O Level Notes : Chemistry - Organic Chemistry

Organic chemistry is the chemistry of the compounds of carbon. Actually, it is the study of ‘hydrocarbons’. Hydrocarbons are compounds containing hydrogen and carbon and no other element. There are many types of hydrocarbons. They are classified into several types according to their structure.

Homologous Series:

A homologous series is a family of organic compounds with similar chemical properties. Examples of homologous series are the alkanes, the alkenes, the alcohols, and the carboxylic acids. Compounds of the same homologous series contain the same functional group. A functional group is an atom or a group of atoms that gives a molecule its characteristic properties.


Let us study the different examples of homologous series now.



The members of this group of hydrocarbons are distinguished by possessing the general molecular formula CnH2n+2, where n is a real number, for successive members of the group. For example, the first member of the group is methane CH4, in which n=1. The second member is ethane C2H6, in which n=2. And so on.

Note that in all the homologous series, whenever n=1, the compound’s name will start with meth- and the next part of the word will indicate the group it falls in. Like in methane, the ‘ane’ shows that it belongs to the alkane group.

We will learn the names and formulas of the first ten members of the alkane group.

Methane CH4

Ethane C2H6

Propane C3H8

Butane C4H10

Pentane C5H12

Hexane C6H14

Heptane C7H16

Octane C8H18

Nonane C9H20

Decane C10H22



Each carbon atom is any molecule can make four bonds. It may bond with less than four elements by making double and triple bonds, but the bonds must equal four. In alkanes, each carbon bonds with four hydrogen atoms. The molecular structures of the first three alkanes are:



Methane CH4

Ethane C2H6


Propane C3H8


And so on. I am sure you get the general idea.

The alkanes are called unsaturated compounds. It is because they cannot take part in addition reactions, as they do not have any double bonds that can be easily broken. They can, however, take part in substitution reactions under the right conditions, which means that any other element or radical can take the place of any hydrogen in the compound to form a new compound.



The alkenes are members of a homologous series of general molecular formula CnH2n. For example, ethene is C2H4. Every carbon in an alkene makes bonds with four other elements, i.e. one carbon and other hydrogens. However, two of the carbons in each alkene molecule are linked by a double bond. It is shown in these molecular structures:


Ethene, C2H4

Propene, C3H6


You see, all carbons are still making four bonds, but there exists one double carbon to carbon bond in each alkene molecule. This double bond is the ‘functional group’ of alkenes that I mentioned earlier. This double bond exists in every alkene and is responsible for the chemical properties of the alkenes.


Alkenes are said to be unsaturated because of the double bond in their structures. This means that one molecule of it can combine with two hydrogen atoms (or their equivalent) in addition reactions.


For example,

  + H2   



This will be treated in more detail in the next chapter about macromolecules. However, unsaturated molecules can, under the right conditions, join together to form giant molecules called polymers. All plastics and man-made fibers are polymers. Each polymer is made up of thousands of identical units called monomers. For example, the polymer polyethene is made up of a very large number (n) of ethene monomer molecules.


This is a monomer of ethene. You can see that the double bond is broken, leaving two free bonds to bond with any other element. So they form chains and each monomer in the chain bonds with the next monomer. It is then called a polymer.


This is the polymer. The zigzag lines show that this continues.



The alkynes are those hydrocarbons that contain one triple bond. The triple bond is their functional group and decides their properties.



These are compounds containing not only the elements carbon and hydrogen, but also oxygen. Like hydrocarbons, they form a homologous series, with the general formula CnH2n+1OH. The OH group is its functional group, and determines its properties. Do not confuse these with hydroxides of the alkalis, as alcohols (or alkonols) are neutral.


Alcohols can take part in the reactions combustion and oxidation. In the combustion reaction, alcohols burn in air to produce carbon dioxide and water. If we oxidise alcohols, they become alcohols. For example, if we oxidise ethanol, it becomes ethanoic acid.

The molecular structures of alcohols are like this:

Methanol CH3OH


Ethanol C2H5OH


Alcohols are used mainly in making alcoholic drinks, as solvents, and as fuels.


Carboxylic Acids:

The carboxylic acids are a homologous series of organic compounds with the carboxyl functional group COOH. They have the general formula CnH2n+1COOH. Carboxylic acids are weak acids. They are a class of organic acids.

The carboxylic acids are all soluble. The more their carbon content, the more is their boiling point.

Let’s take a look at the molecular structure of ethanoic acid.


Ethanoic acid CH3COOH


Note that there are two carbons, but n=1. the other carbon is a part f the functional group of the acid.



Esters are formed when we react acids with carboxylic acids. For example, if we add ethanol and ethanoic acid together, we get water and an ester, ethyl ethanoate, CH3COOC2H5. This reaction of alcohols with carboxylic acid is called esterification. This is a reversible reaction, and we can gain the acid and alcohol back by boiling the ester with sodium hydroxide. This is in fact a reaction with water, and is called hydrolysis.

Esters have a sweet smell, so they are used in making perfumes. They are also used in flavorings, as solvents, etc.


Organic chemistry is not finished, but we will end this lecture here so that it does not get too complicated. Thank you for your attention students.