Chapter 12 - Alkanes Flashcards
What are alkanes? General formula?
Homologous series of saturated hydrocarbons with the general formula C2H2n+2
What is the reactivity of alkanes?
Very unreactive but do burn and react with halogens
Why aren’t alkanes very reactive?
- C-C and C-H bonds are strong
- C-C and C-H bonds are non polar (similar electronegativity so no partial charges to attract any other molecules)
Which isomer of alkanes I more likely to react E or Z?
Z isomer in E isomers the polar bonds cancel out as they act in opposite directions
What is the trend for boiling points in alkanes?
- longer chains means higher boiling point due to stronger London forces between molecules (more electrons in the molecule)
- for isomers the more branched carbon chain has the lower boiling point as they have weaker London forces (molecules have less surface contact)
THEN TALK ABT ENERGY IN RELATION TO BP
How is petroleum (crude oil) formed?
Slow decay of marine animals and plants over millions of years under heat and pressure
How is crude oil separated?
By fractional distillation as different hydrocarbons within crude oil have different boiling points
What is the process of fractional distillation of crude oil?
- crude oil is vaporised
- vapour is passed into a tower which is hot at the bottom and cold at the top
- as the vapour rises it cools
- molecules condense at different heights as they have different boiling points
- larger the molecules, lower down it condenses
What are the different fractions at what temperatures?
< 40° -> FUEL GASES (C - C4)
40° - 90° -> GASOLINE
90° - 160° -> NAPTHA
160° - 250° -> KEROSENE
250° - 350° -> DIESEL
> 350° -> RESIDUE
What is the trend in properties as the C chain gets longer?
- becomes more viscous
- harder to ignite
- less volatile (ability to turn into a gas)
- higher boiling points
What is part of the residue fraction?
Contains fuel oil, lubricating oil, waxes, and bitumen
These decompose at up high temperatures so they’re separated by distillation at lower pressure
What is cracking?
Thermal decomposition of longer alkanes to form smaller alkanes
What are the conditions for thermal cracking?
TEMPERATURE: 900°
PRESSURE: 70atm
CATALYST: none
PRODUCTS: alkenes
What are the conditions for catalytic cracking?
TEMPERATURE: 450°
PRESSURE: 1-2 atm
CATALYST: zeolites
PRODUCTS: motor fuels (aromatics, cyclic alkanes, branched alkanes)
What is reforming?
Process where straight chain hydrocarbons are converted into branched chain alkanes and cyclic alkanes. Both these burn more efficiently and are used in petrol for cars
What is the complete combustion formula?
Alkane + O2 → CO2 + H2O
Highly exothermic, with enough oxygen
What are the 2 formulas for incomplete combustion?
Alkane + O2 -> CO + H2O
Alkane + O2 -> C + H2O
What are the problems with CO2, CO, C and SO2 and how can we avoid them?
CO2 - greenhouse gas -> burn less fossil fuels
CO - toxic -> ensure good supply of oxygen when burning fossil fuels
C - respiratory problems, global dimming, blackens building -> ensure good supply of oxygen
SO2 - acid rain -> remove S from fuel before burning (flue gas desulfurisation)
How can SO2 be removed from waste gases?
By flue gas desulfurisation:
- gases pass through a scrubber containing calcium oxide or calcium carbonate which reacts with the sulfur dioxide
The reaction forms calcium sulfate which is used to make plasterboard
What is the alkane reaction with halogen?
In the presence of sunlight an UV radiation alkanes react with halogens
E.g. CH4 + Br2 -> CH3Br + HBr
Substitution reaction
What is a mechanism?
Series of steps of a reaction showing how electrons are thought to have moved
What is the mechanism for bromination of alkanes called?
Free radical substitution
What are the 1st step of free radical substitution?
Initiation: covalent bond in a bromine molecule is broken by homolytic fission to form two highly reactive bromine radicals (Br-Br -> Br. + .Br)
What is the 2nd step of free radical substitution?
Propagation Step 1: CH4 + Br. -> .CH3 + HBr
- bromine radical reacts with a C-H bond in the methane forming a methyl radical and a molecule of hydrogen bromide
Propagation Step 2: .CH3 + Br2 -> CH3Br + Br.
- each methyl radical reacts with another bromine molecule, forming the organic product bromomethane together with a new bromine radical
Why is the propagation step called a chain reaction?
As the new bromine radical formed after the second propagtion step it can react with another CH4 molecule like the first propagation stage and the two stages continue to cycle in a chain reaction
In theory propagation could continue until all reactants are used up but in practice it is terminated when two radicals collide (estimated up to a million propagation cycles take place before termination)
What is the 3rd step of free radical substitution?
Termination: two radicals collide forming a molecule with all electrons paired, but there are a number of of possible termination steps with different radicals in the reaction mixture
Br. + .Br -> Br2
.CH3 + .CH3 -> C2H6 (unwanted product)
.CH3 + .Br -> CH3Br
What are the limitations of free radical substitution?
Can form numerous organic compounds
