alkanes

• bond angle and bond shape in aliphatic: 109.5 and tetrahedral
• types of bond in alkanes: all sigma bonds (C-C, C-H)
• as hydrocarbon chain increases:

  1. boiling point increases
  2. flammability decreases
  3. viscosity increases (how easily it flows)
  4. volatility decreases (how easily it evaporates)
• general formula for alicyclic alkanes: CnH2n
• general formula for aliphatic alkanes Cn H2n+CnH2n+22
• an alkanes is a saturated hydrocarbon
• effect of chain length (3)

  longer

  1. longer chains have more surface area of contact between molecules
  2. so more dispersion forces/induced dipole-dipole interactions
  3. therefore more energy required to overcome dispersion/induced dipole-dipole interactions
• effect of branching (3)

  more

  1. decrease surface area of contact
  2. fewer London dispersion forces
  3. less energy is needed to break the dispersion forces
• alkanes are generally unreactive:

  • high bond enthalpy
  • AND C-C bonds are non-polar
  • and C-H are very close in polarity,
  • no regions of reduced/increased electron density (delta +/-)
  • that can be attacked by other reagents
• mechanisms: 3 attackers: positive, unpaired electron, lone pair of electron
• nucleophile: lone pair of electron 'attacker'
• electrophile: positive 'attacker'
• radical: unpaired electron 'attacker'
• nucleophile

  • lone pair, attracted to positive charge
  • examples: hydroxide ion, water, chloride ion, ammonium ion
  • most important part is the lone pair not the - charge
  • lone pair donator
  • forms a covalent bond
• electrophile:

  • have delta positive/ positive charge
  • attack electron-rich areas
  • examples: C--C double bonds, N+O2, Br+
  • lone pair acceptor
• radical: only attack alkanes!

  • unpaired electron
  • most reactive alkane
  • Cl - single dot
• breaking covalent bonds (2 ways): heterolytic fission and homolyitic fission
• heterolytic fission: splitting unequally, forms ion, half-curly arrows involved
• homolytic fission: splitting equally, forms radicals which are then exposed to UV or 400 degrees
• radical substitution: replace one atom or small group of atoms with another
• 3 steps of radical substitution: initiation, propagation, termination
• initiation: always produces 2 radicals using UV or 400 degrees
• propagation: 2 steps, has to be a radical on each side of the equation
• termination: always involves two radicals reacting together
• radicals damage the o-zone layer; radicals are uncontrollable, keep attacking until two radicals come together (random event)
• evidence for radical substitution: 1. the presence of a double bond in the product

  CH3 + CH3 -> C2H6 ethane