module 4: alkanes Flashcards
functional group
C-C
general formula
Cn H2n+2
sigma bonds
overlap of orbitals directly between bonding atoms
free rotation
bp straight-chain
carbon chain length increases:
more electrons + more points of contact
stronger London forces
more energy
higher bp
bp branched-chain
branching increases:
points of contact decrease
London forces decrease
less energy
lower bp
solubility
alkanes = polar
do not dissolve in polar solvents
mix with non-polar solvents
shape
tetrahedral around each carbon atom
reactivity
low
strong C-C and C-H sigma bonds hard to break, high enthalpy
C-C and C-H non-polar (little difference in electronegativity between c and H)
complete combustion
unlimited supply of O2
exothermic
C02 + H2O formed
balance
incomplete combustion
insufficient O2
C undergoes partial oxidation forming CO
unburnt C released as soot
CO
toxic: binds to haemoglobin in blood reducing supply of O2 to heart and vital organs
colourless and odourless
radical substitution
reactions of alkanes with halogens in presence of uv
3 steps: initiation, propogation and termination
initiation
cl radicals formed in presence of uv light by photodissociation of cl molecules
Cl2 -> 2Cl*
propogation
chain reaction
radicals react with molecules to form new molecules and radicals
Cl* + CH4 -> HCl + CH3*
CH3* + Cl2 -> CH3Cl + Cl*
termination
free radicals collide and combine
2Cl* -> Cl2
CH3* + Cl* -> CH3Cl
2CH3* -> C2H6
limitations of radical substitution
large mixture of products formed
further substitution
H atoms replaced by further substitution with Cl
e.g. CH3Cl + Cl2 -> CH2Cl2 + HCl
preventing further substitution
use an excess of alkane
use as fuels
readily available
easy to transport
don’t release toxic products
