4.1.2

• It is more energetically favourable for the electrons in 2s and 2p in carbon to be evenly distributed so the 2s electron moves to the empty 2p orbital and 2s + 2p move to the same energy level creating 4 sp3 hybrid orbitals
• Sigma bonds form when a carbon sp3 orbital overlaps with another orbital of a bonding atom
• Carbon atoms in an alkane form a tetrahedral shape with a bond angle of 109.5
• Alkanes generally have low boiling points
• London forced hold alkane chains together
• Alkanes are non polar as the elements have similar electronegativities
• Boiling points increase with alkane chain length as the London forced are stronger due to the increase in electrons
• Straight chain alkane chains have a higher boiling point than branched alkane chains as there is more surface contact between chains causing them to fit together better causing an increase in the London forces
• Alkanes are unreactive as the C-H bond enthalpy is very high and not polar
• C-H bonds can react with sufficient activation energy by combustion or radical substitution
• Complete Combustion: excess Oxygen used, forms water and carbon dioxide, blue flame, releases more energy and less toxins
• Incomplete combustion: not enough oxygen used, forms water, carbon dioxide, carbon monoxide, carbon soot, the carbon soot glows orange in heat causing an Orange flame
• Balanced symbol equations for combustion can be worked out using moles and gas volume
• Bromine and chlorine can ’attack’ alkanes in a radical substitution reaction containing 3 stages using UV light in a photochemical reaction
• Radical substitution stages: initiation, propagation, termination
• Initiation: homolytic fission , a new radical is created ( bromine or chlorine)
• Propagation: a radical will react with an alkane and form a new covalent bond and a new radical
• Termination: two radicals react to form a stable product and new covalent bond
• Radical substitution equations use curly arrows
• Overall radical substitution equation: reactant + reactant -> stable propagation products
• Overall radical substitution equations are based on one substitution
• Nucleophile: electron pair donor
• Why might an ethanol change be different: incomplete reaction, heat energy released into the environment, non-standard conditions
• Limitations of radical substitution: further substitution can occurs, reactions can occur at different positions on the carbon chain
• sigma bond

  A) sp3 orbital

  B) sigma bond

  
• radical substitution

  A) UV light

  B) initiation

  C) propagation

  D) termination

  
• structural isomer boiling point differences:

  • all have same molecular formula
  • boiling points decrease with more branches
  • branches cause less surface contact and weaker London forces
  • weaker London forces require less energy to break bonds
• when explaining difference in alkane boiling points talk about

  length/ size of carbon chain, branching, electrons and London forces
• radical substitution forms

  a haloalkane + a hydrogen halide
• why does radical substitution produces a range of products

  substitution can occur anywhere on a carbon chain and there is more than 1 termination step