alkanes Flashcards
formula for alkanes
CnH2n+2
are alkanes saturated or unsaturated
saturated
what is a sigma bond
a single covalent bond which is the direct overlap of orbitals
properties of sigma bond
-low polarity
-high enthalpy
why are alkanes relatively unreactive
-C-C and C-H bonds require a lot of energy to break
-bonded by sigma bonds which have a low polarity and high enthalpy
what is the homologous series
a group of organic compounds that have the same functional group, but each successive member differs by CH2
what is structural isomerism
compounds that have the same molecular formula but different structural formulae
why is the C-H sigma bond non-polar
The electronegativities of carbon and hydrogen are very similar
what forces do alkanes exhibit
weak temporary induced dipole-dipole forces
what factors affect the boiling point of alkanes
Chain length
Branching
how does chain length affect bp
the boiling points of the alkanes increases with the molecular size, due to the increased temporary induced dipole-dipole forces
how does branching affect bp
-The more branched the chain, the lower the boiling point
Temporary induced dipole-dipole forces can only operate over very short distances
-It is more difficult for short, molecules to be compact compared with long molecules
-The unbranched alkanes have greater london forces of attraction because of their greater surface areas
what is complete combustion
when alkanes are burnt with plenty of oxygen
what is the formula for complete combustion
alkane + O2 –> CO2 + H2O
what is incomplete combustion
When alkanes are burnt in only a limited supply of oxygen and not all the carbon is oxidised
what is the equation for incomplete combustion
alkane + O2 –> CO + H2O
when can alkanes undergo free radical substitution
when a hydrogen atom gets substituted for a halogen
what type of light is used for free radical substitution
UV
why is UV light used in free radical substitution
as alkanes are very unreactive
what are the three steps of free radical substitution
-initiation
-propagation
-termination
what happens in initiation of free radical substitution
-the halogen bond (Cl-Cl or Br-Br) is broken by the energy of the UV light
-This produces two radicals
what happens in propagtion of free radical substitution
-the free radicals are very reactive and will attack the unreactive alkanes
-A C-H bond breaks homolytically (each atom gets an electron from the covalent bond)
-An alkyl free radical is produced
This can attack another chlorine/bromine molecule to form the halogenoalkane and regenerate the chlorine/bromine free radical
-This free radical can then repeat the cycle
what happens in termination of free radical substitution
the chain reaction stops due to two free radicals reacting together and forming a single unreactive molecule
How do CFCs damage the ozone layer?
they produce reactive chlorine radicals that catalyse ozone destruction
