what are alkanes
saturated hydrocarbons containing single, sigma C-C and C-H bonds
why are alkanes not very reactive
- high bond enthalpy
- low polarity of the sigma bonds
what are sigma,σ, bonds
direct overlap of s orbitals
angle around C atom in alkanes and explanation
- 109.5 degrees
- 4 bonding pairs of e- around C repel equally to give a tetrahedral shape
how does chain length affect boiling point
- longer chain length increases London forces between molecules as theres more surface contact and more electrons to interact
why does branching decrease boiling point of alkanes
- branched alkanes can’t pack as clodely together as straight chains
- More branching gives less (surface) contact
- fewer/weaker London forces
why are alkanes used as fuels
- readily undergo combustion
- release a lot of energy due to very exothermic combustion enthalpy changes
when does complete combustion occur
- when there is sufficient oxygen
- burns with a clean, often blue flame
products of complete combustion
- CO2 and H2O
- both greenhouse gases known to contribute towards global warming
when does incomplete combustion occur
when there is limited supply of oxygen and burns with a dirty yellow flame
products of incomplete combustion
- CO and H2O
dangers of CO
- colourless, odourless, highly toxic gas
- combines with Hb, blocking transport of O2 to cells
what do alkanes need to undergo free radical substitution
- halogen
- UV light
what does free radical substitution form
haloalkanes
what are the three steps in free radical substitution
- initiation
- propagation
- termination ( multiple reactions can occur)
free radical substitution of hexane with bromine
1) Br2 —> 2Br*
2) Br * + C6H14 —> C6H13* + HBr
C6H13* + Br2 —> C6H13Br + Br*
3) Br* + Br* —> Br2
C6H13* + C6H13* —> C12H26
C6H13* + Br* —> C6H13Br
