MODULE 4: Alkanes
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What bond is present in alkanes?
Sigma bond (σ)- overlap of 2 sigma bonds causes free rotation
Why do alkanes have a tetrahedral shape?
Due to each carbon atom being surrounded by 4 shared pairs of electrons, electrons repel each other to make the tetrahedral shape
How does fractional distillation work?
The longer the chain length, the more London forces and surface contact between the molecules, so the longer the molecule is the higher the boiling point is
Why does the boiling point increase as the alkane gets longer?
Increased surface contact and increased number of electrons, therefore London forces increases
Why do branched alkanes have a decreases boiling point?
Decreased surface contact, therefore weaker London forces
Why are alkanes unreactive?
Very strong covalent bonds, non-polar bonds
What are the products formed from complete combustion of alkanes?
Carbon dioxide and water
What is the algebraic equation for the complete combustion of alkanes?
CₓHʸ + (x + y/4)O2 –> xCO₂ + y/2 H₂O
What are the products formed from incomplete combustion of alkanes?
Carbon monoxide, carbon and water
Why is carbon monoxide dangerous?
Colourless, odourless gas that bonds irreversibly with haemoglobin in the blood, therefore prevents the transportation of oxygen for respiration
What is free radical substitution?
The reaction of an alkane with a halogen under ultra violet light involving homolytic fission and radical reactions
In free radical substitution, what is initiation?
When one radical is formed from a non-radical: so when UV light breaks up a chlorine molecule into two chlorine radicals (homolytic fission)
In free radical substitution, what is propagation?
Continuing the chain reaction (one radical on both sides of the reaction)
In free radical substitution, what is termination?
Removal of radicals from the reaction (two radicals colliding to form one non-radical)
What are the negative aspects of using free radical substitution?
No control over products, however can be slightly controlled by using excess of certain reactants