Introduction to organic chemistry (A level Chemistry)

Organic chemistry is the study of the organic compounds of carbon.

Living organism are made of organic molecules that are based on carbon compounds. There are a lot of ways in which carbon can bond with other elements to form thousands of organic compounds. Carbon can form rings and very long chains, which may be branched and this is because:

  • a carbon atom has four electrons in its outer shell and so it forms four covalent bonds.
  • carbon-carbon bonds are relatively strong and non-polar.
  • the carbon-hydrogen bond is also strong and relatively non-polar and hydrocarbon chains form the skeleton of most organic compounds.

However, not all carbon compounds are organic compounds. Oxides of carbon, carbonates, hydrogencarbonates and metal carbides are classified as inorganic compounds.

Carbon bonds in organic compounds

Carbon has four valence electrons and so has the ability to form four covalent bonds and to make a full octet. Carbon can form three different types of bonds in organic compounds:

  • single bonds
  • double bonds
  • triple bonds

Single bonds in organic compounds

Carbon can form four single bonds, for example in methane (CH4).


Double bonds in organic compounds

Carbon can form double bonds, for example in ethene (C2H4).


Triple bonds in organic compounds

Carbon can form triple bonds, for example in ethyne (C2H2).


Representing organic molecules using formula

There are a variety of different formula that we can use to represent organic molecules, such as:

  • empirical formula
  • molecular formula
  • structural formula
  • displayed formula
  • skeletal formula

Representing organic molecules using empirical formula

We use empirical formula when we want to know only the simplest ratio of the atoms that make up the organic compound. For example, an organic compound called ethene (C2H4) has the empirical formula CH2. This way the empirical formula tells us that ethene has twice as many hydrogen atoms as carbon atoms in its molecules. One interesting thing to note is that all alkenes have the same empirical formula.

Empirical formulae is also the formula that we get from experimental data on samples of compounds. Knowing the empirical formula and the the relative molecular mass of a substance allows us to calculate its molecular formula.

Representing organic molecules using molecular formula

The molecular formula of an organic compound shows us the actual numbers of each type of atom in one molecule. To find molecular formula from experimental data, we need to know the relative molecular mass of the compound.

For example, the relative molecular mass of propene is 42 and its
empirical formula is CH2, we can find the molecular formula as follows:

We first calculate its empirical mass (total mass of the atoms as shown in the empirical formula).

mass of CH2 = 12 + 2(1) = 14

Then after that we calculate the number of formula units, n.

n = molecular mass ÷ empirical mass

n = 42 ÷ 14 = 3

Then finally multiply the empirical formula by n.

molecular formula = (CH2) × 3

= C3H6

Representing organic molecules using structural formula

Structural formula gives us more detail about how the atoms are bonded to each other in the molecule. For example, the structural
formula of propene (C3H6) is CH3CH═CH2. What this structural formula of propene tells us is that the:

  • first carbon atom is bonded to 3 hydrogen atoms.
  • second carbon atom is bonded to 1 hydrogen atom and also to the third carbon atom by a double bond.
  • third carbon atom is bonded to the second carbon atom by a double bond and also to 2 hydrogen atoms. Carbon–carbon double bonds are shown in a structural formula.

Representing organic molecules using displayed formula

Displayed formula shows all the bonds and atoms within a molecule.

Example – Displayed formula of hexane , showing single bonds in the molecule.


Example – Displayed formula of hexene , showing single bonds and a double bond and in the molecule.


Example – Displayed formula of ethane-1,2-diol , showing all the bonds in the molecule.

Organic molecule

Example – Displayed formula of propenenitrile , showing single bonds, a double bond and a triple bond in the molecule.

Structural formula

Representing organic molecules using skeletal formula

A simplified version of the displayed formula is called the skeletal formula. On a skeletal formula, all the symbols for carbon and hydrogen atoms removed, as well as the carbon to hydrogen bonds. Only the carbon to carbon bonds and the functional groups are left in place. Each bend and line ending in the skeletal formula represents a carbon atom.

The diagram below shows the skeletal formula of propanoic acid.

Skeletal formula

The diagram below shows the skeletal formula of kerosene.

Skeletal formula