Saturated and Unsaturated Carbon Compounds

We have already seen the structure of methane. Another compound formed between carbon and hydrogen is ethane with a formula of C₂H₆. In order to arrive at the structure of simple carbon compounds, the first step is to link the carbon atoms together with a single bond (Fig. 4.6a) and then use the hydrogen atoms to satisfy the remaining valencies of carbon (Fig. 4.6b). For example, the structure of ethane is arrived in the following steps –

C— C Step 1

Figure 4.6 (a) Carbon atoms linked together with a single bond

Three valencies of each carbon atom remain unsatisfied, so each is bonded to three hydrogen atoms giving:

The electron dot structure of ethane is shown in Fig. 4.6(c).

Can you draw the structure of propane, which has the molecular formula C₃H₈ in a similar manner? You will see that the valencies of all the atoms are satisfied by single bonds between them. Such carbon compounds are called saturated compounds. These compounds are normally not very reactive.

However, another compound of carbon and hydrogen has the formula C₂H₄ and is called ethene. How can this molecule be depicted? We follow the same step-wise approach as above.

Each carbon atom gets two hydrogen atoms to give –

We see that one valency per carbon atom remains unsatisfied. This can be satisfied only if there is a double bond between the two carbons giving up –

The electron dot structure for ethene is given in Fig. 4.7. Yet another compound of hydrogen and carbon has the formula C₂H₂ and is called ethyne. Can you draw the electron dot structure for ethyne? How many bonds are necessary between the two carbon atoms in order to satisfy their valencies? Such compounds of carbon having double or triple bonds between the carbon atoms are known as unsaturated carbon compounds and they are more reactive than the saturated carbon compounds.