Saturated compounds

1. A saturated compound has molecules in which all carbon-carbon bonds are single bonds.

2. In a saturated molecule each carbon atom uses its four bonds to join four separate atoms, so no more atoms can be attached to it.

3. Only the bonds between two carbon atoms decide the classification.

   1. Bonds from carbon to hydrogen, oxygen or any other element are ignored when you apply this definition.
   2. A $C=O$ double bond is not a carbon-carbon bond, so it does not make a compound unsaturated.

4. Every alkane is saturated, because alkanes contain only $C-C$ and $C-H$ single bonds.

5. Propane, $C_3{H}_8$, has two carbon-carbon bonds and both are single, so propane is saturated.

   $$\begin{array}{ccccccccc}& & H& & H& & H& & \\ & & |& & |& & |& & \\ H& -& C& -& C& -& C& -& H\\ & & |& & |& & |& & \\ & & H& & H& & H& & \end{array}$$

6. Methane, ${CH}_4$, has no carbon-carbon bond at all, and it is still described as saturated because every bond in it is single.

7. Saturated compounds are not only hydrocarbons.

   1. Ethanol, ${CH}_3{CH}_{2}OH$, has one $C-C$ single bond, so it is saturated.
   2. Ethanoic acid, ${CH}_{3}COOH$, has one $C-C$ single bond and a $C=O$ bond, so it is saturated too.

Unsaturated compounds

1. An unsaturated compound has molecules in which one or more carbon-carbon bonds are not single bonds.

2. At this level the bond that is not single is almost always a carbon-carbon double bond, $C=C$.

3. The two carbon atoms in a $C=C$ bond each use two bonds on each other, so each is joined to fewer atoms than it could hold.

4. Every alkene is unsaturated, because the $C=C$ bond is the functional group of the alkenes.

5. One double bond is enough.

   1. Propene, $C_3{H}_6$, has one $C-C$ single bond and one $C=C$ double bond.
   2. Because one of its carbon-carbon bonds is not single, the whole compound is unsaturated.

6. The displayed formula of propene shows the single bond on the left and the double bond on the right.

   $$\begin{array}{ccccccccc}& & H& & H& & H& & \\ & & |& & |& & |& & \\ H& -& C& -& C& =& C& & \\ & & |& & & & |& & \\ & & H& & & & H& & \end{array}$$

Classifying compounds

1. To classify a compound, find every carbon-carbon bond and decide whether each one is single.
   1. If all of them are single, the compound is saturated.
   2. If at least one is a double bond, the compound is unsaturated.
2. A displayed formula makes this easiest, because a double bond appears as two lines between two carbon symbols.
3. A written formula such as ${CH}_2={CHCH}_3$ shows a double bond with an $=$ sign between two carbon groups, while ${CH}_3{CH}_2{CH}_3$ has none.
4. For a hydrocarbon you can also use the molecular formula.
   1. A hydrocarbon that fits $C_n{H}_{2n+2}$, such as $C_4{H}_{10}$, is an alkane and is saturated.
   2. A hydrocarbon that fits $C_n{H}_{2n}$, such as $C_4{H}_8$, is an alkene and is unsaturated.
   3. The alkene has two fewer hydrogen atoms than the alkane with the same number of carbon atoms, because two of its carbon bonds are used in the double bond.
5. For a compound that contains oxygen, such as an alcohol or a carboxylic acid, look at the carbon-carbon bonds and ignore any $C=O$ or $O-H$ bonds.