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Advanced A/AS Level Organic Chemistry: Crude Oil, fractions and useful products
1.2 Fractional distillation of crude oil, fractions and the uses of the various products
Part 1. ALKANES and the PETROCHEMICAL INDUSTRY
Doc Brown's Advanced A Level Organic Chemistry Revision Notes
Crude petroleum oil is a mixture of hydrocarbons, mainly alkanes, that can be separated by fractional distillation into a variety of useful products.
The Fractional distillation of crude oil and the boiling point of alkanes
This is described on another page fractional distillation of crude oil & uses of fractions
In reality it is a fractional condensation process as each fraction isn't simply distilled over, but each fraction is tapped off at a particular condensation point within the negative temperature gradient up the column.
You should appreciate how the uses of the fractions is often related to its molecular size and hence the intermolecular forces (intermolecular bonding) e.g. boiling point and volatility, ease of combustion - flammability, viscosity ('stickiness'!).
Boiling point of alkanes and intermolecular bonding forces
Space filling diagrams to illustrate the different magnitudes of the intermolecular bonding forces between two alkanes of different molecular sizes (different number of carbon atoms, different numbers of electrons). This gives rise to octane having a boiling point of 174oC and octadecane a boiling point of 317oC.
In the above diagram the alkane molecules have been drawn in a linear manner and they would be described as linear alkanes because there are no branches in the carbon chain. However, as illustrated in the 2nd diagram below, they are flexible from propane onwards and even the two methyl groups of ethane can freely rotate with respect to each other!
Graph 1 red line = alkanes
Note: The red line represents linear alkanes in all the graphs 1-3.
A plot of number of electrons in an alkane molecule versus its boiling point (K) shows a steady rise with a gradually decreasing gradient.
Graph 1 is probably the best graph to look at the relative effects on intermolecular forces (intermolecular bonding) on boiling point because it is the distortion of the electron clouds (e.g. in non-polar alkanes, that gives rise to these, weak, but not insignificant forces.
Graph 2 red line = alkanes
A plot of the molecular mass of an alkane molecule versus its boiling point (K) shows a steady rise with a gradually decreasing gradient.
Graph 3 red line = alkanes
A plot of number of carbon atoms in an alkane molecule versus its boiling point (K) shows a steady rise with a gradually decreasing gradient.
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