a bit on alkanes

RADICAL SUBSTITUTION OF ALKANES

• Alkanes react with halogens in the presence of UV light or at 800c .
• Radical substitution occurs
• e.g, CH4+Cl2=CH3Cl+HCl
•  a radical is a species with an unpaired electron
• . In this case a hydrogen atom has been substituted for a chlorine atom.
• a mechanism is a sequence of steps showing the path taken by the electrons in the reaction
• radical substitution is a type of substitution reaction in which a radical replaces an atom/group of atoms.
•  covalent bonds are broken by homolytic fission to form radicals with an unpaired electron this is intitiation.
• the reaction keeps going, its a self perpetuating reaction, this is propagation
• the reaction will keep on going until the products react with eachother, that is the termination step.

COMBUSTION IS THE REACTION OF BURNING IN OXYGEN

• The products always include water
• if burnt in sufficient O2 then CO2 is produced
• if insufficient O2 then CO or C is produced
• C3H8 + 5O2 = 4H2O + 3CO2
• C2H6 +2 1/2 O2 = 2CO + 3H2O
• CH4 + O2 = C + 2H2O
• Cycloalkanes take longer to burn than their aliphatic counterparts as the ring structure takes longer to burn

VAN DER WAALS FORCES

• electrons are not evenly distributed
• this causes a temporary induced dipole which creates forces of attraction between molecules
• these are van der waals forces
• individual van der waals forces will be weak
• however in a long chain alkane with no branches, where two long chains can be close there are oppertunities for points of attraction to be formed, collectively they are strong and can impact on energy needed to seperate them.
• long chains have higher boiling points than short chains and those with side chains
• the side chains physically prevent close contact resulting is lover van der waals forces
• the boiling points will therefore decrease and volatility will increase
• that means that branched short alkanes make the best fuels

FRACTIONAL DISTILLATION

• is the seperation of components in a liquid mixture into fractions which differ in boiling point by means of distillation- typically using a fractioning column
• takes place in a fractioning column
• crude oil is vapourised by heating
• then passed into a fractioning column
• gases pass up the column through a series of bubble caps
• eventually the gases reach a temperature below boiling point
• so condense into liquid fractions tapped off into storage containers
• short chained hydrocarbons (petroleum gas) are distilled off at the top and long chains (bitumen) at the bottom