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4.1.2 Alkanes

Definitions

Term Definition
Free radical A species with an unpaired electron
Chain reaction A reaction in which the propagation steps release new radicals that continue the reaction
Initiation The first stage in a radical reaction in which radicals form when a covalent bond is broken by homolytic fission
Propagation The steps that continue a free radical reaction, in which a radical reacts with a reactant molecule to form a new molecule and another radical, causing a chain reaction
Termination The step at the end of a radical substitution when two radicals combine to form a molecule

Properties of alkanes

Bonding in alkanes

  • Saturated hydrocarbons
  • Only carbon and hydrogen atoms joined together by single covalent bonds
  • Bond type = \(\sigma\)-bond (sigma bond)
    • \(\sigma\)-bond = heads on overlap of orbitals directly between the bonding atoms
    • One orbital from each bonding atom, each containing one electron
    • Positioned on a line directly between bonding atoms
    • Sigma bond Wikipedia
  • Atoms can rotate freely around the \(\sigma\)-bond

Shape of alkanes

  • Tetrahedral shape around each carbon atom, bond angle 109.5°
  • Each carbon atom surrounded by 4 bonded pairs in four \(\sigma\)-bonds
  • The electron pairs repel each other as far away as possible
  • Organic chemistry the alkanes

Effect of chain length on boiling points

  • Longer chain length = higher boiling point
  • Increased chain length = greater surface area of contact + more electrons
  • Stronger London forces
  • More energy is required to overcome the London forces

Effect of branching on boiling points

  • More branching = lower boiling point
  • More branches = fewer surface area of contact \(\rightarrow\) weaker London forces
  • The branches prevent the branched molecules getting as close together as straight-chain molecules \(\rightarrow\) further decrease intermolecular forces
  • Less energy is required to overcome the London forces

Alkane reactions

Reactivity of alkanes

  • Low reactivity
  • \(C-C\) and \(C-H\) \(\sigma\)-bonds are strong as they have a high bond enthalpy
  • \(C-C\) bonds are non-polar
  • \(C-H\) bond can be considered non-polar as the electronegativities of C and H are very similar

Combustion of alkanes

  • Complete combustion (sufficient oxygen present)
    • Equation: \(C_{x} H_{2 x + 2} + (\frac{3 x + 1}{2}) O_{2} \rightarrow x CO_{2} + (x + 1) H_{2} O\)
  • Incomplete combustion (insufficient oxygen present)
    • Hydrogen atom always oxidised to water
    • Combustion of carbon may be incomplete so carbon (soot) or carbon monoxide is formed instead of \(CO_2\)
    • Carbon monoxide is toxic + colorless and odorless so it is difficult to spot
    • CO combines irreversibly to haemoglobin and replace oxygen so oxygen cannot pass round the body and the person can suffocate
  • Alkane is a good source of fuel
    • Release large amounts of energy when burned
    • Easy to transport

Methane and chlorine reaction

  • Equation: \(R-CH_3 + X_2 \rightarrow R-CH_2X + HX\)
  • Type: free radical substitution
  • Step 1: initiation
    • The halogen-halogen bond is broken by homolytic fission to form 2 free radicals
    • Energy for bond fission is provided by UV radiation
    • e.g. \(Cl_2 \xrightarrow{u.v.} 2Cl\bullet\)
  • Step 2: propagation (halogen radical intermediate react with original reactants)
    • One free radical reacts to produce another different free radical
    • Always in 2 steps
    • First propagation step: \(Cl\bullet + CH_4 \rightarrow \bullet CH_3 + HCl\)
    • Second propagation step: \(\bullet CH_3 + Cl_2 \rightarrow CH_3Cl + Cl\bullet\)
    • A halogen radical acts as an catalyst and is recreated
  • Step 3: termination
    • Two free radicals combine and their unpaired electrons pair up to form a covalent bond between the 2 species
    • \(Cl\bullet + \bullet Cl \rightarrow Cl_2\) / \(\bullet CH_3 + \bullet CH_3 \rightarrow C_2H_6\) / \(\bullet CH_3 + \bullet Cl \rightarrow CH_3Cl\)
    • Both radicals are removed from the reaction mixture so the reaction stops
  • (Same equation for bromine atoms)

Skeletal formulae for free radical substitution

  • Exported image
  • Dot on the carbon atom with hydrogen atom removed

Limitations of radical substitution in synthesis

  • Further substitution
    • The propagation step can continue many times as it is a chain reaction
    • Conditions can be altered to favour the termination step and limit the number of substitutions
  • Substitution at different positions in a carbon chain
    • Longer chain = a mixture of monosubstituted isomers formed by substitution at different positions of the chain
    • Different chains can also undergo further substitution
  • Produces different termination products (more than one possible termination step)