Chapter 12 - alkanes Flashcards
what is an alkane
an alkane is a type of saturated hydrocarbon with the general formula CnH2n+2
what sort of bonds are formed within alkanes
alkanes form sigma bonds
sigma bonds are caused by the overlap of orbitals, (usually a p orbital of a carbon with an s orbital of a hydrogen)
what property do sigma bonds have
free rotation around the bond (unlike double bonds)
shapes of alkanes
the alkanes have a shape of 109.5 degrees around the carbon because there are four bonds so they form a tetrahedral shape
how does the boiling point of alkanes change with chain length and why
the boiling point of alkanes increases with chain length because Mr increases and surface contact between molecules increases. This increases the strength of the London forces between molecules thus meaning more energy is required to separate the molecules.
how does the boiling point of alkanes change with branching
Branched alkanes have lower boiling points because the molecules can’t get as close, this reduces surface contact meaning there are lower London forces and less energy is required to overcome the inter-molecular forces.
how are these boiling point properties used to help separate crude oil.
fractional distillation is used to separate alkanes of different chain length. There is a temperature gradient in the fractionating column, this means that the longest chain molecules condense and drain off first. The shorter chain molecules then rise for longer before condensing. eventually this separates all the fractions.
Are alkanes reactive?
No, the only ways that alkanes react are combustion and with the halogens in the presence of UV light.
Why are alkanes so unreactive?
the strong sigma bonds have a high bond enthalpy, along with this there is low bond polarity.
formula of complete combustion
CH4 + 2O2 —> CO2 + 2H2O
formula of incomplete combustion (CO formed)
CH4 + 3/2 O2 —> CO + 2H2O
formula of incomplete combustion (C formed)
CH4 + O2 —> C + 2H2O
full formula of methane with bromine
CH4 + Br2 —> CH3Br + HBr
what are the three stages of free radical substitution
initiation
propagation
termination
what is a radical
a radical is a species with an unpaired electron
what occurs during initiation
how many radicals?
in initiation the chain is started by UV light breaking the halogen molecule into two radicals through homolytic fission
e.g. Cl2 —> 2Cl*
0 radicals –> 2 radicals
what occurs during propagation
and how many radicals
the reactions which keep the chain going, they always come in pairs
e.g.
CH4 + Cl* —> CH3* + HCl
CH3* + Cl2 —> CH3Cl + Cl*
this Cl* can then go back into the top reaction
1 radical —> 1 radical
what occurs during termination
these are reactions where radicals are removed from the system 2 radicals ---> 0 radicals e.g. 2Cl* ---> Cl2 CH3* + Cl* ---> CH3Cl
variants of substitution
substitution can occur at any point on the carbon chain
substitution can also occur multiple times to give molecules such as CH2Cl2
why are branched chain alkanes considered more useful than straight chain alkanes
their combustion is more efficient
