two radicals collide. 
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Mechanism for bromination of alkanes

Step 3: termination 

Br.+Br. ---> Br2

.CH3+.CH3 ---> C2H6

.CH3+.Br ---> CH3Br

  • In the termination stage, two radicals collide, forming a molecule with all eletrons paired. 
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