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A Level Organic Chemistry Notes: Refining crude oil, definition of cis/trans, geometrical isomerism, hydrocarbon types

[EDITOR'S NOTE: Other exam topics are: Macromolecules and Polymers, Acid/Base reactions, Redox reactions, environmental chemistry (ozone depletion), Molecules/Salts/Metals/Atomic lattice, Equilibrium equations and organic chemistry]

Importance of Carbon in Organic Chemistry

In organic chemistry, carbon (C) is the most important element. It has remarkable properties, which allow carbon a great diversity of combinations and bonds with other molecules.
Carbon has four valence electrons. This means that it can make four covalent bonds with other molecules. Also, it has a moderate electronegativity and a rather small (right) size. So, it can bond to other molecules easily and to itself and make infinite structures. Double bonds or even triple bonds are also possible with the carbon atom.

Definition of an organic molecule (definition of organic chemistry)

An organic molecule consists of many carbons and hydrogen atoms, which form bonds with the carbons. In addition, it has functional groups attached to the whole complex. The carbon-hydrogen bonds are assumed to be neutral as the difference in electronegativity is practically neutral anyway. Functional groups make up the polar part of an organic molecule.

There are different types of hydrocarbon molecules:

Aromatic hydrocarbon: These are hydrocarbon rings, which have double or triple bonds.
Saturated hydrocarbon: The formula of the hydrogen-carbon ratio is given by: CnH2n+2 , assuming a non-cyclic (ring) structure. Saturated hydrocarbons only consist of single bonds and the very easiest appearance of a saturated hydrocarbon is an alkane.
Unsaturated hydrocarbon: The ratio of hydrogen and carbon is given by: CnH2n , where again a non-cyclic structure is assumed. Unlike saturated hydrocarbons, it has double bonds and the simplest appearance of an unsaturated hydrocarbon is an alkene.
There is a second type of unsaturated hydrocarbons which has the ration formula given by: CnH2n-2. Once again, a non-cyclic structure is assumed and this type of hydrocarbon only makes triple bonds. The simplest structure is an alkyne.
Cyclic hydrocarbons are quite similar to aromatic hydrocarbons. Both are rings, but the difference lies in the bonds. Cyclic hydrocarbons only have saturated rings (only single bonds).

Crude oil refinery / refining process of oil / steps in manufacturing crude oil

The process of turning crude oil into a useful form is done in a crude oil refinery. The steps for making crude oil into oil, petrol or whatsoever are fractional distillation, cracking and reforming.

Step One of refining crude oil: Fractional distillation

The mixture is inserted at the bottom, where mostly everything will condense as the temperature is 350°C and more. The condensed crude oil will rise to the next fraction above, which has a very high temperature as well, but a slightly smaller one. Only the part of the mixture, which boiling point is under the temperature of the fraction, will condense and rise to the next fraction. The part of the mixture, whose boiling point is higher than the temperature inside the distillation fraction, will stay there and be pumped out.

Example: Supposing the temperature inside the current fraction is 300°C. The part of the mixture, which boiling point is less than 300°C, will condense. Only the molecules, which have a boiling point over 300°C, will remain in the fraction. That process is repeated about 50 to 60 times in total.

Step Two of refining crude oil: Cracking

After the fractional distillation process, the separated mixtures have to be cracked down. This means that a long molecule will be split up in smaller parts. How does one achieve this?
Firstly, single bonds will be broken down. This results into some lone electrons.
The lone electrons form double bonds. Thus, hydrogen will disconnect from the carbon atom. Hydrogen (H2) remains as a side product. The loss of hydrogen in these smaller organic molecules is logical, because when they are lost, more lone electrons remain with what the previous lone electrons can make a bond.

Step Three of refining crude oil: Reforming

After cracking, the molecules are ready to undergo the reforming process.
This is given by the octane number. The octane number is very important in petrol. It tells what the percentage of pure heptane (in the earlier days it was octane – that is why it is called octane number) in petrol is. This is of great importance for the chemical behaviour. The quality of petrol is improved by adding mixtures to pure heptane. The chains of heptane are heated up (where platinum is used as a catalyst). So they can change. After the heating process, it shows a higher amount of branched chains. This increases the octane number.

Example: When the petrol you buy has a 98 in the name, then it means that 98% are branched chains and 2% of the mixture is pure heptane (or another pure molecule).

Different forms of Isomerism

Isomers are compounds of with the same molecular formula, but with different structural formulas (means: they look different). Isomers do not necessarily share the same chemical properties (like boiling point, reactivity etc.)

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Definition of structural isomerism

Two organic molecules which have the same chemical formula but look different. For example, C2H6O can be drawn as C2H5OH or CH3OCH3 (see picture below). Both look differently and have completely different chemical properties.

Definition of cis / trans isomer pairs

A cis/trans pair has the same molecular formula but their build-up is only slightly different, depending upon the position of the heaviest molecules.
The cis isomer has its heaviest molecules on the same side of a double bond (or a ring system).
The trans isomer has not its heaviest molecules on the same side of a double bond (or a ring system). Those two isomers make up the cis/trans isomer pair.
This does not work on single bonds, since the molecule would be considered as flexible and can so turn around its axis.

Definition of chiral molecules

Chirality is another form of isomerism. If a molecule is mirrored and its mirror image is the same as the original molecule, then it is achiral and therefore the two molecules are identical.
If the mirror molecule is unlike the original molecule, then they are chiral and therefore not identical. This is the case if the molecules have no plane of symmetry (see picture).

Definition of enantiomers

Enantiomers are molecules which look very similar, but are not superimposable. Supposing we have a molecule A which is mirrored, giving us molecule B. If there is a molecule A’ which is moved about 180° and is not superimposable on the resulting molecule B, then those two are enantiomers (see picture). Its difference lies often in the orientation of the bonds whether they are pointing outwards or pointing inwards the room.


For the production of esters only a carboxyl and an alcohol are needed.
carboxyl + alcohol => ester + side product
R-CO2H + R’-OH ó R-CO2-R’ + H2O (or another side product)

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