Chapter 12 Flashcards
How does petrochemical industry work?
Composition of crude petroleum consists mostly of alkanes and can be split up into fractions by fractional distillation - it separates the compounds according to their boiling point (eat each level compounds in a similar boiling range are taken off) ; rough fractions can be distilled further to obtain narrower boiling ranges and some fractions are more important (usually the lower boiling point ones) - higher boiling fractions (longer alkanes) may be broken down into useful lower ones (CRACKING)
SMALLER ONES MORE USEFUL AS THEY ARE EASIER TO IGNITE/PUT FIRE TO - LESS ENERGY REQUIRED
Why have alkanes survived for so long?
Very stable and lack of reactivity means that crude oil deposits could remain in the Earth for millions of years - used mainly as fuels exploiting reaction with oxygen to generate heat (CnH2n+2)
Bonding in Alkanes
Saturated with covalent bonds - each carbon atom is joined to 4 other atoms by single covalent bonds (sigma bonds) - HEAD ON overlap between 2 orbitals (s or p) with each orbital having one electron - shared pair is psoitioned along the bonding axis
Each carbon atom has 4 sigma bonds (C-C or C-H) - electron density is along the bonding axis producing a more stable arrangement
Shape of alkanes
4 single covalent bonds and the repulsion between the pairs of electrons results in a tetrahedral arrangement with the bond angle around each carbon being 109.5 degrees (no lone pairs)
Sigma bonds act as AXIS around which the atoms can rotate freely ; THEREFORE NOT RIGID BUT INSTEAD FLEXIBLE
How to draw 3D alkanes?
Use wedges for tetrahedral - one going in, out and on the same plane
How are alkanes extracted?
Through fractional distillation in a distillation tower - each fraction contains a range of alkanes and this separation is only possible because the boiling points are different - INCREASING AS CHAIN LENGTH INCREASES
Why does boiling point increase as size increases?
This is due to the increase in instantaneous dipole dipole interactions (london forces - CH not polar enough) ; once broken the molecules move apart from each other and the alkane becomes a gas
Larger molecules meaning more electrons meaning more dipoles and larger strength dipoles thus more energy required to overcome them
Bonding in alkanes
They are saturated with single covalent bonds ; each carbon atom is joined to 4 other atoms by single covalent bonds (sigma bonds) ; this is the head on overlap of two orbitals and each overlapping orbital (could be s or p) has one electron - this shared pair is positioned on a line directly between boning atoms
Each carbon atom has 4 SIGMA BONDS (C-C OR C-H) - ELECTRON DENSITY IS ALONG THE BONDING AXIS producing a more stable arrangement
Effect of chain on boiling point?
As chain length increases, molecules increase in surface area therefore there are more London forces available - so more energy is required to overcome the forces (large surface contact)
Effect of branching on boiling point
As branching increases, boiling point decreases - this is becuase there are fewer surface points of contact between molecules of the branched alkanes giving fewer London forces.
Branches also prevent molecules getting as close together as straight chain molecules decreasing London forces further
Why are alkanes not that reactive?
C-C and C-H sigma bonds are strong
C-C bonds are non polar
C-H similar electronegativity - non-polar too
Why are alkanes used as fuels?
Readily available, easy to transport and burn in a plentiful supply of oxygen
Combustion processes are…
Exothermic
Complete combustion of alkanes?
Produce carbon dioxide + water
Incomplete combustion of alkanes?
Not enough oxygen for complete combustion - when oxygen is limited the hydrogen atoms in the alkane are always oxidised to water but combustion of the carbon may be incomplete forming CO or C (+ H2O)
When does incomplete combustion occur?
In a closed space such as a car engine/fault heating system/inadequacy ventilation areas
Reactions of alkanes with halogens
Only with chlorine or bromine - fluorine is TOO reactive and would cause an explosion while iodine is not reactive enough
When does halogenation of alkanes occur?
Presence of sunlight - UV radiation provides initial energy
Overall equation
Alkane + X2 -> Alkyl halide + HX
Substitution reaction
3 steps of radical substitution (example of bromination) -shows how electrons move
Initiation, propagation termination
Initiation
Covalent bond in halogen is broken through homolytic fission forming two highly reactive radicals
Unpaired electron makes it really reactive
Propagation
Chain reaction
First step =
CH4 + Br. -> .CH3 + HBr (product)
Second step =
.CH3 + Br2 -> CH3Br (organic product) + Br.
This Br. Radical then reacts with another CH4 and the cycle continues (in theory until all reactants used up - TERMINATES WHEN TWO RADICALS COLLIDE)
Termination - 3 possible equations
Br. + Br. -> Br2 (both radicals are removed - stopping the mixture)
.CH3 + .CH3 -> C2H6
.CH3 + .Br -> CH3Br
Limitations of radical substitution
Further substitution - another bromine radical could combine with the haloalkane substituting a further hydrogen atom in until all have been substituted
Limitation of different positioning on chain with radical substitution?
If carbon chain is LO her we will get a mixture of Monosubstituted isomers by substitution at different positions in the carbon chain
Why is incomplete combustion more likely with longer chain alkanes?
They need more oxygen to combust completely - thus instead producing CO and H2O
