Alkanes ✂️

• what is the general formula of an alkane?

  general formula of CnH2n+2
• what type of bonding does an alkane have?

  saturated- only contain single covalent bonds

  each carbon atom has four sigma bonds

  sigma bond- overlap of two orbitals, one from each bonding atom

  • each orbital has one electron so therefore each sigma bond has two electrons shared between the bonding atoms
• what is the bond angle of an alkane?

  • has 4 sigma bonds= repulsion causes a bond angle of 109.5 around eavh carbon
• How do boiling points change in alkanes?

  More carbons in alkane= higher boiling point

  • longer chain length =more London forces more energy required to break forces
  • longer chain length= larger surface area and more surface contact therefore more London forces
• How does branching affect the boiling point?

  • branched isomers have lower boiling points
  • fewer points of contact between molecules of branched alkanes = less London forces= less energy required
  • shape also means that the packing of molecules is not tight and the molecules wont be as close together = more further= decreases intermolecular forces= less energy required
• Why are alkanes not that reactive?

  C-C bonds are non polar and the electronegativity between Cand H is so similar they can be considered non polar too and therefore find it difficult to attract molecules

  C-H and C-C sigma bonds are strong
• why are alkanes used as fuels?

  because they react with a plentiful supply of oxygen in a combustion reaction

  producing carbon dioxide and water
• What is the incomplete combustion of alkanes and what does it produce?

  When there is a limited supply of oxygen and carbon monoxide or just carbon is formed alongside water
• how do alkanes react with halogens and what conditions are needed?

  an alkane reacting with a halogen requires UV radiation from the sunlight to provide the initial energy needed for the reaction to occur

  the reaction is a substitution react where the hydrogen atom replaces the bromine atom

  
• what is the mechanism for an alkane reacting with a halogen called and what are the three steps?

  • radical substitution
  • initiation, propagation and termination
• what happens in initiation stage of bromination of an alkane?

  • covalent bond between bromine molecules broken by homolytic fission
  • both atoms take one radical
  • must have UV light present
  • radical is very reactive

  
• what happens in the propagation stage of bromination of an alkane?

  • chain reaction so two steps occur
  • step 1= Br radical reacts with the C-H bond, forming a CH3 radical, and a molecule of hydrogen bromide (HBr)
  • step 2= the methyl radical reacts with another bromine molecule to form CH3Br and a Br radical
  • in theory these two steps can occur till all reactant is used up but in practise the step is terminated when the two radicals collide

  
• What happens in the termination stage of bromination of an alkane?

  • two radicals collide and there are more than one possibility of collisions so there are more than one termination steps

  
• what are the potential weaknesses of using radical substitution to make haloalkanes?

  • the bromine radical can collide with the bromoethane molecule for example to make dibromoethane, this substitution can occur until all hydrogen atoms have been substituted
  • If carbon chain is longer than the substitution of a hydrogen atom by a bromine atom can occur in different positions leading to different isomers