Part 1. ALKANES and the PETROCHEMICAL INDUSTRY

Doc Brown's Advanced A Level Organic Chemistry Revision Notes

1.2 Fractional distillation of crude oil and uses of products

Crude petroleum oil is a mixture of hydrocarbons, mainly alkanes, that can be separated by fractional distillation into a variety of useful products.


Alkanes and Petrochemical Industry INDEX

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A basic introduction to the chemistry of alkanes


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

  • Alkanes are non–polar molecules where the only intermolecular force operating is the weakest possible, that is the instantaneous dipole – induced dipole intermolecular forces. These are sometimes called London–dispersion forces and occur between ALL molecules, even single atoms of the noble gases. Van der Waals forces include all types of intermolecular forces which are not due to an actual chemical bond BUT sometimes this name is used just to mean these instantaneous dipole – induced dipole dispersive forces (sorry but it can be confusing!).
    • The electronegativities are: C (2.5) and H (2.1) and produces a virtually non–polar bond and any very small effects will tend to cancel out e.g. H–C–H situations and so alkanes are the least polar organic molecules i.e. as near non–polar molecules you will get.
  • A transient δ+ in one alkane molecule induces a transient δ– in a neighbouring alkane molecule, so causing a very weak and transient electrical attraction.
    • Note that these partial charges on the alkane molecule are shown as a delta + (δ+) or a delta – (δ–) and they are tiny charges compared to a full single plus charge e.g. on an Na+ sodium ion or a full single minus charge  on a Cl chloride ion.
  • These electrical attractive forces act between ANY atoms or molecules and is primarily a function of the number of electrons in the molecule, though their spatial distribution can be significant.
  • The larger the alkane molecule, i.e. the greater the number of electrons in it, the more polarizable it is and the greater the chance of a random instantaneous dipole occurring to induce a dipole in a neighbouring molecule, so increasing the intermolecular attractive forces.
    • Hence the larger the alkane molecule the higher the boiling point (see diagram and graphs below).
  • The force arises from the instantaneous and random asymmetry of the electron fields in the atomic orbitals because of the random behaviour of electrons in the atomic or molecular orbitals and particle collisions and vibrations.
  • This polarisation can readily occur when particles collide with each other e.g. in liquids or vibrate against each other e.g. in a solid. In this situation electron clouds from neighbouring atoms/molecule will repel each other and the distortion of the charge distribution causes the polarization. Under these circumstances, contact between any two atoms/molecules can produce temporary or transient polarisation.
  • attractions between alkane molecules

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

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 2

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

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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